The Alkaloids: Chemistry and pharmacology Volume 34

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THE ALKALOIDS Chemistry and Pharmacology VOLUME 34

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THE ALKAL Chemistry and Pharmacology Edited by Arnold Brossi National Institutes of Health Bethesda, Maryland

VOLUME 34

Academic Press, Inc. Harcourt Brace Jovanovich, Publishers

San Diego New York Berkeley Boston London Sydney Tokyo Toronto

COPYRIGHT

0 1988 BY ACADEMICPRESS, TNC.

ALL RIGHTS RESERVED NO PART OF THIS PUBLICATION MAY B E REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

ACADEMIC PRESS, INC . San Diego, California 92101

United Kingdom Edition published by ACADEMIC PRESS LIMITED 24-28 Oval Road. London NW1 7DX

LIBRARYOF CONGRESS

CATALOG CARD

ISBN 0-12-469534-5

(alk. paper)

PRINTED IN THE UNITED STATES OF AMERICA

88 89 YO 91

9 8 7 6 5 4 3 2

I

NUMBER: 50-5522

CONTENTS

Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii ix

Chapter 1. Chemistry and Reactions of Cyclic Tautomers of Tryptamines and Tryptophans NAKACAWA TOHRU HINOAND MASAKO I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

...................... III. Cyclic Tautomers of Tryptophan-Containing Dipeptides . . ... IV. 3a-Hydroxypyrrolo[2,3-b]indoleDerivatives. ............................

1 4 17 18

V. 3a-Bispyrrolo[2,3-b]indole Alkaloids: Dimeric, Trimeric, Tetrameric, and Pentameric Tryptamines ............................... VI. 3a-Prenylpyrrolo[2,3-b]indolesand Related Alkaloids .................... VII. Other Pyrrolo[2,3-b]indoles .... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41 49 65 69

11. Cyclic Tautomers of Tryptamines and Tryptophans

Chapter 2. Alkaloids in Cannabis saliva L. RAPHAEL MECHOULAM

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Quaternary Bases, Amides, and Arnines . .

III. Spermidine Alkaloids ............................ . . . . . . . . . . . . . . . . . . . IV. Synthesis of Cannabinoid Spermidine Alkaloids ........................ V. Pharmacology ......................................................

...........................

77 79 80 83 91 92

Chapter 3. Aconitum Alkaloids AND HIDEO BANDO TAKASHI AMIYA

....... .... ...... .................................... 111. Pharmacology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Analytical Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Tabulation of New Diterpenoid Alkaloids. ............................. References . . s

V

95 96 126 132

133 174

vi

CONTENTS

Chapter 4. Protopine Alkaloids TAKAHASHI MASAYUKI ONDAAND HIROSHI I. 11. 111. IV. V. VI. VII. VIII. IX.

Introduction ....................................................... Occurrence ........................................................ Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conformation and Spectroscopy. . . Synthesis .......................................................... Transformation of Protopines to Related Alkaloids ..................... Biosynthesis ....................................................... Callus Culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pharmacology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Addendum .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

181 182 182 190 194 198 201 202 203 203 204

Chapter 5. African Sfrychnos Alkaloids AND CL~MENT DELAUDE GEORCES MASSIOT

Introduction ....................................................... Ethnobotany ....................................................... ... Chemical Scree ’ ......................... Alkaloid Conte ... Biosynthesis an ion. ............................. VII. Synthesis and Chemistry. ............................................ ........................................... VIII. Pharmacology. . . . . IX. Conclusion ........................................................ References ......................................................... I. 11. 111. IV. V.

21 1 215 217 218 288 301 305 319 321 322

Chapter 6 . Cinchona Alkaloids AND THEOVAN DER LEER ROBERT VERPOORTE, JANSCHRIPSEMA,

Introduction ....................................................... Isolation ............ ........................................... Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spectroscopy ....................................................... ............ Chromatography . . Biological Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metabolism Biosynthesis .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biotechnology ........................... References .........................................................

332 333 344 358 37 1 376 378 382 389 391

Cumulative Index of Titles.. .............................................. Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

399 405

I. 11. 111. IV. V. VI. VII. VIII. IX.

CONTRIBUTORS

Numbers in parentheses indicate the pages on which the authors’ contributions begin.

TAKASHI AMIYA(99, Hokkaido Institute of Pharmaceutical Sciences, 7-1 Katsuraoka-cho, Otaru, 047-02, Hokkaido, Japan HIDEOBANDO(99, Hokkaido Institute of Pharmaceutical Sciences, 7-1 Katsuraoka-cho, Otaru, 047-02, Hokkaido, Japan (21 l), Faculte de Pharmacie, Universite de Reims, Reims, CLAMENT DELAUDE France TOHRUHINO(l), Faculty of Pharmaceutical Sciences, Chiba University Yayoi-cho, Chiba-shi 260, Japan GEORGES MASSIOT (21 I), Faculte de Pharmacie, Universite de Reims, Reims, France RAPHAELMECHOULAM (77), Department of Natural Products, Faculty of Medicine, Hebrew University, Jerusalem 91 120, Israel MASAKONAKAGAWA (l), Faculty of Pharmaceutical Sciences, Chiba University, Yayoi-cho, Chiba-shi 260, Japan MASAYUKIONDA(1 81), School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan JAN SCHRIPSEMA (33 l), Department of Pharmacology, Center for BioPharmaceutical Sciences, Gorlaeus Laboratories, University of Leiden, 2300RA Leiden, The Netherlands HIROSHITAKAHASHI (18 1), School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan THEOVAN DER LEER(331), Department of Pharmacology, Center for BioPharmaceutical Sciences, Gorlaeus Laboratories, University of Leiden, 2300RA Leiden, The Netherlands ROBERTVERPOORTE(33l), Department of Pharmacology, Center for BioPharmaceutical Sciences, Gorlaeus Laboratories, University of Leiden, 2300RA Leiden, The Netherlands

vii

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PREFACE

Indole alkaloids represent a major class and were reviewed in a general fashion in Vols. 2 (1952) and 7 (1960) of this treatise, before they were broken up into subgroups. The chapter on “Chemistry and Reactions of Cyclic Tautomers of Tryptamines and Tryptophans” (not including physostigmines) discusses in detail the chemistry of the tricyclic alkaloids derived from biologically important indole precursors, which occur in plants, fungi, and mammals. “Alkaloids from Cannabis sutivu L.,” the source of the cannabinoids present in hashish, are minor constituents of little-known pharmacological actions and are presented here for the first time. The chapter on ‘2conitumAlkaloids” updates information already collected in Vols. 4 (1954), 7 (1960), 17 (1979), and 18 (1981) of this work and summarizes pharmacological and toxicological data on these alkaloids used in herbal compositions in Japan and in China. “Protopine Alkaloids” were first presented in Vol. 4 (1954) and later repeatedly referred to under the title “Papaveraceae Alkaloids” in Vols. 10 (1967), 12 (1970), 15 (1975), and 18 (1981). The information collected here updates the material presented in earlier reviews. More than 240 alkaloids isolated by the end of 1987 from African Strychnos are listed in the chapter on “African Strychnos Alkaloids,” which reviews the biochemistry, chemistry, and pharmacology of these interesting indole alkaloids. This chapter updates material discussed in Vols. 5 (1955), 8 (1965), and 11 (1968) of this treatise. The medically important group of “CinchonaAlkaloids” presented in Vols. 3 (1953) and 14 (1973) is again reviewed here. In addition to chemistry, the chapter discusses important analytical details and brings the pharmacology of these alkaloids up to par. It is pleasing to note that this volume continues to benefit from material collected and presented by an international group of collaborators. Such collaboration is vital in keeping this treatise moving. Arnold Brossi

ix

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-CHAPTER1CHEMISTRY AND REACTIONS OF CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

TOHRU HINOAND MASAKONAKAGAWA Faculty of Pharmaceutical Sciences Chiba University Yayoi-cho, Chiba-shi 260, Japan

I. Introduction 11. Cyclic Tautomers of Tryptamines and Tryptophans

111.

IV.

V.

VI.

VII.

A. Formation and Stereochemistry B. Reactions C. Biological Implications and Applications D. Dehydro Derivatives Cyclic Tautomers of Tryptophan-Containing Dipeptides 3a-Hydroxypyrrolo[2,3-b]indoleDerivatives A. Naturally Occurring 3a-Hydroxypyrroloindoles B. Dye-Sensitized Photooxygenation of Tryptophans C. Other Oxidations of Tryptophans D. Reactions of 3a-Hydroxypyrroloindoles 3a-Bispyrrolo[2,3-b]indole Alkaloids: Dimeric, Trirneric, Tetrarneric. and Pentameric Tryptamines A. Chimonanthine, Folicanthine, and Calycanthidine B. Hodgkinsine, Quadrigernines, and Psychotridine C. 3a-Bispyrrolo[2,3-b]indole Alkaloids Derived from Diketopiperazines D. Tryptophan Dimer Having C-3-N" Linkage 3a-Prenylpyrrolo[2,3-b]indolesand Related Alkaloids A. Flustramines B. LL S490p and Azonalenine C. Roquefortine D. Amauromine E. Synthetic Approaches to Prenylated Indoles Other Pyrrolo[2,3-b]indoles References

1. Introduction

In general, indoles are known to exist in two tautomeric forms: indole (1) (1H-indole) and indolenine (2) (3H-indole). Most indoles exist overwhelmingly in the indole form. The indolenine 3 was first isolated in 1 THE ALKALOIDS, VOL. 34 Copyright 01988 by Academic Press. Inc. All rights of reproduction in any form rescrved

2

TOHRU HINO AND MASAKO NAKAGAWA

2 -

1 -

3 -

crystalline form as 2-ethoxyindole by Harley-Mason. Some other indolenines are observed in an equilibrium mixture with the indolic form ( I ) . O n the other hand, three tautomeric forms are possible in tryptamines: the indole (4), the indolenine (5), and the cyclic tautomer (6). The cyclic H

H

6 -

5 -

4 -

tautomer was not recognized for a long time. The cyclic tautomeric structure 7 was first suggested to represent folicanthine, a calycanthaceaeous

m M e H Me

7 -

alkaloid, by Hodson and Smith in 1956 (2); however, the structure was later revised to the dimeric form (see Section V). Immediately after the proposal of the structure, Sugasawa and Murayama (3) attempted to prepare 7 by Ladenburg reduction of N",Nb-dimethyloxytryptaminebut instead obtained N",Nb-dimethyltryptamine. In 1960 Witkop and coworkers ( 4 ) investigated the presence of cyclic tautomers of tryptamines in neutral solution by NMR spectroscopy in the first application of NMR to indole chemistry. They found the indolic form to be the sole tautomer.

AC

H

COCH 3

OpMe

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

3

SCHEME1

The first example of a cyclic tautomer of tryptamines appeared in 1968 when Witkop’s group (5) prepared 10 from the tryptophan (8) with chlorination followed by catalytic hydrogenation. As the cyclic tautomer of tryptamines is of the indoline type, whose chemical reactivity is different from that of the indole, Baldwin and Tzodikov (6) proposed the cyclic tautomer as a hypothetical intermediate for the enzymatic prenylation of tryptophan at the 4 position (Scheme 1). In 1978, we developed a simple procedure for preparing cyclic tautomers of type 10 directly from Nb-acyl tryptophan esters, enabling cyclic tautomers of tryptamines to be used as versatile intermediates for the preparation of tryptophan derivatives (see Section 11). The concept of the cyclic tautomer of tryptamines may also be applied to the equilibrium between 3-substituted 3-aminoethylindolenines (11) and 3a-substituted pyrroloindoles (12). The cyclic tautomer 12 is the predominant form in this equilibrium, and the indolenine form is characterized in special cases. Many indole alkaloids having a pyrrolo(2,3-bJindole ring system (12) have been isolated and characterized from plants, fungi, and animals. Y

11 -

Y

12 -

In this chapter we discuss the chemistry and reactions of cyclic tautomers (13) derived from tryptamines in the broad sense. When E is a hydrogen, 13 is a true cyclic tautomer of tryptamines. Among indole alkaloids having the ring system 13 where E is other than hydrogen, physostigmine and related

4

TOHRU HINO AND MASAKO NAKAGAWA

alkaloids have long been known, and their chemistry and physiology are discussed in previous volumes of this treatise (7). Therefore, we have excluded physostigmines from this chapter.

11. Cyclic Tautomers of Tryptamines and Tryptophans

A. FORMATION AND STEREOCHEMISTRY Tryptamines exist exclusively in the indolic form as described above. However, the addition of a proton to the indole ring (14) might form the indolenium (15), which may easily cyclize to 16. Protonation of the indole

15 -

14 -

16 -

ring at the 3 position is well known (8,9). Tryptamines in acid media, however, are first protonated at Nb when R in 14 is hydrogen or alkyl. In more acidic media (6-1 1 M H2SO4)the diprotonated form (17) is obtained instead of the cyclic tautomer (16).

A &

H

17

‘NH2

RyJ-----LR*r2 Me

18

R

Me

19 -

Physostigmine analogs (18) undergo opening of the pyrrole ring to form 19 in strong acid (6 M HCl in EtOH) (10). The basicities (pK, values) of indole rings are reported by Hinman (11)as follows: indole, -3.5; skatole, -4.55; l,2-dimethylindole7+0.30; tryptamine, -6.31. 2-Phenylindole derivatives are known to be protonated at the 3 position of the indole ring

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

5

p'

I

DL-2 1

DL-20

DL-22

+

O2Me

2Me L2Me

DL-23

DL-24

in 85% phosphoric acid (12). In order to obtain cyclic tautomers of tryptamines it is necessary to reduce the basicity of Nbbelow that of the indole ring and to retain the nucleophilicity to attack at the 2 position of the indolenine (15). The methoxycarbonyl group was found to be the desired substituent for R in 14. When Nb-methoxycarbonyl-DL-tryptophan methyl ester (DL-20) was dissolved in 85% phosphoric acid at room temperature for 3 hr, after which the mixture was added to an excess of sodium carbonate solution with cooling, the cyclic tautomer (DL-21) was obtained as stable crystals in 85% yield (13). Acids other than phosphoric such as trifluoroacetic acid are also be effective, as shown in Table I. As the cyclic tautomer has new two chiral centers, two diastereomers are possible. The other isomer (DL-22)was observed in the reaction mixture along with 21 but could not be isolated. However, two diastereomers TABLE I FORMATION OF CYCLIC TAUTOMER DL-21in Various Acid Media

Acid

85% H,P04 70% H3P0, Conc H2S04 85% H,SO4 70% H2S04 50% H2S04 85% H,SO,-MeOH 50% H,SO,-MeOH 30% H,SO,-MeOH CF3COOH HCOOH AcOH

Reaction time 3 hr 4 hr 4 hr 30 min 2 hr 3 days 1.5 hr 4 hr 10 hr 2 hr

Yield of ~ ~ - (%) 2 1 85 0 0 61 57 0 60 38 0 75 0 0

6

TOHRU HINO AND MASAKO NAKAGAWA

TABLE I1 YIELDSOF N a - A c ~ 7 CYCLIC y~ TAUTOMERS

Yield (%) Cyclization conditions

DL-23

DL-24

85% H3P04,RT, 3 hr CF,COOH, RT, 2 hr CF,COOH, RT, 60min CF,COOH, RT, 30min CF,COOH, RT, 2-3 min CF,COOH, -1o"C, 30 min

82 79 56 32 2 5

6 8 13 35 38 38

(DL-23 and DL-24) were isolated after acetylation ( 1 4 ) . The yield of N"-acetyl cyclic tautomers DL-23and DL-24varies depending on the cyclization conditions as shown in Table 11. Formation of DL-24increases under mild cyclization conditions, indicating that DL-22,its precursor, is the kinetically controlled product and, therefore, that DL-21is the thermodynamically stable one. The stereochemistry of these cyclic tautomers was determined by comparing their NMR spectra with that of the 3a-hydroxypyrrolo[2,3-b]indole, whose stereochemistry had been established by X-ray analysis (see Section IV,B) (13,14).The characteristic features of the NMR spectra of the pyrrolo[2,3-b]indole-2-carboxylicacid methyl esters are as follows: (1) the methyl signal of the 2-carboxylic acid ester in the trans isomer, with relative stereochemistry of 2-carboxylic acid and 3a substituents (OH, OAc, or H), appears at higher field than that of the cis isomer irrespective of the 3a substituents and (2) two signals are observed for the methyl group owing to hindered rotation of the amide group at the 1 position. DL-21,DL-23,and DL-24are stable as crystals and can be kept at room temperature. DL-21can be reverted to 20 on dissolving in acetic acid, but it is stable in pyridine. On the other hand, Na-acetylatedcompounds DL-23and DL-24)are stable in acetic acid but can be reverted to 20 in 10% sulfuric acid in methanol at room temperature. The pattern of ring opening of the two isomers differs: DL-23gave 20, probably via 21, while DL-24gave 20 via the N"-acetyl derivative (25), which was detected on TLC. These results - ~more ~ susceptible to ring opening than ~ Y U ~ Z S - D L - ~ ~ , indicate that c ~ s - D L is probably owing to greater steric strain in 24. However, DL-23gave 25 in less nucleophilic media (10% H2S04in AcOH). Thus, cyclic tautomers (21,23, and 24) are easily formed and can also be reverted to the indolic tautomers under mild conditions. During these ring

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

(DL-23)

7

C02Me

1O%H 2 SO4 MeOH

closure and openings the chirality of tryptophan is retained. This has been proved by the isolation of optically active L-20 from L-23, obtained from L-20 in 85% phosphoric acid. Similar acid treatment of Nb-acetyl-Ltryptophan ethyl ester (26) gave the cyclic tautomer (27), though in low yield (29%). The difference in yield may be attributed to the lower nucleophilicity of the Nh-acetyl group compared to that of Nhmethoxycarbonyl group.

A similar situation was found in the tryptamine series. NbMethoxycarbonyltryptamine (28a) in 85% phosphoric acid cyclized to 29a, which was detected by NMR but could not be isolated because of its instability. After acetylation, 30 was isolated in 70% yield. 28b, however,

a :R-OCH 3

29 -

30 -

gave mostly dimeric products with a small amount of 29b under similar conditions. Acid-catalyzed dimerization to form 31 is a well-known reaction for indole derivatives (15). Not only simple indole derivatives but also the tryptophan derivative (26) have been known to give dimeric products

8

TOHRU HINO AND MASAKO NAKAGAWA

H

R

H

H

(31), although forcing conditions were necessary for dimerization of tryptophan derivatives (16). Therefore, cyclization to the cyclic tautomer in acidic media competes with the acid-catalyzed dimerization. The likely mechanism of formation of cyclic tautomers of tryptophans is shown in Scheme 2. Protonation of the indole ring may occur from both sides to form A and B at nearly the same rate. The subsequent cyclization of B to D proceeds more rapidly than that of A to C. However, the kinetically controlled product D gradually transforms to thermodynamically stable C through equilibrium between D and C via 32 under the reaction

dimeric p r o d u c t s

t

H

i-

H LOR

H

H

A

a*&, H

COR

33 -

SCHEME2

LOR

B -

-

32 -

H

H 34 L

COR

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

9

conditions. This mechanism is supported by two facts. First, under milder conditions 34, which was isolated as the N"-acetyl derivative, was obtained as the major product. Second, the equilibrium between D and C via 32 was proved by deuterium exchange C-3a and C-8a of 33 in 85% deuterated phosphoric acid. To obtain the cyclic tautomer of 32 efficiently the nucleophilicity of Nb is important. The methoxycarbonyl group is superior to the acetyl group in imparting nucleophilicity to N b . Another factor to be considered is the competition between cyclization and dimerization. Tryptamines are more readily dimerized in acid media than tryptophans, for steric reasons. Therefore, acid-catalyzed dimerization becomes a more important side reaction in the cyclization of tryptamines. 5-Methoxy- and 5chlorotryptophans (35, X = MeO, Cl) cyclized smoothly in trifluoroacetic acid, while the 5-nitro derivative did not (17).These results suggest that a sufficient amount of the protonated form, such as A and B, is necessary to form the cyclic tautomer.

Characteristic features of cyclic tautomers of tryptamines and tryptophans are as follows: (1) protection of the reactive enamine system, which has reactivity typical of the open chain tautomer, the indolic form; (2) activation of the benzene moiety of the indole ring to the aniline derivative; and ( 3 ) facile reversion to the open chain tautomer. Application of cyclic tautomers to the synthesis of 5- or 6-substituted tryptophans is described in the next section. Protection of the reactive enamine of the indole ring is usually carried out by conversion to the indoline (18) by reduction or by N-acylation. However, more severe conditions are required to reproduce the indole form than the cyclic tautomer. For the protection of simple indoles, the sodium bisulfite adduct of indoles reported by Thesing et al. (19) is an attractive device, but few applications have been reported, probably owing to instability of the adduct. B. REACTIONS There are many naturally occurring indole alkaloids that have substituents at the benzene moiety of the indole ring. For the synthesis of these

10

TOHRU HINO AND MASAKO NAKAGAWA

natural products, substituted tryptophans or tryptamines have been prepared from substituted benzene derivatives through indole ring closure. This situation arises from the fact that electrophilic substitution of tryptamines usually occurs at the 3 position of the indole to give 2-substituted derivatives, and a practical method of introducing a substituent at a specific position of the indole ring is not known. The nitration of tryptophan at the 6 position has been reported as an exception (20-22). Cyclic tautomers are suitable intermediates for introducing a substituent at the N " , 5 , and 6 positions of tryptophans, as the benzene moiety has aniline reactivity in the cyclic tautomer and facile reversion to tryptophans.

21 -

Alkylation of cyclic tautomer DL-21 with alkyl halides in acetonepotassium carbonate at room temperature gave N"-alkyl derivatives (36) which can be converted to the tryptophan derivatives (37) in good yields (23). This N"-alkylation may serve as a general method and employs milder conditions than the known method using sodium amide in liquid ammonia (24). Chlorination of DL-23 with N-chlorosuccinimide in acetic acid at room temperature gave the 5-chloro derivative (38, X = Cl) in excellent yield accompanied by a trace amount of the 7-chloro isomer. Acid treatment of 38 (X = C1) smoothly furnished the 5-chlorotryptophan derivative (39, X = Cl). Bromination of DL-23 with N-bromosuccinimide in acetic acid and nitration with fuming nitric acid at -5°C likewise gave the 5-bromoand 5-nitrotryptophans (39, X = Br, NO,) after acid treatment of 38. The preparative value of these reactions is exemplified by the 66% yield of the 5-nitro-DL-tryptophan derivative (39, X = NO2) from Nbmethoxycarbonyl-DL-tryptophan methyl ester (DL-20) (23).

23 -

38 -

X=CI. Br. NO?

39 -

Bromination and nitration of the cyclic tautomer of tryptamine (30) also afforded 5-substituted tryptamines in excellent yields. A different feature

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

30 -

40 -

11

41 -

was observed, however, in the chlorination reaction. In contrast to the chlorination of 23 that gave 38 (X = Cl), the reaction of 30 with N chlorosuccinimide in acetic acid gave a mixture of products such as 40 (X = CI), N"-acetyl-41 (X = Cl), and 42 (23). This result is interpreted as partial ring opening of 30 under the reaction conditions caused by the chlorination reaction being slower than bromination. Structure 42 was once proposed as an intermediate in the biosynthesis of pyrrolnitrin from tryptophan (25).

Oxidation of the indoline derivative (43) with Fremy's salt, ON(SO3K),, was reported to give the indole (44) and the 5-hydroxyindole (45) (26),

43 -

44 -

45 -

12

TOHRU HiNO AND MASAKO NAKAGAWA

and 5-hydroxytryptophan (47) was obtained by the oxidation of 2,3dihydrotryptophan (46) in low yield (27). On the other hand, the pyrroloindole derivative (48) which could not be oxidized to the indole, gave the quinoneimine (49) with Fremy’s salt in good yield (28).From these results and an observation that the cyclic tautomer (23) could not be oxidized to 50

50

with palladium-carbon or DDQ (17),it was thought that oxidation of the cyclic tautomer (23) with Fremy’s salt may give the quinoneimine. The unstable quinoneimine (51) was obtained in 50% yield by Fremy’s salt oxidation of 23. The quinoneimine gave the 5-hydroxytryptophan derivative (53) by sodium borohydride reduction and acid treatment (29,30).

CO 2 Me OzMe

A more practical method, using lead tetraacetate in trifluoroacetic acid as the oxidizing agent, has been reported for the hydroxylation of various methyl benzene derivatives (31-33). Nb-Methoxycarbonyl-DL-tryptophan ester (DL-20)was dissolved in trifluoroacetic acid at room temperature to form the protonated cyclic tautomer (DL-21). This solution was added to lead tetraacetate in methylene chloride at 10°C to form the quinoneimine (51). Zinc powder was added to the solution to give the 5hydroxytryptophan (53) in 60% yield from DL-20 (29,30). This procedure was also applied to the tryptamine derivatives (54) to give the 5-hydroxy derivatives (55) in good yield. Debenzylation of 55b gave serotonin (29,30). These methods allow the first practical and selective synthesis of 5-substituted tryptophan derivatives. Since not only DL-tryptophan but also

13

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

55 -

54 -

a :u-cn3

b :R-CH2 P h

L- and D-tryptophan have become commercially available at reasonable prices, these methods may increase their preparative value. The above examples involve the reaction of DL-tryptophans, but the method is also applicable to the optical isomers. Trimethylsilyl iodide was found to be a particularly good deprotecting reagent for the Nb-methoxycarbonyl group, and several optically active 5-substituted D- and L-tryptophans have been prepared (34). On the other hand, compared to the abundance of methods for preparing 5-hydroxytryptophan derivatives, only a few are known for 6-hydroxytryptophan derivatives. Cyclic tautomers of tryptophans provide a new approach to hydroxylation at the 6 position, although the selectivity of the reaction is not so high as that of the 5-hydroxylation. For example, oxidation of DL-23with lead tetraacetate (1 equiv) in trifluoroacetic acid at 1-2°C gave a mixture of hydroxylated products. After methylation the 6-methoxy derivative (DL-56, 42%) and the 5-methoxy derivative (DL-57, 17%) were obtained, accompanied by a trace amount of the 7-hydroxy and 7-methoxy derivatives. O n acid treatment DL-56and DL-57underwent

miA

+ M ~ o = ~ &H

AcH C 0 2 M e 23

OpMe

0p M e

Me0

AcH C02Me

AcH COpMe

I "

m

Me0

C

O

2

n

M

e M

e

O

m

C

O

H

k0,Me

OOpMe

24 -

-

Me0

T H

M

e

kOOpMe

59 -

58 -

H

p

OpMe

t Ye0

H

+ OpMe

14

TOHRU HINO AND MASAKO NAKAGAWA

ring opening to provide smoothly the 6-methoxy- and 5-methoxytryptophans (DL-58and DL-59)(30,35). Similarly oxidation of DL-24, the less stable cis isomer, gave the Smethoxy derivative (DL-61,30%), the 6-methoxy derivative (DL-60,2S%), and a trace amount of the 7-hydroxy derivative (30,35). Oxidation of the optically active isomer L-23 likewise gave the 6-methoxy-~-tryptophan (L-58) as well as the 5-methoxy derivative (L-59) (36). As the 58, especially the optically active isomer, has not been readily obtainable by other methods, this oxidation may serve as a preparative method for these compounds. 6-Methoxy-~-tryptophanmethyl ester prepared by this method has been utilized as the starting material for the total synthesis of fumitremorgin B (see Section V1,D) (37,38). C. BIOLOGICAL IMPLICATIONS AND APPLICATIONS As described above, the reactivity of cyclic tautomers differs from that of the open chain isomers, the indolic forms, and the S and 6 positions of cyclic tautomers are reactive sites for electrophilic substitution and oxidation. Biological oxidation of tryptophan to 5-hydroxytryptophan by a monooxygenase is well known, and the above finding suggests that cyclic tautomers play an important role in the enzymatic reaction, although the detailed mechanism is not established. Reaction of the cyclic tautomer of tryptophan gave the S-chloro, S-bromo, 5-nitr0, and 5-hydroxy derivatives selectively, but not 6substituted tryptophans except the 6-hydroxy derivatives. For preparation of 6-bromotryptamine derivatives, which are found in many marine natural products, the S-nitro derivative (62) was used as an intermediate. Catalytic hydrogenation of 62 followed by bromination with N-bromosuccinimide in dimethylformamide gave the 5-amino-6-bromo derivative (63) as the major product. Deamination of 63 smoothly gave the 6-bromo derivative (64),

H

Br

Br

H

1. CYCLIC TAUTOMERS OF TRYETAMINES

AND TRYPTOPHANS

15

which afforded the 6-bromotryptamine (65) on acid treatment. Overall yield of 65 was 25% from the tryptarnine (54a) (39). Synthesis of flustramine B from 65 will be discussed in Section VI,A.

D. DEHYDRO DERIVATIVES The dehydro derivative (9) of the cyclic tautomer of tryptophan has been prepared from tryptophan as described in Section I. A similar dehydro derivative was prepared from melatonin by the reaction with terf-butyl hypochlorite (40). Chlorination of 9 with fert-butyl hypochlorite resulted in the unstable chloroindolenine (66), which gave the aromatic pyrrolo[2,3-b]indole (67) on treatment with sodium acetate. The delocalization energy of 67 was calculated to be 6.18 /3 units by the HMO method (5). The fully aromatized pyrrolo[2,3-b]indole is 68, and thus 67 is a 1,8dihydropyrrolo[2,3-b]indole.Ring system 67 was found in an anhydrodethiosporidesmine (see Section IV,A,l).

9 -

66 -

67 4

3

5

6

2 1

8

1

68 -

Sodium borohydride reduction of 3-hydroxyiminoethyldioxindole(69) at 10°C provided the 1&dihydropyrroloindole (70). 70 was also obtained by acid treatment of 71, which was prepared by sodium borohydride reduction of 69 at room temperature (41). Recently the 1,8-dihydropyrroloindole derivative (73) has been prepared from the 3-indolecarboxyaldehyde by reaction with methyl azidoacetate via 72 (42). -

16

TOHRU HINO AND MASAKO NAKAGAWA

N3

I

Compound 9 has been found to be resistant to hydrolysis by achymotrypsin, in contrast to other simple tryptophan derivatives (43). Catalytic oxygenation of 9 followed by reduction gave the 3a-hydroxypyrroloindole derivative (75) (5) via the hydroperoxyindolenine (74). On the other hand, dye-sensitized photooxygenation of 9 in methanol gave the benzoazocine derivative (76) (44).

. & A

-

OfMe

O2Me

H 75 -

Ac

74 -

Ac

CO 2Me UB/hv/O~

-

MeOH

Compound 73 was readily allylated to give 77, which can be rearranged to 78 by irradiation (42). In another reaction, the dehydro cyclic tautomer (9) gave the oxindole derivative (79) on heating with hydrochloric acid (45). Reaction of 9 with mercaptans, including cysteine derivatives, however, furnished 2-alkylthiotryptophan derivatives (80) (46).

17

1 . CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

111. Cyclic Tautomers of Tryptophan-Containing Dipeptides

Formation and reactions of cyclic tautomers of tryptophan can be extended to dipeptides containing tryptophan. Cyclo-L-prolyl-L-tryptophan (81) gave the cyclic tautomer (82a) in excellent yield on dissolving in 85% phosphoric acid or trifluoroacetic acid at room temperature. At lower temperatures the less stable and kinetically controlled compound (83a) became the major product. In contrast to the tryptophan series, the less stable isomer (83a) can be isolated and characterized, and, furthermore, it was found that 83a can be converted to the stable isomer (82a) in phosphoric acid at room temperature. 7

H

H

/h

6 /H

+

L/ H 85 -

& p e

H

84

H

a :5 - M e 0 b :6 - M e 0

The stereochemistry of the stable isomer (82a) differed from that of the tryptophan series (DL-21)and was confirmed by X-ray analysis of the acetyl derivative (82b) (13,47,48). Sammes and Weedon ( 4 9 ) reported the formation of one isomer of the cyclic tautomer (82, 83) when 81 was dissolved in trifluoroacetic acid. Its physical data were not consistent with either 82 or 83, but the isomer may be 82. The stereochemistry of 82a and 83a reflects the reactivity of these compounds. Acetylation of 82a in acetic anhydride in pyridine smoothly gave 82b, but that of 83a gave 83b in poor yield under the same conditions. On the other hand, 82a can be reverted to the diketopiperazine (81) in 0.1 N HCl in EtOH gradually, while the

18

TOHRU HINO AND MASAKO NAKAGAWA

conversion 83a to 81 under the same conditions was rapid. These results indicate that the less stable isomer 83a is more crowded around the N-C-N group than 82a, although Dreiding models did not show the difference clearly. Oxidation of cyclic tautomers 82b and 83b with lead tetraacetate in trifluoroacetic acid gave results similar to those of tryptophan series. The 8-methoxy derivative was obtained as the major product from 82b, while the 9-methoxy derivative was the major product from 83b. These methoxylated compounds can be readily converted to the 5-methoxy- and 6methoxy diketopiperazines (84a and 84b) on acid treatment. Furthermore, hydroxylation at the 5 position of 81 is also possible under conditions similar to those used for the tryptophan series (47,48). Facile N"prenylation of the cyclic tautomer was also reported (49). Further examination of the formation of cyclic tautomers of other diketopiperazines discloses that the stereochemistry of diketopiperazines is reflected in the formation of cyclic tautomers. trans-Diketopiperazine 86 did not form the corresponding cyclic tautomer in trifluoroacetic acid, whereas both the cis and trans isomers of cycloalanyltryptophan (87 and 88) gave the corresponding cyclic tautomers in trifluoroacetic acid (48). Under the same conditions, however, the trans isomer of cyc1o-Nmethylphenylalanyltryptophan (90) gave the cyclic tautomer but not the cis isomer (89).

86 -

87 -

88

-

0

e

IV. 3a-Hydroxypyrrolo[2,3-b]indoleDerivatives A. NATURALLY OCCURRING 3a-HYDROXYPYRROLOINDOLES The 3a-hydroxypyrrolo[2,3-b]indolering system (91) has been found in some natural products such as sporidesmines, brevianamide E, and rhazidine. This ring system may form from tryptamine by oxidation in the

19

I. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

biological system via the hydroxyindolenine or 2,3-epoxyindole intermediates (Scheme 3). 1. Sporidesmins

Sporidesmin was first isolated from Pithomyces chartarum in 1959. This toxic metabolite is known to be the cause of the animal disease called facial eczema in New Zealand (50-52). Taylor's group has conducted extensive studies on the structure of sporidesmin and many other analogs isolated from the same fungus (53-66) (Table 111). Sporidesmins possess not only the 3a-hydroxypyrrolo[2,3-b]indole ring system but also an epidithiadiketopiperazine ring system. The latter ring system had been known only in gliotoxin at the time, but other examples have been recognized since (67). Another characteristic feature of sporidesmins is the presence of a chlorine atom as well as a methoxy group on the benzene ring. Chemical

-~ -

Sporidesmin d iacetate

'H3

anhydrodethiosporldesmin

OHMe

Hoot<

103 SporidesminB acetate

anhvdrosporidesminB

20

TOHRU HINO AND MASAKO NAKAGAWA

TABLE 111 SPORIDESMINS

Name

Y

X

mP

Sporidesmin A (92) Sporidesmin B (93) Sporidesmin C

OH

S2

110-120°C (dec), benzene solvate 183°C

(94)

Sporidesmin D

H

sz

I"" OH

2SMe

Sporidesmin E (96) Sporidesmin F (97)

OH

Si

Sporidesmin G (98)

OH

(95 1

Sa

[.ID

(c,

solvent)

Ref.

-33.5" (1.1, MeOH) +6.9" (1.4, CHCI,) -27" (1.0, MeOH) +12" (0.75, CHC13) -215" (0.46, CHCII)

55 53,58 60 68

105-107"C, ethanolate 110-120"C, etherate 180- 187"C, ethanolate 65-75°C

+58" (0.11, CHCI,)

62,7I

-131" (0.065, C;ICI,)

62,63,70

148-153"C, etherate

-217" (0.023, CHCI,)

195-200°c, diacetate

-

53,54

61

69,71

degradation of sporidesmin diacetate (99) with boron trifluoride etherate produced anhydrodethiosporidesmin (loo), which gave 5-chloro-6,7dimethoxy-l-methylisatin by manganese dioxide oxidation (56). On the other hand, mild treatment of sporidesmine B acetate (101) with boron trifluoride etherate gave anhydrosporidesmin (102), which yielded DL-Nmethylalanine via 103 on alkaline hydrolysis (58).

1. CYCLIC TAUTOMERS OF TRYFTAMINES AND TRYPTOPHANS

21

The structure of sporidesmin A (92) was established by X-ray analysis (72), and its absolute configuration was determined by comparison of its

CD spectrum with that of gliotoxin (58,73). X-ray analysis of sporidesmin G (98) has been carried out to establish the conformation of the tetrasulfide bridge (69). Interconversions among the compounds cIarified the structure of other sporidesmins. Sporidesmin A (92) gave sporidesmin D (95) by sodium borohydride reduction followed by methylation. Sporidesmin E (96) gave sporidesmin A (92) on treatment with triphenylphosphine, and sporidesmin E (96) was obtained from sporidesmin A (92) on treatment with phosphorus pentasulfide-sulfur (70). Sporidesmin G (98) was converted to sporidesmin A (92) by treatment with triphenylphosphine and to sporidesmin D (95) by reduction with sodium borohydride followed by methylation (65). Sporidesmin A (92) Sporidesmin E (96)

1 NaBHJ

Ph,P

sporidesmin D (95) sporidesmin A (92)

u

-

P2SsrSx

Sporidesmin G (98) Sporidesmin G (98)

Ph,P

1 NaBHj

sporidesmin A (92) sporidesmin D (95)

The detailed biosynthetic pathways of sporidesmines are not yet clear, but tryptophan, alanine, and methionine were found to be incorporated (74). Furthermore, hydroxylation at the tryptophan side chain was found to occur with retention of configuration (75).

a * ax&opeH

Ac

H

Ac

02Me

22

TOHRU HINO AND MASAKO NAKAGAWA

For the synthesis of sporidesmins there are two problems: formation of 3a-hydroxypyrroloindoles and preparation of epidithiadiketopiperazines. The 3a-hydroxypyrroloindole ring system (105) was first prepared by lithium aluminum hydride reduction of the dioxindole oxime (104) (76). Catalytic oxygenation of the pyrroloindole (9) followed by reduction gave the 3a-hydroxypyrroloindole (75) (5). A biomimetic conversion of the tryptamine (106) to the 3a-hydroxypyrroloindole (107) was accomplished by photoinduced oxygenation with aromatic amine N-oxide, a model reaction of monooxygenases (77,78). Furthermore, peracetic acid oxidation of tryptophan has been reported to give the 3a-hydroxypyrroloindole (108) (79). Dye-sensitized photooxygenation of tryptophans also gave the 108 after reduction of the hydroperoxide. This is discussed in Section IV,B.

mZwH -a *COOH

H

H

H

The simple epidithiadiketopiperazine (11 1) was first prepared by substitution of the bromodiketopiperazine (109) with potassium thioacetate followed by hydrolysis and oxidation (80). Two other devices to form Br

SAC

0

111

110 __

109

epidithiadiketopiperazines were reported using activated diketopiperazine (112). The first is the reaction of a carbanion with sulfur monochloride (112 + 113) (81,232).The second is decarboxylative C-S bond formation by potassium carboxylate (114) with sulfure monochloride (83). Epidithiadiketopiperazine (117) was prepared from the diketopiperazine (115) Me

0

Me

Me

112 C K 0 2 C f X

0

Me

02K

114

o&T

113

s2C12

Me

111

116

2) ‘2

117 -

Me0

Me0

NaH 123 -

Me

Me0 124

125 -

heat

Me0

127 -

1) HC 1 -CF3 COOH

1) D I BAL

1)NaOH

2) mCPBA ___)

3)BF3- E t 2 0

sporidesminA 92 -

24

TOHRU HINO AND MASAKO NAKAGAWA

via 116 by reaction with sodium amide-sulfur followed by reduction and oxidation (84). These epidithiadiketopiperazines are labile under oxidative , reductive, and basic conditions. Therefore, the epidisulfide bridge should be constructed in the last stage of synthesis of sporidesmins. Furthermore, C-S bond formation requires rather drastic conditions as shown above. Kishi and co-workers developed a protective group for the disulfide (85,86) and succeeded in the synthesis of sporidesmins A and B (87,88). The diketopiperazine part (121) of sporidesmins was prepared as follows. 1,6-Dimethylpiperazine-2,5-dione (118) was protected with a methoxymethyl group, and the thiol derivative was formed by a method similar to that of Trown. Protected epidithiadiketopiperazine (121) was obtained by reaction of 120 with the trithiane derivative of anisaldehyde. The indole part (127) of sporidesmin was prepared from 6,7-dimethoxyindole (122) as follows. 5-Chloro-6,7-dimethoxyindole(125) was prepared from 122 by chlorination, methylation, and reduction. Treatment of 125 with oxalyl chloride followed by heating gave the indole-3-carbonyl chloride (127). The carbanion prepared from 121 and butyllithium was treated with the acid chloride (127) at low temperature to give the ketone (128) as a mixture of diastereoisomers. Removal of the protective group at nitrogen gave the ketone (129), which can be separated into two isomers. Stereoselective reduction of 129a with DIBAL followed by acetylation provided the acetyl compound (130). Oxidative cyclization of 130 to the 3a-hydroxypyrroloindole 131 was carried out with iodosobenzenediacetate. Final steps to racemic sporidesmin A (92) were alkaline hydrolysis of the acetate in 131, oxidation to the sulfoxide, and deacetalization with Lewis acid to form the disulfide bridge. Racemic 131 was identical to a sample prepared from the natural product (87). Sporidesmin B was synthesized from the acetate 130. Reductive removal of the acetoxy group in 130 was achieved with sodium cyanoborohydride to give 132. Oxidative cyclization of 132 was carried out with benzoylperoxide in the presence of a radical inhibitor to give the benzoate (133) and the minor diastereomer. Hydrolysis of 133 followed by deprotec-

1. CYCLK TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

25

tion of the thioacetal by a way similar to the synthesis of sporidesmin A gave racemic sporidesmine B (93) (88).

2. Brevianamide E Inverted prenylated cyclo-L-prolyl-L-tryptophan derivatives were isolated from Penicillium brevi-compactum and named brevianamides A , B , C, D, E, and F (89,90). Among these compounds, only brevianamide E has the 3a-hydroxypyrroloindole ring system. The parent structure, cyclo-L-prolyl-L-tryptophan (135) was isolated as a minor component (brevianamide F). Brevianamide E (134) was isolated as colorless glass, [.ID -30" (EtOH). The structure of brevianamide E was determined by spectral data and biogenetic considerations. Reduction of brevianamide E (134) with zinc in acetic acid gave deoxybrevianamide E (136), which was also isolated from Aspergillus ustus (91) and considered to be the precursor. The stereochemistry and absolute configuration of brevianamide E (134) were later determined by synthesis. Tryptophan, proline, and mevalonic lactone were shown to be incorporated into brevianamide E (89,90).

brevianamideF

135

brevianamideE 134 -

deoxvbrevianamide E 136 -

Synthesis of brevianamide E (134) has been reported by Kametani's group, who applied dye-sensitized photooxygenation to form the 3ahydroxypyrroloindole ring system (92). The indole part (140) of brevianamide E, which has an inverted prenyl group at the 2 position, was prepared by modification of the known method (93) (see Section VI,E,2).

.. H

H

1 38

eme2

CH20-Me2NH v

AcOH

140

139 -

26

TOHRU HINO AND MASAKO NAKAGAWA

Reaction of indole with succinimide-2-(3,3-dimethylallyl)ethylsulfonium chloride gave the sulfonium salt (137), which was converted to 138 by the thio-Claisen rearrangement. Reductive removal of the ethylthio group in 138 and subsequent Mannich reaction gave 140. The diketopiperazine part 143 was prepared from L-proline as follows. (2)-L-Proline chloride was condensed with dimethyl aminomalonate to produce the prolylaminomalonate (141), which gave the diketopiperazine (143) as a single isomer by deprotection of the 2 group and heating in the presence of 2-pyridone. Condensation of 143 with 140 by means of sodium hydride gave two isomers of the diketopiperazine (144), both of which yielded deoxybrevianamide E (136) and its isomer (145) on removal of the ester group by the alkaline hydrolysis followed by heating. The desired isomer (136) was obtained as the minor product (26%), and the stable transdiketopiperazine (145) was obtained as the major product. Dye-sensitized photooxygenation of 136 at -10°C gave brevianamide E (134) (major product) and its isomer (146). This synthesis proved the stereochemistry and absolute configuration [(4a-S, 5a-R, 10a-S, lla-S)]. Deoxybrevianamide E (136) has been also prepared by Ritchie and Saxton (94).

-

l)RB/hu/02 MeOH

7

2) M e 2 S

a

6

OH

OH

brevianami d e E

134

3. Rhazidine Rhazidine was isolated from the bark of Aspidosperma quebrancho blanco as a major alkaloid (95). The same alkaloid was also isolated from Rhazya stricta (96) and Gonioma kamassi (97). Rhazidine is an example of indolenine-pyrroloindole tautomerism. The structure of rhazidine base

1 . CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

quebrachamine

27

rhazidine base 147 -

rhazidine s a l t

I48 -

(147) (formally known as rhazidigenine) has been proved by synthesis from quebrachamine by peracid oxidation. Addition of acid caused the free base to be converted to the quaternary salt, rhazidine salt (148). As both the free base and the salt were isolated as crystals, and their IR and NMR spectra are different, the tautomeric nature of the two compounds was not clear in earlier studies (96). The melting points of rhazidine base (147) and rhazidine chloride (148, X = Cl) are 187 and 285°C (dec), respectively. Equilibrium between the 3a-hydroxypyrroloindole and the hydroxyindolenine is shown by the UV spectra and optical rotation (95). The UV spectrum of rhazidine salt in ethanol (Amax 236 and 293 nm) shows the presence of a Ph-N-C-N+ chromophore but changes to that of 3-hydroxyindolenine (A,,,, 220, 283, 293, and 307 nm) on addition of strong base. The UV spectrum of the free base (147) is of the hydroxyindolenine type in heptane, but changes to that of a Ph-N-C-N+ type in ethanol. The specific rotation of rhazidine salt in ethanol is -37", which changes to -612" on addition of strong base.

B. DYE-SENSITIZED PHOTOOXYGENATION OF TRYPTOPHANS Biological oxidation of tryptophan by dioxygenases to furnish formylkynurenine (153) has been studied extensively by Hayaishi's group (98). Despite intense interest in the mechanism of this oxidation, there remains much to be resolved. Hydroperoxyindolenine (149) has been suggested as the primary intermediate (99,100). Three possible pathways have been proposed for the transformation of 149 to formylkynurenine (153), as shown in Scheme 4 (101,202). The hydroperoxyindolenine (149) may tautomerize to the cyclic tautomer (154), which is the more stable form. To shed light on the mechanism of biological oxidation of tryptophan to formylkynurenine from the viewpoint of organic chemistry, the preparation and reactions of the hydroperoxyindolenine (149) and the hydroperoxypyrroloindole (154) have been studied in the authors' laboratory. Dye-sensitized photooxygenation was the method of choice, because the mild oxidation conditions facilitate isolation of 149 or 154 and molecular oxygen is the oxidizing agent. In the dye-sensitized photooxygenation

28

TOHRU HINO AND MASAKO NAKAGAWA

I formylkvnurenine

153 SCHEME 4

method a substrate solution is irradiated with visible light in the presence of oxygen and sensitizer (103). Jn general, the reactive species of this oxidation is singlet oxygen,'02. The dye-sensitized photooxygenation of simple indole derivatives has been examined. The primary intermediate is the hydroperoxyindolenine, which in some cases has been isolated and characterized, and it transforms to various products (103-106).

rnR

Rsens/hu/oi

H

-e;R 0

+

other

products

H

1. N-Methyltryptamine Dye-sensitized photooxygenation of Nb-methyltryptamine (155) in benzene-methanol in the presence of Rose Bengal and oxygen at around 0°C gave the unstable hydroperoxide (156), instead of the hydroperoxy-

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS OOH

I

29

r,

indolenine, as characterized by its spectral data (MS, NMR, UV). The hydroperoxide (156) was transformed to the oxazinoindole (158) via the N-oxide (157) on standing in methylene chloride, while the hydroxide (159) was obtained on reduction with sodium borohydride. Oxidation of the hydroxide (159) with rn-chloroperbenzoic acid gave the oxazinoindole (158), and this result support the intermediacy of the N-oxide (157) in the transformation of 156 to 158 (107,108). Rapid Meisenheimer rearrangement of this type of N-oxide was also observed in the formation of geneserine, a Calabar bean alkaloid (109). As the formation of 158 and 159 in the oxidation of 155 was observed even in the presence of a radical inhibitor, singlet oxygen may be the reactive species of the reaction (107,108). On the other hand, indole has been shown to be a good quencher of excited Rose Bengal (RB*), and a cation radical of the indole ring might be an intermediate to 156 and 159 (110). The 2,3 bond-cleaved product, the ketoamide (160), was not isolated from the reaction mixture owing to instability, since 160 has a 0-ketoamine moiety. To isolate ketoamides corresponding to formylkynurenine, the nitrogen atom at the side chain must be protected.

2 . Nb-Methoxycarbonyltryptamine Dye-sensitized photooxygenation of Nb-methoxycarbonyltryptamine (161a) in methanol at 0°C gave the hydroperoxide (163a) in 40% yield. The cyclic tautomer (163a) is more stable than the hydroperoxyindolenine (162a), though Nb is acylated. The hydroperoxide (163a) is stable at low temperature but decomposes at room temperature (111).

30

TOHRU HINO AND MASAKO NAKAGAWA

The methylperoxide (166) was obtained as stable crystals, mp 9191.5"C7 on methylation of 163a with diazomethane. This is the first example of a crystalline peroxide in this series (112). When the reaction mixture of this oxidation was reduced with sodium borohydride, the hydroxide (164a) was obtained in 70% yield, but the ketoamides (167a and 168a) were not isolated. This indicates that oxidation proceeds mostly to 163a via 162a. The hydroperoxide (163a) was readily transformed to two ketoamides (167a and 168a) as well as the hydroxide (164a) on treatment with silica gel in methylene chloride. In contrast to the oxidation of Nb-methyltryptamine, 163a gave the formylkynurenine type of compound (167a). However, the Nb-formylated compound (168a) was obtained as the major product. Another characteristic reaction of the hydroperoxide (163a) is BaeyerVilliger rearrangement with methanolic hydrogen chloride to the pyrrolobenzo-l,4-oxazine derivative (169a) (111). Similarly, the 174-oxazine derivative was obtained by the dye-sensitized photooxygenation of tryptamine hydrochloride (113). 3a-Hydroxy-l,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole (165), the parent compound, was obtained by alkaline hydrolysis of the hydroxide (164a) (111). This compound could not be obtained by direct photooxygenation of tryptamine followed by reduction (112).

Dye-sensitized photooxgenation of Nb-methoxycarbonyl-m-tryptophan methyl ester (161b) in methanol gave similar results (114,115). However, the hydroperoxide (163b) and the hydroxide (164b) were obtained as a

1 . CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

31

mixture of two diastereoisomers. The hydroxide (164b)can be separated to yield the trans (170)and cis isomers (171), and the stereochemistry was confirmed by X-ray analysis of 170 (114-1l6). The hydroperoxide(l63b) also decomposed to 164b,167b,and 168b on treatment with silica gel. The L- and D-tryptophan derivatives (161b)similarly gave optically active 164b, 167b,and 168b (115). OH

OH

3. Substituted Tryptamines Dye-sensitized photooxygenation of Nb-methoxycarbonyl-Nb-methyltryptamine (172a)gave only the ketoamide (176a),as participation of N b to form the pyrroloindole ring was not possible (111). Similar oxygenation of the Na-methyl derivative (172b) at low temperature (-70°C) gave the hydroperoxide (175),which was not transformed to 176b on treatment with silica gel but gave the hydroxide (177). On the other hand, on similar oxidation at 5-10°C produced the ketoamide (176b) (40%) and the hydroxide (177) (14%). The ketoamide (176b)may arise from 173 via 174 and not from 175 (111). This temperature dependence of the oxidation has been examined in detail (117,118).

r

a :R'=H.

\

R2=GH3

b :R'=CH 3.

R2-H

I

I OH

C02Me

177

32

TOHRU HINO AND MASAKO NAKAGAWA

Dye-sensitized photooxygenation of tryptophols has been thoroughly examined by the Saito and Matsuura group (117-119), and similar results were obtained to those of the N”-methyltryptamine (172b) and 161. Transformation of the hydroperoxide (163) to the Na-formylketoamide (167) and the Nb-formylketoamide (168) has been explained by combining the results of the oxidation of Nb-methoxycarbonyltryptamines and tryptophols, as follows (120). The homolytic cleavage of the 0-0 bond in the hydroperoxide (163) may produce an oxygen radical which undergoes p-scission to give the eight-membered ring (180) via 179. Cleavage of the eight-membered ring (180) may give rise to 167 (arrow a) and 168 (arrow b). Ionic cleavage, as shown in 181 is also possible to form 180. These reaction paths explain the formation of 167 and 168, but there is no reasonable explanation why 175b does not give 176.

178 -

163 -

QH 167 + -

n,

~ ont y tryptophan but also me .tonin , 82a) ..as been shown to be oxidized to the ketoamide in biological systems, especially in the brain. Me0

,

m s R 1 8H2

r o @ H ]

183

184 H

COR

__ n :R-CH

b :R-OMe

187

H 186

COR

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

33

Dye-sensitized photooxygenation of melatonin (182a) in methanol at -70°C gave the hydroperoxide (184a) which could not be isolated owing to instability but gave the hydroxide (187a) on reduction. The hydroperoxide (184a) transformed to the N-formylketoamide ( M a ) on standing at room temperature in the reaction mixture, but not to Nb-formylketoamide. These results may be explained by the facts that the methoxy group at the 5 position increased the basicity of N" and the cleavage by route b in 180 is depressed. Similar oxidation of 182a at 0°C gave the ketoamide (186), which was produced from the hydroperoxide (184a) or the dioxetane (185). Oxidation of the Nb-methoxycarbonyl derivative (182b) gave similar results (121). Dye-sensitized photooxygenation of 5-methoxytryptamines (182) proceeded at -70°C as above. That of 5-nitrotryptamine (188), however, was very slow, even at room temperature, to give the hydroperoxide (189) (122),indicating that the dye-sensitized photooxygenation has electrophilic character as expected.

188 __

189 -

4. Ring-Chain Tautomerism of the Hydroperoxyindolenine The above examples show that the 3a-hydroperoxypyrroloindole was obtained by dye-sensitized photooxygenation of tryptamine derivatives, but the open chain tautomer, the hydroperoxyindolenine, was not isolated. Direct evidence of ring-chain tautomerism of the hydroperoxypyrroloindole was obtained by dye-sensitized photooxygenation of the 2tert-butyltryptamine(190). When 190 was oxygenated in methanol as above, the hydroperoxyindolenine (191) was isolated as crystals in excellent yield. The spectral data (UV, NMR, and MS) show the hydroperoxyindolenine structure. However, the hydroperoxyindolenine (191) tautomerized to the hydroperoxypyrroloindole (192) on standing in methylene chloride. The cyclic tautomer (192) could be isolated as on oil. The NMR spectrum of 191 in deuteriochloroform after 3 days at 25°C disclosed that the ratio of 191 to 192 was 3 :2. Furthermore, reduction of 191 with dimethyl sulfide provided the alcohol (193) as crystals, which gave the cyclic tautomer (194) on refluxing in methylene chloride (120,123). The NMR spectrum of 193 in deuteriochloroform after 6 days at 25°C showed a ratio of 193 to 194 of 15:85. The same ratio was obtained from 194 after

34

TOHRU HINO AND MASAKO NAKAGAWA

standing in the same solvent for 3 days at 25°C. On the other hand, the equilibrium between 193 and 194 was found to have the ratio of 3 : 1 in deuteriomethanol. These results show the occurrence of tautomerization between 191 and 192, although it may occur only slowly at room temperature. The presence of the 2-tert-butyl group prevents facile cyclization of 191 and destabilizes the cyclic tautomer (192). This is the second example supporting the presence of tautomerism between hydroxyindolenines and 3a-hydroxypyrroloindoles (see Section IV,A,3), and tautomerism of this type may be general, although the cyclic tautomer is usually preferred form (120).

5. Tryptophan Since the early 1950s, when the first report (124) on methylene bluesensitized photooxygenation of tryptophan appeared, this technique has been developed as a tool for the elucidation of the active site of enzymes and reasons for the color change of white silk to yellow on exposure to light, and as a model reaction for the dioxygenase-catalyzed reaction of tryptophan to formylkynurenine. There is, however, considerable variation in the earlier results (104,125), and a complicated mixture of products was obtained in which kynurenine (126), formylkynurenine (127), or dioxindolylalanine (128) was detected or isolated. Improvement of experimental techniques in dye-sensitized photooxygenation, especially cutting off the shorter wavelength by a liquid filter and temperature control, eliminated complicated side reactions and gave clearer results, as described in previous sections. Oxygenation of DL-tryptophan in aqueous solution by irradiation (A > 490 nm) at 0 4 ° C in the presence of Rose Bengal and oxygen gave the 3a-hydroperoxypyrroloindole (154) as a mixture of two diastereomers in 85% yield after careful separation. The hydroperoxide (154) was stable

35

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

& T I -

149

+

QJ----j H

OOH H

196 153

at low temperature (-70°C) as a solid but decomposed rapidly at room temperature. The hydroperoxyindolenine (149), proposed as an intermediate of the biological oxidation, could not be isolated, but the cyclic tautomer (154) was obtained. Immediate reduction of the oxygenation mixture with dimethyl sulfide gave a mixture of alcohols (195 and 196) in 85% yield, and formylkynurenine (153) was not detected. The alcohols were readily separated into trans (195) and cis isomers (196) by fractional crystallization. The stereochemistry of 195 was established by direct comparison with a sample obtained by hydrolysis of trans-Nb-methoxycarbonyl methyl ester 170, whose stereochemistry had been confirmed by X-ray analysis (see Section IV,B,2) (124,115). The same 3a-hydroxypyrroloindoles (195 and 196) were obtained by peracid oxidation of tryptophan (79). L- and D-Tryptophan gave similar results to yield optically active 195 and 196. The hydroperoxide (154) transformed to formylkynurenine (153) under varying conditions of heat, metals, and buffers. The most favourable condition for this transformation was dissolving the hydroperoxide (154) in sodium carbonate-acetic acid buffer (pH 7.0) for 10 min (129,130) to give 153 in 60-70% yield. In phosphate buffer (pH 7.2-8.0) 154 gave 153 (40%), 195, and 196 after 20 hr at room temperature. In HEPES buffer (pH 7 . 3 ) , however, the hydroperoxide (154) afforded the hydroxides (195 and 196) as the major products and formylkynurenine (153) as a minor product. Furthermore, chemiluminescence was observed with the formation of 153, when the hydroperoxide (154) was heated in dimethyl sulfoxide solution. It is interesting to note that Nb-formylkynurenine (197) or kynurenine was not isolated in these transformations of 154. Therefore,

36

TOHRU WINO AND MASAKO NAKAGAWA

the eight-membered ring intermediate proposed for the transformation of the Nb-acyl-3a-hydroperoxypyrroloindole(see Section IV,B,3) may not be effective for the transformation of 154 to 153. Another mechanism for the transformation of 154 to 153 can be considered. The hydroperoxide (154) may tautomerize to the open chain hydroperoxide (149) in the reaction medium, and the hydroperoxyindolenine (149) may cyclize to the dioxetane (150), which decomposes to formylkynurenine (153). The presence of an equilibrium between 3ahydroperoxypyrroloindole (154) and the hydroperoxyindolenine (149) was discussed in the previous section. Moreover, the transformation of 154 to 153 was dependent not on the p H of the buffer but on the properties of the buffer. In sodium carbonate-acetic acid buffer, the open chain tautomer (149) may be stabilized by prevention of recyclization owing to interaction between the amino group in 149 and carbonate ion (formation of a carbamic acid type of compound), and the dioxetane formation may be preferred. Thus, the best yield of 153 from 154 was obtained in sodium carbonate-acetic acid buffer. Support for this hypothesis was obtained by dye-sensitized photooxygenation of DL-tryptophan in sodium carbonate-acetic acid buffer. Oxygenation of tryptophan in this buffer under similar conditions gave formylkynurenine (154) as the major product (54%), and the hydroperoxy or hydroxypyrroloindoles (154, 195, 196) were not isolated. This reaction may serve as a model for the tryptophan 2,3-dioxygenase-catalyzed reaction (129,130). Acid-catalyzed rearrangement of the hydroperoxide (154) was also observed to give o-aminophenol, probably via 198, although this latter compound couldnot be isolated (115,130). CH CH2 COOH

H

H

COOH

199

_ .

The napthalene-l,4-endoperoxide(199) was developed as a source of singlet oxygen in aqueous solutions. Reaction of tryptophan with 199 was reported to give 3a-hydroxypyrroloindoles(195 and 196) and formylkynurenine (153) (131). The p H dependency of dye-sensitized photooxygenation of tryptophan has been investigated by measuring oxygen uptake (124,132-137) and the disappearance of tryptophan (138, 139). In the pH range between 1 and 9, tryptophan was oxidized more rapidly with increasing pH. At lower pH (<1) oxidation practically did not proceed. Product analysis (138,139)

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

COOH

J k

c 9

203 __

37

H

H

ODH

202 -

showed that the hydroperoxypyrroloindole (154) was the major product of oxidation at pH 3.6-6.2 (acetate buffer) and pH 5.9-7.1 (phosphate buffer). However, further oxidation was observed when the reaction was carried out at pH 7.7-8.4 (phosphate or borate buffers). 5Hydroxyformylkynurenine (203) was obtained as the major product along with the 3a-hydroxypyrroloindoles (195 and 196) after dimethyl sulfide reduction and separation on an ion-exchange resin column. When the reaction mixture was reduced with sodium borohydride, the dihydroxypyrroloindole (202) was isolated in excellent yield. p-Quinoneimine (201) may be the first oxidation product at pH 7.7-8.4 and may be derived from the hydroperoxide (154) via 200. The dihydroxypyrroloindole (202) was stable in the solid state, but it was readily oxidized to 5-hydroxyformylkynurenine (203) in phosphate buffer (pH 7.8) by triplet oxygen (138,139).

A similar oxidation was observed for Nb-methoxycarbonyltryptamine, and the dihydroxypyrroloindole (204) was isolated in excellent yield. This dihydroxy derivative (204), however, was not oxidized to the 5-hydroxyformylkynurenine derivative corresponding to 203 in alkaline solution. These results showed that the 3a-hydroperoxypyrroloindole (154) was further oxidized to p-quinoneimine (201) in buffered alkaline solution by dye-sensitized photooxygenation. This is an example of the para hydroxylation of aniline derivatives by dye-sensitized photooxygenation. In conclusion, dye-sensitized photooxygenation of tryptophan gave different products depending on the various media; however, the first intermediate is the 3-hydroperoxyindolenine (149).

38

TOHRU HINO AND MASAKO NAKAGAWA

C. OTHEROXIDATIONS OF TRYPTOPHANS Careful study by Savige (79)of the peracid oxidation of tryptophan has disclosed the following points. (1) One equivalent of peracetic acid oxidized tryptophan to the 3a-hydroxypyrrolo[2,3-b]indole (lOS), benzo1,3-0xazine (205), and formylkynurenine (153). 3a-Hydroxypyrroloindole (108) was obtained in 40% yield as a mixture of diastereoisomers. (2) On the other hand, oxidation of tryptophan by 2-3 equiv peracetic acid gave 205 and formylkynurenine (153) but not 108. (3) Further oxidation of 108 with peracetic acid gave 205 and formylkynurenine (153), while acid hydrolysis of 108 gave oxindolylalanine (79). These observations made it clear that the primary intermediate of the oxidation is 108 and that formylkynurenine or oxindolylalanine are further reaction products of tryptophan in the peracid media (79).

\

108 -

AcOOH

, 205 ~

+

53

(2-3 mol)

Electrochemical oxidation of tryptophan has been reported to give 3a-hydroxypyrroloindole (108) , oxindolylalanine (79), dioxindolylalanine, and kynurenine (140). The y-radiolysis of tryptophan in solution was reported to give 3a-hydroxypyrroloindole (108) among other products (141,142).The oxidation of tryptophan with hydroxy radicals and superoxide anions was reported to give 108 in addition to other products (143).

D. REACTIONS OF 3a-HYDROXYPYRROLOINDOLES Further oxidation of 108 with peracid gave formylkynurenine (153) and 205 as described above. On the other hand, oxidation of 159 with mchloroperbenzoic acid gave the 1,2-0xazinoindole (158) via the N-oxide (107,108). Dye-sensitized photooxygenation of 108 in alkaline phosphate buffer (pH 8) gave 5-hydroxyformylkynurenine (203) and 3a,5-dihydroxypyrroloindole (202), depending on the reaction conditions (139) (Section IV,B,5).

39

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

6= Me

H

-

a

H

157

159 __

H

HCHO

203 -

mm

202

H

"'0

&OOH

H i

e

158

COOH

H

108 -

108

OH

H

H Oxindolylalanlne

79 -

The oxidation state of 108 is the same as the oxindole. In fact, the conversion of 108 t o oxindolylalanine (79) was readily accomplished by acid treatment (79,144). Acid hydrolysis of 108 to oxindolylalanine (79) may proceed via the hydroxyindolenine, to which water attacks at the 2 position. This allows other nucleophiles to add at the 2 position. Savige and Fontana (245,146) have reported the formation of 2-alkylthiotryptophans (206) by the reaction of 108 with alkylthiol in the presence of trifluoroacetic acid. When cysteine was used as the thiol, tryptathionine (207), an important component of the mushroom toxin phalloidin, was

206 -

108

C y s t e i ne

H

SCH27HCOOH

40

TOHRU HINO AND MASAKO NAKAGAWA

obtained. Reactions of 108 with glutathione and reduced ribonuclease gave S-tryptophanylated glutathione or protein, implying its utility in protein modification (145,146). OH

H

-

C02Me

II

H ‘SCH 2 C02Me 20 8 -

164a -

OH

+ H 165 -

H

HSCH2C02Me

-

m

N

H

C

H

O C H 2 SH SCH2C02Me

20 9 -

The reactions of 164a and 165 with methyl thiolglycolate similarly gave the 2-alkylthiotryptamines (208 and 209) (147). Utilizing this procedure, Wieland’s group (148) prepared miniphallotoxin, a model compound of phalloidine. Similar cyclic peptides containing tryptathionine have also been prepared (149-152).

2,2’-Bisindoles such as 210 have been prepared by the reaction of 164a with tryptophol or Nb-methoxycarbonyltryptamine in the presence of aluminium chloride (152). In these reactions the indole acted as a nucleophile and formed the 2,3‘ bond first; the intermediate (211) then

I.

cycLrc

TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

41

rearranged to the 2,2'-bisindoles in the presence of Lewis acid. Similar reactions were reported for the 3a-hydroxyfuranoindole (212) using various nucleophiles (152). OH

Nu

H

Lewis a c i d

212 -

V. 3a-Bispyrrolo[2,3,-b]indoleAlkaloids: Dimeric, Trimeric, Tetrameric, and Pentameric Tryptamines

As described in previous sections, oxidation of tryptamines with singlet oxygen provided 3a-hydroperoxyindolenineswhich cyclized to 3a-hydroperoxypyrroloindoles. On the other hand, one-electron oxidation of tryptamine may give a cation radical (A and B) which may lose a proton to form radicals C, D, E, and F, which are thus available by hydrogen abstraction from tryptamine. Radical F is the cyclic tautomer of the indolyl radical C (Scheme 5 ) . Radical coupling of D, E, and F will form three types of dimeric compounds, G (3a-3a), H (3a-1), and I (3a-7), all of which are found in indole alkaloids. The possible dimeric compound coupled between 3a and 5 , however, not yet been found in nature. Another type of dimer, J, which may be formed by radical addition of F to tryptamine followed by oxidation, has not yet been isolated either. AND CALYCANTHIDINE A. CHIMONANTHINE, FOLICANTHINE,

Chimonanthine (213), folicanthine (214),and calycanthidine (215)have been isolated from plants of the family Calycanthaceae, and their chemistry has been discussed in previous volumes of this treatise (153-155). Chimonanthine (213)has also been isolated from Paficourea fendleri (156) and P. dorningenis (157). Surprisingly, an enantiomer of chimonanthine, d-chimonanthine (216),has been isolated from skin of Phylfobates terribifis

42

TOHRU HINO AND MASAKO NAKAGAWA

B

A

SCHEME 5

AD H M e ! R2

R 1 : Me

i

~ 1 R. ~ - H

C h imonanth i ne

R1, R Z = M e

Folicanthine

RI-H. R 2 = M e

Calycanthidine

21 3 -

214 __ ~

215

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

“I? +280’

(

1-Chimonanthine

43

--329‘ 1

“17

216 -

d-Chimonanthine

(Colombian poison-dart frog) along with many steroidal alkaloids such as batrachotoxin (158). (t)-Chimonanthine and (t)-folicanthine were synthesized in the early 1960s, and their synthesis has been summarized (155,159). After the review by Kutney (159), (?)-folicanthine (214) and chimonanthine (213) were synthesized by dye-sensitized photooxygenation of tryptamines. As described in previous sections dye-sensitized photooxygenation of tryptamines gave oxygenated compounds in methanol or aqueous solutions. However, oxidation (with proflavin as a sensitizer) of Nb-methoxycarbonyltryptamine in formic acid gave oxidative dimers (217 and 218, 30%) along with 3a-hydroxypyrroloindole (219) and a dimer (220). Reduction of 217 with lithium aluminum hydride gave (?)-folicanthine (214) in good yield. Similar reduction of 218 gave rneso-folicanthine. On the other hand, reduction of 217 and 218 after removal of the formyl group gave (“)-chimonanthine and meso-chimonanthine (160). Oxidative dimerization of this type may proceed by electron transfer between tryptamine (ground state) and the excited sensitizer to form the tryptamine cation radical.

d3p F02Me

SKI h v 1 0 2

H

CHO C 0 2 M e

\

CHO C02Me

217 OH

21 8

Pn &,Ope

A H 0 k02Me

219

44

TOHRU HINO AND MASAKO NAKAGAWA

Similar oxidative dimerization of tryptamine occurred via oxidation with thallium tris(trifluor0acetate) in acetonitrile to give the dimer (221) (162). Although the corresponding natural product was not isolated, oxidation of 5-0-benzylbufotenine Grignard reagent (222) with ferric chloride was reported to give the bisbufotenine analog (223) which was reported to show antiacetylcholine activity (162).

221 __

223

222 -

~

B . HODGKINSINE, QUADRIGEMINES, AND PSYCHOTRIDINE Hodgkinsine (244), C33H38N6,[(Y]D + 60" (0.3 N HCl), was isolated as the major alkaloid from leaves of Hodgkinsonia frutlesens (163) and assumed to be an isomer of chimonanthine (155).Later, chemical evidence and mass spectral data (164), X-ray analysis of the trimethiodide (165), and CD spectral data (166) established the trimeric structure of the alkaloid, including the absolute configuration. The lower part of the alkaloid is similar to chimonanthine, but the configuration at the 3aposition is different but still of the meso type.

Hodgkinsine 224 -

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

Quadrisemine

45

A

a

\ I

p

F

H

?

r

i Mn e

226

e

B

Me

Tetrameric tryptamine derivatives, quadrigemine A (225) (0.001% of dry leaf weight), C44H50N6,[aID+ 32" (EtOH), and quadrigemine B (226) (0.01%), C44H50N6,mp 229-234"C, [aID+ 263" (EtOH), were isolated as minor alkaloids from the same plant by counter current fractionation. Hofmann degradation combined with mass spectral and specific rotation data gave the structures of these alkaloids as depicted (167). The absolute configuration of quadrigemine A (225) has been discussed on the basis of its optical rotation, comparing it with that of hodgkinsine and chimonanthine, and it has been concluded tentatively that quadrigemine A is a mixture (1: 1) of quadrigemine A [3 : 3' : 3" : 3"' = (R,R,S,R)]and another isomer [(R,R,S,S) or (R,S,R,S)]. The configuration of chiral centers in quadrigemine B is not established yet (167). Psychotridine (227), C55H62N10, mp 180-181"C, [.ID -38" (0.9, CHCl,), has been isolated from leaves of Psychotria beecurioides Wernh. (Rubiaceae). Its mass and 13C-NMR spectral data and Hofmann degradation studies, including comparison with those of hodgkinsine, disclosed the structure as a pentamer of tryptamine. The stereochemistry, however, has not been determined (168). Isopsychotridine C (228), C55H62N10,[.ID +183.3" (0.05, EtOH), was isolated from Psychotria forsteriana A. Gray Me

Psychotridine

227

a * H

Me

H

18ousychotridine

46

TOHRU HINO AND MASAKO NAKAGAWA

along with psychotridine and quadrigemines A and B (169). The structure was tentatively assigned as 228. These polyindolic alkaloids were found to be potent inhibitors of the aggregation of human platelets induced by adenosine diphosphate (ADP), collagen, or thrombin ( I 70). Cytotoxic activity of these tetrameric and pentameric alkaloids has been reported (171). The structures of these dimeric, trimeric, tetrameric, and pentameric tryptamine alkaloids have some common features. The N b nitrogen of tryptamine is always methylated, whereas an N-methyl group at N” is observed only in dimeric alkaloids. The linkage between the two tryptamine moieties is observed at C-3a-C-3a and C-3a-C-7; however, another possible linkage between C-3a-C-5 is not found in these alkaloids. A synthetic approach to quadrigemine A (225) has been published (172). Compound 229, which has a C-3-C-7 linkage of the two tryptamine moieties, has been prepared.

ae. \

Ac

Bn

-

Me

\

Bn

229

c. 3a-BISPYRROLO[2,3-b]INDOLEALKALOIDS DERIVEDFROM DIKETOPIPERAZINES Some dimeric alkaloids derived from diketopiperazines, which were constructed from tryptophan and serine or alanine, have been found in mp 240°C (dec), [ale fungal metabolites. Chaetocin (230), C30H28N606S4, +37Y (1.OO, pyridine), isolated from Chaetomium minutum, posseses antibacterial and cytostatic activity. Its structure was determined by spectroscopy and X-ray analysis. The absolute configuration was established by comparison of the CD spectrum of the diacetate with that of gliotoxin (173). Similar antibiotics, verticillines A (231), B (232), and C, were isolated from Verticillium sp. (strain TM-759) ( I 74). They possess antimicrobial

47

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

1

Chaetocin 230 Verticilline A 231 B

Melinacidin

lu

Chetracin

A

S

I233 234 235 236

R OH H H H OH OH OH

2 R OH H OH OH OH OH OH

3 R H OH OH H OH OH OH

4

n R H OH OH OH H OH OH

2 2 2 2 2 2 4

CH R -

activity against gram-positive bacteria and mycobacteria. As for cytotoxicity, the values ED5,,of verticillines A and B against HeLa cells were both 0.2 ,u/ml. The two diketopiperazine moieties of verticilline A (231) are cyclo-L-tryptophyl-L-alanine whereas cyclo-L-tryptophyl-L-alanine and cyclo-L-tryptophyl-L-serine are found in verticilline B (232). Molecular formulas and optical rotations are as follows: verticilline A (231), C30H28N606S4, [.ID +703.7"; verticilline B (232), C30H28N607S4, [.ID +704.7"; verticilline C, C30H28N607S5,[&ID +765". Verticilline C is very similar to B, with one of the two epidithiadiketopiperazine moieties in B being replaced by epitrithiadiketopiperazine. The absolute configuration of verticilline A (231) waMetermined from its CD spectrum, which is antipodal to that of gliotoxin and similar to that of chaetocin. The configuration at the carbon atom carrying the hydroxy group was determined by applying the benzoate sector rule to the monobenzoate derivative (174). Melinacidins 11 (233), 111 (234), and IV (235) are produced by Acrostulugrnus cinnabarinus var. inelinacidinus. These compounds inhibit growth of a vareity of gram-positive bacteria in vitro (175,176) and have the , following physical characteristics: melinacidin 11 (233), C30H28N6S406 [.ID +72@ (0.5, CHCI,); melinacidin 111 (234), C30H28N6S407,[&ID +776" (0.5, CHC1,); melinacidin IV (235), C30H28N6S408,[&ID +718" (0.5, CHC1,). By physicochemical methods melinacidin IV (235) was determined to be lla,ll'a-dihydroxychaetocin,and melinacidins I1 (233) and 111 (234) were found to be isomeric to chaetocin and verticillines A and B (177). A similar metabolite, chetracin A (236), has been isolated from Chaetomium ubuense Lodha and C. returdaturn Carter et Khan. Chetracin A (236), [.ID +723.5", amorphous powder, exhibits remarkable cytotoxicity to HeLa cells; it was found, by 'H- and ',C-NMR spectroscopy and X-ray analysis (178), to be the tetrasulfide analog of melinacidine IV. was isolated An oxidative dimer of cyclo-L-tryptophyl-L-phenylalanine from Aspergillus jluvus (strain MIT-M-25, 26, and 27). Ditryptophenaline

48

TOHRU HINO AND MASAKO NAKAGAWA

Ditrvptophenaline

237 __

(237), mp 204-206"C7 [a],, -330°, possesses neither significant toxic (LD,o 200 mg/kg) nor antibiotic properties; the structure and relative stereochemistry were determined by X-ray analysis as well as 'H-NMR (179). Synthesis of ditryptophenaline has been accomplished by oxidative dimerization of cyclo-L-tryptophyl-N-methyl-L-phenylalanine (238) with thallium tris (trifluoroacetate)-boron trifluoride in acetonitrile, as in case of Nbmethoxycarbonyltryptamine (see above), although yields were low. This synthesis established the absolute configuration of the alkaloid, in which both chiral centers at C-3a of the pyrrolo[2,3-b]indole ring are (S), whereas that of chimonanthine and folicanthine are (R). Dye-sensitized photooxygenation of diketopiperazine 238 in formic acid gave two diastereomers of the hydroxywroloindole (239) (161). Ditryptophenaline has also been isolated from Aspergillus f l a w s var. columnaris, and its absolute configuration was deduced from CD spectra of the alkaloid and cyclo-L-phenylalanyl-L-tryptophan derivatives (180).

D. TRYPTOPHAN DIMERHAVINGC-3-Nu Linkage Chetomin (240), C31H30N606S4,[a],, + 257", a toxic metabolite produced by Chaetomium species, is thought to be associated with poor growth in young ruminants. The first isolation was reported by Waksman in 1944 (181-183). Chemical studies of chetomin (184,185) showed the presence of two epidithiadiketopiperazine rings as seen in chaetocin. Later, the I3C- and I'N-NMR spectra of [ '5N6]chetomin, which was biosynthesized by Chaetomium cochliodes, established the structure as a tryptophan dimer having a C-3-N" linkage. Chetomin (240), also isolated from Chaetomium abuens, C. retardatum, and C. tenuissimum Sergejeva, was reported to show strong cytotoxicity to HeLa cells (178). Dethiotetramethylthiochetomin (241) was isolated from Chaetomium globosum Kinze ex Fries, and its structure was determined by X-ray analysis. Reduction of chetomin with sodium borohydride in the presence of methyl

,

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

Chetomin 240

-

49

Dethio-tetramethylthiochetomin 24 1 -

iodide gave dethio-tetramethylthiochetomin. This conversion enabled the establishment of the relative stereochemistry of chetomin (186). VI. 3a-Prenylpyrrolo[2,3-b]indolesand Related alkaloids

Since echinulin (242)was isolated from Aspergillus echznulatus in 1943 (67,187), many indole alkaloids having a C5 substituent on the indole ring have been found in molds and marine organisms. Only normal prenyl

Echinulin 242

groups (3,3-dimethylallyl) were found at the benzene ring of the indole, while both normal and inverted (1,l-dimethylallyl) prenyl groups have been found at the 1, 2, and 3 positions of the indole ring. Formation of 3a-prenylated pyrroloindole alkaloids may be visualized as in Scheme 6, but the exact biosynthetic steps to these alkaloids are not established. The 3-prenylated indolenine (A) may result from direct prenylation of the indole or from other rearrangements from the 1-prenyl precursor, and A may be cyclized to 3a-R-pyrroloindole B or rearranged to C. Forms B and C may be prenylated further to D and E.

50

TOHRU HINO AND MASAKO NAKAGAWA

R

R

R

C

E

D

SCHEME 6

A . FLUSTRAMINES Flustramines A (243) and B (244) have been isolated as major alkaloids from Flustra foliacea, a marine bryozoan, collected from the North Sea, and their structures were established as 3a-prenylated 6-bromopyrrolo[2,3b]indoles in 1979 (188). They represent the first example of the natural occurrence indole alkaloids in marine organisms. As minor alkaloids, flustramine C (245), flustraminols A (246) and B (247) (189), flustrabromine (248) (190), flustramide A (249), and a bromoquinoline alkaloid (250) (191,192) have been isolated from the same source. Dihydroflustramine C (251) was isolated from Flustra foliacea collected from the Minas Basin, Nova Scotia, as the major alkaloid (193,194). Dihydroflustramine C (251) (major), flustramine D (252) (major), and isoflustramine D (minor) were isolated from Flustra foliacea collected from the Bay of Fundy. Interestingly, two N-oxides (253 and 254) of dihydroflustramine C and flustramine D were also isolated (194). Among Calabar bean alkaloids, geneserine was long considered to be the N-oxide, but its structure has been revised to a oxazinoindole, and the N-oxides have not been found (7,109). Flustramines, which do not form crystals, were purified as oils, except dihydroflustramine C (251), mp 8244°C. The structures of these compounds were established by spectral data, especially NMR spectroscopy ('H and 13C).The absolute configurations of these alkaloids have not been determined, however, though the two pyrrolidine rings were found to be cis-fused. Antibacterial activity has been reported for dihydroflustramine C (251) and flustramine D (252) (194). The chemistry of flustramines up to 1982 has been briefly summarized by Christophersen (195).

51

I . CYCLIC TAUTOMERS OF TRYPTAMINES A N D TRYPTOPHANS

TABLE IV FLUSTRAMINES ~

Structure

Br

Name, formula, physical characteristics

Ref

Flustramine A,C,,H,,BrN,, colorless oil

188

Flustramine B,C,,H,,BrN,, colorless oil

2 88

Flustramine C,C 16H,,BrN2, colorless oil

189

Flustrarninol A,C,,H,,BrN,O, brown oil

189

Me

245

Br

a *

J246

(continued)

52

TOHRU HINO AND MASAKO NAKAGAWA

TABLE IV (Continued) Structure

Br

I

Name, formula, physical characteristics

Ref.

Flustraminol B,C16Hz,BrNZ0, impure brown oil

189

Flustrabromine, C,,H,,BrN,O, amorphous solid

I90

Flustramide A,C2,HZ7BrNZO, oil

191

Bromoquinoline alkaloid, C ,,H,,BrN,O

192

Dihydroflustramine C,C,,H2.BrN,, mp 82-84"C, [(~],-110" (1.5, CHZCIZ)

I94

Me

Br

248

Br

Br

H

Me

25 1 -

I . CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

53

TABLE IV (Continued) Structure

Name, formula, physical characteristics Flustramine D, CZ1HZ9BrNZ, oil, [.ID 86.5" (1.03, CHZCI,)

Ref. 193,194

~

Me

H

Br

a H

Ibo

Me

DihydroHustramine C N-oxide, Ci6HnBrNzO oil, [..ID -67.1" (0.32, CH,CI,)

194

Flustramine D N-oxide, C,,H2&rN2O, oil

194

IsoHustramine D (isolated as a mixture with flustramine D)

I94

253

254

Br

Me

255

54

TOHRU HINO AND MASAKO NAKAGAWA

B. LL S4900 AND AZONALENINE The first 3a-prenylpyrrolo[2,3-b]indole alkaloid, LL S490p (256), mp 235-240"C, [aID +425", was isolated from Aspergillus species in 1973 (196). Later the same compound and azonalenine (257), mp 244-247"C, [.ID +53" (1.31, CHCl,), were isolated from Aspergillus zonatus (197). The characteristic structural feature of these alkaloids is the diketopiperazine of tryptophan and anthranilic acid. Azonalenine has been reported to induce the abnormal second cleavage of sea urchin embryos. The stereochemistry and the absolute configuration of azonalenine (257) have been established by the synthesis of dihydroazonalenine (198) (see below).

o%H

LL 54908

258

Azonalenine

257

C. ROQUEFORTINE A neurotoxic mycotoxin, roquefortine (258), C22H23Nj02,mp 195200°C (dec), [.ID -703", was isolated from Penicillium roquefortii (199) and P. crustosum (200). Roquefortine (258) is a diketopiperazine of tryptophan and histidine and possesses an inverted prenyl group at the 3a position of the pyrroloindole ring. This compound showed tremorgenic activity in mice (LDjo 15-20 mg/kg) (199). Incorporation of (2S,3R)[3-3H]histidine and (2RS)-[indole-2-13C, 2-1sN]tryptophan into roquefortine was proved and the pro-S hydrogen of histidine was found to be removed. Oxaline (259), a related alkaloid, was found to be derived from roquefortine (258) (201,202).

0

Roquefortine 258

Oxaline

259

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

55

D. AMAUROMINE Amauromine (260), C32H36N402,mp 156-158"C, [.ID -583" (1.O, CHC13), was isolated from the soil fungus Amauroascus, Sp. No. 6237, and found to have vasodilating activity (203).The structure as well as the absolute configuration were determined by chemical and spectral data (204) and further confirmed by synthesis (see below). Amauromine is the first natural diketopiperazine composed of two L-tryptophans.

Amauromine

2 3

These 3a-prenylated pyrroloindole alkaloids have an inverted prenyl group at the 3a position, with one exception, namely, flustramine B, which has a normal prenyl group. However, positions 8 (N), 5 , and 7 always have normal prenyl groups. The prenyl group at the 2 position of the indole ring or at the 8a position is always inverted. The only exception is found in fumitremorgins such as fumitremorgin B (261), a tremorgenic mycotoxin which possesses a normal prenyl group at this position (205-212). This situation is general for all prenylated tryptophan-derived alkaloids.

Fumitremorgin B % 2

E. SYNTHETIC APPROACHES to PRENYLATED INDOLES Since echinulin was found as a natural product, many attempts to synthesize prenylated indoles have been reported.

56

TOHRU HINO AND MASAKO NAKAGAWA

1. Normal Prenylated Indoles Prenylation of indoles with prenyl bromide in acetate buffer (pH 2.7) to give 3-prenylindoles in good yield was first reported in 1969 (213,214). Prenylation of indole gave the 3-prenyl (262) and the 2,3-diprenyl derivatives (263) as well as 264. 3-Substituted indoles treated with prenyl bromide (4 equiv) gave 2-prenyl derivatives such as 265, via a 1,2-shift of the 3,3-disubstituted indolenine. On the other hand, 3-substituted indoles having a nucleophilic center at the appropriate position such as tryptamine derivatives gave the 3a-prenylpyrroloindoles (266) and the 1,2-diprenyl derivatives (267) (213).

A

\

+

& >. a. c" e t a t e

n

buffer

n 282

b-

5+ 264

n

n 265

287

Similar but slightly different results were reported for the prenylation of Nb-methoxycarbonyltryptamine with 5 equiv of prenyl bromide in acetate buffer. The Na,3a-diprenylated pyrroloindole (269) was obtained via 268 in excellent yield, but the 1,2-diprenyl derivative was not isolated (215). Treatment of the N " , 3a-diprenylpyrroloindole (269) with trifluoroacetic acid in methylene chloride gave the 1,2-diprenyl derivative (271) via 270 in excellent yield. Dye-sensitized photooxygenation of the tetrahydro derivative (272) of 271 gave the 3a-hydroxypyrroloindole derivative (273), which was converted to a P-carboline (274) by acid treatment. (215). Similar prenylation of Nb-trifluoroacetyltryptamine (275) gave the 8, 3a-diprenyl derivative (276) in low yield, which was hydrolyzed by sodium

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

57

27 1 __

OH

272

m

H

275 -

274

273

COCF3 H -----)

276 -

277 -

278 -

borohydride in ethanol to the NH compound (277). Debromoflustramine B (278) was prepared from the NH compound (277) with formalin-sodium cyanoborohydride (216). Flustramine B (244) has been synthesized from the 6-bromotryptamine (see Section II,B) by similar methods. Prenylation of the 6-bromotryptamine (65) with prenyl bromide was slow compared with unsubstituted tryptamine and gave the 8,3a-diprenyl derivative (279) in 70% yield when an excess of prenyl bromide (10 equiv) was used.

58

TOHRU HINO AND MASAKO NAKAGAWA

L

Br

m ' LBr ToLkJ CH0 2 M a eb cu e tf faetreB '

H

65 -

-f

lustramine B 244

I

Me

278

Alkaline hydrolysis of 279 followed by methylation gave flustramine B (244) (39).Flustramine B (244) was obtained in better yield by reduction of 279 with diisobutylaluminum hydride (217). Debromoflustramine B (278) was obtained by lithium aluminum hydride reduction of 279. 2. Inverted Prenyl Indoles Three methods have been reported for introduction of the inverted prenyl group (1,l-dimethylallyl) to the indole ring at the 2 or 3 position. The first is the rearrangement of 1-prenyl, 2-prenylthio, or 3-prenylthio-

mCH> I

4" COCF 3

WCH0 /

286 -

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

59

indole derivatives. l-Prenyl-3-methylindole (281) gave 2-prenyl-3-methylindole (283) and 2-(1 ,l-dimethylallyl)-3-methylindole(282) on treatment with trifluoroacetic acid or a Lewis acid such as boron trifluoride etherate. The 2-inverted prenyl derivative (282) became the major product when the l-prenyl derivative (281) was treated with trifluoroacetic acid at 0°C (218). These results suggested that the 2-inverted prenyl group in echinulin (242) was derived from the l-prenyl derivative in the biosynthetic pathyway; however, the prenyl group of 1-prenyl-3-formylindole (284) did not rearrange under similar conditions, and 285 and 286 were obtained (219). Acid-catalyzed rearrangement of cyclo-A'"-prenyl-~-prolyl-~-tryptophan (287) has been studied (220). With boron trifluoride etherate, 287 gave the pyrrolo[ 1,2-a]indole (288) and the pyrido[l,2,3-hi]indole (289). Only 288 was obtained with boron tribromide, whereas 288 and 290 were obtained with trifluoroacetic acid. Furthermore, 288, 289, and 291 were isolated after reaction with SnCl,. The 2-inverted prenyl derivative was not isolated. Further elaboration of 288 to deoxybrevianamide E (136) was unsuccessful (220). Compound 289 might have arisen from the cyclic tautomer of 287.

Feeding experiments of radio labeled l-prenyltryptophan or cyclo-Lalanyl-N-prenyl-L-tryptophan to Aspergillus amstelodami, which is known to procedure echinulin (242), showed that the l-prenyl derivatives were not precursors of echinulin (242) (221). Rearrangement of 3-(3,3dimethylally1thio)indole (292) prepared from 3-mercaptoindole gave 2(l,l-dimethylallyl)-3-mercaptoindole(293) and the 4-thiacarbazole derivative (294) on heating (222). Facile rearrangement of the sulfonium

60

TOHRU HINO AND MASAKO NAKAGAWA

compound (296), which was prepared from indole and ethyl succinimidyl dimethylallylsulfonium salt (295), was observed to give 2-( 1,ldimethylallyl)-3-ethylthioindole (138). Reduction of this compound with zinc in acetic acid afforded 2-(1,l-dimethylallyl)indole (139) (93). Et

Me

296 __

295 -

This improved method was appGed to the synthesis of brevianamide E (see Section IV,A) and neoechinulin (223). Thio-Claisen rearrangement of 2-allylthio-l , 3-dimethylindole (297) gave 3-allyl-1, 3-dimethyl2-indolinethione (298), while 2-(dimethylallylthio)-l-methylindole (299) gave an equilibrium mixture with l-methyl-3-( 1,l-dimethylally)-2-indolinethione (300) at room temperature (224). R

297 -

299 -

R

298

300

I. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

61

-

303

306 -

307

-

308 __ debromodihydroflustramine C

This thio-Claisen rearrangement was applied to the synthesis of debromodihydroflustramine C (308) and amauromine (260) by Takase’s group (225-227). Methyl 2-indolinethione-3-acetate (301) was methylated to the methylthio derivative (302), which was prenylated with prenyl bromide at room temperature to give a mixture of 3-(1,l-dimethylallyl)indole (303a) and 3-(3,3-dimethylallyl)indole (304a) in an 8 : 1ratio via the sulfonium salt intermediate (305). Hydrolysis of the mixture yielded the carboxylic acids, which could be separated. The acid (303b) was converted to the methylamide (306), which was cyclized to the pyrroloindole (307) by sodium hydride. Reduction of the pyrrolidone (307) with diisobutyialuminum hydride gave debromodihydroflustramine C (308) (225). A similar strategy was applied to the total synthesis of amauromine (260) (226,227). Cyclo-2methylthio-~-tryptophyl-2-rnethylthio-~-tryptophan (313) was prepared by the conventional method from tryptophan. Prenylation of 313 with prenyl bromide in potassium carbonate-dioxane at room temperature for 7 days gave 314 (18%) and 315 (1.5%). Reduction of 314 with lithium aluminum hydride and TiC14 gave amauromine (260) in 1.5% yield. The second method to prepare 2-( 1,l-dimethylally1)indole derivatives is indole ring formation of the appropriate aniline. The first synthesis of 2-( 1,I-dimethyla1lyl)indole (139), reported by Houghton and Saxton

t

0 1

1 8

uX

Im:

m

t

R’s MN

0

4.

f

I

*I t m

=I +

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

63

-a 139

(228), made use of the Bischler indole synthesis from aniline. Starting from 2,4-bis(dimethylallyl)aniline, the Kishi group accomplished the total synthesis of echinulin (242) (229). Saxton’s group also synthesized deoxybrevianamide E from 139 (230). Two other methods for the preparation of 2-( 1,l-dimethylallyl)indole from o-iodoaniline and ethyl 3-hydroxy-2indole carboxylate (316) have been reported (231).

G-? n

A third method of preparing 2- or 3-inverted prenyltryptamines has been reported, involving 1,l-dimethylpropargylationof the tryptamine derivative (232). Reaction of the tryptamine with 2-chloro-2-methyl-3-butyne (317) in the presence of sodium hydride in dimethylformamide gave 3a-(1,l-dimethylpropargyl)pyrroloindole (318, 27%), the allene (319, ll%),and N-(1,l-dimethylpropargy1)indole (320, 23%). Acid-catalyzed rearrangement of 318 resulted in formation of the 2-(1,l-dimethylpropargy1)tryptamine (321) in low yield. The 3a-(l,l-dimethylallyl) derivative (322), however, obtained by partial reduction of 318, gave 2(1,l-dimethylally1)tryptamine(323, 15%) on treatment with trifluoroacetic acid. Major products of this acid rearrangement were the N-dimethylallyl(324) and 2-(3-methyl-3-trifluoroacetoxybutyl)tryptamine (325). Although

n+ Qii; I

U

I ‘

P

I

r, 8

X

fl

r, GI m

Q

IT

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

65

the yield was not satisfactory, this is the first example of the introduction of an inverted prenyl group equivalent to the tryptamine derivative. The 2-( 1,l-dimethylal1yl)tryptophanderivative (331) has been prepared from 3-(3,3-dimethylallyl)indole (326) (233). Acid-catalyzed rearrangement of 327, prepared from 3-prenylindole (326), gave a mixture of 2-prenyl (328) and 2-inverted prenyl derivatives (329) in a 9 : 1 ratio. Reduction of 329 with BH,-NMe, gave the 2-inverted prenyl tryptophan (331). These results showed that [3,5]-sigmatropic rearrangement may compete with the 1,2-shift of the intermediate (330), although the yield was low.

326 ~

327 -

VII. Other Pyrrolo[2,3-b]indoles

Various electrophiles may attack at the 3 position of tryptamines to form 3a-substituted pyrroloindole ring systems. 2-Hydroxy-5-nitrobenzyl bromide, Koshland reagent, has been utilized in quantitative analysis of tryptophan residues in protein, as the reagent attacks at these residues selectively (234).Model reactions of tryptophan derivatives with Koshland reagent were studied by three groups (235-237), and the products were found to be the 3a-(2-hydroxy-5-nitrobenzyl)pyrrolo[2,3-b]indolederivatives (332, 333, 334, 335, and 336). The 3-benzylindolenine intermediate

66

TOHRU HINO AND MASAKO NAKAGAWA

CHpAr R=H. C02Me +

H

kCH3 332 C02Et

CKJJ-L H

cH2Ar

+

ArCHpBr ---+

CO2Et H-

H H H

333

CHpAr C02E-t

H *

H

H

334 __

+ Ar : O2N CHpAr I

H

1337

(337) is trapped by the side chain nitrogen to form the pyrroloindole ring system. The Witkop group demonstrated that the phenolic hydroxyl group participates with the intermediate indolenine to form the pyranoindole derivative (338) when the side chain nitrogen is lacking (235).The reaction of tryptophan ethyl ester and Koshland reagent at pH 4.7 gave 333 and 334 (57 : 43), which were converted to the 2-benzyltryptophan derivative (339) on refluxing in ethanolic hydrogen chloride (236). These results showed that the intermediate (337) formed by the reaction of Koshland reagent with protein may be trapped not only the side chain nitrogen or the phenolic group but also by other adjacent nucleophiles in the protein.

1 . CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

67

338 __

R=H, C H 3

333 - ,334 -+ H

339

As described previously in Section 11, B, chlorination of 1-acetyl-Nmethoxycarbonyltryptamine with N-chlorosuccinimide gave the 3a-chloropyrroloindole derivative (42). The reaction of N-acetyltryptamine (340a) with iodine azide gave 3a-azidopyrroloindoie (341a). The 3a-iodopyrroloindole (344) was the intermediate, and the iodide was replaced by azide ion. On the other hand, the 2-methyltryptamine (340b) gave a mixture of 341b and 342. The latter may arise from the exomethylene intermediate (345). Similar reaction of tryptophols (346) gave 3a-azidofuranolindoles (347). The azidomethyl derivative (348) was not observed in this reaction, but it was obtained by treatment of the azido derivative (347) with acetic acid. In contrast, the 3a-azido derivative (341b) did not rearrange to the azidomethyltryptamine under acid conditions (238,239). 4-Hydroxy-l,4-benzoxazinoneis known to react with several electrophiles, such as tyrosine and histidine derivatives, when the 4-hydroxy group is activated by acetylation. Reaction of a tryptophan derivative with this reagent (349) gave a diastereoisomeric mixture of pyrroloindoles (350 and 351) (240).

68

TOHRU HINO AND MASAKO NAKAGAWA

34 1

340

342

-

-

a :R=H

b :R-CH 3

0TTk-2

341 -

R

R

344

Ac

-

343

-

342

H

347

348 -

-

346

R=H. CH3

H

2 M e +Me

0a.D +

H

AAc

349

1. CYCLIC TAUTOMERS OF TRYPTAMINES AND TRYPTOPHANS

69

Acknowledgments We thank Mr. T. Kawate for assistance in preparation of the manuscript and Mr. A. Hasegawa, Mr. H. Daitoku, and Miss J . Ma for drawing the structures.

REFERENCES 1. J. Elguero, C. Marzin, A. R. Katritzky, and P. Linda, “The Tautomerism of Heterocycles,” p. 216. Academic Press, New York, 1976. 2. H. F. Hodson and G. F. Smith, Chem. Ind. (London), 740 (1956). 3. S. Sugasawa and M. Murayama, Chem. Pharm. Bull. 6, 194 (1958). 4. L. A. Cohen, J. W. Daly, H . Kny, and B. Witkop, J . Am. Chem. Soc. 82,2184 (1960). 5. M. Ohno, T. F. Spande, and B. Witkop, J . Am. Chem. Soc. 90,6521 (1968); M. Ohno, T. F. Spande, and B. Witkop, J . Am. Chem. Soc. 92, 343 (1970). 6. J. E. Baldwin and N. R. Tzodikov, J . Org. Chem. 42, 1878 (1977). 7. B. Robinson, in “The Alkaloids” (R. H. F. Manske and H. L. Holmes, eds.), Vol. 10, Chap. 5 , and Vol. 13, Chap. 4. Academic Press, New York, 1968, 1971, Recent advances in this field will appear in a later volume of this treatise. 8. R. J. Sundberg, “The Chemistry of Indoles.” Academic Press, New York, 1970. 9. W. A. Remers, in “Indoles” (W. J. Houlihan, ed.), Part 1, Chap. 1. Wiley (Interscience), New York, 1972. 10. A. H. Jackson and A. E. Smith, J . Chem. SOC., 5510 (1964). 11. R. L. Hinman and J. Lang, J . A m . Chem. Soc. 86, 3796 (1964). 12. R. M. J . Kamlet and J . C. Dacons, 1. Org. Chem. 26, 220 (1961). 13. T. Hino and M. Taniguchi, J. A m . Chem. SOC.100, 5564 (1978). 14. M. Taniguchi and T. Hino, Tetrahedron 37, 1487 (1981). 15. G. F. Smith, in “Advances in Heterocyclic Chemistry” (A. R. Katritzky, ed.), Vol. 2, p. 287. Academic Press, New York, 1963. 16. Y. Omori, Y. Matsuda, S. Aimoto, Y.”Shimonishi, and M. Yamamoto, Chem. Lett., 805 (1976). 17. M. Taniguchi and T. Hino, unpublished observation. 18. Y. Miyake and Y. Kikugawa, J . Hererocycl. Chem. 20, 349 (1983); M. Kawase, Y. Miyake, and Y. Kikugawa, J . Chem. SOC.Perkin Trans. 1, 1401 (1984). 19. J . Thesing, G . Semler, and G . Mohr, Chem. Ber. 95, 2205 (1962). 20. G. Berti and A. D. Settimo, Cazz. Chim. Ifal. 87, 2238 (1959). 21. T. Nagasaka and S. Ohki, Yakugaku Zasshi 92, 777 (1972). 22. T. Moriya, K. Hagio, and N. Yonedo, Bull. Chem. Soc. Jpn. 48, 2217 (1975). 23. M. Taniguchi, A. Gonsho, M. Nakagawa, and T. Hino , Chem. Pharm. Bull. 31, 1856 (1983). 24. S. Yamada, T. Shioiri, T. Itaya, T. Hara, and R. Matsueda, Chem. Pharm. Bull. 13,88 (1965). 25. E. Leeta, Acc. Chem. Res. 2, 59 (1969). 26. H.-J. Teuber and G. Staigen, Chem. Ber. 89, 489 (1956). 27. Y . Mori and M. Sato, Jpn. Pat. 7432536 (1974); Chem. Abstr. 82, 9 8 3 9 1 ~(1975). 28. H.-J. Teuber and 0. Glosauer, Chem. Ber. 98, 2939 (1965). 29. T. Hino, M. Taniguchi. and M. Nakagawa, Heterocycles 15, 187 (1981). 30. M. Taniguchi, T. Anjiki, M. Nakagawa, and T. Hino, Chem. Pharm. Bull. 32, 2544 (1984). 31. P. E. Partch, J . Am. Chem. Soc. 89, 3662 (1967).

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205. M. Yamazaki, S . Suzuki, and K. Miyaki, Chem. Pharm. Bull. 19, 1739 (1971). 206. M. Yamazaki, K. Sasago, and K. Miyaki, J . Chem. Soc., Chem. Commun., 408 (1974). 207. M. Yamazaki, H. Fujimoto, T. Akiyama, U. Sankawa, and Y. Iitaka, Tetrahedron Lett., 27 (1975). 208. M. Yamazaki, H. Fujimoto, and T . Kawasaki, Tetrahedron Lett., 1241 (1975). 209. N . Eikman, J . Clardy, R. J . Cole, and J . W. Kirksey, Tetrahedron Lett., 1051 (1975). 210. J . Fayos, D. Lokensgard, J. Clardy, R. J. Cole, and J. W. Kirksey, J . Am. Chem. Soc. 96, 6785 (1974). 211. M. Yamazaki, H . Fujimoto. and T. Kawasaki, Chern. Pharrn. Bull. 28, 245 (1980). 212. M. Yamazaki, K. Suzuki, H. Fujimoto, T. Akiyania, U . Sankawa, and Y. Iitaka, Chern. Pharm. Bull. 28, 861 (1980). 213. G . Casnati, M. Francioni, A . Guareshi, and A . Pochini, Tetrahedron Lett., 2458 (1969). 214. V. Bocchi, G. Casnati, and R. Marchelli, Tetrahedron 34, 929 (1978). 215. M. Nakagawa, K. Matsuki, and T. Hino, Tetrahedron Lett. 24, 2171 (1983). 216. P. Muthusbraminiam, J. S. Carlk, and C. Christophersen, Acta Chem. Scand., Ser. B 37, 803 (1983). 217. T. Hino, M. Hasegawa, M . Nakagawa, unpublished results. 218. G. Casnati and A . Pochini, J . Chem. Soc., Chem. Commun., 1328 (1970). 219. K. J. Baird, M. F. Grundon, D. M. Harrison, M. G. Magee, Heterocycles 15, 713 (1981). 220. P. G . Sammes and A. C. Weedon, J . Chern. Soc., Perkin Trans. I , 3053 (1979). 221. M. F. Grundon, M . R. Hamblin, D. M. Harrison, J. N. Logue, M. Maguire, and J. A . McGrath, J . Chern. Soc., Perkin Trans I , 1294 (1980). 222. H. Plieninger, H.-P. Kraemer, and H . Sirowe], Chem. Ber. 107, 3915 (1974). 223. S. Nakatsuka, H. Miyazaki, ap,d T. Goto, Tetrahedron Lett. 21, 2817 (1980). 224. B. W. Bycroft and W. London. J . Chem. Soc., Chem. Commun., 168 (1970). 225. S. Takase, I. Uchida, H. Tanaka, and H . Aoki, Heterocycles 22,2491 (1984); S . Takase, 1. Uchida, H . Tanaka, and H. Aoki, Tetrahedron 42, 5877 (1986). 226. S . Takase, Y. Itoh, I. Uchida. H . Tanaka, and H. Aoki, Tetrahedron Lett. 26, 847 (1585). 227. S. Takase, Y. Itoh, I. Uchida, H. Tanaka, and H. Aoki, Tetrahedron 42, 5887 (1986). 228. E. Houghton and J . E. Saxton, J . Chern. Soc. C, 595 (1969). 229. N. Takamatsu. Y. Inoue, and Y. Kishi, Tetrahedron Lett., 4665 (1571). 230. R. Richie and J. E. Saxton, J . Chem. Soc., Chem. Cornrnun., 611 (1975); R. Richie and J. E. Saxton. Tetrahedron 37, 4295 (1981). 231. H. Plieninger and H. Sirowe, Chem. Ber. 104, 2027, 1863, 1869 (1971). 232. T. Hino, K. Hasumi, H. Yamaguchi, M. Taniguchi, and M. Nakagawa, Chern. Pharm. Bull. 33, 5202 (1985). 233. R. Plate and H. C. J . Ottenheijm, Tetrahedron Lett. 27, 3755 (1986). 234. A . N. Glazer, Annu. Rev. Biochem., 101 (1970). 235. T. F. Spande, M . Milcheck, and B. Witkop, J . A m . Chem. Soc. 90, 3256 (1968). 236. G . M. Loudon. D. Portsmouth, A . Lukton, and D. E. Koshland J . Am. Chem. SOC.91, 2792 (1969). 237. B. G. McFarland, Y. Inoue, and K. Nakanishi, Tetrahedron Lett., 857 (1969). 238. M. Ikeda, F. Tabusa, Y. Nishimura, S. Kwon, and Y. Tamura, Tetrahedron Lett., 2347 (1976). 239 M. Ikeda, K. Ohno, M. Katsuura, M.-W. Chun, and Y. Tamura, J . Chem. Soc., Perkin Trans. I , 3061 (1979). 240. T. Ishizaki, Y. Hashimoto, K. Shudo, and T. Okamoto. Heterocycles 20, 1481 (1983).

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__ CHAPTER 2 __

ALKALOIDS IN CANNABIS SATIVA L. RAPHAEL MECHOULAM Department of Natural Products Faculty of Medicine Hebrew University Jerusalem 91 120, Israel

I. Introduction 11. Quaternary Bases, Amides, and Amines A. Quaternary Bases B. Amides C . Amines 111. Spermidine Alkaloids IV. Synthesis of Cannabinoid Spermidine Alkaloids A . The Weinreb Approach B. The Natsume Approach C . The Wasserman Approach V. Pharmacology References

I. Introduction

Research on the nitrogenous compounds in Cannabis sativa L. and its many herbal preparations (hashish, marijuana, etc.) has a long, though somewhat checkered, history. With the discovery of morphine and other major alkaloids in the early nineteenth century, it was tacitly assumed that most physiologically active plant constituents belonged to this family of compounds. Alkaloids, however, could not be isolated or even detected by early investigators of Cannabis (1,2). It was only in 1876 that Preobraschensky ( 3 ) reported the presence of nicotine in Cannabis sativa resin, which he claimed to have brought from the Far East, where he had accompanied an expedition. However, the resin (apparently hashish) was actually bought in Tashkent in Uzbekistan. From personal inquiries I have recently made, with people who have lived in this Soviet province in Central Asia, hashish mixed with tobacco is still clandestinely smoked there. Its use was probably quite widespread in the nineteenth century. The presence of nicotine in such a product is not 17

THE ALKALOIDS. VOL 34 Copyright 0 1988 by Academic Press, Inc All rights of reproduction in any form reserved

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RAPHAEL MECHOULAM

suprising. Indeed, shortly after the Preobraschensky report was published, Dragendorff ( 4 ) objected to his claim on the basis of the known differences between the pharmacological activities of Cannabis and tobacco. Several groups later tried unsuccessfully to validate Preobraschensky’s claim. Siebold and Bradbury ( 5 ) extracted large quantities of Indian Cannabis but found no evidence of nicotine; however, an unknown oily basic material believed to be an alkaloid (named cannabinine) was isolated. Kennedy (6) also repeated the Russian work and likewise found no nicotine. Again, the presence of other unidentified alkaloids was indicated. Hay (7) also noted the absence of nicotine but was able to isolate a new, biologically active alkaloid, which he named tetanocannabin. His work followed the standard techniques for alkaloid isolation used in the latter part of the nineteenth century: the water infusion of the powdered plant was treated with lead acetate, phosphotungstic acid was added to the acidified filtrate, and a mixture of alkaloids precipitated, from which tetanocannabin could be obtained. It caused strychninelike convusions in tests on frogs. The above, somewhat tenuous, scientific background was apparently sufficient to make possible commercial promotion in those days. The Merck Index of 1896 lists six different Cannabis preparations, one of which relates to an alkaloid: Cannabine Alkaloid Merck , . , . , . . 15 gr. vial 10.00, Also in 10 & 5 grain vials need . .Hypn. without danger. second, effects, -Dose 1.5-4 grains (0.097-0.26 Gm).

Fr. Cannabis sativa, L., var. indica-Fine

One can only wonder what were the fine needles of “Cannabine Alkaloid Merck” that were hypnotic without dangerous secondary effects. Two common nitrogenous bases were also isolated from Cannabis before the turn of the century. In 1887 Jahns (8) extracted Indian Cannabis with water; the dried extract was redissolved in ethanol, and choline (1) was precipitated from the solution as the platinum salt. Shortly thereafter Schulze and Frankfurt ( 9 ) identified trigonelline (2), also as the platinum salt. It was directly compared with synthetic material. With the realization that the active constituent(s) in Cannabis was not an alkaloid (see, for example, Refs. 2, 10, and I I ) , interest in this field waned, and no further progress was made for over 65 years. For modern reviews on the nitrogen-containing constituents, including nineteenth century publications, see Refs. 12, 13, and 14. This chapter covers mostly spermidine-type alkaloids present in Cannabis, including recent syntheses, and includes a short summary on the quaternary bases, amides, and amines found in the plant. Amino acids and proteins are not surveyed.

79

2. ALKALOIDS IN CANNABIS S A T N A L.

11. Quaternary Bases, Amides, and Amines

A.

QUATERNARY

BASES

As mentioned above, choline (1) and trigonelline (2) were identified in Cannabis in the last century. Their presence was confirmed about 50 years later by Merz and Bergner (15). Muscarine (3) was shown to be a

qcoo-

HOCH,CH,N(CH,),OH + -

I CH,

(1) Choline

(2) Trigonelline

HO

H3C

+

'b

CH2kCH3),

CH,CH2 N(CH3), \CHCCOO/ I

CH,

(3) Muscarinr

\

H

(4) L-(+)-isolrucine brtaine

( 8 ) N-(p- hydroxy- p-phrnylrthyl)-p- hydroxy- ( t ~ 8 ) cinnamldr -

HO

D

N

+/

CH3

CH,=CH-N-CH,OH 'CH, (5) Neurinr

( 7 )hrxadrcamidr

( 8 ) Hardrninr

/ \

CH,

CH3

constituent by Kwasniewsky (16). This compound has not been observed by any of the other groups working in this area, however, and its presence is hence questionable. Salemink et al. (17)published a detailed analysis of the nitrogenous bases, using modern techniques, including chromatography on cellulose powder. The Cannabis used was grown in The Netherlands from seeds of numerous countries. Six bases were observed on chromatography. Initially only choline and trigonelline, but not muscarine, were positively identified. A later publication by the same group (18) reported the isolation of an additional base from an ethanol extract of seeds of a French variety. Purification was again on a cellulose column. Initially this base was mistakenly identified, but the structure was definitely

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RAPHAEL MECHOULAM

established as L-( +)-isoleucine betaine (4)by comparison with a synthetic sample (19). Most isolation work on Cannabis has been done on the flowering tops, resin, or seeds. The roots have received scant attention as most research groups, not being allowed (or not willing) to grow Cannabis, have had no access to the plant roots. Turner’s group in Mississippi has taken the opportunity of having an experimental (and legal) Cannabis farm to explore root constituents. Roots were consecutively extracted with a series of solvents of increasing polarity. The aqueous extract was chromatographed on an ion-exchange resin, and after further purification choline (1) and neurine (5) were isolated and identified (20). B. AMIDES

An ethanol extract of Cannabis roots, after further partition in various solvents and chromatography on silica gel, gave N-(p-hydroxy-Pphenylethy1)-p-hydroxycinnamide (6) (21). This amide has been observed previously in only one other plant, namely, Evodiu belahe. The structure of 6 was confirmed by a straightforward synthesis. Smith et al. (22) identified a further amide, hexadecamide (7), from Cannabis resin. C. AMINES

A rather extensive number of simple amines have been detected by capillary gas chromatography (18). They include methyl, ethyl, n-propyl, n-butyl, isobutyl, sec-butyl, as well as other alkyl amines only tentatively identified. Piperidine was isolated by Obata et aE. (23) and later by Salemink’s group (17).The only amine of some structural interest, outside the spermidine alkaloids discussed below, is hordenine (8). It is the only p-arylethylamine in Cannabis observed so far. First isolated in 1975 by El-Feraly and Turner (24) from leaves of an unknown drug type, it was later identified in 15 variants of Cannabis grown in Mississippi from seeds from countries as far apart as Australia, Afganistan, and Jamaica (25).

111. Spermidine Alkaloids

Several groups have reported preliminary (in some cases very preliminary) observations on the presence of alkaloids in Cannabis. Samrah et al. (26) suggested that compounds of an indolic nature are present in Dutch hemp. Their suggestion stems from color reactions with Ehrlich’s reagent

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82

RAPHAEL MECHOULAM

between water and chloroform, and the latter solution was repartitioned between petroleum ether and methanol-water. The polar fraction was chromatographed on silicic acid. The fraction eluted with 8% methanolwater was partitioned anew between chloroform and 1% hydrochloric acid. Standard workup gave, after crystallization, cannabisativine, mp 167-168"C, [.ID +55.1", in a low yield (-0.0004%). The structure of cannabisativine (9) was elucidated by X-ray crystallography, using a direct method program. A map using 258 phases gave positions for 22 of 27 nonhydrogen atoms. Subsequent Fourier maps revealed the positions of the other 5 nonhydrogen atoms. Very high temperature factors for the pentyl side chain were noticed, which indicated thermal motion in this part of the molecule. The X-ray structure also showed the existence of a hydrogen bond between N-5 and N-10 and between the hydroxyl group on C-18 and N-1. The high resolution mass spectrum was interpreted as shown in Scheme 1. A second alkaloid was isolated by the same group from the leaves and roots of a Mexican variant of Cannabis sativa (31). Air-dried leaves, defatted by percolation with hexane, were extracted with ethanol and

n u

OH

N3°

C12H22N3

SCHEME 1. Fragmentation pattern of cannabisativine on high-resolution mass spectrometry (29).

2. ALKALOIDS IN CANNABIS SATIVA L.

83

partitioned between chloroform and citric acid. The acidic layer, after basification and standard workup, was chromatographed on silica gel to yield the alkaloid as a noncrystalline solid, [a],, +18.7", in a 0.00046% yield. The IR spectrum, when compared to that of cannabisativine (9), showed an additional carbonyl peak at 1715 cm-'. The mass spectrum showed a molecular ion at m / z 363 for C21H37N302, which is 18 mass units less than that of 9. A comparison of the mass spectral fragmentation of the two alkaloids showed considerable similarities, except for fragments of the seven-carbon side chain. These data indicated that the new alkaloid is a dehydrated product of 9, and it was appropriately named anhydrocannabisativine (10). The structure was firmly established by dehydration of 9 with oxalic acid at 180-185°C to give 10. Annhydrocannabisativine (10) has been found in plant samples of Cannabis from 15 different geographical locations (25). Several related spermidine alkaloids are known in nature. The structures of two of these, palustrine (11) and palustridine (12), both found in Equisetum species, that are particularly close to the cannabisativines are shown. For a review on spermidine, spermine, and related alkaloids, see the chapter by Guggisberg and Hesse (32) elsewhere in this treatise. IV. SynthesG of Cannabinoid Spermidine Alkaloids

The macrocyclic spermine- and spermidine-derived alkaloids have been the object of numerous synthetic studies aimed at model compounds or at the natural products themselves (33-41). As the object of this chapter is to present an overview of Cannabis alkaloids, I address myself only to the syntheses of cannabisativine (9) and anhydrocannabisativine (10). I also describe the synthesis of dihydropalustrine (13) (an alkaloidal derivative not present in Cannabis), however, as its synthesis by Wasserman's group is germane to the present review.

A. THE WEINREB APPROACH The approach of Weinreb's group (42) is based on an intramolecular imino Diels-Alder reaction. This cycloaddition reaction had been extensively investigated by the same group (42,43). The total synthesis is presented in Scheme 2. The starting material, diene 14a, was prepared from pentadienylsilane (15) described by Seyferth and Pornet ( 4 4 ) . The reaction of 15 with l-hexanal in the presence of titanium tetrachloride led to the desired 14a. The carbamate 14b on reaction with methyl glyoxylate followed by acetylation yielded the methylol acetate (16), which was

84

RAPHAEL MECHOULAM

r

-I

I)methy1 glyoxalate

I

C,%

I

%HI, (16)

(1461

H

H

H

-YCoz OR

homologotion I

I

I

n-CSH,l ({Oak R=H IISbl R ~ ~ - E I I Y ~ ~ S I

1

reaction with triflate (20)

4K2:03

~

'

~

o

M

7s

N e N H T s OR' 1 1

oceto nitrile

-

OSi t- BuMe, I

1

I

n-C5Hll

-

(231 R Ts; R' t - EuM*ZSI (24) R = R ' . H

n-C5HIl

(22)

oxidation

onhydrocannabisativine (10)

CF3S020

-

1211 R.Y.

NHTs

(201

n-CeH,,

SCHEME 2. Synthesis of racemic anhydrocannabisativine. The Weinreb approach ( 4 2 ) .

2 . ALKALOIDS IN CANNABIS S A T N A L.

85

heated with diisopropyl ethylamine in toluene at 215°C for 3 hr in a sealed tube to afford a single bicyclic adduct 17b. The structure and stereochemistry of 17b were unambiguously established by X-ray analysis of the parent acid 17a. It seems plausible that 16 loses acetic acid to produce an intermediate N-acylimine, which then undergoes an intramolecular Diels-Alder reaction. While the trans relationship of the hydrogens flanking the nitrogen in 17a was expected from the known stereochemical route of the reaction, the high stereoselectivity of the formation of the remote chiral center was not. The authors suggested that this is due to formation of an intermediate in a quasi-chair conformation, 18. They also suggested that the intermediacy of such a quasi-chair form may be a general phenomenon in imino Diels-Alder reactions. The acid 17a was converted to 19a through an Arndt-Eistert sequence followed by hydrolysis. Annulation of 19b, the tert-butyldimethyl silyl either of 19a, was achieved by alkylation with the triflate 20 to give the ester 21, which was readily converted to the mesylate 22. Cyclization of 22 afforded the desired lactam 23, which, on removal of the protecting groups, gave the amino alcohol 24; oxidation of 24 led to racemic anhydrocannabisativine (10).

B. THENATSUME APPROACH The Natsume approach (45,46) (Scheme 3) follows a four-stage synthetic plan: (1) formation of compound 26 by stereoselective steps from the starting material 25 (in 26 the seven carbon side chain is already attached to the heterocyclic ring with the correct stereochemistry); (2) introduction of the double bond in the heterocyclic ring (27); (3) formation of the N-containing side chain of the spermidine unit and epimerization of the two-carbon side chain on the heterocyclic ring forming 28; (4) ring closure leading to the 13-membered ring, thus forming cannabisativine. This sequence, though somewhat lengthy, has necessitated the development of some novel reaction procedures which may be of general use. The first stage (45) begins with addition of an acetylene-containing moiety to pyridine and protection of the amine with a carbobenzoxy group (Cbz), forming the starting material, 25. A photooxygenation reaction leads to a presumed endoperoxide (29) (not isolated), which then undergoes a SnCI,-mediated ring opening of the endoperoxide accompanied by introduction of a nucleophile, leading to 30. This “oxygenation nucleophile introduction reaction” has been shown by the Natsume group to be an excellent method for the regio- and stereoselective formation of

1

CHL=CHOEt SnCI, ~n EtOAc

-5ff,work up

with EtOH

1271

1331

101 dl-c~nnabllallvh.

SCHEME3. Synthesis of racemic cannabisativine. The Natsume approach (45.46).

IZOI

2 . ALKALOIDS IN C A N N A B I S S A T N A L.

87

substituted piperidines (for leading references, see Ref. 42). The stereochemistry of 30 was established by conversion to known compounds. Hydrogenation over Lindlar catalyst reduced the acetylene grouping to an olefinic one, producing 31a. The benzoate 31b on reaction with OsO, at low temperature undergoes dihydroxylation on the side chain only, leading to 26. Apparently the bulky benzoate moiety makes discrimination between the two double bonds possible. The second stage involves blocking of the free hydroxyl groups as MOM ethers, hydrolysis of the benzoate, reduction of the double bond, and mesylation, leading to 32. Finally, elimination of the mesylate group with DBU in toluene gives 27. Thus the double bond and all the substituents are properly situated (though the two-carbon side chain is still not in the required stereochemistry) . In the third stage, the carbobenzoxy group is removed and replaced initially by a propylaminotosylate, leading to 33. This side chain is further elongated by an N-butyltrifluoroacetamide group, forming 34. The twocarbon side chain is then epimerized by base treatment of the parent aldehyde and further oxidized to the methyl carboxylate 28. The last stage involves hydrolysis of all ester groupings, formation of the 13-membered ring, and then removal of all remaining blocking groups to form racemic cannabisativine (9), mp 150-151°C.

C. THEWASSERMAN APPROACH In the mid 1980s, the Wasserman group developed a general route to macrocyclic spermidine alkaloids by a p-lactam-imino ether coupling (47-49). This strategy is of good synthetic flexibility and has made possible the preparation of a number of these alkaloids. In addition to the two Cannabis alkaloids 9 and 10, I also review the synthesis of dihydropalustrine (13), which, although not found in Cannabis, is chemically closely related to 9 and 10.

1. Total Synthesis of Dihydropalustrine The total synthesis of dihydropalustrine (13) (47) (Scheme 4) starts from the protected nine-membered amino lactam 35, a cornerstone of numerous Wasserman syntheses. The imino ether 36 derived from 35 is coupled with the known p-lactam 37 at 145°C to give the central intermediate 38. The 13-membered lactam 39 is produced on reduction of 38 with cyanoborohydride in acetic acid (38). By standard procedures 39 is then converted to the phosphonium salt 40. The side chain is attached through a Wittig

88

RAPHAEL MECHOULAM

N-N TROC

eoc

1401

141)

(441

(42)

BOC =COOt-BU TROC =COOCH,CC I, OH (13)

SCHEME 4. Synthesis of racemic dihydropalustrine. The Wasserman approach ( 4 7 ) .

reaction with the epoxide 41 to give 42. Selective removal of the TROC protective group (presumably initially forming the free secondary amine 43) leads directly to 44. This intramolecular ring closure actually produces a mixture of C-13 diastereoisomers, which were separated by chromatography. Hydrogenation of the cis isomer (indicated in Scheme 4) led to racemic dihydropalustrine (13). The same synthetic strategy was followed in the synthesis of the Cannabis alkaloids.

89

2. ALKALOIDS IN CANNABIS SATIVA L.

2. Synthesis of Racemic Anhydrocannabisativine The initial steps of the synthesis of racemic anhydrocannabisativine (10) (Scheme 5) followed the well-established path leading to dihydropalustrine. The above-described intermediate 38 on reduction with NaBH3CN and protection with BOC led to the macrocycle 45, which was oxidized to the saturated aldehyde 46. A Wittig reaction with the synthone 47 gave a mixture of the ( Z , E ) -'and (E,E)-dienols 48; allylic oxidation produced the unsaturated ketone 49. Removal of the BOC groups with trifluoroacetic acid produced the N-deprotected (E,E)-dienone 50. The desired Z,E isomer, which presumably could be thermally cyclized to the tetrahydropyridine ring, was not detected in the reaction products. On UV irradiation at 0

0

BOC

BOC

OAc

OH

CHO

-

2 MnO ' L (49)

0

(46)

C

(48)

( 10)

SCHEME 5. Synthesis of racemic anhydrocannabisativine. The Wasserman approach (48)

90

RAPHAEL MECHOULAM

254 nm, however, cyclization of 50 does take place, presumably by initial isomerization of the appropriate double bond, leading to racemic anhydrocannabisativine (10).

3. Total Synthesis of Racemic Cannabisativine The total synthesis of racemic cannabisativine (9) (Scheme 6) is an ingenious modification of the synthesis of anhydrocannabisativine (10). The central intermediate (40) (see Scheme 4 above) undergoes a Wittig reaction with the aldehyde epoxy acetate 51 leading to 52, in which the olefin is cis as required. Reaction with zinc in tetrahydrofuran-water at pH 5 caused both removal of the trichloroethoxycarbonyl protecting group and cyclization to produce a mixture of two separable diastereoisomers 53a and 53b. Isomer 53a in which the H-17/H-13 relationship was trans led to racemic cannabisativine (9) on cleavage of the acetyl group and removal of the BOC protecting group.

(9) cannabisativine

(53) a. trans-H-I3/H-17; X=BOC; R=Ac b. cir-H-13/H-I7;X=BOC;R=AC

SCHFME 6. Synthesis of racemic cannabisativine. The Wasserman approach ( 4 9 ) .

2. ALKALOIDS IN CANNABIS SATIVA L.

91

V. Pharmacology

Surprisingly, very little is known about the pharmacological actions of the nitrogen-containing compounds of Cannabis. While thousands of publications have appeared on cannabinoid pharmacology, in particular on the various actions of A’-tetrahydrocannabinol (A‘-THC), the active cannabimimetic principle in Cannabis (50) (for recent reviews, see Refs. 51-54), those on the nitrogenous constituents are very few. Gill et al. (55) investigated crude ethanol extracts of Cannabis leaves and flowers. The petroleum ether-soluble fractions, which contain THC and other cannabinoids, showed cannabimimetic activity. The aqueous extract, obtained after exhaustive petroleum ether extractions, contained no cannabinoids. It was subjected to high-voltage paper electrophoresis. One of the eluates showed acetylcholine antagonizing activity, which had a course of action comparable to that of atropine. The active compound was not isolated, but it was shown to be labile to acids and bases. A further eluate exhibited activity comparable to that of the methyl or ethyl esters of trigonelline (but not that of choline). These esters are gut stimulants, the methyl ester resembling acetyl choline but considerably less active. The authors conclude, mostly on the basis of the pharmacological results, that there is “evidence of the presence of an atropinic and two muscarinic substances in the watery extract.” No further work on these substances has been reported. Klein et al. (28) reported preliminary pharmacological studies on the total mixture of cannabarnines isolated by them (see above). Extracts, containing 0.5% total crude alkaloids, were administered to mice in doses of 0.1-0.5 ml subcutaneously, intraperitoneally, and intravenously. There were no gross signs of toxicity, and no deaths were observed. Mice injected subcutaneously with 0.5 ml showed minimal response in an activity cage over 60 min and only slight activity between 60 and 120 min. No other action was observed. A more recent publication deals with the pharmacological activity of the basic fraction of marijuana whole smoke condensate alone and in combination with A’-THC (56). Whole smoke condensate was obtained in a standard smoking machine; the smoke was trapped in acetone at -60°C. The residue obtained on evaporation of the acetone was taken up in dichloromethane, the acids were removed, and the basic fraction was extracted by standard methods. This basic fraction was administered intravenously to mice at doses of 5 , 10, and 20 mg/kg. Symptoms noted included slight impairment in visual placement, increase in tail pinch response, and decrease in spatial locomation, rearing behavior, and

92

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urination incidence. The treatment did not modify body temperature. It also did not alter the hypothermia caused by the THC. The authors conclude that their results offer “little evidence for the presence of highly active compounds.”

REFERENCES

1. T. Smith and H. Smith, Pharm. J . 6, 171 (1847);T. Smith and H. Smith, Pharm. J . 8, 36 (1848). 2. T. Smith, Pharm. J . 44, 853 (1885). 3. W. Preobraschensky, Pharm. Z . Russ., 705 (1876). 4. G. Dragendorff, Jahresber. Pharmacog. Toxicol. 11, 98 (1876). 5. L. Siebold and T. Bradbury, Pharm. J . 41, 326 (1881). 6. G. W. Kennedy, Pharm. J. 46, 453 (1886). 42, 998 (1883). 7. M. Hay, Pharm. .I. 8. E . Jahns, Arch. Pharm. (Weinheim) 25, 479 (1887). 9. E. Schulze and S. Frankfurt, Chem. Ber. 27, 769 (1894). 10. T. B. Wood, W. T. N. Spivey, and T. H. Easterfield, J . Chem. Soc. 69, 539 (1896). 11. C. R . Marshall. Proc. Cambridge Philos. SOC. 9, 149 (1897). 12. L. HanuS, Acta Univ. Palacki. Olomouc 73, 241 (1975). 13. C. E . Turner, M. A. Elsohly, and E. G. Boeren, J . Nut. Prod. 43, 169 (1980). 14. M. A . EISohly, in “Alkaloids: Chemical and Biological Perspectives” (S. W. Pelletier, ed.), Vol. 3, p. 169. Wiley (Interscience), New York, 1985. 15. K. W. Merz and K. G. Bergner, Arch. Pharm,_(Weinheim) 278, 49 (1940). 16. V. Kwasniewsky, Dtsch. Apothekar Ztg. 94, 1177, (1954). 17. C. A. Salemink, E. Veen, and W. A. de Kloet, Planfa Med. 13, 211 (1965). 18. R. J . J . C. Lousberg and C . A . Salemink, Pharm. Weekbl. 108, 1 (1973). 19. C. A. L. Bercht, R . .I.J. C. Lousberg, F. J. E. M. Kuppers, and C. A. Salemink, Phytochemistry 12, 2457 (1973). 20. M. L. Mole and C. E. Turner, Acta Pharm. Jugoslav. 23, 203 (1973). 21. D . J. Slatkin, N. J. Doorenbos, L. S. Harris, A . N . Masoud, M. W. Quimby, and P. L. Schiff, J. Pharm. Sci. 60, 1891 (1971). 22. R. N. Smith, L. V. Jones, J. S. Brennan, and C. G. Vaughan, J. Pharm. Pharmacol. 29, 126 (1977). 23. Y. Obata, Y . Ishikawa, and R. Kitazawa, Bull. Agric. Chem. SOC.Jpn. 24, 670 (1960). 24. F. S. El-Feraly and C. E. Turner, Phytochemistry 14. 2304 (1975). 25. M. A . ElSohly and C. E. Turner, U. N . Secretariat Document ST/SOA/SER. S/54 (1977). 26. H. Samrah, R. J. J. C. Lousberg, C. A. L. Bercht. C. A . Salemink, M. ten Ham, and J. van Noordwijk, U. N. Secretariat Document ST/SOA/SER.S/34 (1972). 27. J. Cabo, J . Jimenez, and M. V. Toro, Pharm. Mediterranea 10, 747 (1974); J . Cabo, J. Jimenez, and M. V. Toro, Pharm. Mediterranea 12, 285 (1978). 28. F. K. Klein, H. Rapoport, and H. W. Elliot, Nature (London) 232, 258 (1971). 29. H . L. Lotter, D . J. Abraham, C. E . Turner, J . E . Knapp, P. L. Schiff, and D. J. Slatkin, Tetrahedron Lett., 2815 (1975). 30. C. E . Turner, M.-F. H. Hsu, J . E. Knapp, P. L. Schiff, and D. J . Slatkin, J . Pharm. Sci. 65, 1084 (1976).

2, ALKALOIDS IN CANNABIS SATIVA L.

93

31. M. A. EISohly, C. E. Turner, C. H. Phoebe, J. E. Knapp, P. L. Schiff, and D . J. Slatkin, J . Pharm. Sci. 67, 124 (1978). 32. A. Guggisberg and M. Hesse, in “The Alkaloids” (A. Brossi, ed.), Vol. 22, p. 85. Academic Press, New York, 1983. 33. H. H. Wasserman and J. S. Wu, Heterocycles 17, 581 (1982). 34. B. M. Trost and J. J. Cossy, J . Am. Chem. SOC.104, 6881 (1982). 35. H. H. Wasserman, R. P. Robinson, and C. G . Carter, J . A m . Chem. Soc. 105, 1697 (1983). 36. B. Nader, R. W. Franck, and S. W. Weinreb, J . Am. Chem. SOC.102, 1153, (1980). 37. B. Nader, T. R. Baley, R. W. Franck, and S . W. Weinreb, J . A m . Chem SOC.103,7573 (1981). 38. H. H. Wasserman and H. Matsuyama, J . Am. Chem. SOC.103, 461, (1981). 39. H. H. Wasserman and G . D. Berger, Tetrahedron 39, 2459 (1983). 40. H. H. Wasserman, R. K. Brunner, J. D. Buynak, C. G. Carter, T. Oku, and R. P. Robinson, J . Am. Chem. SOC.107, 519 (1985). 41. L. Crombie, R. C. F. Jones and D. Haigh, Tetrahedron Lett. 27, 5151 (1986). 42. T. R. Bailey, R. S. Garigipati, J. A. Morton, and S. M. Weinreb, J . Am. Chem. SOC.106, 3240 (1984). 43. S. M. Weinreb and R. R. Staib, Tetrahedron 38, 3087 (1982). 44. D. Seyferth and J. Pornet, J . Org. Chem. 45, 1721 (1980). 45. M. Natsume and M. Ogawa, Heterocycles 20, 601 (1983). 46. M. Ogawa, M. Kuriya, and M. Natsume, Tetrahedron Lett. 25, 969 (1984). 47. H. H. Wasserman, M. R. Leadbetter, and I. E. Kopka, Tetrahedron Lett. 25, 2391 (1984). 48. H. H. Wasserman and B. C. Pearce, Tetrahedron Lett. 26, 2237 (1985). 49. H. H. Wasserman and M. R. Leadbetter, Telrahedrun Lett. 25, 2241 (1985). SO. Y. Gaoni and R. Mechoulam, J.-Am. Chem. SOC. 86, 1646 (1964). 51. B. R. Martin, Pharmacol. Rev. 38, 45 (1986). 52. W. L. Dewey, Pharmacol. Rev. 38, 151 (1986). 53. R. Mechoulam (ed.), “Cannabinoids as Therapeutic Agents.” CRC Press, Boca Raton, Florida, 1986. 54. R. Mechoulam and J. J. Feigenbaum, in “Progress in Medicinal Chemistry” (G. P. Ellis and C. B. West, eds.), Vol. 24, p. 159. Elsevier, Amsterdam, 1987. 55. E. W. Gill, W. D. M. Paton, and R. G. Pertwee, Nature (London) 228, 134 (1970). 56. J. M. Johnson, L. Lemberger, M. Novotny, R. B. Forney, W. S. Dalton, and M. P. Mascarinec, Toxicol. Appl. Pharmacol. 72, 440 (1984).

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ACONITUM ALKALOIDS TAKASHI AMIYAAND HIDEOBANDO Hokkaido Institute of Pharmaceutical Sciences Otaru, Japan

I. Introduction 11. Chemical Reactivity and Synthesis A. Aconitine-Type Alkaloids B. Lycoctonine-Type Alkaloids C. C,o -Diterpenoid Alkaloids D. Synthetic Investigations 111. Pharmacology A. Toxicity B. Arrhythmic Activity C. Cardioactivity D. Analgesic Activity E. Other Biological Activities IV. Analytical Methodology V . Tabulation References

I. Introduction The diterpenoid alkaloids isolated from Aconitum species can be divided into two groups: those based on a hexacyclic C,, skeleton and those based on a C,, skeleton. The C1, diterpenoid alkaloids can be further divided into two types: aconitine-type alkaloids, represented by aconitine, in which

Aconi t i n e

Lycoctonine

95 THE ALKALOIDS, VOL. 34 Copyright 0 1988 by Academic Press, Inc. All rinhts of reproduction in anv form reserved.

96

TAKASHI AMIYA AND HIDE0 B A N D 0

Veatchine

A t i s i ne

the C-7 position is not occupied by any substituent group and lycoctoninetype alkaloids, represented by lycoctonine, in which the C-7 position is always oxygenated. The C2,, diterpenoid alkaloids are divided into atisinetype alkaloids, represented by atisine, in which rings C and D are six membered and veatchine-type alkaloids, represented by veatchine, in which ring D is five membered. Chasmanine is one of the most complex aconitine-type alkaloids. In 1977 the stereospecific total synthesis of this alkaloid was achieved by Wiesner and co-workers. Since then, an improved stereospecific total synthesis has been published. At present time the structures of newly isolated compounds have determined by 13C-NMR spectroscopy and X-ray analysis. Recently, the configuration at the C-1 methoxy group in lycoctonine and related alkaloids has been shown to be the inverse of that previously accepted. The pharmacology of alkaloids from Aconitum species has been considered primarily in terms of toxicity and Oriental medicinal purposes. Earlier work on the chemistry of diterpenoid dkaloids was reviewed in Volumes 17 and 18 of this treatise by Pelletier and Mody (1,2). There are many diterpenoid alkaloids isolated from various genera, Aconitum, Delphinium, Anopterus, and Spiraea. This chapter deals with recent developments in the study of alkaloids obtained from Aconitum species in connection with the earlier work by Pelletier. Other reviews related to diterpenoid alkaloids are available (3). 11. Chemical Reactivity and Synthesis

A. ACONITINE-TYPE ALKALOIDS

1. Structure of Nitronitrosoaconitinic Acid Oxidation of aconitine (1) with nitric acid was first reported by Brady ( 4 ) . The oxidation of aconitine (I), mesaconitine (2), and their derivatives ‘yielded a nitro-N-nitroso derivative (3), which was called nitronitrosoaconitinic acid by Suginome (5).The structure of 3 confirmed by spectroscopic

3. ACONITUM ALKALOIDS

97

TH3 ... OH

1 R 2

=

C2H5 Aconitine

R = CH3

Mesaconitine

3 Nitronitrosoaconitinic acid

methods (6), represents a new type of acid, 2-nitro-2-cyclohexen-l-one, with a pK, value comparable to those of carboxylic acids. The location of the nitro group at C-2 could be determined h i the reaction pathway involving an intermediate compound, isonitroso-mesaconitinone (4) (7). Oxidation of mesaconitine (2) with chromic anhydride in acetone followed by treatment with isoamyl nitrite gave 4, which was oxidized to nitronitrosoaconitinic acid (3) with dinitrogen trioxide followed by nitric acid. OH

*

7

OCH3

OCH,.3

4 Isonitroso-mesaconitinone

2. Oxonitine a. Mechanism of Formation of Oxonitine by Permanganate Oxidation of Aconitine. Aconitine (1) was converted to oxonitine (5) by permanganate oxidation (1). Two mechanisms for this oxidation under different reaction conditions were suggested by isotopic labeling studies (8,9). Oxidation carried out at room temperature in acetone-water (95 : 5 v/v) with potassium permanganate generates an enamine (7) through an intermediate product (6), and oxidative cleavage of 7 may give oxonitine (5) ( 8 ) . On the other hand, hydrolysis of the immonium salt (6) to form the N-desethyl compound (8) might be the preferred reaction when a greater amount of water is present ( 9 ) . Pelletier et al. showed that the formyl group of oxonitine (5) was derived from acetone and the methyl group of the N-ethyl of aconitine (1) ( 9 ) .

98

TAKASHI AMIYA AND HIDE0 B A N D 0

OCH3

5 Oxonitine

6

7 R = -CH=CH2 8 R = H

b. Formation of an N-CHO Group from an N-CH3 Group by Osmium Tetroxide Oxidation. Oxidation of mesaconitine (2) with osmium tetroxide afforded oxonitine ( 5 ) in nearly quantitative yield (92%) (10).

3 . Partial Synthesis of Isodelphinine, Penduline, and Nagarine Conversion of chasmanine (9) to isodelphinine (10) was achieved by two alternative methods (11,12). Diacetylchasmanine (ll),obtained from chasmanine (9) by acetylation, was oxidized with potassium permanganate to a N-desethyl derivative (12), which on treatment with formic acid gave

OCH3

9 Chasnianine

"

dCH3

'

10 I s o d e l o h i n i n e

99

3. ACONITUM ALKALOIDS

an N-formyl derivative (13). Pyrolysis of 13 yielded an olefinic compound (14), which was then converted to epoxide 15 by oxidation with mchloroperbenzoic acid. Epoxide 15 was unexpectedly transformed to cis-diol 16 with formic acid, in a reaction assumed to proceed via a tertiary carbonium at C-8. The alcohol (16) was oxidized by Swern’s method, and the ketone 18 obtained was reduced with lithium aluminum hydride. Thus, isodelphonine (18) was obtained together with 15-epiisodelphonine (19).

’

7

OCH3

OCH3

14

16

15

17

A more efficient synthetic approach to isodelphonine (18) was also reported (12). Compound 12 was methylated with formaldehyde and NaBH3CN to give the N-methyl1 derivative (20). Pyrolysis of 20 yielded olefin 21, which was treated with osmium tetroxide to give cis-diol 22.

100

TAKASHI AMIYA AND HIDE0 B A N D 0

OH

0ch3 18 Isodelphonine

20

22

0ch3

19

21

23

Compound 22 was oxidized to ketone 23 by Swern's method, and reduction of 23 with LiAI(OMe),H gave isodelphonine (18). Selective O-acylation of isodelphonine (IS) to give the naturally occurring alkaloid isodelphinine (10) was studied as follows. Compound 18 was treated with benzoyl chloride in dry pyridine-methylene chloride at -70 to 0°C to give a 14-monobenzoate (24). The C-15 hydroxyl group in 24 was protected with a P,P,P-trichloroethoxycarbonyl group to produce 25, after which the hydroxyl group at C-8 was acetylated with acetyl chloride to give 26. Treatment of 26 with zinc in acetic acid afforded isodelphinine (10). In their synthetic study of chasmanine (9), Wiesner et al. (I3,29,36) also achieved a formal total synthesis of isodelphinine (10). Penduline (27) was

101

3. ACONITUM ALKALOIDS

I

OCH3

OCH3

24

25

dCH3

26

"Ln3 27

Pendul i n e

28

also synthesized from diacetylchasmanine (11) analogously via 28, 31a, 32, 33, and 34. Compound 29 obtained from 28 was transformed to 31a as in the case of the synthesis of chasmanine (9). Compound 31b corresponds to 15epiisodelphonine (19). Isodelphinine (10) has also been prepared from mesaconitine (2) as follows ( 1 4 ) . Treatment of mesaconitine (2) with trifluoromethanesulfonic anhydride gave 35. A solution of compound 35 dissolved in aqueous hexamethylphosphoric triamide was irradiated with a 2.537-A lamp to give dehydro compound 36. On catalytic hydrogenation, 36 absorbed 1 mol of H2 to give isodelphinine (10). Penduline (27) was prepared from aconitine (1) by the same type of reaction (15).

102

TAKASHI AMIYA AND HIDE0 BAND0

0ch3 29 R 1

=

Ac, R 2

= a-OH

32

R 1 = Ac, R 2 ==O 3 1 a R 1 = H, R 2 = a - O H 1 2 31b R = H, R = U-OH

30

'OCOOCH2CC1

0ch3

dCH3

34

33

R

"OH

OCH3 35

R = OS02CF3

36

R = H

Pyrolysis of delphisine (38) afforded pyrodelphisine (39), which was hydrolyzed to pyroneoline (40). Compound 40 was oxidized with osmium tetroxide to give nagarine (37) (16). 4. Partial Synthesis of Aconosine

Cammaconine (41) was oxidized to 42, which was decarboxylated to give 43. Compound 43 was reduced with lithium aluminum hydride to aconosine (44) (17).

3. ACONITUM ALKALOIDS

OCH3 37 Nagarine

38 Delphisine

OCH3 39 R 40 R

= =

Ac Pyrodelphinine H Pyroneoline

R 41 R = CH20H Cammaconine

Aconosine

44 R = H

42 R

=

C02H

43R=H

103

104

TAKASHI AMIYA AND H I D E 0 B A N D 0

5 . Mild Alkaline Hydrolysis of Aconitine Hydrolysis of aconitine (1) with 40 mM K&03 in 90% methanol at room temperature afforded aconine (45), 8-O-methylaconine (46) , desbenzoylpyraconitine (47), and 16-epidesbenzoylpyraconitine (48). The yield of 47 and 48, which were minor products, was increased by heating a solution of aconitine (1) in 40 mM K2C03 in 90% ethanol (18). OH ..-....___ /OCH3

OCH,

1 R1 = 1 45 R = 46 R

1

J

5

2 Ac; R =Bz A c o n i t i n e 2 R = H Aconine

= CH3;

R2

=

47 R 1 1 48 R

=

OCH3;

= H,

R

2

R2 =

=

H

OCH3

H

B. LYCOCTONINE-TYPE ALKALOIDS 1. Some Reactions of Anhydrodiacetyldelcosine

Delcosine (49) was converted to anhydrodiacetyldelcosine (50) on treatment with acetyl chloride, and 50 was reduced to dihydroanhydrodiacetyldelcosine (51) by catalytic hydrogenation (19). Reduction of 50 with lithium aluminum hydride gave 52. Hydrolysis of 5 1 yielded 53, and reduction of 53 with lithium aluminum hydride followed by acetylation with acetic anhydride in pyridine gave 54.

2. Rearrangement of 4-Amino-4-des(oxyrnethylene)anhydrolycoctonam Reactions of lycoctonamic acid (S), a derivative of lycoctonine (56), have been described (20,21). Compound 55 gave anhydrolycoctonamic acid (57) by pinacolic dehydration on treatment with a mixture of dilute sulfuric acid and acetic acid. Acid 57 was transformed to 4-amino-4des(oxymethy1ene)anhydrolycoctonam (58) by Curtius or Hofmann degradation. On treatment with nitrous acid, 58 gave hydroxy keto lactam 59 and aldehydo lactam acid 60 in 50 and 20% yield, respectively, with

105

3. ACONITUM ALKALOIDS

.---. .... .

.

___

..__...-

*.

N

0ch3

OCH3

0ch3

OCH3

50 R1

49 Del cosine

52 R1

=

R3

=

OAc; R2

= R3 = OH; R

2

0

=

= OH,H

53R 1 = R 3 = O H ; R 2 = O

54 R1

=

R3

=

OAc; R 2

=

OAc,H

OCH3

OH

0ch3

55

56 Lycoctonine

0ch3 .1..

'

. I

C

H

._

_...... N

2 5

,

_._.. --..

:

/R

0 OCH3

57 R = COOH 58

OCH3

0

59 R

= CH20H

0

60 R

= CHO

R = NH2

significant skeletal rearrangements. These compounds were converted to the derivatives, the structures of which were determined by X-ray analysis. The X-ray analysis showed that the configuration at C-1 in lycoctoninetype alkaloids was the inverse of that accepted previously.

106

TAKASHI AMIYA AND HIDE0 B A N D 0

C. C2,, DITERPENOID ALKALOIDS

1. Michael Addition of Secondary Amines to Exocyclic a$-Unsaturated Ketones

In the presence of neutral alumina, atisinone (61) added diethylamine to afford 62. Addition of diethylamine to dihyroatisinone (63) proceeded with quantitative yield in the same way (22).

61

62

63

2. Degradation of Atisine to C2()Aminocarbinol Reduction of atisinium chloride (64) with sodium borohydride yielded dihydroatisine (65), which when treated with cyanogen bromide afforded 66a. Compound 66a was hydrolyzed to aminoalcohol 67b (23).

3 . Formation of the Oxazolidine Ring in C2()Diterpenoid Alkaloids Dihydroatisine (65) possessing the N-CH2CH20Hgroup, was converted to isoatisine (67) and atisine (68), which contain oxazolidine rings, by oxidative cyclization. Other C2,, diterpenoids with N-CH,CH20H groups were shown to react with silver oxide in the same way (24).

4. Conformational Analysis of the Oxazolidine Ring of C20 Diterpenoid Alkaloids The behavior of the oxazolidine ring of atisine (68) in hydrogen-bonding and nonhydrogen-bonding solvents was studied by chemical and spectral methods. A solution of 68 in methanol exists in equilibrium with 67 and 69-72 (25).

3. ACQNITUM ALKALOIDS

64 Atisinium chloride

65 Dihydroatisine

68 Atisine

107

66a R = CN 67b R = H

67 Isoatisine CH30H

108

TAKASHI AMIYA AND HIDE0 B A N D 0

5. Novel Rearrangement Products of Hetisine Acid-catalyzed rearrangement of hetisine (73) was studied. The structures of four products were determined by spectroscopic methods including X-ray crystallography. A mechanism for the formation of these products (76, 77, 82, and 83) was proposed (26,27). The reactions proceeded through intermediates 74, 75, 78, 79, 80, and 81.

76

74

HO

73 R = a-OH H e t i s i n e

77

75

+

74

HO.,

$

78 CHo\\ ...

e

Ho3.

I..

,;':

N .......

.<

H

'.

3

t -

79

HO \

3H: $

HO.

"..... , ,

N

,.,, ~

I

. .___

R

-H+

t -

+H+

80

81

>,

........

!

HO..,

N

.......

..

82 R = a-OH

8 3 R = B-OH

H3

3. A C O N l T U M ALKALOIDS

109

6. Transformation of Pseudokobusine to Kobusine Pseudokobusine (84) was converted to kobusine (91) (28). Pseudokobusine (84) was first treated with trichloroethyl chloroformate to give ketocarbamate 85. Compound 85 was acetylated with acetic anhydride and pyridine to the diacetyl derivative 86. Sodium borohydride reduction of 86 yielded alcohol 87, which was converted to diacetylsecodihydropseudokobusine (88) by treatment with zinc in acetic acid at room temperature. Dehydration of 88 with thionyl chloride gave cyclic sulfinyl derivative 89 and diacetylkobusine (90), this reaction proceeding through 89 to give diacetylkobusine (90), which was hydrolyzed to kobusine (91).

OR2

1

2

= H R2 = Ac 1 2 91R = R = H

84 R 90 R

1

=OH; R = H;

85R=H 86 R = A c

87 R = C02CH2CC13 88R=H

D . SYNTHETIC INVESTIGATIONS

1. Total Synthesis of Chasmanine Starting from intermediate 92 the total synthesis of chasmanine (93) was achieved by Wiesner and co-workers (29). This synthesis was preliminarily studied by using model compounds (30). Photoaddition of vinyl acetate to 94 yielded a mixture of 95 and 96 in 96% yield. Compound 95, purified by

110

TAKASHI AMIYA AND HIDE0 BAND0

93

92

94

95

96

Br

98

97

crystallization, was brominated to 97 quantitatively. Dehydrobromination of 97 gave 98 in 82% yield. Treatment of 98 with dilute methanolic potassium hydroxide gave a mixture of epimeric alcohols 99 and 100 by retro-aldol cleavage of the hydrolysis product followed by aldol condensation of the resulting unsaturated keto aldehyde. Benzoylation of 99 and 100 gave a mixture of 101 and 102. Hydrogenation of the epimeric mixture (101 and 102) gave hexahydrobenzoates 103 and 104. The keto ester (103) was

99 R

100 R 101 R 102 R

1 1 1

1

=

OH; R 2

=

H; R 2 = OH

+

= H

= OCOC H5; R = H;R

2 = H

= OCOC6H5

111

3. ACONlTUM ALKALOIDS

103

104

105 R = COC6H11 106

R

107

= H

converted to 105, which was saponified to 106. Compound 106 was oxidized to 107. The overall yield of 107 from 94 was 13.7%. Reduction of 107 with sodium borohydride proceeded stereospecifically and yielded 108 quantitatively. Methylation of 108 gave the previously obtained methoxyl ketal(lO9). This compound was used as an intermediate in the synthesis of the aconitine model (110) (31). Compound 107 was also obtained by an alterate route from 94. The vinyl acetate adduct mixture of 95 and 96 was brominated to give a mixture of 97 and 111, which were converted to 98 and 112 by dehydrobromination. Base treatment of 98 and 112 followed by benzoylation gave the benzoates 101, 102, and 113, which were purified by chromatography. The conversion of both 101 and 102 to 107 was carried out as described above.

OR 108 R = H 109 R = CH3

OA c

110

112 R = Ac

111

113 R = COC6H5

A stereospecific synthesis of racemic chasmanine was also studied (29). The racemic intermediate (92) was reduced with lithium in a mixture of tetrahydrofuran and liquid ammonia. The dihydro compound was acetylated, and then the product was treated with 0.6 N methanolic hydrochloric acid. A series of these reactions gave compound 114 exclusively in 72% yield. Photoaddition of allene to compound 114 gave the stereospecific adduct (115) (86%), which was converted quantitatively to ketal 116 with ethylene glycol and p-toluenesulfonic acid. Compound 116 was ozonized, and the resulting product was reduced with sodium borohydride to alcohol 117. This alcohol (117) was acetylated to 118, which was treated with 0.1 N methanolic hydrochloric acid to yield ketone 119 in an overall yield of 72% from 116.

112

TAKASHI AMIYA AND HIDE0 BAND0

115 R 116 R

114

=

=

0

]:I

117 R1

=

118 R 1 119 R 1

=

]:I ]:I

=

; R2 = H ;

R

0; R 2

=

2

= Ac

Ac

Br

120

Compound 119 was brominated to yield 120 in 80% yield. Compound 120 was then heated with LiBr and Li,CO, to give the a,P-unsaturated ketone (121) in 87% yield. When 121 was treated with 0.3 N aqueous methanolic sodium hydroxide, a mixture of epimeric aldols (122) was obtained in 90% yield. Acetylation of 122 gave the acetate (123) quantitatively. Stereospecific hydrogenation of the acetate (123) was carried out

121

122 R = H 123 R

=

OAc

113

3 . ACONITUM ALKALOIDS

with rhodium on alumina at room temperature &d 95 psi. The hydrogenated products were oxidized with chromium trioxide in pyridine to give the epimeric acetates (124) in an overall yield of 86%. The epimeric acetates (124) were transformed to 125 in 80% yield by (1) formation of the acetals, (2) saponification with dilute potassium hydroxide solution, and ( 3 ) oxidation with chromium trioxide in pyridine. OA c

CH3C0

OCH3 OCH3

124

125

Stereospecific reduction of 125 was accomplished with sodium borohydride to yield 126 quantitatively. Compound 126 was methylated with sodium hydride and methyl iodide to give the methoxyacetal (127) in 82% yield, and then 127 was converted to the ketone (129) by heating in 80% acetic acid. Bromination of 129 gave the bromoketone (130) in 82% yield. The latter was converted to the acetal (128) quantitatively with diethylene orthocarbonate and p-toluenesulfonic acid in chloroform. Rearrangement of the bromoketal (128) yielded the 0x0-pyrochasmanine derivative (131) in 85% yield. H

//

OR1 H

OCH3

V

CH3CO-

dCH3

126 R 1 = H ; R 2 = H 127 R 1 = CH3; R 2 = H

128 R 1 = CH3; R 2

= Br

129 R

= H

130 R = Br

114

TAKASHI AMIYA AND HIDE0 B A N D 0

The transformation of 128 to 131 was accomplished by refluxing in a mixture of xylene and dimethyl sulfoxide (1: 1) in the presence of large excess of 1.5-diazabicyclo[3.4.0]non-4-ene. Oxymercuration of 131 gave 132 (65% yield), which was identical by IR, NMR, mass spectrometry, and TLC to the corresponding optically active derivative prepared from natural

CH3CO--

CH3CO-.

131

132

chasmanine. Compound 132 was heated in 80% acetic acid to give racemic 14-dehydro-a-oxochasmanine (133). Reduction of optically active 14dehydro-a-oxochasmanine (133) with lithium aluminum hydride gave chasmanine (93), which was found to be identical to the natural alkaloid by comparison of IR, TLC, mass, and NMR characteristics and by mixed melting point determinations.

6CH3 133

By 1978 Wiesner and co-workers (13,32,33) had developed a fundamentally different synthesis of chasmanine (93). In this approach they studied a model system starting with compound 134. Treatment of 134 with

134

135

136 R = CH3 137 R = H

115

3. A C O N Z T U M ALKALOIDS

triethyl phosphite followed by reduction with sodium in liquid ammonia gave 135. Hydrogenation of 135 in the presence of palladium yielded 136. Boron tribromide cleaved the methoxyl group in 136 to yield the phenol (137) in 89% yield. Compound 137 was converted to the dithian derivative (138) in 89% yield by treating with N-chlorosuccinimide and 1,3-dithian. Compound 138 was alkylated to 139 on treatment with methyl bromoacetate. When treated with mercuric oxide, 139 gave 140 in high yield.

142 R1 = OH; R 2 = C H 2 C 0 2 H

RO

3.

@$ 144 R

0 = CH2-C6H5

Oxidation of 140 with rn-chloroperbenzoic acid provided 141 in 86% yield. The latter was saponified to 142. Oxidation of 142 by N-bromosuccinimide gave spirolactone 143, which was treated with benzyl vinyl ether to give the epimeric adducts (144) in an overall yield (from 142) of more than 80%. Adducts 144 were hydrolyzed by potassium carbonate to yield epimers 145, which were converted to 146. Epimers 146 were reduced by lithium borohydride to a mixture of the epimeric alcohols (147), which were

145 R = CH2-C6H5

146 R = C H -C H

2 6 5

147 R = CF2-C6H5

treated with acetylacetone. The products obtained (148) were transformed to the corresponding mesylates (149), which were then reduced to the aldols (99 and 100) mentioned above. Compounds 99 and 100 were subjected to a sequence of reactions analogous to the one described above.

116

TAKASHI AMIYA AND HIDE0 B A N D 0

148 R1 = CH2-C6H5; 1 149 R = CH2-C6H5;

R2 = OH R

2

= OMS

In 1978 Wiesner and co-workers reported the direct synthesis of 13-desoxydelphonine (150) and a formal synthesis of chasmanine (93) by the new method related to the above-mentioned model system (13). The starting material was aromatic intermediate 151, which was prepared from vanillin by the aziridine rearrangement method (see Ref. 3 4 ) . Compound 151 was treated with sodium thioethoxide in dimethylformamide to give the phenol (152) in 95% yield. Phenol 152 was reacted with methyl

150

151 R = CH3

152 R = H 153 R = CH2-COOCH3

bromoacetate to provide ester 153 in 90% yield. Hydrolysis of 153 with hydrochloric acid followed by oxidation with N-bromosuccinimide gave the corresponding masked o-quinones, which were converted to adducts 154a and 154b on treatment with an excess of benzyl vinyl ester, in an overall yield of 70%. The adducts were separated by preparative TLC. The mixture of 154a and 154b was reduced with zinc in glacial acetic acid to 155 in 85% yield. Hydrogenolysis removed the benzyl group in 155 to give a mixture of epimeric alcohols (156) in 96% yield. On treatment with acetic anhydride in pyridine the alcohols (156) were acetylated to the acetates (157) in 80% yield after crystallization. Compounds 157 were transformed to 158 by stereospecific hydrogenation with rhodium on alumina at 85 psi, followed by oxidation with chromium trioxide in pyridine (88% yield).

117

3. ACONITUM ALKALOIDS

154a R = H 154b R

=

155

Br AcO

RO

t.

i.

H

156 R

=

157 R

= Ac

I58

Following reflux with p-toluenesulfonic acid and ethylene glycol in benzene, hydrolysis with methanolic sodium hydroxide, and oxidation with chromium trioxide in pyridine, compounds 158 gave the keto acetal (159) in over 88% yield. Reduction of 159 with sodium borohydride quantitatively yielded the alcohol (160), which was converted to 161 with sodium

OCH3 159

160 R 1 = H ; R 2

=

H

161 R1 = CH ; R 2 = H 3 2 162 R 1 = CH3; R = Br

118

TAKASHI AMIYA AND H I D E 0 B A N D 0

hydride and methyl iodide. On heating in 80% acetic acid 161 gave the corresponding ketone (162) (94% yield), which was brominated to 163 in 90% yield. The bromoketone (163) was converted to 164 (80%) by reflux with ethylene glycol and p-toluenesulfonic acid. Compound 164 was heated with DBN in mixture of dimethyl sulfoxide and xylene./The rearranged compound (165), obtained in 89% yield, was subjected to oxymercuration followed by sodium borohydride reduction. The alcohol (166), obtained in 65% yield, was heated in 80% acetic acid to give 167 quantitatively. The keto lactam (167) was reduced with lithium aluminum hydride to 150 in 64% yield after recrystallization from hexane. The racemic synthetic 13-desoxydelphonine (150) was identical with a compound of the same structure derived from the natural product (35). In 1979 the aromatic intermediate (151) was prepared from o-cresol (168) by the preferred route as follows (36). Compound 168 was converted

dCH,

J

OCH3 162 R

=

163 R

=

165

166 R , R = Ethyleneacetal 167 R , R = Carbonyl

H H 3 q OC 168

Fi2

CH2-CO-0 P c H 3 169

@CH3 O R

170 R = H 171 R = CH3

119

3 . ACONITUM ALKALOIDS

to the 3-chloropropionyl ester (169) on treatment with 3-chloropropionyl chloride. The latter was treated with aluminum chloride to give the indane (170). Methylation of 170 with dimethyl sulfate yielded 171. On treatment with trimethyl orthoformate, 171 was converted to the dimethyl acetal, which was transformed to the enol ester (172) by pyrolytic elimination of methanol. By carboxylation with n-butyl lithium and carbon dioxide and subsequent hydrolysis of the enol ester group, 172 provided the keto acid (173). Compound 173 was reduced with sodium borohydride to which was heated with phosphoric acid followed by esterification with methanolic hydrogen bromide to give the ester (175a), which exists in an equilibrium mixture with 175b. This mixture was added to maleic anhydride to give 176 quantitatively. Decarboxylation of 176 with bis(tripheny1phosphine)nickel carbonyl gave 177 in 85% yield.

d4,

HOOC,

/c\

" I72

OCH3

173

175a

I

I

OH

OCH3

174

176

CH300C

Q

C OCH3 H

3

175b

On treatment with trimethylsilyl azide followed by acetic acid and acetic anhydride, 177 gave the acetylaziridine (178). Compound 178 was rearranged by heating to give 179 in 70% yield. Compound 179 was oxidized with ceric ammonium nitrate in aqueous acetic acid to give the aldehyde

120

TAKASHI AMIYA AND HIDE0 B A N D 0

117

178

179

(180) in 75% yield. On treatment with methanol in the presence of potassium carbonate, 180 gave the alcohol (181), which was benzylated to 182. Oxidation of 182 with rn-chloroperbenzoic acid produced the formate ester (183), which was hydrolyzed in the presence of potassium carbonate to yield 184; the two steps were carried out in 96% yield. Alkylation of 184 with chloromethyl methyl ether yielded 185 in 93% yield.

OCH3

CH300C#cHo CH,$HN'

0 O - C H z O

bR 180 R = Ac

183 R = CHO

181 R = H

184 R = H

182 R = CHz-C6H5

185 R

=

CH2-O-CH3

By reduction with lithium borohydride, followed by reoxidation with dimethyl sulfoxide and dicyclohexylcarbodiimide, compound 185 was converted to 186. The overall yield of these reaction products was 86%. Compound 186 was reacted with 3-benzoyloxy-4-methoxy-n-butylmagnesium bromide to give the epimeric alcohols (187) in 87% yield. Alcohols 187 were then acetylated to the acetates (188), which were hydrogenolyzed over palladium to the diols (189). Oxidation of 189 with the pyridinechromium trioxide complex in dichloromethane gave the epimeric ketones (190) in 85% yield. On treatment of 190 with boiling methanol containing potassium carbonate, the a,P-unsaturated ketones (191) were obtained in 90% yield. Photoaddition of vinyl acetate to 191 gave the adducts (192) in 95% yield. Hydrolysis of 192 with base was followed by retro-aldol cleavage. The products (193) were obtained in 97% yield. Compounds 193 were converted to 194 and then acetylated to 195, which were heated to eliminate methanol. The yield of the products obtained (196) was 92%.

121

3. ACONITUM ALKALOIDS

0-CH2-C6H5

0-CH2-C6H5 186

187

R

=

H

188 R = Ac

CH30

190 -CH2-O-CH3

@

CH3

CH3C@--. "NH CH30

'OAc 192

1

0

C HO

193

Oxidation of 196 with permanganate-periodide followed by esterification with diazomethane gave the esters (197) in 81% yield. Epimers 197 were heated with dilute methanolic sodium methoxide under reflux to give 198 in 85% yield. Oxidation of the epimeric hydroxylactams (198) with the

122

TAKASHI AMIYA AND H I D E 0 B A N D 0

OCH3

'OCH,

196

194 R = H 195 R -

=

OAc

CH3d

19 7

198

pyridine-chromium trioxide complex yielded 199. Reduction of 199 with tri-tert-butoxyaluminum hydride gave 200, which was methylated with sodium hydride and methyl iodide to provide the aromatic intermediate (151).

199

200

2. Stereospecific Synthesis of Napelline Wiesner and co-workers (32) carried out a study aimed at the total synthesis of napelline (201). On treatment of 145 with trimethylsilylmethylmagnesium chloride, the epimeric alcohols (202) were obtained. Epimers 202 gave 203 on warming with methanolic perchloric acid. Reduction of 203 with sodium borohydride in methanol yielded 204 in 74% yield. On acetylation of 204 with acetic anhydride in pyridine, the acetates (205) were obtained. Hydrogenolysis of 205 over palladium on charcoal gave the

123

3. A C O N I T U M ALKALOIDS

OH

C6H5-CH -0

\,~

202

CH3

20I

alcohols (206), which were transformed to the mesylates (207). Epimers 207 were heated with glacial acetic acid, and a mixture of the rearranged epimers (208) was obtained in 90% yield. Saponification of 208 gave 209, which were oxidized with the chromium trioxide-pyridine complex to the diketone (210). Compound 210 was identical to the same compound previously synthesized by the other method (37).

203

204 R = H 205 R = Ac

RO

&CH3

“OAc

206 R = H 207 R = Ms

b.,,... & ‘

.

.

*:

-

.,CH3

“OR

208 R = Ac 209 R = H

0 2 10

Alternatively, the aromatic intermediate (211) (38) was reduced with lithium borohydride to the alcohol (212), which was heated with 6 N HC1 to give 213. These reactions proceeded quantitatively. Treatment of 213 with CaC03 and T1(N03)3 in tetrahydrofuran gave the quinone acetal (214) in 95% yield. Compound 214 was heated with benzyl vinyl ether to yield adducts 215. Epimers 215 were converted to the tetrahydropyranyl derivatives (216) by treatment with dihydropyran and pyridinium p toluenesulfonate quantitatively. On treatment of 216 with trimethylsilylmethylmagnesium chloride, the two epimeric alcohols (217) were

124

TAKASHI AMIYA AND H I D E 0 B A N D 0

211 R 1 OR1

R3

OR^

= CH20CH3

R~ = C H ~ C O ~ C H ~ = THP

212 R1 = CH20CH3

2

R = CH2CH20H R 3 = THP 1 3 213R = R = H

CH, J

CH3

R L = CH2CH20H

2 14 c6H5-cH2-0\.

215 R 216 R

= H

OH

217

= THP

obtained in 84% yield. These compounds were heated with 70% HC104 to give the a$-unsaturated ketones (218) in 85% yield. Compounds 218 were reduced with lithium borohydride and then acetylated with acetic anhydride and pyridine to give the diacetates (219). C6H5-CH -0

2

1

219 R 1 2 R 1 220 R

= CH2-C6H5 = COCH3 = H

R L = COCH3

221 R1 R2

=

Ms

= COCH3

218

Hydrogenolysis of 219 with palladium on charcoal in methanol gave the alcohols (220), which were mesylated with mesyl chloride and triethylamine to 221. By refluxing 221 in glacial acetic acid, the rearranged products (222) were obtained in 95% yield. Epimers 222 were saponified with 5% methanolic potassium hydroxide to 223, which were oxidized with the chronium trioxide-pyridine complex to 224. These two reactions

125

3. ACONITUM ALKALOIDS

222 R = COCH3 223 R = H

224

0

225

226

were carried out in an overall yield of 90%. Hydrogenation of 224 with palladium on calcium carbonate in ethanol gave 225 quantitatively. Compound 225 was reduced with lithium aluminum hydride to dihydronapelline (226) as shown previously (39). Dihydronapelline (226) has been transformed to napelline (201) (40). 3. Synthetic Approach to Kobusine

Synthesis of racemic 6,15,16-iminoprocarpane-8,11,13-triene (227), which constitutes a partial structure of kobusine (228), was reported (41). Kobusine is a C20 diterpenoid alkaloid and has been obtained from Aconitum species. On catalytic hydrogenation of 229 with palladium on 12

227

228

126

TAKASHI AMIYA A N D H I D E 0 B A N D 0

230 R = 0

229

231 R

=

232

OH,H

carbon, 230 was obtained in 73% yield. Reduction of 230 with sodium borohydride gave the epimeric alcohols (231). On treatment of 231 with Raney nickel in ethanol, epimers 232 were obtained in 63% yield.

233

234

235 R = C02CH2-C6H5

Dehydration of 232 with hydrochloric acid in ethanol gave 233. Treatment of 233 with lead tetraacetate gave the aziridine (234), which was treated with benzyl chloroformate to provide the chlorocarbamates (235) in 45% yield. These compounds were reduced with Raney nickel in ethanol to afford the amine (236) in 56% yield. Compound 236 was reacted with N-chlorosuccinimide to give the N-chloramine (237) in 85% yield. Photolysis of 237 in trifluoroacetic acid gave 227 in 38.7% yield.

236

237

111. Pharmacology

The roots of some Aconitum species are one of important herbal drugs that have long been in China and Japan. The roots, however, must be carefully applied in clinical settings because of the high toxicity. There are many kinds of treatments for reducing the toxicity, such as soaking in

127

3. ACONITUM ALKALOIDS

saline solution, heating, and covering with lime ( 4 2 ) . In Japan the drug is generally prepared by autoclaving at 120°C for 30 min. (43).In an Oriental medicinal remedy, herbal drugs are used necessarily in combination with other drugs. In particular, it has been stated empirically that Aconiturn roots improve hypometabolism and have cardiotonic, anodyne, febrifuge, and sedative effects. Recently, study of those pharmacological effects has been accelerated, and pharmacological activities have been reviewed with respect to diterpenoid alkaloids including aconitine (44-46). Bisset has also reported on the botany of Aconitum species, components of their alkaloids, and pharmacology (47). Toxicity and Oriental medicinal purposes are reviewed in this chapter.

A. TOXICITY Aconitine is a well-known toxic compound (see Table 11) responsible for the characteristic intoxication called aconitine syndrome. In mice, aconitine intoxication causes at first promotion of respiration followed by increased salivations, emesis, urination, paralysis of hindlegs, convulsion, paralysis of forelegs, and death. Table I shows the acute toxicity of raw and processed Aconiturn roots (48).The toxicity of processed samples, with the exception of Shirakawa-bushi, was obviously reduced, and Hikino et al. reported that the content of the major alkaloids, hypaconitine, aconitine, and mesaconitine, decreased but the content of benzoylaconines increased, based on quantitative determinations (48). TABLE I ACUTETOXICITY OF Aconiturn ROOTSI N MICE(48)

LD5" (g crude drug/kg)

Material Original Plant

Location

Preparation

PO

sc

iP

A . japonicum A . japonicum A . japonicum Aconitum sp. A . carmichaeli A . carmichaeli A . carmichaeli A . carmichaeli A . curmichaeli

Niigata Niigata Niigata

Raw Processed" Processed' Processed" Raw Processed" Raw Processed" Processed'

0.54 195 1.8 13 1.61 116 5.49 161 290

0.12 23.1 0.20 10.9 0.57 11.9 146

0.11 13.9 1.1 2.2 0.19 9.17 0.71 11.5 61.3

a

-

Hokkaido Hokkaido China China China

KukZ-bushi (Japan).

" Shirakawa-bushi (Japan). ' Ha-bushi (Hong Kong).

iv 0.06 4.9

0.14 1.3 0.49 2.8 16

-

128

TAKASHI AMIYA AND HIDE0 B A N D 0

TABLE I1 ACUTETOXICITY O F ACONlTlNE AND RELATED COMPOUNDS IN M I C E ~ Alkaloid

c-3

C-8

C-14

LDSo (mg/kg)

Ref.

iv 0.12, ip 0.380, sc 0.270, PO 1.8 iv 0.10, ip 0.213, sc 0.204, po 1.9 ip 0.35 iv 0.470 iv 0.47, ip 1.10, sc 1.19, PO 5.8 sc 5.2, PO 56.5 sc 100-200 iv 23, ip 70 iv 1160

49

Aconitine

OH

OAc

OBz

Mesaconi tine

OH

OAc

OBz

Jesaconi tine 3-Acetylaconitine Hypaconitine

OH OAc H

OAc OAc OAc

OAs OBz OBz

Aljesaconitine A

OH

OMe

OAs

Lipoaconitine Benzoylaconine

OH

OH

OOCR OH

OBz OBz

Aconine

OH

OH

OH

49

44 50 49

51 52 49

50

a OAc, OOCH,; OBz, OOC-C6Hs; OAs, OOC-C6H4-OCH3 (para); OCR, mixture of lineoyl, palmitoyl, oleoyl, stearoyl, and linoleoyl in the ratio 64 : 20 : 16 : trace : trace.

Table I1 shows toxicities of aconitine and related compounds. Two ester groups, an acetyloxy at C-8 and a benzoyloxy at C-14, seem to be responsible for toxicity, which decreases by a factor of 200 and 1000 in the cases of a partial hydrolysate at C-8 and a hydrolysate at both of C-8 and C-14, respectively. It is recognized that the toxicity of C-8 methoxy and lip0 compounds decreases to some extent. Judging from the tendency for such a decrease in toxicity, traditional processing methods of the herbal drug are considered to be performed mainly for the purpose of hydrolysis. The toxicity of 3-acetylaconitine does not decrease much but the analgesic activity of the compound has been reported to better by a factor of around 100 that of cocaine (53);qualitative differences in pharmacological action were recognized for even slightly changed substitution (50). As for neurotoxicity, some investigators have reported that the distance between nitrogen and oxygen atoms in substituents at C-8, C-14, and C-16 is important for the drug association with the same sodium channel

129

3. ACONITUM ALKALOIDS

receptors that also bind other popular toxins, such as batrachotoxin, veratridine, and grayanotoxin (54,55). In a neurophysiological study, Schmidt and Schmitt showed that aconitine altered sodium channel kinetics, eliminating inactivation, and lowering the threshold for activation by approximately 50 mV (56).The depolarizing effect on sodium channels of aconitine as well as as batrachotoxin and veratridine was found to be inhibited by tetrodotoxin (57,58). Aconitine is a popular reagent in the study of sodium channel kinetics, and a binding site on the channel receptor has been investigated (59-62). Interestingly, lappaconitine, which was about 40 times less toxic than aconitine on intravenous administration to mice, reportedly, blocked the calcium channels in Helix pomatia neurons without activating sodium currents (63,64).

B . ARRHYTHMIC ACTIVITY The arrhythmia induced by aconitine has been ascribed mainly to an effect of acetylcholine (65), and the mechanism of inhibition by atropine has been fully investigated (66). It has been also shown that antihistamine in isolated frog heart (67) and propranolol and lidocaine in cat (68) were effective inhibitors of the arrhythmia. OH

Denudatine

Lucicul ine

A matter of interest is that denutadine, Czo atisine-type alkaloid, showed prophylactic inhibition of arrhythmia (69). Luciculine at smaller doses (5-20 mg/kg, iv) also showed an antiarrhythmic effect on CaC1,- and aconitine-induced arrhythmia (70). In mice, intraperitoneal administration of 25-200 mg/kg luciculine before administration of a lethal dose of

130

TAKASHI AMIYA AND H I D E 0 B A N D 0

aconitine prevented death of the animals (70). Lappaconitine showed arrhythmic activity and caused a marked decrease in heart rate. The results of testing several diterpenoid alkaloids related to lappaconitine led to the proposal that substitution at C-4 must be important for arrhythmic activity (64). Aconitine-induced arrhythmia has been widely used in the development of antiarrhythmic agents ( 7 I ) , including prostaglandins (PGF2 and PGIJ (72,73), disopyramide (74), androstane derivatives ( 7 9 , ethylenediamine derivatives (76),quinidine derivatives (77), ethmosine derivatives (78), alpherol (79), 1,3-benzodioxazole (80), trimecaine (81), procaine amide derivatives (82), and verapamil (83).

C. CARDIOACTIVITY During recent years improved techniques in researching biologically active principles in combination with pharmacological screening have also been applied to Aconitum roots as well as other herbal drugs. Kosuge and Yokota isolated higenamine [ (*)-demethylcoclauline] from the aqueous portion of a crude extract of Aconitum japonicum, on the basis of its cardiac activity as tested by the Yagi-Hartung method (84). Higenamine has been also been also isolated from embryos of Netumbo nucifera (85), leaves and stems of Annona squamosa (86), and radices of Asiasarum heteropoides (87). It was reported that optically active (S)- (-)-higenamine has potent p-adrenergic activity and that the ( R ) - (+) compound has an antitussive effect (87,88).

Higenamine

Corynein c h l o r i d e

S a l s o l inol

Konno et al. isolated corynein chloride, a compound with hypertensive activity, from Aconitum carmichaeli (89). An interesting study on blood pressure and neuromuscular junctions has been reported for catecholamines including corynein bromide by Cuthbert (90). Salsolinol, possessing hypertensive activity (91-93), has been isolated from the same species ( A .carmichaeli)by a Chinese group (94).In connection with catecholamine activity, the following aminophenols were reported: N-methyladrenaline

131

3. A C O N I T U M ALKALOIDS

from tubers of Aconitum nusutum (95), noradrenaline, dopamine, and tyramine from tubers of A. nupellus ( 9 6 ) , and hordenine from whole plants of A . tanguticum (97).

D. ANALGESIC ACTIVITY In studies of the analgesic activity, of Aconitum alkaloids, mesaconitine was isolated from the active fraction of a crude extract (98). Its activity was related to responses involving the central catecholaminergic system (99) and was promoted through activation of the p-adrenergic system followed by an increase in cyclic AMP levels (100). Mesaconitine is more effective than aconitine and benzoylaconine (100). Kitagawa et al. also reported on the analgesic activity of aconitine, mesaconitine, and lipomesaconitine (52).Saito et ul. reported that ignavine, a C2" diterpenoid alkaloid, showed analgesic activity without inhibition of the mortality induced by mesaconitine (101). Finally, there was a interesting report that intraperitoneal administration of aconitine induced a painful writhing syndrome and was useful in evaluating analgesic activity (102). Such a syndrome may be affected by local responses according to the manner of administration.

E. OTHERBIOLOGICAL ACTIVITIES Regarding antiinflammatory activity, aconitine alkaloids at low doses showed inhibition of the increased vascular permeability induced by acetic acid in mouse peritoneal cavity or by histamine in rat skin as well as inhibition of edema induced by carrageenan, but these alkaloids showed no inhibition of adjuvant arthritis (103). Lipomesaconitine (0.5-2 mg/kg) (52) and ignavine (100 mg/kg) (101) also showed inhibition of carrageenan-induced edema. Mesaconitine was deduced to be effective in improving hypometabolism in feeble patients, as judged from activation of protein synthesis (104) and increase in incorporation of [5-3H]-orotic acid into polysomal RNA in mouse liver (105). Glaucine

: R

1

I

= R

3 3

= OCH ;

3 2

R

2

R

= H;

4

4

= CH3

Isoboldine : R = R = OH; R = H; R = CH3 3 4 + Magnoflorine: R1 = R2 = OH; R = H; NR = N ( C H 3 ) 2

R

132

TAKASHI AMIYA AND H I D E 0 B A N D 0

Glaucine, an aporphine alkaloid isolated from Aconitum yesoense (106), is known to have antitussive activity (107). Isoboldine, obtained from aerial parts of A . karakolicum (108), has been reported to possess antifeeding activity in Trimerisia miranda and Prodenia litura (109). Nijland reported the detection of magnoflorine ( I I O ) , which is known to show hypotensive activity through blocking ganglias, in tubers of A. carmichaeli, A . nappelus, and A . vulparia. A number of biologically active compounds will be available from Aconitum species in the future according to development of the means to bioassay them. It is important, however, for medicinal purposes to require constant quality and quantity of active components in herbal drugs when using traditional crude preparations.

IV. Analytical Methodology

Adequate analytical methods are required to study components of traditional herbal drugs, both processed and raw materials. In particular, for quantitative determination of the toxic alkaloid aconitine, UV spectroscopy (111), paper electrophoresis (112),thin-layer chromatography (113), and multibuffered paper partition chromatography (114) have been developed. Kurosaki et al. (115) determined the content of several aconitine, lycoctonine, atisine, and veatchine type alkaloids in tubers of Aconitum mitakense with dual wavelength TLC scan and gas chromatography. It was found, however, that some of aconitine-type alkaloids were decomposed by gas chromatography. Kurosaki et al. examined seasonal variation in alkaloid content of some Japanese Aconitum species in connection with the appropriate harvest period for the herbal drug. Hikino et al. (48) reported an improved gas chromatographic procedure to determine the content of trimethylsilylated aconitine-type alkaloids in processed and raw materials of A . japonicum and A. carmichaeli. Kosuge and Yokota (116) applied gas chromatography to determine the content of higenamine, a cardioactive isoquinoline alkaloid, in tubers of Aconitum species and commercial preparations. Since the first report on the application of high-performance liquid chromatography for quantitative determination of aconitine in tubers of some Aconitum species and commercial preparations was made by S.-J. Sheu et al. (117), many reports dealing with analysis by HPLC have been published (51,118-121). Commercially available preparations of aconitine have been evaluated with HPLC on CIS reversed-phase columns with a

3. ACONITUM ALKALOIDS

133

mixture of phosphate buffer (pH 2.7) and tetrahydrofuran (89 : ll), using the ion pair reagent, sodium hexanesulfonate, as the mobile phase (118). Recently, vacuum liquid chromatography (122,123) and a centrifugally accelerated radial thin-layer chromatographic instrument (Chromatotron) have been efficiently applied for preparative-scale isolation of diterpenoid alkaloids (123-125). The rotors of the Chromatotron were coated with a mixture of aluminum oxide gel and calcium sulfate hemihydrate, and the layer thickness was 1 mm. Commercial “Aconitine Potent Merck” (250 mg) gave deoxyaconitine (9 mg), aconitine (190 mg), and mesaconitine (4 mg) with the Chromatotron, using gradient elution with hexane, hexaneether, ether, and ether-methanol. This method demonstrates a significant advantage over classic time-consuming preparative-scale separation of diterpenoid alkaloids.

V. Tabulation of New Diterpenoid Alkaloids The configuration of C-1 group of base I (septentriodine), base V (puberaconitine), gigactonine, puberaconidine, and septentriodine has been revised on the basis of correlation with lycoctonine and derivatives. New Aconitum alkaloids discovered since 1978 are presented in Tables 111 and IV. Pelletier et al. have recently reported C19 diterpenoid alkaloids and derivatives obtained from Aconitum and Delphinum species together with ‘H- and 13C-NMR spectral data (126).

TABLE I11 CATALOG OF CI9 DITERPENOID ALKALOIDS FROM Aconitum SPECIES

Compound

3-Acetylaconitine

OH

14-Acetyltalatiramine

Physical characteristics; source; means of identification

Ref.

C36H49N012; 196-197"C, [&ID +18.6" (CHC13);A . Feavum;A. flavum; A . pendulum; spectral and chemical data

127-129

CZhH4,N06; amorphous, [.ID i-19.7' (CHCI,); A . japonicum; A . carmichaeli; A . colurnbianum; spectral data and correlation with talatizamine

130-134

14-Acetylneol i n e ( B u l l a t i n e C )

C26H41N07, amorphous, [a],+18.6“ (MeOH); A . yesoense; A . nagarum;

69,106,128,

135

A . jinyangense; spectral and chemical data

Aconif ine

C34HqNO12; 195-197”C, A . karakolicum

Aljesaconitine A

C34H4YNOl, ; amorphous, [.ID +7.5” (EtOH); A . juponicum; spectral and chemical data

[a]D-;

128,136, 137

51

(continues)

TABLE 111 (Continued)

Compound

Physical characteristics; source; means of identification

Ref.

~

Aljesaconitine B

Anisezochasmaconi t i n e

C35H5,NOll;amorphous, [.ID +5.8" (EtOH); A . japonicum; spectral and chemical data

51

C,,H,,NO,; 136-138.5"C, [.ID-; A . yesoense; spectral and chemical data

106,138

C,,H,2N2011;amorphous, [a],, f45"

13Y--141

(MeOH); A . gigus; A . seprenirionale; A . barbaturn; spectral and chemical data

139,141

C ~ - $ I ~ ~ N196-198"C, O~Z; [.ID-; A . kusnezofii; A . carmichaeli; spectral data

128,142

137

C36H5H50Nz0,,; amorphous, [.ID +34.0" (CHCI,); A . gigus; A.barbatum; spectral and chemical data

TABLE 111 (Conrinued)

Compound Benzoylheteratisine

Physical characteristics; source; means of identification

Ref.

C,gH,,NO,; 214-216°C. [.ID -; A . fanguticum; spectral and chemical data

97

C ~ ~ H ~ ~ 206-208"C, N O S ; [a] -; A . karakolicum; spectral and chemical data

143

C3,Hd3N07;amorphous, [.ID f9.1" (MeOH); A . subcuneatum; spectral data and correlation with neoline

1s

CH3 OBz 1-Ben z o y 1 k a r a s a ni in e

C2H5

14-Benzoyl neol i n e

OCH3

Colurnbianine

C,,H,,NO,;

202-205"C,

[ N ]-6" ~

I34

(EtOH); A. colurnbianurn; spectral and chemical data

6H Col umbi d ine

C26H43N05;amorphous, [.ID -6.4" (CHCI,); A . calurnbianurn; spectral and chemical data

144

Crassicaul ine A

C3,H,9NO,o ; 162.5-164.5"C, [.ID +31.5"(CHCI,); A. crassicaule; A . forrestii; A. pseudogeniculaturn; spectral and chemical data

145-147

(continues)

TABLE 111 (Continued)

Compound Crassicaulidine

OCH3

Physical characteristics; source; means of identification

Ref.

C24H39N08; 206-209"C, [.ID -; A . cvassicauk; spectral and chemical data

148,149

C30H42N209; 121-123"C, [.ID +34.9" (CHCI,); A . finetianurn; spectral data

150

'

N-Deacetylfinaconitine

N-Deacetyllappaconitine (Puberanidine)

C-,oH4~NZ07; 120-121"C, [ a ]+42" ~

(MeOH); A . ranuncalaefolium; A . finetianurn; A . barbatum;

64,141,150,

IS1

Delphinium cashmiranurn; spectral data and correlation with lappaconine

0 -c=o

N-Deacetylranaconitine

C ~ O H ~ ~ ;N125-127"C, ZO~ [.ID +43.7" (CHCl,); A. finetianurn; spectral and chemical data

150

N-Deacetylscaconitine

C3,HUNZO6; amorphous, [.ID -; A. scaposurn; spectral and chemical data

152

(continues)

TABLE 111(Continued) Compound

Physical characteristics; source; means of identification

Ref. ~

8-Deacetylyunaconitine

C33H47NOli; 101-105"C, [(~]j3-; A . forrestiz, spectral and chemical data

153

C,,H,,NO,, 128-I30"C, [o]D-, A suposhnikovzz, spectral data

154

C29H39N06;[oID-: A . deluvyi; spectral and chemical data

155,156

OCH3

14-Dehydrotalatisamine L

N e

OCH3 Delavaconitine ----_.___

___---

H

Delphinifol ine

CZ3H37N07; 218-220°C, [aID-;

157

A . delphinifoliurn; spectral and X-ray data

OCH3 Deoxydel sol ine

CZ5H41NO6; 134-135"C, [(YID-; A monticolu; spectral data

158

C35H49NOll;174-176"C, [a],, +52' (MeOH); A . subcuneatum; spectral and chemical data

159,I60

OCH, J

Deoxyjesaconitine

C2H5

(continues)

TABLE 111 (Continued)

Compound

Dihydromonticarnine

Physical characteristics; source; means of identification

Ref.

C ~ ~ H ~ S N 156-157"C, OS; [.ID -; A . monficola;spectral data

158

C34HJ7N010; 168-169"C, [.ID +23.8" (CHCI,); A . duclouxii; spectral data

161

CZ9H39N06; amorphous, [a],,-11.7" (EtOH); A . epkcopale; spectral data

162

OH

CL

P

dCH3 OCH3

Episcopal isine

E p i s c o p a l isinine

C,2H3sNOs, 152-154"C, [a]D-3.8"

I62

(EtOH); A . episcopale; spectral and

chemical data

Episcopal i t i n e

CZ4H37N05; amorphous, [.ID -0.9" (EtOH); A. episcopale; spectral and chemical data

I62

8-0-Ethylbenzoylmesaconine

C33H47NOIO; amorphous, [.ID +5.8" (MeOH); A. ibukiense; spectral and chemical data

163

"0B z __--OH

HO" OCH3

(conrinues)

TABLE 111 (Continued)

Compound

Ezochasmaconitine

Physical characteristics; source; means of identification

Ref.

C34H47N08; 163-165"C, [(YID-; A yesoense: spectral and chemical data

106,139

Ezochasmani ne

C25H41N07; 115-118"C, [.ID +40.3" (CHCl3); A. yesoense; spectral and chemical data

106,139

Finaconi tine

C3zH44N2010,220-22 I T , [ (Y]D-; A . finetianurn; correlation with rannaconitine

128

OCH3

C 2 H 5 - - -&- r ; N1 @ c H 3

I.

0-c=o

___----

OH

Flavaconitine

C 3 , H 4 , N 0 , , ; 165-166"C, [a],+36" (CHCI,); A . flavum; spectral data

I64

C35H49N09; 153-154"C, [.ID +30.5" (CHCI,); A . forrestii, A . vilmorianum; A . pseudogeniculatum; spectral and chemical data

146,147, 165-167

C Z ~ H ~ ~79-80"C, N O ~ ; [.ID -1.9" (CHCI,); A . forresfii; spectral data and correlation with chasmanine

168

O'CH3 Foresaconitine (Vilrnorrianine C)

OCH3

0 As = - ! D 0 C H 3

Foresticine QCH3

(continues)

TABLE 111 (Continued) Compound

Forestine

OCH3

Physical characteristics; source; means of identification &Hd7N09; amorphous, [ ( Y ] ~ - ; A. forrestii; spectral data

Ref. 168

0 As = -@)-OCH,

Franchetine

IOCH3

C3,H41N06;amorphous, [a],,-106.4” (CHCQ; A. franchetii; spectral and chemical data

169

Geniconitine

C32H*5NOR ;235--237.5-C (hydrochloride),

170

[a],-; A . geniculutum; spectral and

"OA s

chemical data

___----

')

OH

OCH3

Gigactonine

Guayewuanine B

nu

CZdH39N07; 168-169"C, [.ID +49" (EtOH); A . gigas; spectral and chemical data

139

C31H43N09; 120"C, [.ID +31,8"(CHCI-,); A . hemsleyanum; spectral and chemical data

171

0

OCH3 AS =

-!-@OCH3

(continues)

TABLE III(Continued)

Compound

Physical characteristics; source; means of identification

Ref

Gymnaconi t i ne

C34H47N04; llO-lll°C,[(Y]D + 18.2”, A . gymundrum; spectral data

172

Hokbusine A

C3ZH45N0,1; amorphous, [.ID +11.4” (MeOH); A . curmichaeli, A . juponicum; spectral and chemical data

51,173

“OBz

OH

OCH3

Hokbusine B

C ~ Z H I ~ N O183-l85"C, ,; [a],--; A . curmichueli; spectral and chemical

I 73

data

15n-Hydroxyneol ine (Fuzil i n e , Senbusine C )

c-

C24H39N07;206.5-207"C, [.ID +19.3" (CHC13),A. japonicum; A . ibukiense; A . carmichaeli; spectral and chemical data

132,163, 174-1 78

C23H3sN07;243-246°C (dec), [.ID +71.7" (MeOH); A . ibukiense; spectral and X-ray data

163,179

OH

E OCH3 I bukinami ne

OCH3

(continues)

TABLE 111 (Continued)

Compound

lsoaconitine

C2SH49NO,,; 144-146"C, [a]o-; A. deluvyi; spectral and chemical data

OH

6CH3

Ref. 128.155, 156

0

As

Karasamine

Physical characteristics; source; means of identification

=

-!@OCH,

112"C, [(Y]D-; A . karukolicum; spectral data

C23H37N04; 110-

143

Lipoaconitine

Oil, [aID+6.0” (CHC1,); A . carmichaeli;

‘‘3

131,133

spectral and chemical data

OH

i= a mixture

o f linoleoyl, palmitoyl, oleoyl, stearoyl, and linolenoyl (64:20:16:trace:trace)

Lipodeoxyaconitine

OCH3 R =

Oil, [aID+12.4“ (CHC1,);A. carmichaeli; spectral and chemical data

131,133

a mixture o f linoleoyl, palmitoyl, oleoyl, stearoyl, and 1 inolenoyl (61:19.5:19.5:trace:trace) (continues)

TABLE 111 (Continued)

Compound

Physical characteristics; source; means of identification

Ref.

~~~~~

Lipohypaconitine

Oil, [a],,+13.5" (CHC1,); A . carmichaeli; spectral and chemical data

131,133

Oil, [a],, +13.8" (CHCI,); A. carmichaeli; spectral and chemical data

131,133

OH

+ VI P

OCH3

R = a m i x t u r e o f 1 inoieoyi I palmitoy1 , oleoyl , stearoyl , and 1 inol enoyl (58:19: 23: trace: trace)

Lipomesaconitine

OCH3

R

=

a m i x t u r e o f linoleoyl, palmitoyl, oleoyl, s t e a r o y l , and linolenoyl (57:32:ll:trace:trace)

Liwaconitine

C4,HS3NO,,;201--202.5"C,1 ~ 1 .t133.3" 1~

146

(CHC1,); A . forresrii; spectral data and

correlation with bikhaconine

Methyl oyrnnaconi t i ne

C35H49N09; amorphous, [a]D-; A. gymundrum; spectral data

I72

8-0-Methyl t a l a t i z a m i n e

CZ5H4,N0,;amorphous, [ a ] D -4" (CHC13);A . columbianum; spectral data

134,144

and correlation with talatizamine

OCH3

(continues)

TABLE I11 (Continued)

Compound Monticamine

Physical characteristics; source; means of identification

Ref.

C ~ ~ H ~ T N163-164"C, OS; [a]D-; A . monticola; spectral and chemical data

180

CzZHi3N06; 166-167"C, [.ID-; A . monticola; spectral and chemical data

I80

1

Monticol ine

Nagarine

(Crassicaulisine)

CZ4H39N07;

1YO-19l0C,[ a ] D +20.3"

(CHCI,); A . nagarum; A. crassicaule; spectral data and correlation with delphisine and 15P-hydroxyneoline

128,148,174, 181,182

Nevadenine

C,,H3,N05 ;resin, [aID-; A . nevadense;

183

spectral data and correlation with isotalatizidine

Nevadensine

C23H3sN06;resin, [aID-; A . nevadense; spectral data and correlation with virescenine

183

OCH3 Pendul i n e

166-167"C, [a]D-; A . pendulum; A . japonicum; spectral data and correlation with jesaconitine and chasmanine

C34H47N09;

15,128,I29

(continues)

TABLE 111 (Conrinued)

Compound

Polyschistine A

Physical characteristics; source; means of identification C3hH51NOll;265-266“C, [a]D-;

Ref. 184

A . polyschistum; spectral data

OCH3

Polyschistine B

OCH3

C ~ ~ H ~ 1;~ 182-185”C, NOI [a]D-; A . polyschistum; spectral data

184

Polyschistine C

C31H41NOI~; amorphous, [elD-; A . polyschistum; spectral data

284

C32H52N2011 ; amorphous, [.ID +22.4” (CHCI,); A . barbarum; spectral data and correlation with septentrionine

141

OCH3

Puberaconidine

O-c=o

-

0

aNH-c (continues)

TABLE III(Conrinued)

Cornpou nd Puberani ne

Physical characteristics; source; means of identification

[.ID

Ref.

+16.6"

141

CZ5H3,NOS ; 124.5-127"C, [ L Y ] D+251" (MeOH); A . yesoense; spectral and chemicai data

106

C32H44NZ09; amorphous,

I

c=o

@- N H C O C H ~ Pyrochasman ine

Ranaconi t i n e

G ~ H ~ ~ ;N132-134"C, z O ~ [a]D+33.2" (CHCI,); A . ranunculaefolium;

128,141,

18.5

A . finetianurn; A . barbatum; spectral and chemical data

Scaconi ne

C2,H,,NOS; amorphous, [aID-; A . scaposum; spectral and chemical data

I52

OH

(continues)

TABLE I11 (Conlinued) Compound Scaconi t i ne

Physical characteristics; source; means of identification

Ref.

C33H46N20,;amorphous, [a]D-; A . scuposum; spectral data

152

C,,H,,NO,; amorphous, [@ID-; A . carmichueli; A . ibukiense; spectral data

132,163

0-c=o

NHCOCH3

Senbusine A

OCH3

Senbusine B

C23H37N06;amorphous, [a],--; A . carmichaeli; spectral data

I32

C38Hs4NZ0,1 ; 123-125"C, [a],,+21.2" (CHC13);A . septentrionale; A . barbaturn; spectral and chemical data

140,141

OCH3

Septentrionine

c W m

0-c-0

(continues)

TABLE 111 (Continued)

Compound Takaonine

Physical characteristics; source; means of identification

Ref.

C ~ ~ H ~ ~ N 0 ~ , 1 8 6 - 1 8 7 . 5 " C+52" , (CHCI,); A . japonicum; A . ihukiense; spectral and chemical data

130,163

CZ3H37N07; 174-175"C, [.ID +61.2" (CHCI,); A . japonicum; spectral and chemical data

130

Y

P ch

OCH3 Takaosamine

,

OCH3

'ZH5

a

i

m

u I

m I u 0

n t W .? t

m

m

I

u

m I

V

: i n :

; !

B

N 0

;r

L

=.

P-

.,.-

L

0

165

m I

0

o=v

m

TABLE IV CATALOG OF CzoDITERPENOID ALKALOIDS FROM Aconitum SPECIES

Compound

Physical characteristics; source; means of identification

Ref ~

1 -Acetyll uci cul ine

OH

12-Acetylnapelline N-oxide OAc

C24H35N04;amorphous, [.ID -; A . yesoense; spectral data and

106

C&,sNOs ; 235°C [.ID -; A . karakolicurn; spectral data

154

W

E

.r C

01 0 C

0 VI

W

+J

h

F

5

0

Y-

o\

Y-

x 1

167

TABLE IV (Continued) Physical characteristics; source; means of identification

Compound

Dehydrolucidusculine

Ref.

C24H33N04; 186-189"C, [(Y]D +2.6" (EtOH); A. yesoense; spectral data and correlation with lucidusculine

I90

C22H,,N0,; amorphous, [.ID -; A . finetianurn; spectral and chemical data

I91

C21H29NOZ; 236-238"C, [.ID -143.7" (EtOH); A . finetianurn; spectral data

191

OAc

1-Dehydrosongorine

n

Fineti anine n

Episcopalidine

0

I62

X-ray data

Guan-fu base A :OH

AcO

C30H3,N06;21O-22O0C, [ a ]-80.0" ~ (CHCI,); A . episcopale; spectral and

+49" (CHCI,); C24H3jN06; 199"C, A . bullatijolium; A . koreanum; spectral and chemical data and correlation with guan-fu base G

,.

Guan-fu base G ,OAc

C26H33N07; 178"C, [.ID +97.3" (CHC1,); A. bullatifolium; A . koreanum: spectral and chemical data; X-ray analysis

128,I92

(confinues)

TABLE IV (Continued) Physical characteristics; source; means of identification

Compound Hanamisine

9-Hydroxynominine

Ignavine

BzO..

OH

Ref.

C,,H,,NOs; 124-127"C, [.ID +122.6" (MeOH); A . sanyoense; spectral data

193

C2,HuNO,; 287-291°C (dec), [.ID +68.5" (MeOH); A . ibukiense; spectral data

163

C,7H31N05; A . japonicum; A . carmrchaeli; A . ibukiense; structure revised on the basis of X-ray analysis

163,173, 194,195

Jynosi ne

C24H3sN03; 254-256°C (dec, perchlorate), [a],,-37.4" (CHCI,);

69

A . jinyangense; spectral data and correlation with denudatine

HO

Nomi ni ne

Ryosenamine e

C20H27NO;251-254"C, [@]a +53.4"; A . sanyoense; spectral data and correlation with kobusine

I 96

CZ7H31N04; 213-215"C, [.ID +96.8" (MeOH); A . ibukiense; spectral data and correlation with ryosenaminol

163,179

(continues)

TABLE IV (Continued) Compound

Ryosenaminol

Sadosine

Physical characteristics; source; means of identification

Ref

C20HZ7NO3;287-290"C, [(Y]D +66.8" (MeOH); A . ibukiense; spectral and X-ray analysis

163,179

CZ7H31NO6; 222-224"C, [cY]D+53.1" (MeOH); A . japonicum; spectral and X-ray analysis

195,I97

Sanyonamine

C20H27N02;276-278"C, [aID+ 62.9";

I98

A . sanyoense; spectral and X-ray analysis

Talatisine

C,,H,7N0, ; [a],,-; A . talussicum;

199

X-ray data

Tanwusine

CzoH27N03, 144-15OoC, [.ID A . tanguticum

-;

97

174

TAKASHI AMIYA AND HIDE0 BANDO

Acknowledgments We wish to express our thanks to Mr. Koji Wada, who collected many references.

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3 . ACONITUM ALKALOIDS

175

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165. 166. 167. 168. 169. 170. 171. 172.

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-CHAPTER 4-

PROTOPINE ALKALOIDS MASAYUKI ONDAAND HIROSHI TAKAHASHI School of Pharmaceutical Sciences Kitasato University Tokyo, Japan

I. Introduction 11. Occurrence 111. Structure A . Izmirine B. Protothalipine C. Thalictricine D. (-)-Oreophiline E. Protopine Methohydroxide F. . Protopine N-Oxide IV. Conformation and Spectroscopy A. Conformation in the Solid State B. Conformation in Solution V. Synthesis A. Synthesis from Tetrahydroprotoberberines B. Synthesis from Phthalideisoquinolines C. Total Synthesis via Benz[d]indeno[l,2-b]azepines VI. Transformation of Protopines to Related Alkaloids VII. Biosynthesis VIII. Callus Culture IX. Pharmacology Addendum References

I. Introduction

The protopines were first covered in Vol. 4 (p. 147) of this treatise in 1954 as a separate chapter. Since then, complementary information on protopines has been reviewed as a group of Papaveraceae alkaloids (Vol. 10, p. 467; Vol. 12, p. 333; Vol. 15, p. 207; Vol. 17, p. 385). The latest supplementary review appeared in 1981 in Volume 18 (p. 217) as a section of a review covering isoquinoline alkaloids. In addition, three reviews (1) on protopines were published in the 1970s. This chapter supplements the previous reviews in this treatise by incorporating recent 181

THE ALKALOIDS, VOL 34 Copyrlght 0 1988 by Academic Press, Inc All rights of reproduction in any form reserved

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MASAYUKI ONDA AND HIROSHI TAKAHASHI

advances in this field and updating literature citations through mid 1987 as well as by covering valuable earlier data that have not yet been mentioned.

11. Occurrence

The protopines are widely distributed in the families Berberidaceae, Fumariaceae, Papaveraceae, Ranunculaceae, and Rutaceae. Table I covers the period from 1977 to mid 1987 and supplements previous data that appeared in this treatise (Vol. 4, p. 77; Vol. 9, p. 41; Vol. 10, p. 467; Vol. 12, p. 333; Vol. 17, p. 385).

111. Structure

The structures of protopines are characterized by the 7-methyl5,6,7,8,13,14-hexahydrodibenz[c,g]azecinering system containing a 140x0 group, except for one which has a 14-hydroxyl group. (The trivial numbering system is used throughout this chapter.) The benzene rings contain four or five oxygen functions, two or three in ring A and two in ring

1

312 \ y j

10 V

11

C. The variety and the number of substituents in each ring can be confirmed by ions in the mass spectrum arising from retro-Diels-Alder fragmentation (124). The substituted positions can be assigned from the absorption pattern of aromatic protons in the 'H-NMR spectrum. Protopines with a methyl group or oxygen functions at the 13 position are also known. The protopines that have been reported so far in the literature are shown in formulas 1-26. Among them, protopines 6, 7, 8, 18, 24, and 25 have not yet been mentioned in this treatise series. A. IZMIRINE

Izmirine (6), C20H21N05,amorphous, was isolated as a phenolic base [IR (CHC13) : 3540 cm-I (OH)] along with cryptopine (2) and hunnema-

183

4. P R O T O P I N E ALKALOIDS

TABLE I PLANTS AND THEIRPROTOPINE ALKALOIDS Plant Berberiaceae Berberis darwinii stems B. cordaia Wild.

B. frustescens L. B. gracilis Hartw. Nandia dorneslica Fumariaceae Fumaria bella P. D. Sell F. bracteosa Pomel F. capreolata L. F. densiflora DC.

F. gaillardoiii Boiss F. indica (Haussk.) Pugsley F. judaica Boiss

F. macrocarpa Parlatore F. oficinalis F. parvipora Lam. F. rostellata

F. schleicheri Soy-Will F. schrammii F. vaillanfii Papaveraceae Argemone mexicana L. A . orchroleuca Chelidonium japanium Thumb. C. majus Corydalis bulbosa C. cava (L.) Schw. et Koerte C. cheilantifolia Hemsl.

Alkaloid Protopine Allocryptopine, protopine Allocryptopine, protopine Cryptopine, pro topine Protopine Protopine Protopine Protopine Cryptopine, protopine Protopine Protopine Allocryptopine, protopine Protopine Cryptopine, protopine Cryptopine, hunnemanine, izmirine, protopine Cryptopine, protopine Cryptopine, protopine Protopine Cryptopine, protopine Allocryptopine, protopine Allocryptopine, protopine Protopine Protopine Protopine Allocryptopine, protopine Allocryptopine, protopine

Ref.

2 3 4 5 6

7 8 /

9.10 11 12,13 14,15

16,17 18,19

20-23

24 25 26 24,27, 28 29,30 31

32 33,34 35,36 37 38

(conrinues)

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MASAYUKI ONDA AND IlIROSHI TAKAHASHI

TABLE I (Continued) ~

Plant ~

C. cornuta Royle C. decumbens C. delarayi Franch C. giganta C. gortschakovii Schrenk. C. hendersonii C. ledebouriana Kar et Kir.

C. lineariodes C. lutea (L.) DC. C. maius L. C. marshalliana C. rneifolia Wall. C. ochotensis var. raddeana C. ophiocarpa Hook et Thorn. C. palfida var. speaose Kom. C. paniculigera C. rasea C. remota C. repens C. rutifolia C. saxicola C. sheareri C. slivenesis C. solida (L.) Swarz.

C. stricta Steph C. suavelens C. taliensis Fr. C. tashiroi Makino C. turtschaninovii Yanhusuo

C. vaginanus C. yanhuso Dicentra macrocapnos Prain D. spectabilis L. D . leptopodium (Maxim.) Fedde Eschscholtzia californica

~~~

Alkaloid

Ref

~~~

Protopine Protopine Protopine Protopine Protopine Protopine Allocryptopine, cryptopine, protopine Protopine Protopine Allocr yptopine Pro topine Protopine Protopine Allocryptopine, protopine Protopine Protopine Protopine Protopine Protopine Allocryptopine, protopine Protopine Protopine Allocryptopine, protopine Allocryptopine, protopine Protopine Allocryptopine, protopine Protopine Protopine Allocryptopine, protopine Protopine Protopine Protopine Protopine Protopine Allocryptopine, protopine

39 40 41 42 43,44 45 46,47

48,49 50 51 42 52 53 54,5 56 57 42 42 58 59 60 61 62

63.64 49,65,66 67 68 69 70,71 42 72 39 73 74,75 76

185

4. P R O T O P I N E ALKALOIDS

TABLE I (Continued) Plant

E. californica Cham. E. douglusii (Hook et Am.) Walp. E. glauca Greene Glaucium corniculatum

G. corniculatum (L.) Rudolph. subsp. refractum (NAB) Cullen G. jimbrilligerum

G. flavum Grantz G. grandiflorum var. torguatum

C. oxylobum Boiss er Buhse C. pulchrum Staf. G. vitellium Boiss et Buhse

G. vitfinum Boiss et Buhse Hunnemania fumariaefolia Sweet Hypecoum erectum

H . lactiflorum H . leptocarpum H . ponticum Mowt.

H . procumbens Macleaya cordata (Wild.) R . Br. Meconopris rudis Prain Pupaver albiflorum

P. armeniacum P. atlaniicum Ball

Alkaloid Protopine Allocryptopine, protopine Allocryptopine, protopine Allocryptopine, protopine Allocryptopine Allocryptopine, protopine Allocryptopine, protopine Allocryptopine, cryptopine, protopine Allocryptopine, protopine Protopine Allocryptopine, protopine Protopine Allocryptopine, hunnemanine, protopine Allocryptopine, protopine Allocryptopine, protopine Protopine Allocryptopine, cryptopine, protopine Hunnemanine, 13-oxoprotopine Allocryptopine, protopine Allocryptopine, protopine Allocryptopine, protopine Cryptopine Cryptopine, muramine, pro topine

Ref 76,77 77 77 78,79

80 81 82-84 85,86

87,88 89 9# 91

90 92, 93

94 94 95 96

97

98 99 100 101

102

(continues)

186

MASAYUKI ONDA AND HIROSHI TAKAHASHI

TABLE I (Continued) Plant

P. hracteatum P. curviscapum Nabk P. decaisnei Hochst P. glaucum Boiss et Hauskn. P. kernevi Hayek

P. lateritium P. lecoguii Lamotte protopine P. lisae P. litwinowii Fedde ex Bornm.

P. macrostomum Boiss et Huet P. oreophilum P. pavonium Schrenk P. pseudo-orientale (Fedde) Medw. P. rohoeas L.

P. tatricsim (Nyar.) Ehrend P. tauricola Stylophorum diphyllum (Michx.) Nutt. Ranunculaceae Thalictrum revolutum T. revolutum DC. T. rugosum Ait. Rutaceae Xanthoxylum integrifoliolum (Merr.) Merr. (Fagara integrifoliolum Merr.) X . nitidum (Roxb.) DC.

Alkaloid

Ref.

Muramine, protopine Allocryptopine, protopine Protopine Allocryptopine, protopine Allocryptopine, cryptopine, protopine Protopine Allocryptopine, cryptopine, protopine Protopine Allocryptopine, cryptopine Protopine Allocryptopine, protopine Allocryptopine, protopine Allocryptopine, protopine Alocryptopine, protopine Allocryptopine, pro topine Cryptopine, protopine Allocryptopine, cryptopine, protopine

103-105 106

Allocryptopine Allocryptopine Protopine

119 120 121

Allocryptopine

122

Allocryptopine

123

107 102

I 0s

109 100

110 Ill

106 112,113 114 115 116 93,106, 117 108 101 118

187

4. PROTOPINE ALKALOIDS

1

R ' + R'= CH,

, R2=Me

allocryptopine

hunnemanine

u-fagarine

izrnirine protothalipine

p-, v-homochelidonine

thalictricine vaillantine

cryptopine cryptocavine thalisopyrine muramine cryptopalmatine protopine I

RO

corydinine

10 R r M e 1 1 R + R = CH,

fumarine

fagarine II pseudoprotopine

rnacleyine

12

R ' + R ' = R'+ R*=CH,

13

R'. R ' = C H ,

14

R'= Me

15

R ' + R'=R'. R >= C H ,

,

,

R'=Me

R'+ R'= CH,

coulteropine

17

1-methoxycryptopine

R ' * R ' = C H , , R'=Me

18

(+)-corycavamine

(+)-

,

R2:H

(+)-ochrobirine (+)-13-hydroxyprotopine

(?)-form = corycavine 16

R ' + R ' =CH,

1-rnethoxyallocryptopine

and ( i ) - c o r y c a v i d i n e

R'=R'.M~

(-)-areaphiline

188

MASAYUKI ONDA AND HIROSHI TAKAHASHI

OR' OR'

19

R'= H , R'= R'= Me

20

R' = O M e , R'

alipinone

24

protopine methohydroxide

25

protopine N - o x i d e

13-oxornuramine +

R'=cH,, oreonone

R'=M~ 21

R'=H , R ' + R ~ . c H , , 13-oxoa11ocryptopine R' = M~

22

R'=H

,R'=M~,

13-oxocryptopine

R3 * R'=CH,

23

R'=H

,

R ' * R'=

13-oxoprotopine

a:>

R 3 + R' =CH,

26

dihydroprotopine

nine (5) from Fumaria parviJEora by Shamma et al. (23). The 'H-NMR spectrum (CDCl,) of 6 revealed the presence of a methoxyl (6 3.90), an N-methyl (6 1.87), and a methylenedioxy group (6 5.94) in addition to four aromatic protons [6 7.01 (s), 6.75 (s), 6.71 (d, J = 7.9 Hz), and 6.67 (d, J = 7.9 Hz)]. Treatment of 6 with diazomethane gave 2, suggesting the presence of a hydroxyl group at either the 2 or 3 position. The 3-hydroxyl group was assigned by comparison of the IH-NMR chemical shifts due to the C-4 protons in 2 and 6 (A- = 0.08 ppm).

B. PROTOTHALIPINE Protothalipine (7), C21H25N05,mp 195-196°C (dec) (MeOH), was isolated as a phenolic base [IR (CHC13) : 3540 cm-' (OH)] from Thalictrum rugosum by Wu et al. (125). Treatment of 7 with diazomethane afforded muramine (3). The 'H-NMR spectrum (CDC1,) showed the presence of a hydroxyl (6 4.07, exchangeable with D20), three methoxyl (8 3.90), and an N-methyl group (6 1.87) in addition to four aromatic protons (6 7.05-6.68). Mass fragment ions ( m / z 222 and 150) arising from retro-Diels-Alder fragmentation (124) indicated a possible location of the hydroxyl group at either the 9 or 10 position. The hydroxyl group at C-9 was confirmed by aromatic solvent-induced shifts (ASIS) experiments in the 'H-NMR spectrum. The ASIS using pyridine indicated

4. P R O T O P l N E ALKALOIDS

189

M' , m / z 371 (5.6%)

that the C-8 protons (A = +0.25 ppm) and the 10-methoxyl protons (A = -0.25 ppm) in 7 are considerably shifted in comparison with those in 3. C. THALICTRICINE Thalictricine (8), C20H21N05, mp 261-263°C '(dec) (MeOH), was isolated as a phenolic base [IR (CHC13) : 3640 cm-' (OH)] along with allocryptopine (1) from Thalictrum simplex and T. amurense by Yunusov et al. (126). Treatment of 8 with diazomethane provided 1. The pattern of mass fragmentation ( m / z 206 and 150) demonstrated that a methylenedioxy group is located on ring A and that a hydroxyl and a methoxyl group occur on ring C (124). Since it was confirmed that 8 is different from hunnemanine (5) by comparison of the physiocochemical properties, it was concluded that 8 is an isomer of 5 containing the 9-methoxyl and 10-hydroxyl groups on ring C. D. ( -)-OREOPHILINE

(-)-Oreophiline (IS), CzzHzsN06, mp 177-178°C (MeOH), [a]'," -254 ? 5" (1, CHC13), was isolated along with protopine (4) from Papaver oreophilum and P. feddei by Pfeifer and Mann (127). The presence of three methoxyl, a methylenedioxy, and an N-methyl group was confirmed by means of chemical analysis and mass spectroscopy. The structure 13-methoxyallocryptopine was tentatively assigned to 18 by comparison of spectral properties with those of protopines.

E. PROTOPINE METHOHYDROXIDE Protopine methohydroxide (24), C21H23N06,mp 231-233"C, was isolated as a quaternary base [IR (KBr) : 3360 cm-' (OH)] along with protopine (4) from Fumaria indica by Satish and Bhakuni (128). The

190

MASAYUKI ONDA AND HIROSHI TAKAHASHI

‘H-NMR spectrum (CF,COOH) of 24 revealed the presence of two methylenedioxy (6 5.84 and 5.60) and two N-methyl groups (8 2.73 and 2.68) in addition to four aromatic protons [S 7.08 (d, J = 9 Hz), 6.79 (d, J = 9 Hz), 6.47 (s), and 6.32 (s)]. The structure of 24 was confirmed to be protopine methohydroxide by comparison of the ‘H-NMR data with those for 4. F. PROTOPINE N-Oxide Protopine N-oxide (25), CzoH19N06,mp 144-145°C (dec) (Me,COMeOH), was isolated along with 1 and 4 from Bocconia cordatu by Takao et al. (3).The ‘H-NMR spectrum (CDCl,) of 25 suggested the presence of two methylenedioxy (6 6.07 and 6.02) and an N-methyl group (6 3.16) in addition to four aromatic protons [6 7.23 (d, J = 7.9 Hz), 7.13 (s), 6.99 (d, J = 7.9 Hz), and 6.77 (s)] which are similar to those of allocryptopine N-oxide. Conclusive structure proof was obtained by direct comparison with an authentic sample prepared by oxidation of 4 with m-chloroperbenzoic acid.

IV. Conformation and Spectroscopy A. CONFORMATION I N THE SOLIDSTATE

Hall and Ahmed (129) reported an X-ray analysis of cryptopine (2) and protopine (4). It was shown that their crystal structures adopt the most stable conformations with following geometrical features. (1)The carbonyl group is at an angle of 39 t 2” out of the plane of ring A . This is responsible for a high-frequency shift of the carbonyl group in the IR spectrum owing to reduced conjugation. (2) The internuclear distance between the nitrogen atom and the carbonyl carbon is 2.57 I+_ 0.01 A, and the nitrogen lone pair is directed toward the carbonyl carbon. It is anticipated that the transannular (“amide-type”) interaction exists between these atoms and causes a low-frequency shift of the carbonyl group (130). The conformation of 4 in the solid state can be shown as the Dreiding model drawing 4a on the basis of the X-ray data (129) (see below). Onda et al. (131) investigated the conformations of allocryptopine (1) and 4 by means of spectroscopic studies. The difference = -6 cm-’) in carbonyl frequency between 4 (1654 cm-’, KBr) and acetopiperone (27) (1660 cm-l, KBr) suggested that the transannular inter-

191

4. PROTOPZNE ALKALOIDS

oO-comc

\

'Me 0

27

28

action is more important than the high-frequency shift arising from reduced conjugation due to noncoplanarity of the carbonyl group and ring A. Comparison of the carbonyl absorption of 1 (1640 cm-', KBr) with that of oxyhydrastinine (28) (1636 cm-', KBr) suggested that the carbonyl group in 1 is nearly coplanar with ring A in the conformation similar to 4a.

B. CONFORMATION IN SOLUTION

1. IR Spectroscopy The identification of two carbonyl absorptions (1686 and 1670 cm-', CCl,) of 4 indicated the existence of two interconverting conformations with different types of carbonyl groups (131). From comparison of these absorption frequencies with that of 27 (1683 cm-', CC14), it was assumed that the absorption at 1670 em-' is due to the perturbed carbonyl group in conformation 4a (corresponding to the crystal structure) of the interacting form 4A and that the other at 1686 cm-I is due to the carbonyl group in conformation 4b of the noninteracting form 4B, in which the carbonyl group is slightly out of ring A (132). The IR spectrum of 1 also showed two carbonyl absorptions (1662 and 1654 cm-', CC14). It was assumed that these absorptions are due to the perturbed carbonyl group in two conformations of the interacting form 1A and that one of the conformations corresponds to the 4a type and the other to conformation l a containing the carbonyl group noncoplanar with respect to ring A (28: 1658 cm-', CC14) (131).

2. NMR Spectroscopy It is well known that variable-temperature 'H-NMR spectra of protopines show slow interconversions between two conformations at low temperature and conformational equilibria at 60°C (131,133). Onda el al. (131) have recently made highly accurate assignments of protons and carbons in 1 and 4 by means of gated decoupling and selective proton

192

MASAYUKI ONDA AND HIROSHI TAKAHASHI

I

13

4a

4b

L d la

decoupling experiments as well as two-dimensional 'H-13C shift correlations at 60°C (Tables 11 and 111). An upfield shift of the C-12 proton, an NOE (15.5%) between the C-1 and C-13 protons, and an NOE (3.5%) between the C-5 and C-13 protons observed for 4 cannot be expected from conformation 4a only. It was deduced that the 'H-NMR spectrum of 4 is subject to a large contribution from conformation 4b with the following geometrical features: (1) the C-12 hydrogen atom lies in the vicinity of the

4.

PROTOPZNE ALKALOIDS

193

TABLE I1 PROTONCHEMICAL SHIFTS( 6 ) OF ALLOCRYPTOPINE (1) AND PROTOPINE (4) Proton

1" (131)

1-H 4-H 5-Hz

6.92 s 6.60 s 2.88 m

6-H,

2.54 m

8-Hz

3.68 s

11-H

6.73 d , J = 8.5 Hz 6.89 d, J = 8.5 Hz 3.68 s 1.85 s 5.92 s 3.80 s 3.76 s

12-H 13-Hz 7-CH3 2,3-OCHz0 9,10-OCH20 9-OCH3 10-OCH2

4" (131)

6.89 s 6.63 s 2.89 s, W I I= 18 Hz 2.52 t , J = 5 3.57 d , J = 1.5 Hz 6.67 d, J = 8.5 Hz 6.65 d , J = 8.5, 1.5 Hz 3.79 s 1.93 s 5.93 s 5.91 s -

'' Spectrum was taken in CDC13 at 90 MHz. Spectrum was taken in CDCI? at 300 MHz

shielding region of ring A; (2) the carbonyl group is at an angle of about 45", and (3) the internuclear distance between the nitrogen atom and the carbonyl carbon is approximately 3.5 A and the nitrogen lone pair is directed away from the carbonyl carbon. This geometry is nearly strain free and is in accord with the data for 4 in solution. The NOE data (131)observed for 1 as well as the IR data were explained by contributions of conformations of the types 4a and l a possessing the following geometry: (1) the carbonyl group is at an angle of about 70" out of ring A and (2) the internuclear distance between the nitrogen atom and the carbonyl carbon is around 2.5 A and the nitrogen lone pair is directed toward the carbonyl carbon. The observed differences in the IR and 'H-NMR spectra between 1 and 4 were ascribed to conformational changes caused by steric interaction between the 7-methyl and 9-methoxyl groups in 1.

194

MASAYUKI ONDA AND HIROSHI TAKAHASHI

TABLE 111 CARBON CHEMICAL SHIFTSOF ALLOCRYPTOPINE (1) A N D PROTOPINE (4) 1' (131)

4h (131)

Carbon

6

Splitting (Hz)

6

1 2 3 4 4a 5 6 8 8a 9 10 11 12 12a 13 14 14a 7-CH3 2,3-OCH,O 9,10-OCH20 9-OCH3 10-OCH?

109.4 146.3 148.3 110.6 133.1 32.4 57.8 50.3 128.9 147.8 151.9 110.0 128.0 129.9 46.5 193.7 136.3 41.3 101.3 55.8 60.8

D(165)s Sm Sm D (165)t (5) Sm T(12%(5) T( 135)m T(128)q(5) Sdt(7, 3) Sm Sm D(162)s D( 162)t (5) Sm T( 128)d(S) W5) Sdt(7, 4) Q( 136)m T(175)s

108.0 145.7 147.8 110.3 132.6 31.7 57.7 50.6 117.7 146.2 145.8 106.6 124.9 128.8 46.3 194.8 136.0 41.3 101.1 100.7 -

"

-

Q(146)s Q(146)s

Splitting (Hz) D(164)s Sm Sm D( 164)t(6) Sdt(7, 5) T(127)q(5) T(135)m T( 134)q(5.5) Sdt(7, 3.5) Sm Sm D( 163)s D( 163)t(5) Sm T(126)d(5) Sq(5) Sdt(7, 5 ) Q(135)m T( 173)s T( 173)s -

Spectrum was taken in CDCI, at 25.2 MHz.

' Spectrum was taken in CDCI, at 75.4 MHz.

V. Synthesis A. SYNTHESIS FROM TETRAHYDROPROTOBERBERINES Total syntheses of cryptopine (2) (I34), protopine (4) (134), and pseudoprotopine (11) (13.5) have been achieved by several groups. This synthetic method, called Perkin's method, includes tetrahydroprotoberberines as key intermediates. Allocryptopine (1) (136), fagarine I1 (10) (137),hunnemanine (5) (138),and muramine (3) (139)were prepared from the corresponding tetrahydroprotoberberines. The synthesis of 11, which was carried out by Sotelo and Giacopello (135), illustrates Perkin's method (Scheme 1). The amide 31, which was

195

4. P R O T O P I N E ALKALOIDS

29

30

32 0-J

HCI

-

AcOH

>

'0-

<:% 35

0

4

0 11

0.i

SCHEME1. Total synthesis of pseudoprotophine by Perkins's method (135).

prepared from homopiperonylamine (29) and homopiperonylic acid (30), was converted to the tetrahydroprotoberberine 34 by a reaction sequence involving the Bischler-Napieralski reaction (31 + 32), reduction with sodium borohydride (32 + 33), and treatment with formaldehyde (33 + 34). Hofmann degradation of the methochloride 35, which was derived from 34, afforded the methine base 36, which was converted to the N-oxide 37 on treatment with perbenzoic acid. The N-oxide 37 rearranged to 11 on acid treatment. Bentley and Murray (140) prepared carbinolamines 40 by oxidation of the N-oxides 39, which were derived from the tetrahydroprotoberberines 38, with potassium chromate. Treatment of 40 with methanol yielded the O-methyl ethers 41, which were converted to the protopines by the action of methyl iodide. Allocryptopine (I), 2, and 3 were prepared from tetrahydroberberine (38, R' = Me, R2 + R2 = CH2), anhydrotetrahydroepiberberine (38, R' + R' = CH2, R2 = Me), and norcoralydine (38, R1 = R2 = Me), respectively, according to this method (140) (Scheme 2). Hanaoka et al. (141) carried out a simple synthesis of 1 by photooxidation of tetrahydroberberine methiodide (42). Rodrigo et al. (142) prepared 13-oxoallocryptopine (21) by air oxidation of 13-oxotetrahydroberberine methosulfate (43) in the presence of sodium hydride and potassium iodide (Scheme 3 ) . On the other hand, a simple transformation of 1 and 4 to 21

196

MASAYUKI ONDA AND HIROSHI TAKAHASHI

39

38

1

2 3

40

R3-H

41

R'=Me

R'+R'=cH,, R ' = M ~ R ' = M ~R'+R'=cH, , R'=R'=M~

SCHEME 2. Synthesis of protopines by oxidation of the tetrahydroprotoberberine N-oxides (140).

42

<:q 1

<,.%:;:

-2L,<:%oMe]NaH/KI

,

OMe

\

OMe

43

\

OMe

\

OMe

21

SCHEME 3. Synthesis of protopines by oxidation of the tetrahydroprotoberberine metho salts ( I 41,142).

and 13-oxoprotopine (23), respectively, by oxidation with mercuric acetate or iodine was achieved by Rodrigo et a f . (142), Leonard and Sauers (143), and Castedo et al. (144). B. SYNTHESIS FROM

PHTHALIDEISOQUINOLINES

Brossi et al. (145) reported the preparation of isoindolo[2,3-~]benz[3]azepines45 from the phthalideisoquinolines 44 via several

197

4. P R O T O P I N E ALKALOIDS

steps. Hofmann degradation of the methobromide of 45 furnished the methine bases 46, which were converted to the protopines by a procedure similar to Perkin's method (Scheme 4). a- and p-hydrastine (44, R = H), and a-narcotine (44, R = OMe) afforded 1 and l-methoxyallocryptopine (13), respectively, according to this method (146).Trojanek et al. (147) also prepared 45 (R = OMe) from the opium alkaloid narceine imide (47); this compound yielded 13 in a similar manner (148) (Scheme 4).

c. TOTALSYNTHESIS VIA BENZ[d] INDENO [1,2-b]AZEPIN€S Kametani et al. (149) and Orito et af. (150) prepared benz[d]indeno[1,2-b]azepines 48, and the latter group carried out the conversion of 48 to pseudo-type protopines (150). Photooxidation of 48 in the presence of methylene blue yielded the dibenzazecinediones (8-oxoprotopines) 49, which were converted to the protopines by reduction with lithium aluminum hydride followed by oxidation with manganese dioxide. 9,10-Dimethoxy-7-methy1-1-2,3-methy1enedioxy-5,6,7,12-tetrahydrobenz[d]indeno[172-b]azepine (48, R = Me) and 2,3,9,10-bis(methylenedioxy)-

<:% H'.

liil'.Pr,EtN/Me,SO iCICOOPh/r-Pr,EtN

0

,<EQ''oph

, 'OMe OMe

<:%

, ,,, ii, AcOH N~OH I

0

' OMe

Hooc

\

OMe

OMe

OMe

44

45 Me

~

<

\ O

a ~

o

M

e

iiil HCi I A c O H Iilm-CIC,H.CO,H

1iiilRA-400(6H) m 8i MeBr

<:%

R

OMe

46

lilKOH

1 R=H

OMe

13 R = O M e iil

H,/PtO,

A

OMe

45

R=OMe

liil LiAIH.

OMe OMc

47

SCHEME 4. Synthesis of protopines from the phthalideisoquinolines (145-148).

198

MASAYUKI ONDA AND HIROSHl TAKAHASHI

w

RO

48

OR

49

10 R = M e 11

RrRzCH,

SCHEME 5. Total synthesis of protopines via the benz[d]indeno[l &blazepines (150).

7-methyl-5,6,7,12-tetrahydrobenz[d]indeno[l,2-b]azepine(48, R + R = CH,) furnished 10 and 11, respectively, according to this method (Scheme 5 ) .

VI. Transformation of Protopines to Related Alkaloids

It is well known that treatment of protopines with phosphoryl chloride yields the dihydroprotoberberine methochlorides 50, which furnish the methine bases 51 on Hofmann degradation (Vol. 4, p. 147). Onda et al. (151,152) studied the conversion of 51 to the benzo[c]phenanthridines 53. Photolysis of 51 produced the tetrahydrobenzo[c]phenanthridines 52, which were converted to 53 by dehydrogenation with palladium-carbon followed by oxidation with 2,3-dichloro-5, 6-dicyano-1,4-benzoquinone (DDQ). Chelerythrine (53, R1 + R' = C H 2 , R2= Me) and sanguinarine (53, R1 + R1 = R2 + R2 = CH,) were obtained from allocryptopine (1) and protopine (4), respectively, according to this method (151).Photolysis of isocarbostyril 43, which was obtained from 51 (R1 + R' = CH2, R2 = Me) by oxidation with DDQ, in the presence of nitrosobenzene, afforded the photoadduct 55. Hydrogenation over palladium-carbon followed by treatment with D D Q furnished oxychelerythrine (57) via the 12-anilino compound 56 (152) (Scheme 6). Hanaoka et al. (153) reported an alternative approach to 57 from 54. Hydroboration of 54 with diborane followed by oxidation with hydrogen

199

4. P R O T O P I N E ALKALOIDS

50

51

53

52

N,Ph

51

-

PhNO

DDQ

R'+R'=CH, R2 =Me

A

hv Me0

0

Me0

0

55

54 NHPh

56

--

58

57

59

57 SCHEME6. Transformation of protopines to the benzo[c]phenanthridines (151-153,155).

200

50

MASAYUKI ONDA AND HIROSHI TAKAHASHI

A

I in vacuo

R'+R'=cH, R2+R2=CH,

60

61

62

SCHEME 6 (Codnued)

peroxide gave the alcohol 58, which was oxidized with pyridinium chlorochromate to provide 57 via the aldehyde 59 (Scheme 6). Dihydrocoptisine (60) was obtained by pyrolysis of isoprotopine chloride (50, R' + R1 = R2 + R2 = CHJ (154). Jeffs and Scharver (155) found that oxidation of 60 with rn-chloroperbenzoic acid followed by reduction with sodium borohydride afforded (?) - 13P-hydroxystylopine (62) via 13hydroxycoptisine chloride (61) (Scheme 6). Photochemical transformation of 1, cryptopine (2), and 4 to berberine (63), epiberberine (64), and coptisine (65), respectively, was achieved by Dominguez et al. (156). Shamma et al. (157) reported that photolysis of 13-oxoallocryptopine (21) with sunlight in the presence of potas-

'OMe

63

R'+ R'= CH,, R * = M ~

, R 2 +R2=CH,

64

R'= Me

65

R'+ R ' = R 2 +R ~ = C H ?

66

4. P R O T O P l N E ALKALOIDS

201

Zn,/AcOH

/

ii 68

t

70

25

-

,hv

67

+

53

SCHEME 7. Pyrolysis and photolysis of protopine N-oxide (158,159)

sium tert-butoxide stereoselectively afforded (?z)-czs-8,9-dimethoxy5,6,7,7a, 12,12a-hexahydro-12a-hydroxy-7-methyl-2,3-methy1ened~oxy-12oxobenz[d]indeno[l,2-b]azepine (66). Gozler and Shamma (158) studied pyrolysis of protopine N-oxide (25). Pyrolysis of 25 afforded the dibenzoxazacycloundecine 67, which furnished (2)-corydalisol (68) on hydrogenation over palladium-carbon. Alternatively, reduction of 67 with zinc in acetic acid provided 68 and ( 2 ) hypecorine (69) (Scheme 7). 5,6-Didehydro-7-hydroxy-6,7-secoprotopine (70) and 2-(2-methyl-3,4-methylenedioxyphenyl)-6,7-methylenedioxynaphth-1-01 (71) were isolated from the pyrolysate of 25 by Takao et al. (159) (Scheme 7). Alternatively, photolysis of 25 furnished 67 and 53 (R' + R' = R2 + R2 = CH2) (159).

VII. Biosynthesis

It has been shown that (S)-(+)-reticuline (72) is an efficient precursor in the biosynthesis of protopine (4). The pathway from 72 to 4 via (S)-(-)stylopine a-N-methochloride (73) has been reviewed in detail in this treatise (Vol. 12, p. 333; vol. 17, p. 385) by Santavg. Buzuk et al. (160)

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MASAYUKI ONDA AND HIROSHI TAKAHASHI

OH

72

0

73

recently demonstrated that glaucine prevents the incorporation of tyrosine into 4 in Glaucium fiavum Crantz and that it plays an important role in regulating the biosynthesis of 4.

VIII. Callus Culture A number of studies on callus c u h r e s of a variety of plants in the family Papaveraceae have been reported by several groups, who compared the alkaloids in callus tissues with those in the original plants. Allocryptopine (1) (55,161,162), cryptopine (2) (55,163,164), and protopine (4) (161-163,165-167) were among the alkaloids produced from the callus tissues. Takao et al. achieved the biotransformation of (*)-tetrahydrocoptisine a-N-methochloride (73) and (k)-tetrahydroberberine a-N-methochloride (74) by callus cultures of Corydalis ophiocarpa (162) and Mackaya cordata (167) in order to clarify the role of the intermediate in the biosynthesis of 1 and 4. They found that (-)-73 and (-)-74 are, respectively, converted to 4 and 1, whereas the (+)-enantiomers are not metabolized. Administration

OMe

74

R = H

75

R =----Me

76

R=-Me

77

R=----OH

78

R=-OH

79

4. P R O T O P I N E ALKALOIDS

203

experiments with callus tissues of C. platycarpa indicated that (?)mesothalictricavine a-N-methochloride (75) and (5)-thalictricavine a-Nmethochloride (76) are transformed to corycavidine (16) (168). These results are in accord not only with those obtained by feeding experiments in C. incisa plants (169) but also with the findings on biosynthesis. Biotransformation of the 13-hydroxytetrahydroprotoberberinemethochlorides in callus tissues of C. ophiocarpa and C. ochotensis var. raddeana was also examined by Takao et al. (168,170). (*)-Epiophiocarpine a-A'-methochloride (77) and (?)-ophiocarpine a-N-methochloride (78) were transformed to 13-oxoallocryptopine (21) via 13-hydroxyallocryptopine (79), while the P-N-methochlorides were not metabolized. In addition, it was observed that (-)-77 is transformed to 21, whereas (+)-77 is not. IX. Pharmacology

Pharmacological data for protopines that appeared before 1975 have been covered and reviewed in detail in this treatise (Vol. 15, p. 207) by Preininger. Akbarov et al. (171) reported that allocryptopine (1) has an antiarrhythmic effect and that it is more effective than nicotinamide for aconitineinduced arrhythmia in rats. It was also found that 1 prevents fibrillation in animal hearts subjected to electric stimulation of the auricle. Burtsev et al. (172)found that protopine (4) also has antiarrhythmic activity and that it is more effective than quinidine or novocainamide for calcium chlorideinduced and aconitine-induced cardiac arrhythmias in rats. They suggested that the mechanism of the antiarrhythmic effect of 4 is due to suppression of the foci of heterotropic stimulation, decrease in the excitability of the myocardial cells, and normalization of the catecholamine content in the myocardium. Yue (173) stated that 4 exhibits analgesic activity in mice and inhibits duodenal contractions in rabbits. It was also observed that 4 is an antagonist against acetylcholine and barium chloride-induced spasm in isolated rat ileum. Recently, Karados et al. (174) reported that I, cryptopine (2), and 4 enhance y-[3H]-aminobutyric acid binding to rat brain synaptic membrane receptors, suggesting that these protopines have diazepam-like activity. Addendum

In 1987, Takao et al. (175) reported an X-ray analysis of (?)-corycavine (15). It was shown that its crystal structure adopts a conformation similar

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to those of cryptopine (2) and protopine (4) (129). By NOE experiments in the ‘H-NMR spectrum, they also demonstrated that the most stable conformation of 15 in solution is the same as that in the solid state (175).

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137. D. Giacopello, V. Deulofeu, and J. Comin, Tetrahedron 20, 2971 (1964). 138. D . Giacopello and V. Deulofeu, Tetrahedron 23, 3265 (1967). 139. R . D . Haworth, J . B. Koepfli, and W. H. Perkin, Jr., J . Chem. SOC.,2261 (1927); D. Giacopello and V. Deulofeu, Tetrahedron Lett., 2859 (1966). 140. K. W. Bentley and A. W. Murray, J . Chem. Soc., 2497 (1963). 141. M. Hanaoka, C. Mukai, and Y. Arata, Heterocycles 4, 1685 (1976). 142. B. Nalliah, R . H. F. Manske, and R. Rodrigo, Tetrahedron Lett., 1765 (1974). 143. N. J. Leonard and R . R . Sauers, J . Org. Chem. 22, 63 (1957). 144. L. Castedo, A. Peralta, A. Puga, J. M. Saa, and R. Suau, Heterocycles 24, 5 (1986). 145. W. Klotzer, S. Teitel, J. F. Blount, and A . Brossi, J. A m . Chem. SOC.93, 4321 (1971); W. Klotzer, S. Teitel, and A . Brossi, Helv. Chim.Acta 54, 2057 (1971); S. Teitel, W. Klotzer, J. Borgese, and A . Brossi, Cun. J. Chern. 50, 2022 (1972). 146. S. Teitel, J. Borgese, and A. Brossi, Helv. Chim. Acta 56, 553 (1973). 147. Z. Vesely, J . Holubek, and J . Trojanek, Chem. Ind. (London), 478 (1973); J . Trojanek, Z. Koblicova, Z. Vesely, V. Suchan, and J. Holubek, Collect. Czech. Chem. Commun. 40, 681 (1975). 148. Z. Vesely, J. Holubek, H. Kopecka, and J. Trojanek, Collect. Czech. Chem. Commun. 40, 1403 (1975). 149. T. Kametani, M. S. Premila, S. Hirata, H. Seto, H . Nemoto, and K. Fukurnoto, Can. J . Chem. 53, 3824 (1975). 150. K. Orito, S. Kudoh, K. Yamada, and M. Itoh, Heterocycles 14, 11 (1980). 151. M. Onda, K. Yonezawa, and K. Abe, Chem. Phurm. Bull. 19, 31 (1971). 152. M. Onda and H . Yamaguchi, Chem. Pharm. Bull. 27, 2076 (1979). 153. M. Hanaoka, T. Motonishi, and C. Mukai, J . Chem. Soc., Chem. Commun., 718 (1984). 154. W. H . Perkin, Jr., J. Chem. SOC.,815 (1926); R. D. Haworth and W. H . Perkin, Jr., J . Chem. Soc., 1783 (1926). 155. P. W. Jeffs and J . D. Scharver, J . Org. Chem. 40, 644 (1975). 156. X. A. Dominguez, J. G . Delgado, W. P. Reeves, and P. D . Gardner, Tetrahedron Lett., 2493 (1967). 157. G . Blasko, V. Elango, N. Murugesan, and M. Shamma, J . Chem. Soc., Chem. Commun., 1246 (1981). 158. B. Gozler and M. Shamma, J. Chem. Soc., Perkin Trans. 1, 2431 (1983). 159. K . Iwasa, M. Sugiura, and N. Takao, Chem. Pharm. Bull. 33, 998 (1985). 160. G . N. Buzuk, M. Ya. Lovkova, and N. I. Grinkevich, lzv. Akad. Nuuk SSSR, Ser. Biol., 458 (1981). 161. J. Hodkova, Z. Vesely, Z. Koblicova, J . Holubeck, and J . Trojanek, Lloydia 35, 61 (1972). 162. H. Koblitz, U . Schumann, H . Bohrn, and J. Franke, Experientia 31, 768 (1975). 163. T . Furuya, A . Ikuta, and K. Syono, Phytochemistry 11, 3041 (1972). 164. E. J. Staba, S. Zito, and M. Amin, 1. Nat. Prod. 45, 256 (1982). 165. A. Ikuta, K. Syono, and T. Furuya, Phytochemistry 13, 2175 (1974). 166. S . Kamimure and M. Nishikawa, Agric. Biol. Chem. 40, 907 (1976). 167. N. Takao, M. Kamigauchi, and M. Okada, Helv. Chim. Acta 66, 473 (1983). 168. K. Iwasa, A . Tomii, N. Takao, T. Ishida, and M. Inoue, J. Chem. Res., Synop., 16 (1985). 169. C. Tani and K. Tagahara, Chem. Pharm. Bull. 22, 2457 (1974); N. Takao, K. Iwasa, M. Kamigauchi, and M. Sugiura, Chem. Pharm. Bull. 24, 2859 (1976). 170. K. Iwasa, A. Tomii, and N. Takao, Heterocycles 22, 33 (1984).

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171. 2 . S. Akbarov, K. U . Aliev, and M. B. Sultanov, Farmakol. Prir. Veschestv, 11 (1978). 172. V. N. Burtsev, E. N. Dormidontov, and V. N. Salyaev, Kardiologiyu, 76 (1978). 173. K.-L. Yue, Chung-kuo Yuo Li Hsueh Pao, 16 (1981). 174. J . Karados, G. Blasko, and M. Simonyi, Arzneim. Forsch. 36, 939 (1986). 175. M. Kamigauchi, K. Iwasa, N. Takao, T. Ishida, and M. Inoue, Helv. Chim. Acta 70, 1482 (1987).

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-CHAPTER 5-

AFRICAN STRYCHNOS ALKALOIDS GEORGESMASSIOTAND CLI~MENT DELAUDE Faculte de Pharrnacie Universite de Reirns Reims, France

I. Introduction 11. Ethnobotany 111. Chemical Screening Alkaloid Content of African Strychnos Species Biosynthesis and Biogenetic Relationships Recent Advances in Structural Elucidation Synthesis and Chemistry Pharmacology Conclusion References

IV. V. VI. VII. VIII. IX.

I. Introduction

The family Loganiaceae comprises some 470 species of plants classified into 29 genera. Among these, the numerically most important is the genus Strychnos, which is characterized by the special property of producing indole alkaloids. The genus Strychnos was created by Linnaeus in 1753, after examination of specimens from the tree which produces nux vomica, Strychnos nuxvomica, and Strychnos colubrina (1). Strychnos grow in equatorial and tropical areas; they may be trees, climbing shrubs, or even true lianas of large size and great strength. They display typical botanical characteristics, which make them easy to recognize, such as opposite leaves, limbs with three veins from the base, and single, double, or quadruple curled tendrils in the case of lianas. Identification beyond genus is an arduous task, however, and this has been the cause of much confusion, for example, the creation of new taxa for already known vegetables or simply for ecological varieties. Modern authorities describe 200 species of Strychnos worldwide, and Leeuwenberg, in his botanical revision of African Strychnos, has reduced the number of taxa from 300 to 75 (2). A similar reduction was also performed on Asian (3)and American Strychnos (4). Species on the three 211 THE ALKALOIDS. VOL 34 Copyright 0 1988 by Academic Press, Inc All rights ot reproduction in any form reserved

212

GEORGES MASSIOT AND CLEMENT DELAUDE

continents are almost totally segregated, and only S. potatorum grows in Asia and in Africa. Previous botanical monographs have allowed Leeuwenberg to classify Strychnos into 12 sections, whose occurrence is summarized in Table I. The number of species and varieties makes the African Srrychnos a unique set, with representatives of 11 sections out of 12 as opposed to 3 and 5 in America and Asia, respectively. The origin of the use of Strychnos by mankind has been lost with the passage of time and in the depth of the continents. They were brought to the West in the course of successive Arab invasions, by the discovery of the New World, and also through colonial expansion. The inhabitants of southern Asia had an ancestral knowledge of the toxicity of nux vomica, which they passed on to Arab conquerors. The first reference to nux vomica is found in the renowned Kitab al-sumum (Book on Poisons) which was written in the second half of the ninth century by a learned Aramean named Ibn Wahshiya, after transcriptions into Arabic of now lost Persian and Indian texts ( 5 ) . This book mentions nux vomica as a deadly poison but also, when administered in small doses, as part of a remedy for demonic possession. Confirmation of the use of nux vomica by the Islamic world is found in the oldest known medical treatise written in Arabic by a physician named Serapio (6). From this time onward, by means of caravan trade nux vomica were progressively exported from Asia to North Africa, to the Mediterranean basin, and finally to the Western world, to England, and to Germany. TABLE I WORLDWIDE DISTRIBUTION OF Strychnos Continent Section

America

Africa

Asia

Strychnos Rouhamon Breviflorae Penicillatae Aculeatae Spinos a e Brevitubae Lanigerae Phaeotrichae Densiflorae Dolichantae Scyphostrychnos Number of species

35 9 20

11 12 9

18 1 8 11 20

1 4 7 12 1 8 9 1 75

58

5. AFRICAN STRYCHNOS ALKALOIDS

213

In the fifteenth century, nux vomica were available in European markets and used in bait to kill dogs, cats and rodents, and they might have been employed as a redoubtable poison in the hands of skillful poisoners. St. Ignatius beans have properties similar to those of nux vomica; they were known in the Philippines for their aphrodisiac and stimulative properties and also as a violent lethal poison. They were brought to Europe in 1699 by father Camelli, a Jesuit missionary in the Philippines who was devoted to botany. He gave the beans the latin name Faba sancti ignatii in honor of Saint Ignatius, the founder of the Society of Jesus; the liana was later named Strychnos ignatii. St. Ignatius beans were too rare to compete with nux vomica, but both of these exotic, dreaded materials struck popular imagination and aroused the curiosity of chemists. Some of the earliest chemical works on Strychnos alkaloids were done by a German chemist named Jean Rodolphe Glauber (1604-1670), who treated nux vomica with solutions of nitric and sulfuric acids and then precipitated, by means of potassium carbonate, a powder supposed to contain the “concentrated virtue” of the vegetable material. Looking back, it is possible that Glauber was the first to glimpse strychnine and brucine in the laboratory (7). The history of chemistry, however, justly records that the honor of the discoveries of brucine and strychnine, the major alkaloids of nux vomica and St. Ignatius beans, belongs to Pelletier and Caventou ( 8 ) . In 1818, the French scientists isolated strychnine as a white powder which they called vauqueline; they reported that the substance was responsible for the action of nux vomica on “animal life.” The preparation of pure strychnine was a pioneering work and a landmark in the chemistry of natural products. Strychnine was among the first alkaloids to be a reward of research into the vegetable kingdom. After strychnine’s isolation, nux vomica, a crude material with variable properties, could be replaced by a pure substance with constant characteristics usable for research in the fields of chemistry, pharmacology, and toxicology. The Asian poison was mastered and was to become one of the major substances in the pharmacopeia. At the end of the nineteenth century, Burgraeve, a medicine professor in Ghent, proudly qualified strychnine as “the cornerstone of dosimetry” (9). Not only a therapeutic, commercial, and industrial material, strychnine was for more than a century the subject of an impressive number of investigations aimed at unraveling its molecular structure. The ultimate steps in the structural elucidation work were performed by Sir R. Robinson and R. B. Woodward in the years 1945-1947. Later, in 1954, Woodward and co-workers achieved what seemed an impossible task, the total synthesis of strychnine (10,Il). These studies have been summarized in chapters on the Strychnos alkaloids of Vols. 1, 2, 6, and 8 in this treatise

214

GEORGES MASSIOT AND CLEMENT DELAUDE

(12-15), and it is worth mentioning here the most frequently quoted names of Wieland, Leuchs, Prelog, Robinson, and Woodward. Asiatic Strychnos are not the sole species to provide ethnography, chemistry, and medicine a fascinating subject with which to obtain glory. During the pre-Columbian era, in the depth of Amazonian forests, hunters had discovered that the deposit of concentrated extracts from these plants on arrowheads paralyzed big game, which otherwise would have only been slightly injured by primitive darts. Indian poisons, known as curares, are remarkable hunting poisons, toxic in minute amounts when they reach the bloodstream, but harmless when ingested. The conquistadors’ chronicles leave no doubt about the awe inspired by the indians’ chemical weapon, but, when describing the preparation of curare by hunting parties or its action, they mask the real facts with beliefs in the supernatural and magic. Curare kept its mysteries for more than 300 years, and progressively, after many controversies, observations of botanists, naturalists, and ethnologists revealed that there was not a curare, but curares. Plants responsible for the activity of curare are Menispermaceae (Chondodendron) and diverse Strychnos species exclusively. Despite the simplicity of the recipe, curare composition was variable, and different Strychnos were used in different proportions depending on their abundance at the place of collection. From a chemical standpoint, curares were analyzed by the groups of Wieland, Karrer, Schmid, King, Marini-Bettolo, and many others. Little by little, the secrets of curare were unraveled, and it was shown that active constituents were ammonium salts of isoquinolines (from Menispermaceae) and of indole alkaloids (from Strychnos spp.). These alkaloids were named calebassines and tubocurarines (16) after the names of the recipients in use: calabash gourds or bamboo tubes. This chemistry has been the subject of several chapters in this treatise (27-19). As an oversimplification, it may be stated that Asian Strychnos have provided a deadly poison, strychnine, marvelous exercises in chemistry, but almost no practical permanent applications. American Strychnos are also sources of poisons, but, as predicted by Claude Bernard, the curares would be useful in medicine; this has indeed proved to be true since synthetic curares are currently used in surgery as auxiliaries to anesthetics. No such features belong to the African Strychnos, whose study was deferred until the second half of the twentieth century. African Strychnos are found south of the Sahara, in areas which were unknown to Europeans until the recent colonial conquests. In 1860, five of the most widely occurring species of Strychnos had been seen by botanists, and the 70 remaining species were discovered around 1900. People from Africa, who lived in close contact with nature, had an ancestral knowledge of the vegetables growing in their immediate environment. For example,

5 . AFRICAN STRYCHNOS ALKALOIDS

215

the root bark of a plant known as Icaja or N’caja was used in western Africa as a poison for arrows as well as an ordeal poison; it was a Strychnos, named S. icuja by Baillon in 1879 (20). Ninety years later, Sandberg and co-workers isolated strychnine from the root of S. icuju; it was obviously the cause of the activity of the African poison (21). In Rwanda, the Banyambo used to hunt with poisoned arrows, and it was pointed out by Angenot in the 1970sthat S. usambarensis had a part in the preparation of the poison (22). From the alkaloid mixture, a new quaternary ammonium afrocurarine was extracted as well as curarizing alkaloids previously identified from American Strychnos (23). Thus, it has only relatively recently been delnonstrated that Asian Strychnos do not have the exclusive occurrence of strychnine and American Strychnos are not the only source of “curares;” it may be regretted that these findings come too late to give African Strychnos the fame they deserve. Chemical investigations of African Strychnos started with species which were in use as traditionnal poisons and were extended to numerous other species. Despite the lack of any ethnobotanical indications, several Strychnos were shown to be rich sources of alkaloids, starting materials for biological studies. With the exceptions of proposals of hypothetical structures for three of the four alkaloids of S . henningsii in 1951 (24), of a preliminary screening of Strychnos from Zaire (25),and of the isolation of three alkaloids from S. icaju (26), both in 1953, most of the work on African Strychnos results from present investigations. Paradoxically, the fields of Asian and American Strychnos are now dormant, and African Strychnos are becoming the subject of an increasing number of studies. The studies have been sporadically reported in this treatise as sections of chapters dealing with Strychnos and curares (14,19), with miscellaneous classes of alkaloids (27-31), as examples in structural X-ray investigations (32), and also in the famous chapter, “Alkaloids Unclassified and of Unknown Structure” (33,34).In the 1980s, two monographs o n the subject have been published (35,36), and it is also worth mentioning a review entitled “Strychnos Alkaloids” (37) and an entire article dedicated to strychnine (38).This chapter aims to fill gaps caused by rapid advances in the field and gathers and discusses results scattered in the chemical literature. 11. Ethnobotany

The first information on the uses of Strychnos by African people appeared in the scientific literature of the nineteenth century and soon after 1900. Two main uses were described. Strychnos cocculoides, S.

216

GEORGES MASSIOT AND

CLBMENT

DELAUDE

innocua, S. pungens, and S. spinosa are shrubs or small trees, growing in savannas, which are frequently reported to be fruit-producing species. The most famous one is S. cocculoides, whose fruit is the object of limited small trade. When, in 1879, Baillon gave the first description of S. icaja, this large liana was well known for its high toxicity. An infusion of its red root bark was one of the most powerful ordeal poisons in central and western Africa, called icaja, N’casa, or Mboundou. Until the beginning of the twentieth century, it was responsible for a large number of victims among the people who submitted themselves to its law. Even though recourse to this god’s judgment has fallen into obsolescence, in areas where S. icaja grows no one would fail to identify the species and comment on its toxicity. Symptoms of poisoning by Mboundou were well described and strongly suggested poisoning by strychnine. From these early ethnobotanical reports, Strychnos were classified into toxic species, supposed to contain strychnine and brucine, and species with edible fruit, devoid of any of these alkaloids. Their characteristics of toxicity or innocuousness have been retained in the treatises on “useful and medicinal plants from tropical Africa” written in the first half of the twentieth century. Comparison of the violent actions of Mboundou, St. Ignatius bean, and nux vomica as well as the absence of curarizing effects in African Strychnos have led the false but widespread assimilation of African and Asian Strychnos. It was also believed that American Strychnos only could elaborate curares. These considerations were condemned after more and more ethnobotanical and chemical studies were performed and especially following publication of the work of Bisset. Bisset is a contemporary author who gathered information from a great number of sources, including written inscriptions on herbarium specimen sheets, and who compiled the data in a single article based on the now accepted classification of Leeuwenberg (39). Both authors also collaborated on an article which dealt solely with the use of Strychnos in the preparation of poisons for arrows and ordeals (40). Modern ethnobotanical reports state that S. angolensis, S. cocculoides, S. decussata, S. densifora, S. floribunda, S. gossweileri, S. innocua, S. kasengaensis, S. lucens, S. madagascariensis, S. mitis, S. nigritana, S . panganensis, S. potatorum, S. pungens, S. schefieri, S. spinosa, S. usambarensis, and S. variabilis yield fruits which are eaten by men or animals, whereas S. aculeata, S. angolensis, S. camptoneura, S. cocculoides, S. decussata, S. densiftora, S . dinkiagei, S. henningsii, S. icaja, S. innocua, S. johnsonii, S. malchairii, S. mellodora, S. mitis, S. phaecotricha, S. potatorum, S . pungens, S. samba, S . spinosa, and S. variabilis are toxic. The distinction between toxic and edible species is thus not as clear-cut as was previously thought, since at least nine species belong to both categories. Bisset also reported uses of S. aculeata, S. afzelii, S. boonei,

5 . AFRICAN S T R Y C H N O S ALKALOIDS

217

S. cocculoides, S. congolana, S. dinklagei, S. joribunda, S. gossweileri, S. henningsii, S. icaja, S. innocua, S. longicaudata, S. lucens, S. madagascariensis, S. melastomatoides, S. panganensis, S. potatorum, S . pungens, S. schefleri, and S. spinosa in the preparation of remedies against a large variety of ailments. The most frequently cited prescriptions were for venereal diseases, sexual impotence, stomach and intestinal aches, pulmonary diseases, fevers, malaria, and snakebites. They also found use against parasites and worms and could be used as aphrodisiacs as well as abortifacients. Soon after the publication of Bisset’s article, Angenot revealed that the Banyambo hunters from Rwanda used a curarizing arrow poison made with S. usambarensis (22). This observation called into question the tenacious fallacy that Strychnos could be divided into curarizing American species and tetanizing Asian and African species. As a final remark, it is important to point out as Bisset did (39) that the most frequently used species (S. potatorum, S. henningsii, S. innocua, S. madagascariensis, S. pungens, and S. spinosa) are very commonly encountered in savannas. Exceptions are S. icaja and S. aculeata, which are lianas growing in dense forests. Generally speaking, Africans have used the species which they could find easily, thus excluding rare plants from practical utility. At present, ethnobotanical information is available on about 40 species. The species for which no special uses are reported are either rare, narrowly distributed dense forest species or plants which are difficult to identify. The choice of ethnobotanical criteria in the decision to conduct chemical investigations on a plant species may be an unnecessary limitation of the field, and we feel that more credit should be given to the systematic botanists. Just recognition, however, should also be given to the African empiricists for focusing our attention on the most interesting plants.

111. Chemical Screening

In a quest for species with high alkaloid content, Denoel was the first scientist to examine a large number of Strychnos species growing in Zaire and to attempt to determine their content of brucine and strychnine (25). The weak points of this early study are the botanical identifications, which do not follow the modern classification; a merit of this work is its drawing of attention to species rich in alkaloids, such as S. angolensis, S. gossweileri, S. henningsii, S. icaja, S. longicaudata, S. schefleri, S. usambarensis, and S. variabilis, all of which have been the object of later fruitful investigations. This study reported the positive identification of strychnine in S. icaja and the putative presence of brucine in the same vegetable.

218

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

A similar investigation was performed by Bouquet on 19 Strychnos species from the Ivory Coast, among which 9 had been seen before by Denoel (41). This study showed that S . camptoneura, S . diizklagei, S. floribunda, and S . splendens were rich in alkaloids. The complexity of the alkaloid mixtures was later demonstrated by thin-layer chromatography by Sandberg (42) and Rolfsen et al. (43); all of their material had been previously investigated by Denoel or Bouquet. Studies on herbarium specimens of most of the African Strychnos were later performed by Bisset and Phillipson (44). As necessitated by the circumstances, investigations were limited to minute amounts of leaves, twigs, or, rarely, seeds, with TLC as the only tool. Despite their limitations, these early investigations have been and still are precious guidelines for the orientation of future research.

IV. Alkaloid Content of African Strychnos Species

Table I1 lists African Strychnos species that have been screened for the presence and content alkaloids. Species are listed in alphabetical order, and mention is made of their botanical sections. Also given are the parts of the plant from which alkaloids were isolated (e.g., Rb is root bark) and estimated yields to crude alkaloid mixtures. In most cases, the name and numbering given by the original authors are reported; when they conflict with the commonly used biogenetic numbering (176), or when the same alkaloid has been given several names, synonyms appear. For example, the name of 4-hydroxystrychnine (systematic nomenclature) has been changed to 12-hydroxystrychnine, but this does not imply that the OH group has been moved. The formulas of these 249 alkaloids are presented below in alphabetical order (with the exception of 243-248) along with a list of sources. A large majority of the alkaloids (169 of them) were isolated only one time, but this fact might be related to the limited number of compounds that were separable in the early investigations. The most frequently isolated alkaloids are the symmetrical dimers: bisnordihydro toxiferine (57) (12), caracurine V (6), and bisnor-C alkaloid H(22) (6). Angustine (12) and its congeners angustidine (11) and angustoline (13) have been detected in many species but in the course of screening aiming only at verifying their occurrence. Among other monomers, akagerine (3) (10 isolations), 11methoxydiaboline (45) (8), desacetyl-isoretuline (158) (6), and WielandGumlich aldehyde (240) (6) present a certain character of ubiquity. (Text continues on p. 230)

219

5 . '4FRICAN STRYCHN0.T ALKALOIDS

TABLE I1 SELECTED AFKICAN Strychnos A N D THEIRALKALOIDS Species and alkaloids

Strychnos aculeata Solered. (Aculeatae) [Rb: 6-9 g/kg (45)J Strychnofendlerine (181) N'-Acetylstrychnosplendine (192) N'-Acetyl-O-methylstrychnosplendine(193) Spermostrychnine (166) Isosplendine (101) afzelii Gilg (Breviflorae) Bisnordihydrotoxiferine (57) Bisnor-C alkaloid H (22) Caracurine V (29) Wieland-Gumlich aldehyde (240) Diaboline (44) Bisnordihydrotoxiferine mono-A-oxide (58) Longicaudatine (110) angolensis Gilg (Breviflorae) [Mixture Rb-Sb: 7 g/kg; L: 0.9 g/kg (25,50)] Tubotaiwine (207) 1I-Methoxy Wieland-Gumlich aldehyde (241) 17-0-Methyl-1I-methoxy Wieland-Gumlich aldehyde (242) 1I-Methoxydiaboline (45) Epi-17-0-methyl-11-methoxydiaboline (47) Caracurine V (29) 11-MethoxymacusineA (115) Angustine (12) Angustidine (11) barteri Solered. (Dolichantae) [R: 9.2 g/kg; Sb: 13 g/kg; S: 0.6 g/kg (53)l Akagerine (3) 18,19-Dehydronigritanine (usambarine) (225) 10-Hydroxynigritanine (lO-hydroxy-18,19-dihydrc~usambarine) (229) I0-Hydroxy-18,19-dehydronigritanine (10-hydroxyusarnbarine) (226) Barterine (19) 10-Hydroxybarterine (20) Nigritanine (18,19-dihydrousambarine) (230) S. camptoneura Gilg (Scyphostrychnos) [Sb: 8.5 g/kg; L: 2.8 g/kg (55,56)] Serpentine (165) Alstonine (10) Retuline N-oxide (150) Camptoneurine (28)

Source"

Ref.

Rb Rb, Sb Sb Sb Sb

45 45,46 46 46 46

Sb Sb Sb Sb Sb Sb Sb

47 47 47 47 48 48 49

Mixture Rb-Sb Mixture Rb-Sb Mixture Rb-Sb

50 50 50

Mixture Rb-Sb Mixture Rb-Sb Mixture Rb-Sb Mixture Rb-Sb L L

50 50

R , Sb Sb, L L

53 53 53

L

53

L L S

54 54 54

Rb, Sb Rb, Sb Sb Sb

57 57 55 55

50

51 52 52

(continues)

220

GEORGES MASSIOT AND C L ~ M E N T DELAUDE

TABLE I1 (Continued) Species and alkaloids Akagerine (3) Sb Retuline (147) Sb Kribine (105) Sb Anthirine (14) L Anthirine methobromide (16) L Angustine (12) L S. chrysophylla Gilg (Lanigerae) [Sb: 0.16 g/kg (60)] Longicaudatine (110) Sb Wieland-Gumlich aldehyde (240) Sb Caracurine V (29) Sb Longicaudatine N-oxide (111) Sb dale De Wild. (Rouhamon) [Sb: 12 g/kg ( 6 I ) ] Akagerine (3) Sb 17-0-Methylakagerine (61) Sb Kribine (105) Sb 17-0-Methylkribine (106) Sb Epi-17-0-methyl-kribine (107) Sb Decussine (36) Sb 3,14-Dihydrodecussine (38) Sb L 10,10‘-Dimethoxy-3~u,17a-(Z)-17,4’,5’ ,6’tetrahydrousambarensine (218) L 10,10’-Dimethoxy-N-methyl-3cu17a-(Z)-17,4’,5‘,6‘tetrahydrousambarensine (219) Harmane (83) Sb Dihydroharmane (85) Sb Sb Usambarensine (208) 5’ ,6’-Dihydrousambarensine (212) Sb 17,4‘,5‘,6’-Tetrahydrousambarensine (214) Sb Sb Usambarensine N-oxide (210) 5‘ ,6’-DihydrousambarensineN-oxide (213) Sb 10,10‘-Dihydroxy-17,4’,5’,6’-tetrahydrousambarensine L (221) 10-Hydroxy-10’-methoxy-N4’-methyl-17,4’,5’,6’-L tetrahydrousambarensine (222) L lO‘-Hydroxy- 10-methoxy-N4’-methy1-17,4!,5‘ ,6‘tetrahydrousambarensine (223) L lO-Hydroxy-10’-methoxy-17,4’,5’ ,6’tetrahydrousambarensine (220) L 10,10‘-Dihydroxy-N4’-methyl-17,4’ ,5’,6‘tetrahydrousambarensine (224) decussata (Pappe) Gilg (Rouhamon) [Sb: 5.4 g/kg (66)] Akagerine (3) Sb

Source“

Ref 58 59 36 56 56 52

49 60 60 60

61 61 61,62 61,62 61,62 63 63 64 64

65 65 65 65 65 65 65 65 65 65 65 65

66

(continues)

221

5. AFRICAN STRYCHNOS ALKALOIDS

TABLE I1 (Continued) Species and alkaloids 17-0-Methylakagerine (6) 10-Hydroxy-17-0-methylkribine(108) 10-Hydroxyepi-17-0-methylkribine (109) 10-Hydroxyakagerine (4) Akagerine lactone (8) Decussine (36) 3,14-Dihydrodecussine (38) lO-Hydroxy-3,14-dihydrodecussine(39) Rouhamine (5,6-dehydrodecussine) (37) Bisnordihydrotoxiferine (57) Macusine B (116) 0-Methylmacusine B (117) Malindine (120) Glucoalkaloid (82) 10-Hydroxy-17-0-methylakagerine( 5 ) S. diizklagei Gilg (Lanigerae) [Sb: 5 g/kg; L: 1 g/kg (71,72)] Ellipticine (66) 17-Oxoellipticine (70) Ellipticine N-oxide (72) 3,14-Dihydroellipticine (67) 3,14,4,21-Tetrahydroellipticine(68) 10-Hydroxyellipticine (69) 17-Oxoellipticine N-oxide (71) 18-Hydroxyellipticine (249) Brafouedine (25) Isobrafouedine (26) Venoterpine (247) Gentianine (245) Dinklageine (243) Cantleyine (244) Strychnovoline (246) Strellidimine (248) S. dolichothyrsa Gilg ex Onochie et Hepper (Breviflorae) Bisnordihydrotoxiferine (57) Bisnor-C-curarine (35) Bisnordihydrotoxiferine N-oxide (58) Bisnordihydrotoxiferine di-N-oxide (59) Bisnor-C alkaloid D (21) 18-Deoxy Wieland-Gumlich aldehyde (42) Caracurine V (29) Caracurine V N-oxide (30) Caracurine V di-N-oxide (31) Bisnor-C alkaloid H (22) Bisnor-C alkaloid H di-N-oxide (24)

Ref.

Source" Sb Sb Sb Sb Sb Sb Sb Sb Sb

Sb Sb Sb Sb L Sb

Sb Sb Sb Sb Sb Sb Sb Sb Sb Sb Sb Sb, L L

L L Sb Sb Sb Sb Sb Sb Sb Sb Sb Sb Sb Sb

66 66,62 66,62 67 67 63,68 63 63 63 63 69 69 69 70 66

71 73 74 74 74 74 74 74 75 75 74 74,72 76,72 72 72 105

77 77 77 77 77 77 78 78 78 79 79 (continues)

222

GEORGES MASSIOT AND CLEMENT DELAUDE

TABLE I1 (Continued) Species and alkaloids Dolichocurine (64) Dolichothine (dolichothyrine) (65) Wieland-Gumlich aldehyde (240) 1LMethoxydiaboline (45) Tubotaiwine (207) Condylocarpine (33) Nor-C-fluorocurarine (78) Norrnacusine B (119) Longicaudatine (110) Bisnor-C alkaloid H N-oxide (23) S. elaeocarpa Gilg ex Leeuwenberg (Rouhamon) [Sb: 10 g/kg (61)] Akagerine (3) 17-0-Methylakagerine (6) Kribine (105) 17-0-Methylkribine (106) Epi-17-0-methylkribine (107) Strychnocarpine (180) Decussine (36) 3,14-Dihydrodecussine (38) Bisnordihydrotoxiferine (57) S. floribunda Gilg (Rouhamon) Bisnordihydrotoxiferine (57) Akagerine (3) Decussine (36) Rouhamine (5,6-dehydrodecussine) (37) Strychnocarpine (180) Desacetylisoretuline (158) Isorosibiline (164) Angustine (12) S. gussweileri Exell. (Dolichantae) [Rb: 28 g/kg (82)] Dolichantoside (62) Alstonine (10) Diploceline (60) Isodolichantoside (63) Strychnofluorine (182) 16-Epidiploceline (61) Strychnoxanthine (197) S. henningsii Gilg (Breviflorae) [Rb: 35 g/kg (87)] N'-Acetylstrychnosplendine (192) N'-Acetyl-11-methoxystrychnosplendine(194) Diaboline (44) Holstiine (91)

Source" Sb Sb Sb Sb Sb Sb

Ref.

Sb Sb Sb

79 79 79 79 79 79 79 79 49 79

Sb Sb Sb Sb Sb Sb Sb Sb Sb

61 61 61,62 61,62 61,62 80 63 63 63

Sb Sb Sb Sb Sb Sb Sb L

81 81 81 81 81 81 81 52

Rb Rb Rb Rb Rb Rb Rb

82,83 82 82,84 85 85 85 86

Rb Rb Rb, Sb Rb, Sb, L

87 87 87,88 87,89-92

Sb

(continues)

223

5 . AFRICAN STRYCHNOS ALKALOIDS

TABLE I1 (Continued) Species and alkaloids Rindline (162) Henningsamine (86) Henningsoline (89) 0-Acetylhenningsoline (90) 11-Methoxyhenningsamine (87) 2,16-Dehydrodiaboline (48) 11-Methoxydiaboline (45) ll-Methoxy-2,16-dehydrodiaboline (49) Holstiline (92) Condensamine (32) Retuline (147) 0-Acetylretuline (148) Tsilanine (202) 10-Methoxytsilanine (203) 0-Demethyltsilanine (204) 10-Methoxy-0-demethyltsilanine (205) N'-Desacetyl-17-O-acetyl-18-hydroxyisoretuline (161) N'-Desacetylisoretuline (158) 18-Hydroxyisoretuline (156) N'-Desacetyl-lS-hydroxyisoretuline (159) icuju Baill. (Breviflorae) [Rb: 102 g/kg; Sb: 30 g/kg; L: 18 g/kg (25)] Bisnordihydrotoxiferine (577) Sungucine (199) N4-Methylstrychninium (171) Strychnine (169) 12-Hydroxystrychnine (4-hydroxystrychnine) (170) 19,20-cu-Epoxynovacine (21,22-epoxy-N-methyl-sec-pseudobrucine, 21,22-epoxy-2,3-dimethoxy-N-methyl-secpseudostrychnine, or 21,22-a-epoxynovacine) (139) 19,20-cu-Epoxy-15-hydroxynovacine (21,22-a-epoxy-14-hydroxy-2,3-dimethoxy-Nmethyl-sec-pseudostrychnine) (140) Icajine (N-methylpseudostrychnine) (94) Pseudostrychnine (16-hydroxystrychnine) (141) 19,20-~-Epoxyvomicine (21,22-a-epoxy-4-hydroxy-N-methyl-secpseudostrychnine) (236) 19,20-a-Epoxy-l0-methoxyicajine (21,22-a-epoxy-2-methoxyicajine) (98) 19,20-a-Epoxy-l5-hydroxyicajine (21,22-a-epoxy-14-hydroxy-N-rnethyl-secpseudostrychnine) (96)

Source"

Ref

Sb Sb Sb Sb Sb Sb Sb Sb Sb, L Sb, L Sb, L Sb, L T T Mixture T-L Mixture T-L Mixture T-L Mixture T-L Mixture T-L Mixture T-L

88,92,93 88,94 88,95 96 96 96 96 96 90,92 90,97 90,97,98 99

Rb Rb Rb Rb, Sb, L Rb, Sb, L Rb, L, F

102 102,103 102 104,21 104,21 21,102,106

Rb, L, R

102,1O6,107

Rb, L, F L L

102,107-109 108 108

L

108

L

108,I I I

100 100 100 100

101 101

101 101

~

(continues)

224

GEORGES MASSIOT AND CLEMENT DELAUDE

TABLE I1 (Continued) Species and alkaloids

Source"

15,Hydroxyicajine 1 (14-hydroxy-N-methyl-see-pseudostrychnine) (95) Novacine (138) 1 19,20-Epoxy-15-hydroxy-ll-methoxyvomicine 1 (21,22-a-epoxy-4,14-dihydroxy-3-methoxy-Nmethyl-see-pseudostrychnme) (237) 19.20-a-Epoxy-ll,12-dimethoxyicajine 1 (21,22-a-epoxy-3,4-dimethoxy-N-methyl-secpseudostrychnine) (100) Vomicine (235) L, F 19,20-a-Epoxy-ll-methoxyvomicine L, F (21,22-a-epoxy-4-hydroxy-3-methoxy-N-methylsee-pseudostrychnine) (239) 19,20-a-Epoxy-15-hydroxyvomicine L, F (21,22-a-epoxy-4,14-dihydroxy-N-methyI-secpseudostrychnine) (238) 19,20-a-Epoxy-l2-methoxyicajine F (21,22-a-epoxy-4-methoxy-N-rnethyl-secpseudostrychnine) (99) 19,20-cu-Epoxy-l5-hydroxy-l2-methoxyicajine F (21,22-a-epoxy-14-hydroxy-4-methoxy-N-methylsee-pseudostrychnine) (97) S . johnsonii Hutch. et M. B. Moss. (Brevitubae) [Rb: 2.5 g/kg; Sb: 2 g/kg (112)] Angustine (12) Rb 0-Ethylakagerine (7) Rb 0-Ethylakagerine lactone (9) Rb Dihydrodecussine (38) Rb Rb Normalindine (122) Oxojanussine (104) Rb Norepimalindine (123) Rb Norharmane (84) Rb Harmane (83) Rb Akagerine (3) Rb Janussine A (103a) Rb, Sb Janussine B (103b) Rb, Sb Dernethoxycarbonyl-3,14-dihydrogambirtannine (40) Rb, Sb Dihydrocycloakagerine (54) Rb, Sb Tetr ahydroalstonial(201) Rb, Sb Ajmalicinial(2) Rb, Sb Akagerine lactone (8) Rb, Sb Tetrahydroakagerine (200) Rb, Sb Demethoxycarbonyl-3,14,15,16,17,18~ Sb hexahydrogambirtannine (41) Dihydrocorynantheol(53) Sb

Ref. 108 107 107

107

107,109 107,I08

107,108

107

107

112 112 112 112 112 112 112 112 112 112 112,113 112,113 112 112 112 112 112 112 112 112

(continues)

225

5. AFRICAN STRYCHNOS ALKALOIDS

TABLE I1 (Continued) Species and alkaloids Anthirine lactone (18) Anthirine (14) Isoanthirine (17) S. kasengaensis De Wild. (Lanigerae) [Rb: 13.8 g/kg; Sb: 6 g/kg; L: 1.3 g/kg (14411 Isoretulinal(144) Desacetylisoretuline (158) N'-Desacetylretuline (151) 16,17-Dehydrostrychnobiline (177) Matopensine (125) Retuline (147) Matopensine N-oxide (126) Bisnordihydrotoxiferine (57) (16-R)-Isositsirikine (102) Isoretuline (154) N1-Desacetyl-18-acetoxyisoretuIIne (160) 11-Methoxyretuline (149) 11-Methoxyisoretuline (157) Wieland-Gumlich aldehyde (240) Wieland-Gumlich aldehyde diol (18-hydroxyisoretuline) (156) 18-Deoxy Wieland-Gumlich aldehyde (desacetylisoretulinal) (42) 0-Acetylretuline (148) 0-Acetylisoretuline (155) 18-Hydroxy-N'-desacetylisoretuline(159) S. longicuudata Gilg. (Penicillatae) [Rb: 18 g/kg; Sb: 2 g/kg (116)] Bisnor-C alkaloid H (22) Longicaudatine (110) Longicaudatine F (113) Longicaudatine Y (11.2) Bisnordihydrotoxiferine (57) Wieland-Gumlich aldehyde (240) Diaboline (44) 1,2-DehydrodesacetyIretuline(152) N1-DesacetyI-18-acetoxyisoretuline (160) N'-Desacetyl- 18-hydroxyisoretuline (159) 23-Hydroxy-2,16-dehydroretuline (153) Flavopeirerine (73) S. ntalucoclados C. H. Wright (Brevifloraej 11-Methoxydiaboline (45) Normacusine B (119) 19,20-Dihydro-ll-methoxydiaboEne (50) 11-Methoxydiaboline N-oxide (46)

Source" Sb Sb Sb

Rb Rb Rb Rb Rb, Sb Rb, Sb Sb Sb Sb

Sb Sb Sb Sb Sb Sb

Ref. i12 112 112

114 114 114 114 114,115 114 114 114 114 114

114 114 114 114 114

Sb

114

Sb, L

114 114 114

L Sb

Rb Rb, Sb Rb, Sb Rb, Sb Rb, Sb Rb, Sb Sb

Sb Sb Sb Sb Sb Sb Sb

Sb Sb

49 49,116 116 116 116 116 116 116 116 116 116 116 117 118 118 2 75

(continues)

226

GEORGES MASSIOT AND CLEMENT DELAUDE

TABLE I1 (Continued) Species and alkaloids Bisnor-C alkaloid H (22) Caracurine V (29) Bisnordihydrotoxiferine (57) S. matopensis S. Moore (Penicillatae) [Rb: 12.7 g/kg (119)] Wieland-Gumlich aldehyde (240) 18-Deoxy Wieland-Gumlich aldehyde (42) N-Formyl-18-deoxy Wieland-Gumlich aldehyde (43) Nor-C-fluorocurarine (78) N-Desacetylretuline (151) N-Desacetylisoretuline (158) Isorosibiline (164) (16R)-Isositsirikine (102) 1I-Methoxydiaboline (45) Diaboline (44) Matoperisine (125) 16-Methoxyisomatopensine (128) 16-Ethoxyisomatopensine (129) 18-Hydroxymatopensine (130) 18,18'-Dihydroxymatopensine(127) Matopensine N-oxide (126) Strychnofuranine (185) Bisnordihydrotoxiferine (57) Bisnor-C-curarine (35) Bisnor-C alkaloid H (22) Bisnor-C alkaloid D (21) Longicaudatine (110) Longicaudatine F (113) Longicaudatine Y (112) Longicaudatine Z (114) Longicaudatine N-oxide (111) S. rnernecyloides S. Moore (Lanigerae) Usambarensine (208) S. ngouniensis Pellegr. (Lanigerae) [Rb: 7 g/kg; Sb: 2.4 g/kg (116)] Glucosylngouniensine (136) Epiglucosy Ingouniensine (137) Ngouniensine (134) Epingouniensine (135) Tubotaiwinal(206) Longicaudatine (110) 17-Hydroxyalloibogamine (93) 4',17-Dihydro-17a-tchibangensine (17,4',5',6'-tetrahydro-l7a-~sambarensine) (214)

Source''

Ref.

Sb Sb Sb

2 75 2 75 2 75

Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb Rb

119 119

119 119 119 119 119 119 119 119 115,119 119 119 119 119 119

119 119 119 119 119 119 119 119 119 119

Sb

120

Rb Rb Rb, Sb Rb, Sb Rb, Sb Rb, Sb Rb, Sb Rb. Sb

116 116 I 16,120 116 116 116,49 122 116 (continues)

227

5 . AFRICAN S T R Y C H N O S ALKALOIDS

T A B L E I1 (Continued) Species and alkaloids 10‘-Hydroxy-4’, 17-dihydro-17a-tchibangensine (lO’-hydroxy-17,4’,5‘,6‘-tetrahydro-l7ausambarensine) (216) lO’-Hydroxy-4’ ,17-dihydro-17P-tchibangensine (lO’-hydroxy-4’,5’,6’,7‘-tetrahydro-l7Pusambarensine) (217) 4‘,17-Dihydro-17P-tchibangensine (17,4’,5‘,6’-tetrahydro-l7~-usambarensine) (215) 18-Hydroxynor-C-fluorocurarine(79) 18-Acetoxynor-C-fluorocurarine (80) S. nigritana Bak. (Densiflorae) [L: 1 g/kg (Wl Akagerine (3) Kribine (105) 18,19-Dehydronigritanine (usambarine) (225) Nigritanine (18,19-dihydrousambarensine) (230) 10-Hydroxynigritanine (10-hydrox y- 18,19-dihydrousam barensine) (229) 18,19-Dehydro-lO-hydroxynigritanine (10-hydroxyusambarensine) (226) S.odoruta A . Chev. (Dolichantae) Angustine (12) Angustoline (13) Angustidine (11) S.potatorum L. (Rouhamon) [Sb: 0.9 g/kg; L: 0.9 g/kg; S: 3 g/kg] Diaboline (44) Acetyldiaboline (52) Angustine (12) S. samba Duvign. (Brevitubae) Angustine (12) Angustoline (13) Angustidine (11) S.schefleri Gilg (Lanigerae) [Sb: 28 g/kg; L: 3 g/kg (27)] Desacetylisoretuline (158) Bisnordihydrotoxiferine (57) Mavacurine (131) Fluorocurine (81) Angustine (12) Angustoline (13) Angustidine (11) N’-Acetylstrychnosplendine (192) N’-Acetyl-0-methylstrychnosplendine (193)

Source“

Ref.

Rb, Sb

116

Sb

116

Sb

116

Sb Sb

116 116

Rb Rb Sb, L Sb, L, S L

124 124 53,123 53,123,125 123

L

123

L, F L, F L. F

52 52 52

Sb, L, S Sb, L, S L

126 126 52

L L L

52 52 52

Sb Sb Sb Sb L L

127 127 127 127 52 52 52 127 127

L L L

(continues)

22s

GEORGES MASSIOT AND CLEMENT DELAUDE

TABLE I1 (Continued) Species and alkaloids Strychnobrasiline (178) Strychnofendlerine (181) S. soubrensis Hutch. et Dalz. (Lanigerae) [Sb: 1 g/kg ( U S ) ] Isosplendine (101) Strychnofendlerine (181) Strychnobrasiline (178) 14-P-Hydroxystrychnobrasiline (179) S. spinosa Lam. (Spinosae) [Sb: 0.9 g/kg; L: 0.6 g/kg (130)l 11-Methoxydiaboline (45) 11-Methoxy-12-hydroxydiaboline(51) Akagerine (3) Kribine (105) 10-Hydroxyakagerine (4) S. splendens. Gilg (Lanigerae) [Sb: 8.3 g/kg; L: 6.6 g/kg; F: 1.9 g/kg (132,133)] Strychnosplendine (191) Isostrychnosplendine (195) N’-Acetylisostrychnosplendine(196) Isosplendoline (168) Splendoline (167) Isosplendine (101) S. staudtii Gilg (Densiflorae) [Rb: 5.4 g/kg; Sb: 2.9 g/kg; L: 1 g/kg (136)l 11-Methoxyhenningsamine (87) 12-Hydroxy-11-rnethoxyhenningsamine (88) 11-Methoxydiaboline (45) 11-Methoxy-12-hydroxydiaboline(51) S. tchibarzgensisPellegr. (Penicillatae) [Rb: SO g/kg; Sb: 64 g/kg; F: 4.7g/kg (137)l Tchibangensine (5’,6’-dihydrousambarensine)(212) S. tricalysioides Hutch. et M. B. Moss. (Dolichantae) [Sb: 0.3 g/kg; S: 1.4 g/kg (138,139)] Dolichantoside (62) Vallesiachotarnine (233) Isovallesiachotamine (234) S. trichoneura Leeuwenberg (Penicillatae) Angustine (12) Angustoline (11) Angustidine (13) S. urceolatu Leeuwenberg (Breviflorae) [Sb: 3 g/kg (140)] Bisnordihydroxytoxiferine (57) Bisnor-C alkaloid H (22)

Source“

Ref

L L

127 127

Sb Sb Sb Sb

128 128 128 128,129

Sb Sb Sb, L L L

131 131 130 130 130

Sb, L, F Sb, L, F Sb, L, F Sb, L, F L F

132-134 132,133 132,133 132 135 132

Rb, Sb, L Rb, Sb, L Rb, Sb, L Rb, Sb, L

136 136 136 136

Rb, Sb, L

127

Sb S S

138 139

L L L

52 52 52

Sb Sb

140 140

139

(continues)

229

5. AFRICAN STRYCHNOS ALKALOIDS

TABLE I1 (Continued) Species and alkaloids Caracurine V (29) Longicaudatine (1 10) 11-Methoxydiaboline (45) S . usambarensis Gilg. (Rouhamon) Usambarensine (208) N4-Methylusambarensine (209) 5,6-Dihydrousambarensine (3,J-dihydrousambarensine) (212) N4'-Methyl-5,6-dihydro-usambarensine

Source"

Ref.

Sb Sb Sb

140 4Y 2 75

Rb Rb Rb

141,142 141 141,143

Rb

141

Rb Rb

144 145,I52

(N4'-methyl-3,4-dihydrousambarensine)(211) Harmane (83)

5,6-Dihydroflavopereirine(6,7-dihydroflavopereirine)

(76) Rb Dihydrotoxiferine (55) Rb C-calebassine (27) Rb C-curarine (34) Rb Akagerine (3) Rb Macusine B (1 16) Rb 0-Methylmacusine B (117) Rb 0-Methyldihydromacusine B (1 18) Rb Fluorocurarine (77) Rb Melinonine F (132) Rb Normelinonine F (133) Rb Malindine (120) Rb Isomalindine (121) Rb N4-Methylanthirine (15) L Usambarine (225) L Angustine (12) L 18,19-Dihydrousambarine (230) L Usambaridine Vi (10-hydroxy-usambarine) (226) Usambaridine Br(ll-hydroxy-18,19-dihydrousambarine) L (227) L Strychnobaridine (172) Rb Afrocurarine (1) L Strychnopentamine (186) L Demethoxycarbonyl-3,14-dihydrogambirtannine (Descarbomethoxy-3,14-dihydrogambirtannine) (49) Strychnofoline (183) Isostrychnofoline (184) Isostrychnophyline (189) N4-Methyl-10-hydroxyusambarine (231) N4-Methyl-11-hydroxyusambarine (232) Isostrychnopentamine (187) Strychnophylline (188)

146 146 146,147 148,149 150 150 150 151 152 152 153 153 153 154-156 52 157 157.158

157 157 146,147 157,159 158 160,161 160 160 162 162

163 160

(continues)

230

GEORGES MASSIOT AND CLEMENT DELAUDE

TABLE I1 (Continued) Species and alkaloids

Source"

S. variabilis De Wild. (Rouhamon) [Rb: 16.3 g/kg; Sb: 0.4 g/kg; F: 0.2 g/kg; S: 0.17 g/kg (164)] Desacetylretuline (2p,16p-dihydroakuammicinal) (151) Rb Isoretulinal (N'-acetyl-2p,16a-dihydroakuammicinal) Rb (144) Nordihydrotoxiferine (56) Rb Rb Strychnobiline (173) Rb Isostrychnobiline (175) 12'-Hydroxyisostrychnobiline (176) Rb Retulinal(l42) Rb 12-Hydroxyretulinal(l43) Rb 12-HydroxyisoretuIina1(145) Rb Strychnopivotine (190) Rb 16-HydroxyisoretulinaI(l46) Rb Rb Rosibiline (163) Rb Mavacurine (131) Fluorocurine (81) Rb 16,17-Didehydroisostrychnobiline(177) Rb 12'-Hydroxystrychnobiline (174) Rb Rb Strychnozairine (198) Rb, L Retuline (147) Isoretuline (154) Rb, L 0-Acetylisoretuline (155) L N'-Desacetylisoretuline (158) Rb S. xantha Leeuwenberg (Dolichantae) L Angustine (12) Angustoline (13) L L Angustidine (11) ~~

~

Ref.

164,165 164,165 167 167 167 168,169 166 166 166 170 170 170 171 171 172 173 174 168,I75 168,I75 175 168

52 52 52 ~

~~

Plant part from which the alkaloid was isolated: Rb, root bark; Sb, stem bark; S, stem; L, leaf; F, fruit; T, twigs; S. seeds. a

(Text continued from p. 218)

Longicaudatine (110), a complex dimer which was identified only in 1983, has already been reported in 7 species. Finally, so far there does not seem to exist a correlation between the alkaloids and the botanical section of the source plant. (Text continues on p. 228)

5 . AFRICAN S T R Y C H N O S ALKALOIDS

231

AFROCURARINE

1 Afrocurarine

Rouhamon

S. usambarensis

AJMALICINIAL

2 Ajmalicinial

Brevitubae

S. johnsonii

232

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

AKAGERINE

o=c

\ R

RI

H

H

H

~

3 Akagerine

Rouhamon

Spinosae Brevitubae Densiflorae Dolichantae Scyphostrychnos

S. dale S. decussata S. elaeocarpa S.poribunda S. usambarensis S. spinosa s. johnsonii S.nigritana S. barteri S. camptoneura

4 10-Hydroxyakagerine

OH

H

Rouhamon Spinosae

S. decussata S. spinosa

5 10-Hydroxy-17-0-

OH

CH3

Rouhamon

S. decussata

H

CH3

Rouhamon

S. dale

methylakagerine 6 17-0-Methylakagerine

S. decussata S. elaeocarpa 7 17-0-Ethylakagerine

H

CZH5

Brevitubae

S. johnsonii

5 . AFRICAN STRYCHNOS ALKALOIDS

233

AKAGERINE LACTONE

8 Akagerine lactone 9 17-0-Ethylakagerine lactone

Rouhamon Brevitubae

S. decussata

Brevitubae

S. johnsonii

S . johnsonii

ALSTONINE

H’

10 Alstonine

Dolicanthae Scyphostrychnos

‘cn

S. gossweileri S. carnptoneura

234

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

ANGUSTIDINE

11 Angustidine

Breviflorae Penicillatae Brevitubae Lanigerae Dolichanthae

S. S. S. S.

angolensis trichoneura samba schefleri S.odorata S. xantha

ANGUSTINE

12 Angustine

Rouhamon

Breviflorae Penicilla tae Brevitubae Lanigerae Dolicanthae Scyphostrychnos

S. floribunda

S. potatorum S. usambarensis S. angolensis S. trichoneura S.johnsonii S.samba S. schefieri S. odorata S. xantha S. carnptoneura

5 . AFRICAN STRYCHNOS ALKALOIDS

235

ANGUSTOLINE

13 Angustoline

S. trichoneura S. schefPeri S.samba S. odorata S. xantha

Penicillatae Lanigerae Brevitubae Dolicanthae

ANTHIRINE

14 Anthirine

15 N4-Methylanthirine

rN4-CH,

16 Anthirine methobromide 17 Isoanthirine

A19-20

Brevitubae Scyphostrychnos

S. johnsonii S. camptoneura

Rouhamon

S. usambarensis

Scyphostrychnos

S. campioneura

Brevitubae

S.johnsonii

236

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

ANTHIRINE LACTONE

18 Anthirine lactone

S.johnsonii

Brevitubae

BARTERINE

R 19 Barterine

H

Dolichantae

S. barteri

20 10-Hydroxybarterine

OH

Dolichantae

S. barteri

5 . AFRICAN STRYCHNOS ALKALOIDS

237

BIS-NOR-C ALKALOID D

21 Bis-nor-C alkaloid D

S. dolichothyrsa

Breviflorae Penicillatae

S. matopensis

BIS-NOR-C ALKALOID H

22 Bisnor-C alkaloid H

Breviflorae

Penicillatae

S. afzelii S. dolichothyrsa S . malacoclados S. urceolata S. longicaudata S. matopensis

238

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

23 Bisnor-C alkaloid H N-oxide

~ 4 - 0

Breviflorae

S. dolichothyrsa

24 Bisnor-C alkaloid H di-N-oxide

~ 4 - 0

Breviflorae

S. dolichothyrsa

~ 4 ' - 0

BRAFOUEDINE

25 Brafouedine

Lanigerae

S. dinklagei

ISOBRAFOUEDINE

26 Isobrafouedine

Lanigerae

S. dinklagei

239

5 . AFRICAN S T R Y C H N O S ALKALOIDS

C-CALEBASSINE

0-4, OH

27 C-Calebassine

Rouhamon

S. usambarensis

CAMPTONEURINE

~

28 Camptoneurine

Scyphostrychnos

S. carnptoneura

240

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

CARACURINE V

29 Caracurine V

Breviflorae

Lanigerae

S . afzelii S . angolensis S. dolickotkyrsa S. malacoclados S. urceolata S. ckrysopkylla

30 Caracurine V N-oxide

~ 4 - 0

Breviflorae

S . dolichothyrsa

31 Caracurine V di-N-oxide

~ 4 - 0

Breviflorae

S. dolcckothyrsa

~ 4 ' - 0

CONDENSAMINE

32 Condensamine

Breviflorae

S. kenningsii

5. AFRICAN STRYCHNOS ALKALOIDS

241

CONDYLOCARPINE

33 Condylocarpine

Breviflorae

S. dolichothyrsa

C-CURARINE

34 C-Curarine

35 Bisnor-C-curatine

N(4,4')

Rouhamon

S. usambarensis

Breviflorae Penicillatae

S. dolichothyrsa S . matopensis

242

GEORGES MASSIOT AND CLEMENT DELAUDE

DECUSSINE fi

R 36 Decussine

H

37 Rouhamine 5,6-Dehydrodecussine

H

38 3,14-Dihydrodecussine

H

39 10-Hydroxy-

3,14-dihydrodecussine

OH

Rouhamon

S. dafe S. decussata S. elaeocarpa S. Poribunda

A5-6

Rouhamon

S. decussata S. Poribunda

3,14dihydro

Rouhamon

Brevitubae

S. dale S. decussata S. elaeocarpa S. johnsonii

Rouhamon

S. decussata

3,14dihydro

243

5. AFRICAN STRYCHNOS ALKALOIDS

DEMETHOXYCARBONYL-3,14-DIHYDRO-GAMBIRTANNINE

40 Demethoxycarbonyl-3,14dihydrogambirtannine 41 Dernethoxycarbonyl-3,14, 15,16,17,18 hexahydrogambirtannine

15,16,17,18tetrahydro

S. usambarensis

Rouhamon Brevitubae

S. johnsonii

Brevitubae

S. johnsonii

18-DEOXY WIELAND-GUMLICH ALDEHYDE

R 42 18-Deoxy Wieland-Gumlich aldehyde

H

Breviflorae Penicillatae Lanigerae

S. dolichothyrsa S. matopensis S. kasengaensis

43 N-Formyl-18-deoxy

HCO

Penicillatae

S. matopensis

Wieland-Gumlich aldehyde

DIABOLINE

19 R

w

R

RI

RZ

R3

H

H

OH

H

P P

44 Diaboline

Rouhamon Brevitlor ae Penicillatae

S. potatorum S. afzelii S. henningsii S. longicaudata S. matopensis

~~

45 11-Methox ydiaboline

OCH3

H

OH

H

Spinosae Densiflorae

S. angolensis S. dolichothyrsa S. henningsii S. urceolata S. malacoclados S. matopensis S. spinosa S. staudtii

Breviflorae

S. malacoclados

Breviflorae

Penicillatae

46 11-Methoxydiaboline N-oxide

H

H

OH

H

N(4) + 0

47 Epi-17-O-methyl-11-

OCH,

H

H

OH

H

H

OH

H

Breviflorae

S. angolensis

methoxydiaboline 48 2,16-Dehydrodiaboline

82-16

Breviflorae

~~

S. henningsii ~

~-

OCH,

H

OH

H

A2-16

Breviflorae

S. henningsii

OCH,

H

OH

H

19,20dihydro

Breviflorae

S. malacoclados

51 ll-Methoxy-12-hydroxyd1abol1ne

OCH,

OH

OH

H

52 Acetyldiaboline

H

H

CH,

49 11-Methoxy-2,16-

dehydrodiaboline 50 19,20-Dihydro-l1methoxydiaboline ~

N

VI e

~-

~~

~~

~

I

coo

Spinosae Densiflorae

S. spinosa S. staudtii

Rouhamon

S. potatorum

246

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

DIHYDROCORYNANTHEOL

O H

53 Dihydrocorynantheol

Brevitubae

S. johnsonii

DIHYDRO-CYCLO-AKAGERINE

54 Dihydro-cyclo-akagerine

Brevitubae

S. johnsonii

5 . AFRICAN STRYCHNOS ALKALOIDS

247

DIHYDROTOXIFERINE

w H -C

C-H

/

u3c /

~~

55 C-Dihydrotoxiferine 56 Nordihydrotoxiferine

k

'

4'

Rouhamon

S. usambarensis

N(4)

Rouhamon

S. variabilis

N(4,4')

Rouhamon

S. decussata S. elaeocarpa S. Poribunda S. afzelii S. dolichothyrsa S. icaja S. malacoclados S. urceolata S. matopensis S. longicaudata S. kasengaensis S. schefleri

~

57 Bisnordihydrotoxiferine

Breviflorae

Penicillatae Lanigerae

58 Bisnordihydrotoxiferine mono-N-oxide

N(4) + 0, N(4)'

Rouhamon Breviflorae

S. Poribunda S. afzelii S . dolichothyrsa

N(4,4') + 0

Breviflorae

S. dolichothyrsa

~

59 Bisnordihydrotoxiferine

di-N-oxide

248

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

DIPLOCELINE

60 Diploceline 61 16-Epidiploceline

16-P-H

Dolichantae

S. gossweileri

Dolichantae

S. gossweilen'

DOLICHANTOSIDE

OH

62 Dolichantoside 63 Isodolichantoside

3-P-H

Dolichantae

S. gossweileri S. tricalysioides

Dolichantae

S. gossweileri

5 . AFRICAN STRYCHNOS ALKALOIDS

249

DOLICHOCUWINE

64 Dolkhocurine

Breviflorae

S. dolichothyrsn

BreviRorae

S.dolichothyrsa

DOLICHOTHYRINE

65 Dolichothyrine

250

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

ELLIPTICINE R

I

67 3,14-Dihydroellipticine ~

~~

CH,

CH,

66 Ellipticine

CH,

~

CH,

H H

Lanigerae S. dink/agei 3,14-dihydro Lanigerae

~

S. dinklugei ~

~

68 3,14,4,21Tetrahydroellipticine

CH,

CH,

H

3,14,4,21Lanigerae S dinklugei tetrahydro

69 10-Hydroxyellipticme

CH?

CH,

OH

70 17-Oxoellipticine

CH,

CHO H

71 17-Oxoellipticine

CH,

CHO H

N(4) + 0

Lanigerae S. dinklagei

CH,

CH,

H

N(4) + 0

Lanigerae S. dinklagei

CHzOH CH,

H

Lanigerae S dink/ugei Lanigerae S. dinklugei

N-oxide 72 Ellipticine N-oxide

249 18-Hydroxyellipticine

Lanigerae S. dinklugei

FLAVOPEIRERINE

73 Flavopeirerine 76 5,h-Dihydroflavopeirerine

5,h-dihydro

Penicillatae

S. longicaudata

Rouhamon

S. usambarensis

5 . AFRICAN S T R Y C H N O S ALKALOIDS

25 1

FLUOROCURARINE

R Rouhamon

S. usambarensis

N(4)

Breviflorae Penicillatae

S. dolichofhyrsa S. matopensis

OH

N(4)

Lanigerae

S. ngouniensis

CH3-CO0

N(4)

Lanigerae

S. ngouniensis

77 Fluorocurarine

H

78 Norfluorocurarine

H

79 18-Hydroxynorfluorocurarine 80 18-Acetoxynorfluorocurarine

FLUOROCURINE

81 Fluorocurine

Rouhamon Lanigerae

S. variabilis S. schefleri

252

GEORGES MASSIOT AND CLEMENT DELAUDE

GLUCOALKALOID

82 Glucoalkaloid

S . decussata

Rouhamon

HARMANE

d

R

Brevitubae

S. dale S.usambarensis S.johnsonii

Brevitubae

S. johnsonii

Rouhamon

S. dale

Rouhamon

83 Harmane

84 Nonharmane

H

85 Dihvdroharmane

CH1

5,h-dihydro

5 . AFRICAN STRYCHNOS ALKALOIDS

253

HENNINGSAMINE

R

86 Henningsamine

H

H

Breviflorae

S. henningsii

87 11-Methoxyhenningsamine

OCH3

H

Breviflorae Densiflorae

S. henningsii S. staudtii

88 12-Hydroxy-11methoxyhenningsamine

OCH3

OH

Densiflorae

S. staudtii

HENNINGSOLINE

H3C 0

I

1-1

R 89 Henningsoline

H

Breviflorae

5’. henninxsii

90 O-Acetylhenningsoline

CH,-CO

Breviflorae

S. henningsii

254

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

HOLSTIJNE

91 Holstiine

Breviflorae

S. henningsii

Breviflorae

S. henningsii

HOLSTILINE

92 Holstiline

17-HYDROXY -ALLOIBOGAMINE

93 17-Hydroxyalloibogamine

Lanigerae

S . ngouniensis

255

5 . AFRICAN S T R Y C H N O S ALKALOIDS

ICAJINE

94 Icajine

H

H

H

H

Breviflorae

S. icuju

95 15-Hydroxyicajine

H

H

H

OH

Breviflorae

S. icuju

96 19-20-a-Epoxy-15-

H

H

H

OH 19,20-a- Breviflorae epoxy

S. icuju

97 19,20-a-Epoxy-l5hydroxy-12methoxyicajine

H

H

OCH,

H

19.20-aepoxy

Breviflorae

S. icuju

98 19,20-a-Epoxy-10methoxyicajine

OCH3 H

H

H

19,20-a- Breviflorae epoxy

S. icuju

99 19,20-a-Epoxy-12methoxyicajine

H

H

OCH,

H

19,20-a- Breviflorae epoxy

S . icuju

H

OCH3 OCHi H

19,20-a- Breviflorae epoxy

S. icuju

hydroxyicajine

~

100 19,20-a-Epoxy-11,12-

dimethoxyicajine

256

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

ISOSPLENDINE 0

101 Isosplendine

Aculeatae Lanigerae

S. aculeata

S.soubrensis S. splendens

ISOSITSIRIKINE

102 16-R-isositsirikine

Penicillatae Lanigerae

S.matopensis S. kasengaensis

257

5 . AFRlCAN STRYCHNOS ALKALOIDS

JANUSSINE

103 Janussine A and B

Brevitubae

S johnsonu

104 Oxojanussine

Brevitubae

s

]ohnsonll

KRIBINE

105 Kribine

H

OH

H

S. dale S . elaeocarpa Spinosae S. spinosa Densiflora S . nigritana Scyphostrychnos S. carnptoneura

Rouhamon

25 8

GEORGES MASSIOT AND CLEMENT DELAUDE

106 17-0-Methylkribine

H

107 Epi-17-0-methylkribine H

108 lO-Hydroxy-17-0-

OCH,

H

Rouhamon

S. dale S. elaeocarpa

H

OCH,

Rouhamon

S. dale S. elaeocarpa

OH

OCH3 H

Rouhamon

S. decussata

OH

H

Rouhamon

S. decussata

methylkribine ~

109 10-Hydroxyepi-17-0-

OCHi

methylkribine

LONGICAUDATINE

LONGICAUDATINE Y

Breviflorae

110 Longicaudatine

S. afzelii

S. dolichothyrsa S. urceolata Penicillatae Lanigerae 111 Longicaudatine N-oxide

112 Longicaudatine Y

N - 0

S. longicaudata S. matopensis S. chrysophylla S. ngouniensis

Penicillatae Lanigerae

S. matopensis S. chrysophylla

Penicillatae

S. longicaudata S. matopensis

259

5 . AFRICAN STRYCHNOS ALKALOIDS

Longicaudatine F

Longicaudatine

z

113 Longicaudatine F

Penicillatae

S. longicaiidata S. matopensis

114 Longicaudatine Z

Penicillatae

S. matopensis

115 11-Methoxymacusine A

Breviflorae

S. angolensis

260

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

R 116 Macusine B

H

117 0-Methylmacusine B

Rouhamon

S. decussata S. usambarensis

Rouhamon

S. decussata S . usambarensis

OCH3 118 @Methyl- 19,20-

OCH,

19,20-dihydro

Rouhamon

S. usambarensis

H

N (4)

Breviflorae

S. dolichothyrsa S. malacoclados

dihydro macusine B 119 Normacusine B

MALINDINE

R

RI

120 Malindine

H

CH,

Rouhamon

S. decussata S. usambarensis

121 Isomalindine

CH3

H

Rouhamon

S. usambarensis

26 1

5 . AFRICAN S T R Y C H N O S ALKALOIDS

122 Normalindine

H

CH3

=N4

Brevitubae

S. johnsonii

123 Norisomalindine

CH,

H

=N4

Brevitubae

S. johnsonit

MATOPENSINE

R

RI

RZ

125 Matopensine

H

H

H

126 Matopensine N-oxide

H

H

H

N(4)

+0

Penicillatae Lanigerae

S. matopensis S. kasengaensis

Penicillatae Lanigerae

S. mafopensis

Penicillatae

S. matopensis

S. kasengaensis

127 18, 18’-Dihydromatopensine

H

OH

OH

128 16-Methoxyisomatopensine

OMe

H

H

16‘-aH

Penicillatae

S. matopensis

129 16-Ethoxyisomatopensine

OEt

H

H

16’-(wH

Penicillatae

S. mafopensis

130 18-Hydroxymatopensine

H

OH

H

Penicillatae

S. matopensis

262

GEORGES MASSIOT AND CLEMENT DELAUDE

MAVACURINE

131 Mavacurine

S. variabilis S. schefleri

Rouhamon Lanigerae

MELINONINE F

R ~~~~

~

132 Melinonine F

CH,

Rouhamon

S. usambarensis

133 Normelinonine F

H

Rouhamon

S. usambarensis

NGOUNIENSINE H

134 Ngouniensine

Lanigerae

S. ngouniensis

135 Epingouniensine

Lanigerae

S. ngouniensis

136 Glucosylngouniensine (C,,, or CI1)

Lanigerae

S. ngouniensis

137 Epiglucosylngouniensme

Lanigerae

S. ngouniensis

~~~~~~

5 . AFRICAN STRYCHNOS ALKALOIDS

263

138 Novacine

Breviflorae

S. icaja

139 19, 20-a-Epoxynovacine

Breviflorae

S . icaja

140 19, 20-cu-Epoxy-l5-hydroxynovacine

Breviflorae

S. icaja

NOVACINE

PSEUDOSTRYCHNINE

141 Pseudostrychnine

Rouhamon

S. icaja

264

GEORGES MASSIOT AND CLEMENT DELAUDE

RETULINAL

CH3

142 Retulinal

H

H

H0

Rouhamon

S. variabilis

Rouhamon

S. variabilis

H

Rouhamon Lanigerae

S. variabilis S. kasengaensis

H

Rouhamon

S. variabilis

OH

Rouhamon

S. variabilis

C

\H 0

143 12-Hydroxyretulinal

OH

H

C@

\H 0

144 Isoretulinal

145 12-Hydroxyisoretulinal

@

H

- c.

OH

-

‘H

0 //

c.

‘H

146 16-Hydroxyisoretulinal

H

-C

0 4

‘H

w z

153 23-Hydroxy-2,16-dehydroretuline

R

RI

Rz

R3

H

H

CHzOH

H

N ( l ) -CO

Penicillatae

S. longicaudala

I CH2

I

OH

42-16 154 Isoretuline

H

CHzOH

H

H

Rouhamon Lanigerae

S. variabilis S. kasengaensis

155 0-Acetylisoretuline

H

CH,OAc

H

H

Rouhamon Lanigerae

S. variabilis

S. kasengaensis

156 18-Hydroxyisoretuline

H

CH20H

H

OH

Breviflorae Lanigerae

S. henningsii S.kasengaensis

157 11-Methoxyisoretuline

OCH-,

CHZOH

H

H

Lanigerae

S. kasengaensis

158 N'-Desacetylisoretuline

H

CH,OH

H

H

Rouhamon

S. variabilis S. fioribunda S. henningsii S. matopensis S. kasengaensis S. schefleri

NU)

Breviflorae Penicillatae Lanigerae ~~~

~

~

159 18-Hydroxy-N'desacetylisoretuline

H

CHzOH

H

OH

N(l)

Breviflorae Penicillatae Lanigerae

S. henningsii S. longicaudata S.kasengaensis

160 18-Acetoxy-N'desacetylisoretuline

H

CH,OH

H

OAc

N(1)

Penicillatae Lanigerae

S. longicaudata S. kasengaensis

161 17-0-Acetyl-18-hydroxy-

H

CH,OAc

H

OH

N(1)

Breviflorae

S. henningsii

N'-desacetylisoretuline .~

5 . AFRICAN STRYCHNOS ALKALOIDS

267

RINDLINE

CH,O

-cti3

HO 162 Rindline

Breviflorae

S. henningsii

ROSIBILINE

163 Rosibiline

164 Isorosibiline

16-a-H

Rouhamon

S. variabilis

Rouhamon Penicillatae

S. Poribunda S. matopensis

268

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

SERPENTINE

165 Serpentine

Scyphostrychnos

S. camptoneura

SPERMOSTRYCHNINE

166 Spermostrychnine

Aculeatae

S. aculeata

SPLENDOLINE

167 Splendoline

CH3

H

Lanigerae

S. splendens

168 Isosplendoline

H

CH,

Lanigerae

S. splendens

269

5 . AFRICAN StryChnOS ALKALOIDS

STRYCHNINE

R 169 Strychnine

H

Breviflorae

S. icaja

170 12-Hydroxystrychnine

OH

Breviflorae

S. icaja

171 N4-Methylstrychninium

H

Breviflorae

S. icaja

N(4)--CH,

H

172 Strychnobaridine

Rouhamon

S. usambarensis

270

GEORGES MASSIOT AND CLEMENTDELAUDE

STRYCHNOBILINE

R 173 Strychnobiline

H

Rouhamon

S. variabilis

174 12’-Hydroxystrychnobiline

OH

Rouhamon

S. vuriabilis

175 Isostrychnobiline

17-a-H

H

Rouhamon

S. variabilis

176 12’-Hydroxyisostrychnobiline

17-a-H

OH

Rouhamon

S. variabilis

177 16,17-Dehydroisostrychnobiline

17-a-H A16-17

H

Rouhamon Lanigerae

S. variubih S. knsengaensis

5 . AFRICAN STRYCWNOS ALKALOIDS

27 1

STRYCHNOBRASILINE

R 178 Strychnobrasiline

H

Lanigerae

S. schejleri S. soubrensis

179 14-13-Hvdroxv-strvchnobrasiline

OH

Lanigerae

S. soubrensis

STRYCHNOCARPINE

180 Strychnocarpine

Rouhamon

S. elaeocarpa

S.Poribunda

STRYCHNOFENDLERINE

181 Strychnofendlerine

Aculeatae Lanigerae

S. aculeata S. schefleri S. soubrensis

272

GEORGES MASSIOT AND C L ~ M E N T DELAUDE

STRYCHNOFLUORINE

CH3

182 Strychnofluorine

CHO

Dolichantae

S. gossweileri

STRYCHNOFOLINE

HO

183 Strychnofoline 184 Isostrychnofoline

(7R)

Rouhamon

S. usambarensis

Rouhamon

S. usambarensis

5 . AFRICAN S T R Y C H N O S ALKALOIDS

273

STRYCHNOFURANINE

185 Strychnofuranine

Penicillatae

S. matopensis

STRYCHNOPENTAMINE

186 Strychnopentamine 187 Isostrychnopentamine

2"-ff-H

Rouhamon

S. usambarensis

Rouhamon

S. usambarensis

274

GEORGES MASSIOT AND CLEMENT DELAUDE

STRYCHNOPHYLLINE

188 Strychnophylline 189 Isostrychnophylline

(7R)

Rouhamon

S. usambarensis

Rouhamon

S. usambarensis

STRYCHNOPIVOTINE

190 Strychnopivotine

Rouhamon

S. variabilis

STRYCHNOSPLENDINE

Lanigerae

S. splendens

N '-Ac

Breviflorae Aculeatae Lanigerae

S. henningsii S. aculeata S. schefleri

H

N'-Ac

Aculeatae Lanigerae

S. aculeata S. schefleri

OH

OCH3

NI-AC

Breviflorae

S. henningsii

CH,

OH

H

Lanigerae

S. splendens

CH,

OH

H

Lanigerae

S. splendens

191 Strychnosplendine

CH3

H

OH

H

192 N'-Acetylstrychnosplendine

CH3

H

OH

H

193 N'-Acetyl-0-methylstrychnosplendine

CH3

H

OCH,

194 N'-Acetyl-11-methoxystrychnosplendine

CH3

H

195 Isostrychnosplendine

H

196 N'-Acetylisostrychnosplendine

H

~

~

N'-AC

276

GEORGES MASSIOT AND CLEMENT DELAUDE

STRYCHNOXANTHINE

197 Strychnoxanthine

Dolichantae

S. gossweileri

STRYCHNOZAIRINE

198 Strychnozairine

Rouharnon

S. variabilis

5 . AFRICAN S T R Y C H N O S ALKALOIDS

277

SUNGUCINE

199 Sungucine

Breviflorae

S. icaja

TETRAHYDRO-AKAGERINE

HO

200 Tetrahydroakagerine

-4 Brevitubae

S. johnsonii

27 8

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

TETRAHYDROALSTONIAL

OH

201 Tetrahydroalstonial

S. johnsonii

Brevitubae

~~

TSILAMINE H

~~

~

202 Tsilamine

H

CH3

Breviflorae

S. henningsii

203 10-Methoxytsilamine

OCH,

CH,

Breviflorae

S. henningsii

204 0-Demethyltsilamine

H

H

Breviflorae

S. henningsii

205 10-Methoxy-0-demethyltsilamine

OCH,

H

Breviflorae

S. henningsii

5 . AFRICAN STRYCWNOS ALKALOIDS

279

TUBOTAIWINAL

CHO 206 Tubotaiwinal

Lanigerae

S . ngouniensis

TUBOTAIWINE

207 Tubotaiwine

Breviflorae

S. angolensis

S. dolichothyrsa

USAMBARENSINE

208 Usambarensine

TCHIBANGENSINE

R

RI

H

H

Lanigerae

S. dale S. usambarensis S. memecyloides

Rouhamon

Go

0

209 N4'-Methylusambarensine

H

H

-N4'-CH3

Rouhamon

S. usambarensis

210 Usambarensine N-oxide

H

H

=~4-0

Rouhamon

S. dale

211 N4'-Methyl-5',6'-dihydrousambarenslne

H

H

5',6' dihydro =N~'-cH~

Rouhamon

S. usambarensis

212 5',6'-Dihydrousambarensine (Tchibangensine)

H

H

5'-6' dihydro

Rouhamon

S. dale

Penicillatae

S. usambarensis S. tchibangensis

Rouhamon

S . dale

Rouhamon

S. dale S. ngouniensis

-

~

213 5' ,6'-Dihydrousambarensine N-oxide

H

H

5,6'dihydro -~4-0

214 4',17-Dihydro-17a-tchibangensine 17,4',5',6'-Tetrahydro-l7a-

H

H

17,4',5',6'tetrahydro 17a

Lanigerae

215 4',17-Dihydro-17p-tchibangensine

H

H

17,4',5',6'-tetrahydro-l7/3usambarensine

1I ,4'3' ,6'-

Lanigerae

S. ngouniensis

Lanigerae

S. ngouniensis

Lanigerae

S. ngouniensis

tetrahydro 17p

216 10'-Hydroxy-4', 17-dihydro-17atchibangensine 10'-Hydroxy- 17,4',5',6'tetrahydro17a-usambarensine

H

217 10'-Hydroxy-4',17-dihydr0-17/3tchibangensine 10'-Hydroxy- 17,4' ,5',6' tetrahydro17P-usambarensine

H

OH

17,4',5',6' tetrahydro 17a

OH

17,4',5',6'tetrahydro 17P

~~

OCHi

OCH,

17,4'.5',6'te trahydro

Rouhamon

S dale

OCH3

OCH3

17,4',5',6'tetrahydro

Rouhamon

S dale

220 lO-Hydroxy-lO'-methoxy17,4' .5',6'-tetrahydrousambarensine

OH

OW,

17,4',5',6'tetrahydro

Rouhamon

S . dale

221 10,10'-Dihydroxy-l7,4',5'6'tetrahydrousambarensine

OH

OH

17,4',5',6'tetrahydro

Rouhamon

S. dale

222 10-Hydroxy-10'-methoxy-N4 -methyl-

OH

OCH,

17.4',5'.6'tetrahydro N4 -CH3

Rouhamon

S. dale

OCH?

OH

17,4',5',6'tetrahydro N4'-CHi

Rouhamon

S. dale

OH

OH

17,4',5',6'tetrahydro =N''-cH~

Rouhamon

S. dale

218 10,10'-D1methoxy-3a,l7aN

ca

(2)-17,4',5',6'-tetrahydro-~sambarensine

t

219 10,10'-Dimethoxy-N4 methyl-

3a,17a-(Z)-17,4',5'6'-tetrahydrousambarensine

17,4'.5',6'-tetrahydrousambarensine 223 10'-Hydroxy-10-methoxy-N4'-rnethyl-

17,4',5',6'-tetrahydrousambarensine 224 10,10'-Dihydroxy-N4'-methyl-

17,4',5',6'-tetrahydrousambarensine

NIGRITANINE

USAMBARINE

N m N

C H,-N

&

225 Usambarine

H

H

Rouhamon Densiflorae Dolichantae

S. usambarensis S. nigritana S. baiteri

OH

H

Rouhamon Densiflorae Dolichantae

S. usambarensis S. nigritana S. barteri

18,19-Dehydronigritanine 226 10-Hydroxyusambarine

Usambaridine VI IO-Hydroxy-l8,19-dehydronigritanine

Rouhamon

S. usambarensis

18,19dihydro

Rouhamon

S. usambarensis

18,19dihydro

Rouhamon Densiflorae Dolichantae

S. usambarensis

18,19dihydro

Rouhamon Densiflorae Dolichantae

S. usambarensis

H

sN4-CH3

Rouhamon

S . usambarensis

OH

sN4-CH3

Rouhamon

S. usambarensis

H

OH

228 11-Hydroxy-18,19-dihydrousambarine 18,19-Dihydrousambaridine Br

H

OH

229 10-Hydroxy- 18,19-usambarine 18,19-Dihydrousambaridine Br 10-Hydroxynigritanine

OH

H

230 18,19-Dihydrousambarine Nigritanine

H

231 N4-Methyl-10-hydroxyusambarine

OH

232 N"-Methyl-1 1-Hydroxyusambarine

H

227 11-Hydroxyusambarine

Usambaridine Br

N

w

c/j

H

S. nigritana S. barteri S. nigritana S. barteri

284

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

VALLESIACHOTAMINE

233 Vallesiachotamine

Dolicanthae

S. tricdysioides

234 Isovallesiachotamine (cis isomer)

Dolicanthae

S. triculysioides

VOMICINE

R

R1

H

H

236 19,20a-Epoxyvomicine

H

H

237 19,20a-Epoxy-15-hydroxyvomicine

OH

238 19,20a-Epoxy-15-hydroxy 11-methoxyvomicine

235 Vomicine

Breviflorae

S. icuju

19,20a-epoxy

Breviflorae

S. icuju

H

19,20a-epoxy

Breviflorae

S. icuju

OH

OCHi

19,20a-epoxy

Breviflorae

S. icuja

H

OCH3

19,20a-epoxy

Breviflorae

S. icuju

~~

~

239 19,20a-Epoxy-llmethoxyvomicine

5 . AFRICAN STRYCHNOS ALKALOIDS

285

WIELAND-GUMLICH ALDEHYDE

R

240 Wieland-Gumlich aldehyde

H

H

Breviflorae Penicillatae

S. afzelii S. dolichothyrsa S. longicaudata

S. matopensis Lanigerae

S. chrysophylla S. kasengaensis

241 11-Methoxy Wieland-Gurnlich aldehyde

OCH3

H

Breviflorae

S. angolensis

242 17-0-Methyl-11-methoxy Wieland-Gumlich aldehyde

OCH,

CH,

Breviflorae

S. angolensis

CANTLEYINE OH

''C H,

243 Cantleyine

Lanigerae

S. dinklagei

286

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

DINKLAGEINE I

OH

I

OH ~~

244 Dinklageine

Lanigerae

S. dinklagei

GENTIANINE

245 Gentianine

Lanigerae

S. dinklagei

5. AFRICAN STRYCHNOS ALKALOIDS

287

STRYCHNOVOLINE

,H

/OH

H3C 0 O C

H

246 Strychnovoline

Lanigerae

S . dinklugei

VENOTERPINE OH /

241 Venoterpine

Lanigerae

S. dinklugei

288

GEORGES MASSIOT A N D CLEMENT DELAUDE

STRELLIDIMINE

248 Strellidimine

P

Lanigerae

S . dinklugei

(Text continued from p. 230)

V, Biosynthesis and Biogenetic Relationships Experimental biosynthetic investigations of Strychnos are sparse, and the only reported results concern the biosynthesis of strychnine in S. nux-vomica (177,178). The results are in full agreement with the secoiridoid pathway that has been investigated in whole plants, cultured cells, and cell extracts from plants of the family Apocynaceae (179,180).It is believed that, since indole alkaloids made by African Strychnos species are not basically different from those made by other plants of the families Loganiaceae, Apocynaceae, or Rubiaceae, they must share the same biosynthetic origin if not the enzymes responsible for their formation. Along these lines, the most important established steps of the biosynthesis are recalled in this section, and hypotheses for some unusual alkaloids are put forward. It is now admitted, after much controversy, that strictosidine (or epivincoside) (250) is the first biosynthetic intermediate which retains all the carbon atoms of the monoterpene indole alkaloids. Reference to vincoside in Ref. 178 must therefore be placed in the context of that time.

5 . AFRICAN STRYCHNOS ALKALOIDS

289

Strictosidine has not been found to accumulate in any Strychnos species, but isolation of dolichantosides 62 and 63 in S. gossweileri (82,83) and S. tricalysioides (138) may be taken as evidence of strictosidine intermediacy. After Pictet-Spengler condensation of secologanin (251) with tryptamine to yield strictosidine (250), the next event in the biosynthesis is formation of ring D of tetracyclic intermediates such as 4,21-dehydrogeissoschizine (252)and geissoschizine (253) (Scheme 1). These alkaloids have not been detected in Strychnos, but it is worth mentioning the isolation of ( E ) -and (Z)-isositsirikines in S. kasengaensis (114)and S. minfiensis (65),respectively, as well as the methylated analogs, diplocelines 60 and 61 in S. gossweileri (82,83,85). The heteroyohimbines are formed along similar lines from strictosidine via 4,21-dehydrogeissoschizine; they are not found in any Strychnos species, but the series is represented by the oxidized forms, serpentine (164) and alstonine (lo), isolated from S. camptoneura (57) and S. gossweileri (82) and by the demethoxycarbonyl analogs, ajmalicinial (2) and tetrahydroalstonial (201), from S. johnsonii (112) and S. matopensis (119). Among alkaloids of this type, one must mention dihydrocorynantheol(53), which has lost C-22, as have 2 and 201, the gambirtannines (40 and 41), and geissoschizal (254).Although being too labile to accumulate, this latter compound, which is the demethoxycarboxylated analog of geissoschizine (253), has been isolated among alkaloids from S. nux-vomica seedlings and is a component of many dimers and semidimers discussed later in this chapter. In parallel with the mainstream of biosynthesis, formation of alkaloids with N-4 to C-17 bonds also occurs (Scheme 2). This may be due either to failure of glucosidases to liberate the reactive C-21 or to efficiency of enzymes able to link N-4 and C-17. The most representative alkaloids of this type are the vallesiachotamines (233 and 234) from S. tricalysioides (139), anthirine (14) from S. johnsonii (112) and S. camptoneura, and N4-methylanthirine (15) from S. usambarensis (153). This group must be completed by the recently described isoanthirine (17)and anthirine lactone (18)from S. johnsonii (112). Glucoalkaloid (82)from S. decussata (70) is the most primitive intermediate isolated along this pathway; compared to strictosidine, it has retained the glucose acetal and the vinyl group but with reduction of the ester function. All these alkaloids share the common feature of having H-15 in the p position, an unorthodoxy, at first glance, which is, however perfectly accountable in light of their biosynthesis. Also related to this group are angustine (12), angustidine (ll), and angustoline (13), which are ubiquitous in African Strychnos, and strychnoxanthine (197)from S. gossweiteri (86). In cases where N-4 is methylated as in dolichantosides 62 and 63,the first two pathways are inoperative, and, once freed, C-17 must either react with

290

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

OH

40

k 25 4 -

OH

53

SCHEME 1. Early biosynthetic steps from secologanin to heteroyohimbines.

N-1 or be reduced (Scheme 3 ) . This is what happens with akagerine (3), akagerine lactone (S), their carbinolamine ether analogs 5, 7, and 9, tetrahydroakagerine (200), and dihydrocycloakagerine (54). Kribine (105), decussine (36), rouhamine (37), and the janussines (103) are examples of the addition of N-1, in a Michael fashion, on the other unsaturated

29 1

5 . AFRICAN STRYCHNOS ALKALOIDS

a n t i r h i ne

SCHEME 2. Biosynthesis of vallesiachotamine-anthirine-type alkaloids.

-CH,

0

SCHEME3. Biosynthesis of akagerine-type alkaloids

292

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

aldehyde system. The isolation of camptoneurine (28) from S. camptoneura (55)shows that N4-rnethylation is not a sine qua non condition for the formation of akagerine-type bases. In this example, it is not known if aromatization of rings C and E occurs prior to N-1 to C-20 cyclization. More elaborate alkaloids further down the biosynthesis path possess a fifth ring formed by cyclization of the nucleophilic C-16 on a electrophilic center, N-1 (a) or C-7 (b), in cases where the indole nucleus is oxidized, or C-5 (c) when N-4 is oxidized (Scheme 4). In African Strychnos species only type (a) and (c) cyclizations produce alkaloids, although path (b) corresponds to a possible mode of formation of alkaloids with the anilinoacrylic chromophore (181) such as akuammicine (255) from S. minfiensis (65). Paths (a) and (c) are illustrated by the isolation of rnavacurine (131), the corresponding pseudoindoxyl C fluorocurine (81), and the macusines (115-119). During the cyclization process, one of the C-16 substituents is lost, and it may be presumed that this occurs through a decarboxylation step which could well be the driving force for the reaction.

SCHEME4. Three routes to pentacyclic alkaloids.

293

5. AFRICAN STRYCHNOS ALKALOIDS

The next sophisticated alkaloid isolated in biosynthetic experiments on

S. nux-vomica ( I 78) is Wieland-Gumlich aldehyde (240), representative of a group of alkaloids often associated with the name Strychnos alkaloids. Although this has not been totally proven in Strychnos species, WielandGumlich aldehyde arises from geissoschizine (253) through dehydropreakuammicine (256) (Scheme 5 ) . This latter alkaloid may lose one of its (2-16 substituents, either the formyl group to yield akuammicine (255) or the methoxycarbonyl group to yield nor-C-fluorocurarine (78) [or strychnofluorine (182)l. It is remarkable that the most facile chemical decomposition (loss of formic acid) does not readily happen since akuammicine has been isolated only once from an African Strychnos (65);this “not so chemical” behavior may be held as evidence of the involvement of enzymes in the process. A parallel pathway leads to condylocarpine (33) via dehydroprecondylocarpine (257). At variance with the preakuammicine route, products of deformylation are common in Strychnos (tubotaiwine), whereas there is but one known example of an aldehyde with the same skeleton [tubotaiwinal (206) from S. ngouniensis (116)]. This generally accepted scheme raises two general questions regarding

1

1

J

\

A

240

42

SCHEME 5. Biosynthesis from geissoschizine (253) to Wieland-Gumlich aldehyde (240) and condylocarpine (33).

294

GEORGES MASSIOT AND CLEMENT DELAUDE

the first and final steps of the sequence: How are the starting indolenines made, and at what stage does oxidation of C-18 occur? The first question was the subject of research by Scott, who proposed three reasonable pathways which are more or less supported by chemical evidences (182) (Scheme 6). Oxidation of C-18 seems to operate in the last stages of biosynthesis since most 18-hydroxy compounds are pentacyclic [ 18hydroxy-nor-C-fluorocurarine (79), Wieland-Gumlich aldehyde (240).] From a chemical standpoint, oxidation of C-18 may proceed with assistance of the oxygenated C-17 function, although this has not yet been demonstrated to be feasible. Oxidation of C-18 may also be a consequence of the intermediacy of 4,21-dehydrogeissoschizine. Whereas 2,16-dehydro alkaloids (anilinoacrylic derivatives) such as nor-C-fluorocurarine (78) are widely dispersed in several genera of the families Loganiaceae (Strychnos) and Apocynaceae [Rhazya (183), Vinca (1849, Alstonia (185), Tabernaemontana (186)], the corresponding in-

q

/

H,C 0 , C y C H O

\

I

HO3

1

25 6 SCHEME 6. Hypotheses for the formation of dehydropreakuammicine (256) from geissoschizine (253).

295

5 . AFRICAN STRYCHNOS ALKALOIDS

dolines such as 42 belong only to Strychnos species and may be considered chemotaxonomic markers (Scheme 7). They are found under a variety of forms with H-2 p and H-16 a or p and with different oxidation levels at C-16, C-17, and C-18. The most frequently encountered avatars of these bases are N'-acetyl derivatives. Since the two extra carbon atoms of strychnine (C-22 and C-23) arise from acetic acid (177),it had long been thought [though later disproved (178)] that N-acetyl derivatives such as diaboline (44) could be intermediaries in the biosynthesis of strychnine (169). The final stages of the biosynthesis of strychnine (169) have not yet been fully elucidated; N-acetylated derivatives do not seem to be involved, but C-acetylated compounds, such as the putative amino acid prestrychnine

169 -

147,154

SCHEME7. Transformations of Wieland-Gumlich analog (42).

146 aldehyde (240) and its deoxy

296

GEORGES MASSIOT AND CLEMENT DELAUDE

(258), probably are (Scheme 8). As far as African Strychnos are concerned, the biosynthetic route does not end with 169, and more oxidized forms are produced. Without making any hypothesis about the moment at which oxidation occurs, one may say that 169 is oxidized in the aromatic nucleus, in the 19,20 double bond, and at carbons adjacent to N-4. Although brucine (10,ll-dimethoxystrychnine)has not been detected in African Strychnos, one can find 12-hydroxystrychnine (170) in S. icuja (21,104), several 11, 12-dioxo derivatives (vomicines and icajines in S. icuju), as well as 10,ll-dimethoxy compounds in the novacine series, also from S . icaju. Oxidation of the 19,20 double bond occurs in the vomicines and icajimes; epoxides are formed on the less hindered a side of the molecules. The most frequent oxidations of the strychnine molecule occur near the reactive N-4; they probably involve N-oxides which are readily transformed to carbinolamines such as pseudostrychnine (141) by a Polonovski-type reaction (187). Usually these carbinolamines undergo N-methylation and ring opening to aminoketones. The archetype of these alkaloids is vomicine (235), which at the time of the “Herculean assaults against ‘Everest’ strychnine” was the second highest and most challenging peak of the Strychnos chain (14). Vomicine (235) is an alkaloid from S. icaja (107,109)which displays a ketone function at C-3, an N-4 methyl, and a phenol at C-12. Strychnos icuja also contains vomicine derivatives

191 -

235

t4

141 -

SCHEME 8. Final steps in the biosynthesis of strychnine.

297

5 . AFRICAN STRYCHNOS ALKALOIDS

oxidized in the double bond, at C-15 and C-11. A curious feature of these bases is that their oxepine ring may be replaced by a pyran with an oxygen atom on C-19. This occurs in the S. splendens alkaloids, splendoline (167), strychnosplendine (191), in N-acetylstrychnosplendine (192) from S. henningsii and S. soubrensis, and in strychnobrasiline (178) and strychnofendlerine (181) from S. schefleri. Among African Strychnos species, two, S. dinklagei and S. ngouniensis, produce alkaloids of unexpected structures. Brafouedines 25 and 26 are vallesamine-type alkaloids from S. dinklagei characterized by the presence of a single carbon atom between C-7 and N-4 (75). They are accompanied in the same plant by several derivatives of ellipticine (66) in which three carbon atoms separate C-7 from N-4. These series of alkaloids are usually found in more highly evolved genera of the family Apocynaceae, Alstonia (188) or Ochrosia (189). Although structurally different, these alkaloids share a common biosynthetic feature, i.e., C-5-C-6 bond fission arising from oxidation of N-4or the indole nucleus. A tentative rationale for their formation based on biosynthetic incorporation and chemical evidences (190-192) is presented in Scheme 9.

*-* HO

HO

l

i

I

SCHEME 9. Biosynthetic hypothesis for the formation of ellipticine (66) and brafouedine ( 2 5 ) from a common precursor.

The novel alkaloids ngouniensine (134) and 17-hydroxyalloibogamine (93), from S. ngouniensis, possess the rare C-3-C-16 bond (216,122) (Scheme lo). Compound 93 is racemic, and one may suppose its formation through achiral intermediates 259 and 260, which is comparable to a known degradation product of akuammicine (261) (193). Ngouniensine (134) may

298

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

*-p H

H

cop3

co;

261

SCHEME10. Tentative explanation for the biosynthesis of ngouniensine (134) and 17hydroxy alloibogamine (93).

derive from 93 by fragmentation of one of the quinuclidine rings; in this case 134 would be racemic, and this cannot be totally excluded on the basis of published structural arguments. In addition to monomeric alkaloids, Strychnos species are able to generate higher molecular weight compounds composed of several substructures; they are real symmetrical dimers, “dimers” made of two different units, or quasi-dimers (or semidimers) comprising a monomer and of tryptamine unit. Most quasi-dimers have a skeleton with the partial formula C,,N, and minimum structure 262; they differ in the configurations of their two variable asymmetric centers, in the presence, stereochemistry, and location of their double bonds, and in possible oxidations. The most important quasi-dimers are the usambarensines (208) and tchibangensines (212) from S. usambarensis and S. tchibangensis (ethylidene side chain), the usambarines (225) from S. usambarensis (vinyl side chain), and nigritanines (230) from S. nigritana (ethyl side chain). Some derivatives are oxindoles (in the tetracyclic part), such as the strychnofolines (183) and strychnophyllines (188) from S. usambarensis and the barterines (19) from S. barteri. Strychnophylline (188) and

5. AFRICAN STRYCHNOS ALKALOIDS

299

SCHEME11. Possible mechanism of formation of strychnopentamines.

strychnopentamine (186) are the only known indole alkaloids adorned with a supplementary pyrrolidine ring. The presence in 186 and 188 of a hydroxyl group ortho to the pyrrolidine ring suggests an intramolecular delivery of the new ring (Scheme 11). The janussines (103) from S . johnsonii are the only reported quasi-dimers with two bonds between the monomer and the tryptamine; their structures are so intricate that it is not possible to distinguish the second tryptamine unit from the first. Strychnos unsymmetrical dimers are usually built by condensation of an aldehyde with an amine function, both being parts of complex molecules. Thus, the strychnobilines (173,175) are carbinolamine ethers made of desacetylretuline (151) and isoretulinal (144), and the longicaudatines (110-114) are condensation products of geissoschizal (254) and WielandGumlich aldehyde (240) or its deoxy counterpart 42 (Scheme 12). Afrocurarine (I) is a longicaudatine in which ring D of geissoschizine is aromatized. Although nature can best display its caprice in this kind of molecule, there are not many surprises to be found in Strychnos alkaloids dimers; however, some originality is observed in sungucine (199) from S. icuja and strychnofuranine (185) from S. matopensis. The largest group of Strychnos symmetrical alkaloids possesses skeletons of the types of alkaloid referred to as toxiferines or calebassines in Section I. Some are bis-ammonium salts, but most in African Strychnos are bases. The simpler base is bisnordihydrotoxiferine (57) which is composed of two deoxy Wieland-Gumlich aldehydes; oxygenated forms are bisnor-C alkaloid H (22) and caracurine V (29). These alkaloids are prepared in vitro from the monomers (It?), and they are so easily formed that the monomers

300

GEORGES MASSIOT AND CLEMENT DELAUDE

& ' ' /

254

4 ( ( -

113 SCHEME 12. Formation of strychnobiline (173) and longicaudatine F (113) from monomers. ~

may rightly be thought of as artifacts. The central eight-membered ring of the toxiferines is characterized by two enamine functions that are subject to oxidations to yield compounds with an extra bond between two C-16 atoms [dolichocurine (64), bisnor-C alkaloid D (Zl)]. Oxidation of C-2 also occurs to give the bisoxazoline C-curarine (35). Recently, the toxiferines have been augmented by the isolation from S. matopensis and S. kasengaensis of the matopensines (125-130) a group of alkaloids with an extra oxygen between the two (2-17 atoms. Comparison of the alkaloids produced by Strychnos species and by plants in the family Apocynaceae merits several remarks. The main biosynthetic trends are shared, even though the end products may be different. At the crucial moment when one of the C-16 substituents is lost, the routes diverge: in Strychnos, the ester C-22 disappears, whereas, in the Apocynaceae, the formyl C-17 is removed. Is this the reason why the fragmentation mechanisms effecting the formation of Iboga and Aspidos-

5 . AFRICAN S T R Y C H N O S ALKALOIDS

301

perma alkaloids are inoperative? No satisfactory answer to this question has been given yet, and a rationale must await further isolation and biosynthetic work. VI. Recent Advances in Structure Elucidation

Until the late 1960s, structural elucidation of complex molecules relied predominantly on chemical degradations and correlations and occasionally on X-ray diffraction analysis. Spectroscopic methods such as IR, UV, and NMR were also in use and, in combination with MS, allowed the establishment of many alkaloid structures from a minute amount of material. These tools are still in use, but the increased speed and availability of diffraction techniques and of modern 2D NMR techniques have somewhat changed the picture. As in other investigations, strychnine gave impetus to the structural elucidation, and it was one of the first organic compounds to have been successfully subjected to X-ray crystallography (194,195);it still remains a pet substance for many spectroscopists as well as a test compound for the development of new methodology. Whatever the class of natural product, it is important that some structures be determined by X-ray diffraction. The first reason is that X-ray determined structures are safe (provided the correct crystal has been submitted to analysis) and by starting from them it is possible to construct a network of reliable elements by means of chemical correlations or careful spectroscopic comparisons. The second reason is that X-ray data provide a wealth of information on bond lengths and angles which are useful for understanding NMR data, chemical reactivity, or structure-activity relationships. Despite three reported X-ray analyses of strychnine (194-196), it has recently been felt that older data lacked adequate accuracy and therefore investigations on the free base and the nitrate were conducted (197). From these considerations, it was deduced that quaternarization scarcely affected the molecular geometry. The piperidine rings are in a boat conformation, the cyclohexane rings are chairs, and the 2-0x0 piperidine rings are twisted boats. These conclusions are in full agreement with 'H-NMR deductions (198). The crystal structure of strychnine was later compared to the crystal structure of icajine (94), also from S . icuju (110). In the crystal, there was found a nonbonded interaction between N-4 and the carbon atom of the carbonyl group, C-3, and a N-C distance of 2.24 A. A similar distance (2.46 A) was measured in 19,20-epoxy-15-hydroxyicajine(96) (111). This determination allowed unambiguous assignment of the a configuration to

302

GEORGES MASSIOT AND CLEMENT DELAUDE

the epoxide; in a similar structure Bisset had remarked that the epoxides opened in an unconventional syn fashion owing to a chimeric assistance of the nitrogen atom (106). In the same series, the structures of strychnobrasiline (178) (128) and 14P-hydroxystrychnobrasiline (179) (129) were resolved, and analogous trans annular interactions appeared. In the latter case, the configuration of the “unexpected” hydroxy group was determined, which could not be easily done on the basis of interproton coupling measurements (the previous proposal was based on the observation of hydrogen bonds in the IR spectrum). In the quasi-dimer series, three molecules have been subjected to X-ray analysis: usambarensine (208) (142), strychnofoline (183) (161), and strychnopentamine (186) (159). In these alkaloids, the main structural problems were determinations of the relative configurations of C-3, C-15, and (2-17; in 183 the stereochemistry at (2-17 also had to be determined, and in 186 the configuration of the asymmetric center of the pyrrolidine ring was of interest, even though the two isomers coexist in the plant. X-Ray crystallography demonstrated that in 208 and 183 the two indole nuclei are far apart, whereas the structure of strychnopentamine (186) is folded with the indole rings on top of each other; this observation may be related to the interesting biological properties of 186 (199). Alkaloids of the akagerine group have also been the objects of X-ray investigations in order to establish their planar formulas and configurations of the asymmetric centers of the seven-membered ring. They are akagerine (3), (148,149) and decussine (35) (68). Also related to the field is the structural establishment of an alkaloid from Mostuea brunonia named mostuenine (200,201) and also known as dihydrodecussine (38), which was isolated from S. decussata. It is worth pointing out that the original of 38 stereochemistry of 38 deduced from a total synthesis and from an X-ray structure of an intermediate was wrong because of an unpredictable double inversion (202). As a final example, (~)-17-hydroxyalloibogamine(93) remains one of the only alkaloids with a C-3 to C-16 bond and is also a rare case of a natural racemate (122).Although the value of the optical rotation of 93 was too low to be measured, X-ray analysis provided a unique argument for the presence of the two antipodes in the crystal. 13C N M R was introduced at a time when information obtained by ‘H N M R was limited to small molecules or to parts of spectra. For the first time, the high resolving power of 13C N M R gave a complete picture of complicated molecules, and present literature provides data on about 100 alkaloids from African Strychnos. Strychnine was of course, submitted to the technique many times and there are at least 10 reports on analysis of 13C-NMR spectra of strychnine and brucine. The most important data on the subject are found in the articles of Wenkert and co-workers (203) and

5. AFRICAN STRYCHNOS ALKALOIDS

303

Verpoorte et al. (204),which describe 33 and 15 alkaloids and derivatives, respectively. In these articles, I3C assignments rely on comparison of the spectra of similar compounds and on selective and off-resonance decouplings. An original method for the assignment of I3C-NMR spectra was proposed by Martin (205), but it has fallen into obsolescence because of recent development of 2D NMR. These techniques have been applied to brucine by Bernstein and Hall (206), but they have not brought any dramatic change in the classic assignments. Data on alkaloids with strychnine- or retuline-type skeletons are found scattered in many articles: Wieland-Gumlich aldehyde (240), diaboline (44), 10-methoxy-O-demethyltsilanine (205), and derivatives (203), icajine (94), vomicine (235), and derivatives (204),strychnobrasiline (178) ( I 2 8 ) , isoretulinal (144), and ll-methoxyisoretuline (157) (114), ll-methoxyhenningsamine (87) and 11-methoxydiaboline (95) (136), and strychnozairine (198) ( I 74). Truncated alkaloids such as the melinonines and harmane derivatives have been described by Angenot et al. (152,207). 13C NMR has been of great help in the structural elucidation of semidimeric alkaloids since the 'H NMR spectra of these compounds are too complicated to allow determination of configurations. The first report on the 13C-NMR spectra of quasi-dimers dealth with the nigritanines from S. nigritana and S. barteri ( 5 3 ) , which was followed by two articles on the usambarensine-usambarine alkaloids (207,208). At a later date were published data on the quasi-dimers of S. ngouniensis ( I 1 6 ) , S. johnsonii (113), and S. dale (64), and on the strychnopentamines from S. usambarensis (163). The structural elucidation of longicaudatine (110) could not have been achieved without comparison of its 13C-NMR spectrum with those of strychnine (169) and geissoschizine (253) (49). Reference 49 also describes the I3C-NMR spectrum of a symmetrical dimer, bisnor-C alkaloid H (22). Comparison of these data with those pertinent to toxiferine I (203) shows the expected consequences of N-4 quaternarization; deshielding of (2-3, C-5, and C-21 (10 ppm) and shielding of C-14 and C-6). Data on other bisnortoxiferine-type dimers are presented in Ref. (119) on the alkaloids of S. matopensis [bisnor-C-curarine (35), bisnor-C alkaloid D (21), and several matopensines]. Whereas it is difficult to obtain well-resolved 'H-NMR spectra of water-soluble quaternary ammonium alkaloids, I3CNMR yields a complete image of these compounds. 'H-NMR data are available for the malindines (120 and 121) (69,153) fluorocurarine (77) (15I),fluorocurine (81) (208), mavacurine (131) (208), 11-methoxymacusine A (115) (51), diploceline (60) (207), and strychnoxanthine (197) (86). Although 13C NMR has not been very helpful in the stereochemical assignments of these bases, there are many data available on the akagerine-type alkaloids akagerine (3),

304

GEORGES MASSIOT AND

CLBMENT DELAUDE

(61,130), akagerine lactone (8) (67), decussine (36), and dihydrodecussine (38) (63,68) kribine (105) and 10-hydroxy-17-methoxykribine(108) (66). Among miscellaneous alkaloids that have been described, there are tubotaiwine (207) (116,215), tubotaiwinal (206) (116), the ngouniensines (134 and 135) (116,Z21), sarpagine [lo-hydroxynormacusine B (119) (208)],17-hydroxyalloibogamine (93) (122) and dinklageine (243) (76). 'H-NMR spectroscopy is presently the most important tool in structural elucidation since it yields information on each single proton of even complicated molecules from minute amounts material. The power of the technique has recently been magnified by the introduction of multipulse sequences. The analysis of the 'H-NMR spectrum of strychnine seems to form an endless saga, and, even if there is a consensus on 'H-NMR assignments, there remain enough discrepancies regarding couplings and newly observed long-range couplings to justify new investigations. The original assignments of Carter et al. (198)and Chazin et al. (209)have been reinvestigated using 2D NMR techniques by Bernstein and Hall (206) and Craig and Martin (210,211). Special mention must be made of the latter two investigations by Martin, who proposes new methodology to establish short- and long-range connectivities via zero and double quantum transitions. High-field NMR is now routinely used to describe new alkaloids, and a listing of the pertinent references would exceed the scope of this chapter. As an example, it is worth mentioning the first application of supercon NMR on a Strychnos alkaloids, desacetylstrychnospermine, in 1968 (222). For the first time the *H-NMR spectrum of a large size alkaloid was completely analyzed, and the measurement of coupling constants allowed complete determination of the relative configuration of six asymmetric centers. Angenot and the Ghent group have provided the most advanced analysis of the 'H-NMR spectra of alkaloids belonging to the retuline and isoretuline series (213).They show how to distinguish the series and how a simple substituent change at one end of the molecule may cause conformational variations in the other rings. The spectra of some of these alkaloids and many others (131,136,166) are complicated by the presence of rotamers arising from slow rotation around the amide bond; this phenomenom was studied by variable-temperature NMR by Anet in 1965 (214). Among the new tools offered by NMR spectroscopy, the most promising is the nuclear Overhauser effect, observed either in 1D spectra (NOEDS) or in 2D spectra (NOESY). NOEs are useful in demonstrating connectivities through space or in the cases where coupling constants do not provide reliable information. Establishment of the 2 configuration for the double bond of the S. dale semidimers was permitted by observation of NOEs

305

5 . AFRICAN STRYCHNOS ALKALOIDS

219 128

PH3 PH H

HF

k r i b i n e ( o t d formula)

SCHEME13. Some important NOES in structural elucidation. Arrows correspond to transfers of magnetization.

between H-19 and H-17 and H-16 (64) (Scheme 13). Similar transfers of magnetization are observed between the two halves of unsymmetrical dimers in the matopensine series (119). Care must be taken, however, in assigning the signals before interpretation of an NOE; the interested reader may examine in detail how three different ‘H-NMR assignments and three different sets of NOEs led to the same configurational assignment for tubotaiwine (207) (215-217). As a final example, NOE measurements led to the structural revision of kribine and derivatives. In this case, only structure 105 would account for the observation of NOEs between a vinylic singlet proton and the aliphatic methyl doublet (62).

VIL. Synthesis and Chemistry The last chapter on Strychnos alkaloids published in this treatise (19) was written after completion of most of the structural and synthetic work on strychnine. Since that time some important Strychnos alkaloids have been synthesized, but, despite several assaults, Woodward’s route to strychnine remains unchallenged.

306

GEORGES MASSIOT AND CLEMENT DELAUDE

* I

OCH,

CH20H

SCHEME14. Harley-Mason’s route to tubotaiwine (207), fluorocurarine (77), and geissoschizoline (268) (218-222).

Beside Van Tamelen’s approach, which has already been discussed in this treatise (15), the first general access to the Strychnos alkaloids was developed by Harley-Mason’s group at Cambridge. Their route was based on the fortuitous observation that lithium aluminum hydride reduction of lactam 263 (the condensation product of tryptamine and a-ketoglutaric

5. AFRICAN STRYCHNOS ALKALOIDS

307

acid) led to pyrrolidinocarbazole 264. Treatment of this tetracyclic base with anhydrides brought about N-C bond fission and formation of nine-membered rings through Emde-type chemistry. As an example, treatment of 264 with &,a’-dichlorobutyric anhydride gave amide 265, which was saponified and smoothly oxidized to ketone 266 by means of manganese dioxide or lead tetraacetate (218). Cyclization of 266 to tetracyclic lactam 267 was completed by base treatment (sodium tertamylate in benzene). Compound 267 was used as an intermediate in the synthesis of several Strychnos alkaloids: (t)-geissoschizoline (268) (219), (5)-tubotaiwine (207) (220), and (+)-fluorocurarine (77) (221). These syntheses, which are summarized in Scheme 14, have been the subject of a review (222). Beside being an efficient route to a challenging ring system, the Harley-Mason approach introduces several important new transformations worthy of highlighting. As stated in Section V, akuammicine and condylocarpine ring systems arise from common precursors; this is illustrated here by the conversion of 270 to tubotaiwine by the BischlerNapieralski reaction and to geissoschizoline by reduction followed by air oxidation (on the other side of the nitrogen atom). The second problem, which was brilliantly solved by Harley-Mason, deals with the introduction of the C-16 substituent. In a classic approach, the allylic acetate derived from 267 is solvolyzed in the presence of sodium cyanide in dimethyl suifoxide to yield nitrile 269, which is hydrolyzed to an ester at a subsequent stage; although the yield of this step is only 40%, this solvolysis is by no means a trivial reaction and probably involves assistance of the indole nucleus. Introduction of a C-17 aldehyde has also been performed by the use of methoxy-Wittig reagent or dimethyl sulfonium methylide. More modern techniques involve photo-Fries rearrangement of an enaminocarbamate (223) and methoxycarbonylation of anions with dimethylcarbonate (224); the scope of this latter reaction, however, is limited to N-methylindoles. Ban and co-workers have devised another preparation of HarleyMason’s intermediate 267 (225). Their route (Scheme 15) involves photoFries rearrangement of N-acyl indole 271 to tricyclic lactam 272; activation of the C-16 position is brought about by iodine pentoxide oxidation. This route offers the advantage of allowing preparation of Strychnos as well as Aspidosperma alkaloids. Ring opening of a pentacyclic ammonium salt and use of the thioClaisen rearrangement are the salient features of Takano’s synthesis of the Strychnos alkaloid skeleton (226) (Scheme 16). Other contributions to the synthesis of the basic skeleton of Strychnos alkaloids are due to Bosch and his group in Barcelona. Annulation of a piperidine ring on a

308

GEORGES MASSIOT AND CLEMENT DELAUDE

H _ .

H

SCHEME15. Ban's preparation of Harley-Mason's intermediate (225).

5 . AFRICAN STRYCHNOS ALKALOIDS

309

SCHEME17. ABE to ABCE route to tetracyclic intermediates using Hg(I1) as the cyclization agent (227).

2-methylindole or of a 4-methylpiperidine on an indole has been achieved in all possible fashions. In the first approach (Scheme 17), N-methyl-4piperidone is elongated by a Wittig-Horner reaction; the adduct is then hydrogenated and submitted to Fischer indole synthesis with polyphosphoric acid as the cyclization agent. The 2-substituted indole is separated from the reaction mixture and cyclized under mercuric acetate oxidation in the presence of EDTA (227). The property of N-alkylated pyridinium salts to add stabilized anions has been developed in the field of indoloquinolizidine chemistry by Wenkert et al. (228) and extended to the synthesis of Strychnos alkaloids (229). For example, quenching of the anion of methyl 2-indole acetate with pyridinium salt 273 yields dihydropyridine 274, which is subsequently cyclized under acidic conditions to 275 (230) (Scheme 18). R'

VH3

273

274

275 SCHEME 18. ABE to ABCE route to tetracyclic intermediates using activated pyridinium salts (230).

310

GEORGES MASSIOT AND CLEMENT DELAUDE

SCHEME 19. ABE to ABCE route to tetracyclic intermediates using 2-cyanopiperidines (232).

2-Cyanopiperidines are masked dehydropiperidines (231) whose use avoids the oxidation-cyclization steps in the crucial formation of the C-7 to C-21 bond (Scheme 19). Displacement of the nitrile by indole takes place under acidic conditions or by the use of the Grignard reagent derived from indole. If the piperidine ring is suitably substituted, the second cyclization with the C-2 of indole may be achieved (224,232). Introduction of the five-membered ring C is by no means an easy task, and Bosch has shown that solutions proposed for the construction of ring C of Aspidosperma alkaloids were complete failures when applied to Srrychnos alkaloids. Thus, photocyclization of chloroacetamide 276 (the Sundberg-Snieckus process) unexpectedly led to the seven-membered ring 277 (233) (Scheme 20). Also inefficient was the Pummerer rearrangement of sulfoxide 278 (Magnus process), which yielded thioacetal 279 instead of sulfide 280 (234). Finally, success was met after generating the thionium ion from the dithioacetal 281, in the absence of nucleophile, by action of dimethyl(methy1thio)sulfoniumtetrafluoroborate (DMTSF) (234). Overman et al. similarly experienced severe difficulties in trying to extend to Srrychnos alkaloids methodology developed for Aspidosperma alkaloids (235). Starting material 283 is synthesized in stereoselective manner from cyclopentadiene, ethyl acetoacetate, and o-aminobenzaldehyde (Scheme 21). When 283 is heated in toluene in the presence of acid, fragmentation of the oxazolidine ring occurs, and the molecule rearranges to indolenine 284 through aza-Cope and Mannich reactions. The intramolecular Mannich reaction is highly stereoselective, and the reaction affords a single indolenine with H-3 cis to H-15 as required for further elaboration. Despite many attempts, however, 284 or derivatives thereof could never be transformed to pentacyclic systems (236).

&

& : & & a H

h3

*

L

&277

&

-

279

H

28p

H

B-

278

DMTSF-

H

28 I -

SCHEME 20. Successful and unsuccessful routes to ring D of pentacyclic Strychnos alkaloids (233,234).

I

I

C02tBu

cPh

OTlPS

eHz _I

SCHEME21. Overman's aza -Cope-Mannich approach to Strychnos alkaloids (236).

312

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

CozH

& 1 & 2 4

H H

I

HNH

c

&3

H ’

H

285 SCHEME22. Teuber’s synthesis of a pentacyclic strychnine analog (237).

A t least three groups have announced their intention of challenging Woodward’s route to strychnine. Teuber has described the synthesis of intermediate 285 (Scheme 22) with five of the seven rings of strychnine, which was made in no more than five steps (237). Compound 285 has features suitable for further elaboration of the last two rings. The weak point of this approach is that 285 possesses an angular methyl group which is not able to be easily removed. Since the rearrangement to 286 works only with 2-substituted indoles, this route is limited to the construction of 3-substituted strychnine derivatives. The simplicity of the approach, however, makes it an attractive source of nonnatural, potentially active substances. Vollhardt’s cobalt-mediated [2 + 2 + 21 cycloadditions provide another promising route to Strychnos alkaloids (238). Reaction of a tryptamine substituted by a chain bearing an alkyne substituent with trimethylsilyl methoxyacetylene in the presence of cyclopentadienylcobalt dicarbonyl yields intermediate 287 in 50% yield (Scheme 23). Oxidative decomplexation and protodesilylation affords 288, which contains four of the seven rings of strychnine. The third projected access to strychnine, proposed by Vercauteren, uses Kuehne-type cycloadditions of in situ generated enamines and indolofuma-

313

5 . AFRICAN STRYCHNOS ALKALOIDS

H

I

H I N P C

i

H

I

SCHEME 23. Vollhardt's colbalt-mediated construction of ring C of strychnine (238).

rates. In separate experiments, the tetracyclic products of these reactions have been transformed to pentacyclic compounds containing rings I11 and VI of strychnine respectively. Thus reduction of the 2,16 double bond and N-acetylation gives an esteramide which is readily cyclized to ketolactam in a Dieckmann fashion (239) (Scheme 24). Construction of the piperidine

SCHEME 24. Vercauteren's cycloaddition route to pentacyclic lactams of the strychnine series (239).

314

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

ring has been painstaking since none of the usual bifunctional conjunctive reagents gave a satisfactory yield of an adduct suitable for ring expansion. Success was met by first reacting amide 289 with methyllithium and then transforming the methyl ketone 290 to ketolactam 291 by a series of Se(1V)-mediated oxidations (240). This oxidation cascade is similar to the one developed by Woodward at a similar stage of synthesis. This sequence was run in the tubotaiwine series; use of acetaldehyde instead of butyraldehyde should lead to the strychnine series. The facility of the cyclization of 290 to 291 must be compared with the inertness to cyclization of some of Bosch’s and Overman’s intermediates. These results show that, in order to achieve complete cyclization of a complex polycyclic alkaloid, rings must be closed in a set order with substituents and double bonds correctly placed. Surprisingly, the field of asymmetric synthesis of Strychnos alkaloids remains almost unexplored, except for two articles published by the French group (240,241). In an initial approach, tryptophan methyl ester was used for the cycloaddition instead of tryptamine, and a 85 : 15 diastereoselection was obtained (241) (Scheme 25). Selective removal of the amino acid ester carbonyl was performed through the intermediacies of primary amide 292 and aminonitrile 293. Single-crystal X-ray structural determination of 294 allowed determination of the relative and absolute configurations of 294 and related compounds. The other route, whose success was less predictable, started with an a-phenylethylamine derivative of tryptamine (295) (240). Diastereoselection in the cycloaddition was almost total, and removal of the chiral auxiliary was simply done under catalytic hydrogenation conditions. These strategies have been applied to the total synthesis of tubotaiwine (207), a ubiquitous alkaloid and a subject of controversy because of the putative existence of its stereoisomer, so-called dihydrocondylocarpine (215-217). In Legseir’s approach (Scheme 26), the asymmetric centers are fixed in tetracyclic intermediate 296, and the two remaining carbon atoms are introduced as shown above (242). Reduction of the ketone and lactam functions is performed stepwise without interference of the anilinoacrylate function. The compound so obtained was identical to natural tubotaiwine. Although longer than Harley-Mason’s (221), this latter approach cannot be considered so free from risk of epimerization at C-20 since an iminium ion is involved. To complete the description of the field of strychnine chemistry, one must mention the publication of two articles on the chemistry of retuline (147) and Wieland-Gumlich aldehyde (240) by Schmid, Karrer, and coworkers (243,244). These articles contain experimental details on the degradation of strychnine to Wieland-Gumlich aldehyde and retuline.

& C 02NH2

N

H

CO,CH,

'C02CH, C02CH,

292

R=CN R=H

CH3

SCHEME25. Enantioselective entries to Sfrychnus alkaloids starting from tryptophan and L-phenylethylamine (240,241).

-

H

E

SCHEME26. Total synthesis of tubotaiwine (207) (242)

0 H

E

316

GEORGES MASSIOT ,4ND C L i M E N T DELAUDE

.. 297 U

YLCHz - TCH3 - %?--$ HO..--

W,‘

-

CHO

OH

-

SCHEME27. Total synthesis of akagerine (3) (245).

The biogenetic and structural relationships between akagerine (3) and geissoschizine (253) were demonstrated in the first total synthesis of akagerine by Benson and Winterfeldt (245) (Scheme 27). The “chemically” equivalent carbonyls of acid lactam 297 are distinguishable by the reactivity of their corresponding N-1 and N-4 lactams. Thus, 297 is transformed to dilactam 298 whose functions are clearly differentiated, the C-21 carbonyl being prone to electrophilic addition whereas that at C-17 is subject to nucleophilic attack. In synthetic sequence, treatment of 298 with Meerwein’s salt gives an iminium ion, which is readily opened in acidic medium to amino ester 299. The rest of the sequence depicted in Scheme 27 led to two interesting observations. First, the same lactam 298 is obtained from the 2 or E double bond isomers of 297; this facile epimerization of the 19,20 double bond may have biosynthetic implications. The second observation is that the nucleophilic reagent diisobutylaluminum hydride and electrophilic lithium triethylborohydride (Super-Hydride) give stereochemically different carbinolamines. Treatment of akagerine with ammonia does not lead to a positive identification of dihydrodecussine (38) among the reaction products (63). As mentioned above, dihydrodecussine (38) was synthesized twice along similar lines (201, 202). The difficulty in the synthesis of such molecules is preparation of a suitably 3,4-disubstituted pyridine, and this is a general problem for indolomonoterpene alkaloids. As shown in Scheme 28, the

317

5. AFRICAN STRYCHNOS ALKALOIDS

-? -CH

CN

-3

38 -

302

0

OH

30 I -

SCHEME 28. Total synthesis of dihydrodecussine (38) (202)

routes to 38 start with 3-cyano-4-methylpyridine, to which is added one carbon by Mannich-type chemistry. Condensation of the resulting masked aldehyde with tryptamine yields cyanopyridine 300, which is unevenfully transformed to methylketone 301. Reduction of 301 with sodium borohydride at low temperature offers alcohol 302 as a major product, whose configuration was determined by X-ray diffraction (201).Displacement of a derivative of this alcohol by indole N-1 led to a compound identical with mostuenine or dihydrodecussine (38). A careful reexamination of the reaction products and intermediates together with NOE measurements led to the conclusion that solvolysis of the “French alcohol” proceeded with double inversion of configuration (i.e., retention of configuration) (202). The chemical transformation of kribine (105) to akagerine (3) was used as an argument in the structural elucidation of 105. Unfortunately, a primary interpretation of the reaction mechanism was adopted that led to a wrong formula (61) (Scheme 29). The hydrolysis of kribine involves not only hemiketal opening but also enamine hydrolysis and carbinolamine formation. Ngouniensine (134) was synthesized simultaneously by two groups, using the same sequence of reactions (246). Both routes use the methyl ester of

318

GEORGES MASSIOT AND CLEMENT DELAUDE

i 303

H 3 SCHEME29. Hydrolysis of kribine (105) to akagerine (3) (61).

5-ethylpipecolic acid (304) and tryptophyl bromide as starting materials. Coupling of the two moieties is uneventually achieved under basic conditions, and the C-2 to C-16 bond is formed by a Friedel-Crafts reaction. The last carbon atom (the exomethylene) could not be introduced by means of Wittig chemistry, and harsher conditions had to be used at the expense of yield (Scheme 30).

I 134 -

SCHEME30. Total synthesis of ngouniensine (134) (246).

5 . AFRICAN STRYCHNOS ALKALOIDS

319

In the field of structural elucidation quasi-dimers, total synthesis has often been used to help resolve the configuration of the asymmetric centers and to secure a large quantity of material for biological evaluation. Provided the two halves of the quasi-dimers are available, there is no practical problem in devising a synthetic scheme; the only reactions required are Pictet-Spengler and Bischler-Napieralski condensations (247-249).

VIII. Pharmacology

Early studies on African Strychnos were concerned mainly with research on classification between Asiatic-type species (characterized by convulsant properties) and American-type species (muscle relaxant). In this respect, Sandberg et al. examined approximately 50 plants whose alkaloids were extracted and submitted to biological assays (250-254). Evaluation of muscle-relaxant properties was performed on whole mice, in the screen grip test, and on rat diaphragm preparations. Strychninelike activity was determined by observation of clonic and tonic convulsions. The first series of experiments revealed that extracts from S. aculeata, S. angolensis, S. asterantha, S. boonei, S. camptoneura, S. chrysophylla, S. dale, S. decussata, S. dolichothyrsa, S. elaeocarpa, S. floribunda, S. gossweiieri, S. schefleri, S. splendens, S. staudtii, and S. usambarensis had pronounced muscle-relaxant properties. Moderate activity was found in S. afzelii, S. barteri, S. cocculoides, S. dinklagei, S. innocua, S. longicaudata, S. lucens, S. minjiensis, S. phaeotrycha, S. potatorum, S. samba, S. spinosa, S. ternata, S. tricalysioides, S. urceolata, and S. xantha. Strong tonic convulsions were observed with an extract from S. icaja; at high doses, tonic convulsions were obtained with S. cocculoides, S. dinklagei, S. soubrensis, S. tricalysioides and S. xantha. It has also been recently shown that decoctions from S. henningsii induce convulsions, and it was therefore suggested that this commonly used appetizer be consumed in moderation (264). In S. icaja, Strychnine (169) and 12-hydroxystrychnine (170) are responsible for the convulsant effects (255). The convulsant activity of 20 Strychnos alkaloids was determined and it was found mainly in those alkaloids which had the intact skeleton of strychnine; seco alkaloids such as icajine (94), vomicine (235) or diaboline (44) were far less active (256). Despite preparation of a large number of derivatives of strychnine, the activity of this alkaloid remains unmatched (257-259). Modern authors have shown that the activity of strychnine results from interaction with

320

GEORGES MASSIOT AND C L ~ M E N TDELAUDE

glycine receptors (260);interaction of strychnine with small size acids and amino acids has been the subject of X-ray diffraction studies (261). Curare-type activity arises from inhibition of acetylcholine. In African Strychnos, curarelike activity was first identified in the bark of S. usambarensis (262).This activity is mainly related to three alkaloids, C-curarine (34), dihydrotoxiferine (55), and C-calebasine (27) and to a lesser extent to afrocurarine (1) (146). After the selection of bioactive Strychnos species was complete, evaluation of the properties of pure alkaloids was undertaken. Thus, the tonic convulsions observed with extracts from S. dale and S. efaeocarpa were found to be related to the presence of akagerine (3) and 17-methoxyakagerine ( 6 ) (61); the muscle-relaxant effects originated in polar fractions of the extracts. The simple alkaloid strychnocarpine (180) from S. elueocarpa was shown to have a weak muscle-relaxant effect and the ability to stimulate 5-hydroxytryptamine receptors (80). Decussine (36) from S. decussata was a muscle relaxant at a concentration of 5.1 pg/ml in the rat diaphragm test (68). This activity is remarkably high for a tertiary base; the same reduction in amplitude of muscular contraction of the diaphragm was obtained with 0.9 ,ug/ml alcuronium (Alloferine). Unlike the case for alcuronium and the Erythrina alkaloids, the blocking effects of decussine was not antagonized by synstigmine (Neostigmine). Dihydrodecussine (38) was much less active in the same test (63). The quaternary ammonium salt malindine (120) from S. decussata was also a muscle relaxant but 500 times less active than alloferine (69). N1-Acetylstrychnosplendine (192) is probably responsible for the muscle-relaxant properties of extracts from s. acufeutu; isosplendine (168) the major alkaloid from the plant, is only weakly active (46). Another muscle-relaxant alkaloid is 11-methoxymacusine A (115), from S. angolensis; the pure alkaloid is less active, however, than the total bases (51). The related macusine B (116), investigated in-depth from a pharmacological standpoint, was found to act on both adrenergic and tryptamine receptors; it also caused convulsions when injected into rats (263). Diaboline (44), the major alkaloid from S. potatorum and a ubiquitous Strychnos alkaloid, was found to be hypotensive (265);the total alkaloids from this plant were convulsant, but they also displayed the peculiar feature of enhancing convulsions caused by strychnine (266). Observation that the stem of S. afzelii is used in western Africa as chewing stick-probably in order to prevent dental caries-led to an investigation of the antimicrobial properties of its alkaloids. Three of them, bisnordihydrotoxiferine (57) , bisnor-C alkaloid H (22), and caracurine V (29), were demonstrated to be active against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and several Streptococ-

5. AFRICAN STRYCHNOS ALKALOIDS

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cus strains (47). In a subsequent investigation, 40 extracts from different Strychnos species were assayed against Bacillus subtilis, S. aureus, and P. aeruginosa; some of the activities found could be correlated with the presence of the aforementioned alkaloids, but clearly more work with pure alkaloids is needed to explain all the activities (267). Some activity against gram-positive bacteria was found in a study of the properties of around 30 quasi-dimers, including those of S . dale, S . ngouniensis, and S . tchibangensis (268); the most bioactive alkaloids were dihydro derivatives of tchibangensine (212). Tchibangensine (5’ ,6’-dihydrousambarensine) has been shown to exhibit cytotoxic action in cultures of L1210 lymphatic tumor cells; the high toxicity of the molecule, however, has hampered any further development (35).The same kind of activity is displayed by several quasi-dimers from S . usambarensis, including strychnofoline (183), 18,19-dihydrousambarine (230), and strychnopentamine (186) (269,270). In the case latter of the alkaloid, the N-methylpyrrolidine group increases the antimitotic activity of the alkaloid. Interaction of the planar alkaloids from S. usambarensis (melinonine F and normelinonine F ) with DNA has been studied by physicochemical methods (271). The ellipticine derivatives of S . dinklagei are expected to have the same antitumor activities as their congeners from other plant species (272).

IX. Conclusion By the end of 1987, 38 African Strychnos species (out of 75) had been investigated from a chemical standpoint. Approximately 250 different alkaloids have been identified, among which one-third belong exclusively to these plants. Chemistry and pharmacology in this field of research have steadily progressed, although, as often happens, many of the isolated compounds have yet to be assayed by biologists. Difficulties in providing reliable sources of many of these forest lianas are probably the cause of this lack of biological experimentation. As a consequence, only two industrial patents in the field of African Strychnos chemistry have been filed so far (273,274). Future research will lead to identification of new compounds, investigation of new plants, and, it is hoped to the discovery of alternative sources of material. As a final remark, it might be worth pointing out that the list of references does not contain any reports on cell cultures of Strychnos species, and it is the hope of the authors that the next review on the topic will see this gap partly filled.

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Acknowledgments This work has benefitted from the help of several individuals and from the knowledge of a large number of scientists active in the field; may they find here the expression of our deepest gratitude. Funds were provided by the CNRS, the Universiti de Reims-ChampagneArdenne, and by the Ministere de la Coopiration au Development de Belgique.

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M. Tits, L. Angenot, and D. Tavernier, Tetrahedron Lett. 21, 2439 (1980). M. Tits and L. Angenot, Planta Med. 34, 57 (1978). M. Tits and D. Tavernier, Plant. Med. Phytother. 12, 92 (1978). M. Tits, D. Tavernier, and L. Angenot, Phytochemistry 18, 515 (1979). M. Tits, D. Tavernier, and L. Angenot, Phytochemistry 19, 1531 (1980). M. Tits, M. Franz, D. Tavernier, and L. Angenot, Planta Med. 42, 371 (1981). M. Tits, L. Angenot, and D. Tavernier, J . Pharm. Belg. 38, 241 (1983). M. Tits, L. Angenot, and D. Tavernier, J . Nut. Prod. 46, 638 (1983). M. Tits, D. Tavernier, and L. Angenot, Phytochemistry 24, 205 (1985). M. Tits and L. Angenot, Plant Med. Phytother. 14, 213 (1980). J. Le Men and W. I. Taylor, Experientia 21, 908 (1965). C. Schatter, E. E. Waldner, H. Schmid, W. Maier, and D. Groger, Helv. Chim. Acta 52, 776 (1969). 178. S. I. Heimberger and A. I. Scott, J . Chem. SOC., Chem. Commun., 217 (1973). 179. R. B. Herbert, in “Indoles: Monoterpenoid Indole Alkaloids” (J. E . Saxton, ed.), p. 1. Wiley, New York, 1983. 180. J. Stockigt, in “Indoles and Biogenetically Related Alkaloids” (J. D. Phillipson and M. H. Zenk, ed.), p. 113. Academic Press, London, 1980. 181. L. Olivier, J. Lkvy, J. Le Men, M.-M. Janot, C. Djerassi, and H. Budzikiewicz, Bull. Soc. Chim. Fr., 868 (1965). 182. A. I. Scott and A. A . Qureshi, J . A m . Chem. Soc. 91, 5874 (1969). 183. Y. Ahmad, Y. Fatima, Atta-ur-Rahman, J . L. Occolowitz, B. A. Solheim, J . Clardy, R. L. Garnick, and P. W. Le Quesne, J . A m . Chem. Soc. 99, 1943 (1977). 184. B. Pyuskyulev, I. Ognyanov, and P. Panov, Tetrahedron Lett., 4559 (1967). 185. C. Caron, Y. Yachaoui, G. Massiot, L. Le Men-Olivier, J. Pusset, and T. SCvenet, Phytochemistry 23, 2355 (1984). 186. T. A. van Beek, R. Verpoorte, and A. Baerheim Svendsen, Tetrahedron 40, 737 (1984). 187. A. S. Bailey and R. Robinson, J . Chem. Soc., 703 (1948). 188. M. Zkches, T. Ravao, B. Richard, G. Massiot, L. Le Men- Olivier, and R. Verpoorte, J . Nat. Prod. 50, 714 (1987). 189. S . Goodwin, A. F. Smith, and E. C. Horning, J . Am. Chem. SOC. 81, 1903 (1959). 190. A. I. Scott, C. L. Yeh, and D. Greenslade, J. Chem. Soc., Chem. Commun., 947 (1978). 191. P. Potier and M.-M. Janot, C. R. Acad. Sci. Paris 276, 1727 (1973). 192. R. Besselikvre, C . Thal, H.-P. Husson, and P. Potier, J . Chem. Soc., Chem. Commun., 90 (1975). 193. P. N. Edwards and G. F. Smith, J . Chem. Soc., 1458 (1961). 194. H . J. Robertson and C. A . Beevers, Acta Crystallogr. 4, 270 (1951). 195. C. Bokhoven, J. C. Schoone, and J . M. Bijvoet, Acta Crystallogr. 4, 275 (1951). 196. S. Doddahalli, Sake Gowda, L. Cartz, and S . Natarajan, Acta Crystallogr., Sect. B 29, 2760 (1973). 197. A . Mostad, Acta Chem. Scand. B 39, 705 (1985). 198. J. C. Carter, G. W. Luther 111, and T. C. Long, J . Magn. Reson. 15, 122 (1974). 199. M. Tits, C. Desaive, J. M. Marnette, R. Bassleer, and L. Angenot, J . Ethnopharmacol. 12, 287 (1984). 200. M. Onanga and F. Khuong-Huu, C. R. Acad. Sci. Paris, Ser. C 219, 191 (1980). 201. M. Onanga and F. Khuong-Huu, Tetrahedron Lett. 24, 3627 (1983). 202. L. R. Mc Gee, G. S. Reddy, and P. N. Confalone, Tetrahedron Lett. 25, 2115 (1984). 203. E. Wenkert, H. T. A. Cheung, H. E. Gottlieb, M. C. Koch, A. Rabaron, and M. M. Plat, 1. Org. Chem. 43, 1099 (1978).

166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177.

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204. R . Verpoorte, T. A. van Beek, R . L. M. Riegman, P. J. Hylands, and N. G. Bisset, Org. Magn. Reson. 22, 345 (1984). 205. G. E. Martin, J . Pkarm. Sci. 70, 81 (1981). 206. M. A . Bernstein and L. D. Hall, Can. J . Ckem. 63, 483 (1985). 207. C. A. Coune, L. J. G. Angenot, and J. Denoel, Pkytockernistry 19, 2009 (1980). 208. C. A. Coune, M. Tits, and L. Angenot, J . Pharm. Belg. 37, 189 (1982). 209. W. J. Chazin, L. D. Colebrook, and J. T. Edward, Can. J. Ckem. 61, 1749 (1983). 210. D. A. Craig and G. E. Martin, J. Nut. Prod. 49, 456 (1986). 211. A . S. Zektzer and G. E. Martin, J . Nut. Prod. 50, 455 (1987). 212. M. Plat, M. Koch and J. Le Men, C. R. Acad. Scz. Paris, Ser. C 267, 1419 (1968). 213. D. Tavernier, M. J. 0. Anteunis, M. J. G . Tits, and L. J. G. Angenot, Bull. Soc. Ckim. Belg. 87, 595 (1978). 214. F. A. L. Anet, Can. J . Ckem. 41, 883 (1963). 215. J . Schripsema, T. A . van Beek, R. Verpoorte, C. Erkelens, P. Perera, and C. Tibell, J . Nut. Prod. 50, 89 (1987). 216. Atta-ur-Rahman, K. A. Alvi, and A. Muzaffar, Planta Med., 325 (1986). 217. M. Lounasmaa, A. Koskinen, and K. O’Connell, Helv. Chim. Acta 69, 1343 (1986). 218. B. A. Dadson, J . Harley-Mason, and G. H. Foster, J . Ckem. Soc., Ckem. Commun., 1233 (1968). 219. B. A. Dadson and J . Harley-Mason, J . Ckem. Soc., Chem. Commun., 665 (1969). 220. B. A. Dadson and J. Harley-Mason, J . Ckem. Soc., Ckem. Commun., 665 (1969). 221. G. C. Crawley and J. Harley-Mason, 1. Chem. SOC., Ckem. Commun., 685 (1971). 222. J. Harley-Mason, Pure Appl. Ckem. 41, 167 (1979). 223. E. Wenkert, B. Porter, D. Simmons, J. Ardisson, N. Kunesch, and J. Poisson, J . Org. Ckem. 49, 3733 (1984). 224. M. Alvarez, R. Lavilla, C. Roure, E. Cabot, and J . Bosch, Tetrahedron 43,2513 (1987). 225. Y. Ban, K. Yoshida, J . Goto, and T. Oishi, J . A m . Ckem. SOC. 103, 6990 (1981). 226. S. Takano, M. Hirama, and K. Ogasawara, Tetrahedron Lett. 23, 881 (1982). 227. J. Bosch, J. Bonjoch, A. Diez, A. Linares, M. Moral, and M. Rubiralta, Tetrahedron 41, 1762 (1985). 228. E. Wenkert, G. Kunesch, K. Orito, W. A . Temple, and J. S. Yadav, J . A m . Ckem. Soc. 100, 4895 (1978). 229. M. L. Bennasar, R. Lavilla, M. Alvarez, and J . Bosch, Heterocycles 27, 789 (1988). 230. M. Alvarez, R. Lavilla, and J. Bosch. Tetrukedron Lett. 28,4457 (1987). 231. D . S. Grierson, M. Harris, and H. P. Husson, J. Am. Chem. SOC. 102, 1064 (1980). 232. M. Felix, J. Bosch, D. Mauleon, M. Amat, and A. Domingo, J. Org. Chem. 47, 2435 (1982). 233. J . Bosch, M. Amat, E. Sanfelin, and M. A. Miranda, Tetrahedron 41, 2557 (1985). 234. J. Bosch and M. Amat, Tetrahedron Lett. 26, 4951 (1985). 235. L. E . Overman, M. Sworin, and R. M. Burk, J . Org. Chem. 48, 2685 (1983). 236. L. E. Overman and S. R. Angle, J . Org. Ckem. 50, 4021 (1985). 237. H. J . Teuber, K. Schumann, U. Reinehr, and A. Gholami, Liebigs Ann. Chem., 1744 (1983). 238. D . B. Grotjahn and K. P. C. Vollhardt, J . Am. Ckem. Soc. 108, 2091 (1986). 239. J. Vercauteren, A. Bideau, and G. Massiot, Tetrahedron Lett. 28, 1267 (1987). 240. B. Legseir, J. Henin, G. Massiot, and J. Vercauteren, Tetrahedron Lett. 28, 3573 (1987). 241. J. Henin, G . Massiot, J . Vercauteren, and J. Guilhem Tetrahedron Lett. 28, 1271 (1987). 242. B . Legseir, Ph.D. Thesis. Uriiv. of Reims, 1987.

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243. J . R. Hymon and H. Schmid, Helv. Chim. Acta 49, 2067 (1966). 244. J . R . Hymon, H. Schmid, P. Karrer, A. Boller, H. Els, P. Fahrni, and A. Fiirst, Helv. Chim. Acta 52, 1564 (1969). 245. W. Benson and E. Winterfeldt, Heterocycles 15, 935 (1981). 246. J. Bosch, M.-L. Bennasar, E. Zulaica, G . Massiot, and B. Massoussa, Tetrahedron Lett. 28, 231 (1987). 247. K. Yamada, K. Aoki, and D. Uemura, J . Org. Chem. 40, 2572 (1975). 248. C. Mirand-Richard, L. Le Men-Olivier, J. Ltvy, and J. Le Men, Heterocycles 12, 1409 (1979). 249. E. Seguin, T. M. Chau, M. Koch, S. Andrt, N. Farjaudon, C. Pareyre, C. Tempete, M. Robert-Gero, C. Bourut, E. Chenu, and R. Maral, Ann. Pharm. Fr. 43,301 (1985). 250. F. Sandberg, E. Lunell, and K. J. Ryrberg, Acta Pharm. Suec. 6, 70 (1969). 251. F. Sandberg, R. Verpoorte, and A. Cronlund, Acta Pharm. Suec. 8, 341 (1971). 252. L. Bohlin, Y. Ali, and F. Sandberg, Acta Pharm. Suec. 11, 233 (1974). 253. R. Verpoorte and L. Bohlin, Acta Pharm. Suec. 13, 245 (1976). 254. M. Geevaratne, W. Rolfsen, and L. Bohlin, Actu Pharm. Suec. 14, 46 (1977). 255. K. Kambu, S. Kaba, E. Cambier, K. Nzuzi, and L. Angenot, Planta Med. 40, 356 (1980). 256. F. Sandberg and K. Kristianson, Acta Pharm. Suec. 7, 329 (1970). 257. L. Bohlin, Y. Ali, and G. M. Iskander, Acta Pharm. Suec. 12, 461 (1975). 258. G. M. Iskander and L. Bohlin, Acta Pharm. Suec. 15, 431 (1978). 259. L. Bohlin and G. M. Iskander, Acta Pharm. Suec. 16, 41 (1979). 260. H. Betz, Angew. Chem. Int. Ed. Engl. 24, 365 (1985). 261. R. 0. Gould and M. D . Walkinshow, J. A m . Chem. SOC.105, 7840 (1984). 262. L. Angenot, A. Denoel, and M. Goffart, J . Pharm. Belg. 25, 73 (1970). 263. B. E. Leonard, J. Pharm. Pharmacol. 17, 566, 788 (1965). 264. J. 0. Ogeto, F. D. Juma, and G. Muriuki, E. Afr. Med. J . 61, 427 (1984). 265. H. Singh and V. K. Kapoor, Planta Med. 29,226 (1976). 266. H. Singh and V. K. Kapoor, Planta Med. 38, 133 (1980). 267. R. Verpoorte, T. A. Van Beek, P. H. A. M. Thomassen, J . Aandewiel, and A. Baerheim-Svendsen, J. Ethnopharmacol. 8, 287 (1983). 268. C. Caron, C. Choisy, M. J . Hoizey, E. Le Magrex, L. Le Men-Olivier, G. Massiot, R. Verpoorte, and M. Ztches, Planta Med. (submitted). 269. R. Bassleer, M. C. Depauw-Gillet, B. Massart, J. M. Marnette, P. Wiliquet. M. Caprasse, and L. Angenot, Planta Med. 45, 123 (1982). 270. M. Tits, C. Desaive, J . M. Marnette, R. Bassleer, and L. Angenot, J . Ethnopharmacol. 12, 287 (1984). 271. M. Caprasse and C. Houssier, Biochimie 65, 157 (1983). 272. V. K. Kansal and P. Potier, Tetrahedron 42, 2389 (1986). 273. K. Bernauer, P. Karrer, and H. Schmid, Hoffmann-La Roche, U.S. Pat. 3,073,832 (1/15/ 1963). 274. Omnium Chimique, S . A , , Belg. Pat. 4 953590 (8/1/1977). 275. R. Verpoorte, personal communication.

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-CHAPTER6 -

CINCHONA ALKALOIDS ROBERTVERPOORTE, JANSCHRIPSEMA, AND THEOVAN

DER

LEER

Department of Pharmacognosy Center for Bio-Pharmaceutical Sciences Gorlaeus Laboratories University of Leiden P.O. Box 9502 2300RA Leiden, The Netherlands

I. Introduction 11. Isolation A. Quinoline Alkaloids B. Indole Alkaloids C. Nonalkaloids 111. Synthesis A. Total Synthesis of Cinchona Alkaloids B. Conversion of Quinine to Quinidine C. Miscellaneous IV. Spectroscopy A. 'H-NMR Spectroscopy B. I3C-NMR Spectroscopy C. "N-NMR Spectroscopy D. Mass Spectroscopy E. X-Ray Crystallography F. Miscellaneous Techniques V. Chromatography A. Thin-Layer Chromatography B. Gas Chromatography C. High-Performance Liquid Chromatography VI. Biological Activities A. Antimalarial Activity B. Amebicidal Activity C Cytotoxicity D. Antimicrobial Activity E. Miscellaneous Activities VII. Metabolism VIII. Biosynthesis IX. Biotechnology References

33 1

THE ALKALOIDS, VOL. 34 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

332

ROBERT VERPOORTE

et al.

1. Introduction

Since the last review on Cinchona alkaloids in this series by UskokoviC and Grethe (1) quite a few papers have appeared on various aspects of these compounds, although the number of alkaloids in this group is still rather limited compared to others. Quinine and quinidine are, however, among the alkaloids with the highest commercial production; estimations run 300-500 metric tons a year, for which 5000-10,000 metric tons of bark are extracted (2). Quinine is used against malaria, and has received renewed interest in recent years as it is the only antimalaria compound to which resistance of the malaria parasite Plasmodium falciparum has not yet been reported. It has thus been used successfully in past years to combat chloroquine-resistant malaria (2-7, and references cited therein). Quinidine has also shown strong antimalaria activity (4). New studies of the therapeutic uses of these drugs are therefore required, e.g., on pharmacokinetics, formulations, activity, and toxicity of other Cinchona alkaloids (8). Quinidine is also employed as an antiarrhythmia drug (9, and references cited therein), and for this purpose about 30-50% of quinine production is converted chemically to the diastereoisomer quinidine (2). About 60% of Cinchona production is used for production of pharmaceuticals; most of the remaining 40% is used in the food and beverage industry as bitter agent, tonic water being the largest product (2). Other applications are in the asymmetric catalysis of chemical reactions. Cinchona alkaloids are widely employed for this purpose, both as such or immobilized in various polymeric matrices. The use of these alkaloids in catalysis of chiral syntheses has been reviewed by Wijnberg (10). The large volume of the market for Cinchona alkaloids has led to a number of efforts aimed at commercially feasible syntheses. Although several successful approaches to total synthesis have been reported (see below), it has not led to any industrial process. More recently, studies of possible biotechnological production of the alkaloids with plant cell cultures have been initiated (11,12) (see below). So far this has not led to any large-scale process either. Therefore, plant material will remain the source of the alkaloids in the next decade. Aspects of the history of the cultivation of Cinchona and its present status have been reviewed (1,1345). In this chapter we deal with aspects of the production of these alkaloids (isolation, synthesis, biosynthesis), their spectral analysis (e.g., MS and NMR), separation (TLC, GLC, and HPLC), biological activities other than antimalarial and antiarrhythmic, and their metabolism in animals and man. Two methods of numbering quinoline alkaloids are presently being

333

6 . C I N C H O N A ALKALOIDS

15

19

b

a

FIG.1. Numbering of Cinchona quinoline alkaloids according to Rabe (a) and Le Men and Taylor (b).

used. The one according to Rabe (Fig. la) is the most widely used in chemical and pharmacological literature. The biogenetic numbering proposed by Le Men and Taylor for terpenoid indole alkaloids, common in phytochemical literature, may also be applied to quinolines (Fig. lb). Here we use the first method to comply with the numbering employed in most of the literature reviewed in this chapter.

11. Isolation

Cinchona bark is an important commodity as the raw material for extraction of quinine (la) and quinidine (lc). In addition to these alkaloids, the nonmethoxylated analog cinchonidine (le) and cinchonine (lg) as well the dihydro derivatives (lb, Id, If, and lh) are also found in the bark. The most important cultivated species among others in Central America, central Africa, and several Asian countries, are C. pubescens (C. succirubra) and C. ledgerianu (15). A century of Cinchona cultivation and selective breeding makes it difficult at present to clearly distinguish the various cultivars, the situation being exacerbated by the complex botany of the genus and by lack of knowledge of from which species the cultivars originated (16). A revision of the genus is needed. In this chapter we do not deal with isolation of quinoline alkaloids from the bark; only isolations from other parts of the plant and from other plant species are reviewed. Furthermore, the isolation and structure elucidation of indole alkaloids from Cinchona and related genera are discussed. For the occurrence of alkaloids in Cinchona plant cell cultures, see Section IX. A summary of the various isolations of alkaloids and references to spectral data are given in Table I.

334

ROBERT VERPOORTE

et al. in

R1

la. -

quinine

OMe

c. quinidine

OMe

d. dihydroquinidine

OMe

e. cinchonidine

H

f. dihydrocinchonidine

H

cinchonine

H

i. epiquinine

OMe

j. epidihydroquinine

OMe

k . epiquinidine

OMe

1. epidihydroquinidine

OMe

m. epicinchonidine n. epidihydrocinchonidine

H

epicinchonine

c-3

l0,ll-dihydro l0,ll-dihydro l0,ll-dihydro

H

h. dihydrocinchonine

0.

c-9

OMe

b. dihydroquinine

g.

c-g

H

l0,ll-dihydro l0,ll-dihydro l0,ll-dihydro 10,ll dihydro

H

p. epidihydrocinchonine

H

q . cupreine

OH

r. dihydrocupreine

OH

s.

cupreidine

OH

t.

dihydrocupreidine

OH

u. epivinyl quinine

OMe

v. epivinyl quinidine

OMe

w. epivinyl epiquinine

OMe

x. epivinyl epiquinidine

OMe

10,ll-dihydro 10,ll-dihydro l0,ll-dihydro

TABLE I ISOLATIONS OF Cinchona ALKALOIDS AND REFERENCES TO SPECTRAL DATA Isolation" Alkaloid Quinolines Quinine (la) Dihydroquinine (lb) Quinidine (lc)

Molecular weight

[aID

ORD, CD

UV

IR

MS

' HNMR

61,62

b

61.62

b

324 326 324

60 60 60

Dihydroquinidine (Id)

326

60

Cinchonidine ( l e )

294

61,62

Dihydrocinchonidine (If) Cinchonine (lg)

296 294

61.62

Dihydrocinchonine (lh)

296

Quininone/quinidinone (2a)

322

'C NMR

C . ledg.

C. pub.

Others

63,64' 63,64' 63,64c

I (32,36) I (35,36) I (35,36)

1 (33,36,59) 1 (36,59)

1 (33,36,59)

Aspidosperma marcgravianum

63,64'

I (35,36)

l(36)

A . marcgravianum

b

64'

l(35)

1 (33,59)

Olea europea (1) (20,21) Ligustrum vulgare (1) (21)

b

C

1 (35)

I (33,59)

0. europeae (1) (24, L . vulgare (1) [21)> Anthocephalus chinensis [I) (17) 0. europeae (1) ( 2 0 ,A . chinensis

(1) (19) (1) (19)

(continues)

TABLE I (Continued) Isolation"

w

ORD , CD

Alkaloid

Molecular weight

N-Methyldihydroquinicine (3)

340

23

23

23

N-Methyldihydroquinicinol(4a)

342

23

23

342

23

496

28,30, 31 30,31

2-Epi-N-methyldihydroquinicinol(4b) lndoles 3,a,l7a-Cinchophylline (6a) 3a,l7P-Cinchophylline (6b)

496

3p,l7a-Cinchophylline (6c) 3P,17P-Cinchophylline (6d)

496 496

18,19-Dihydro-3&17p Cinchophylline (6e) 17,4'-Didehydro-3acinchophylline (6f)

[a]D

30,31 28,30, 31

498 494

30

31 31 31 31

'HNMR

"CNMR

23

23

23'

23

23

23

23

23

23

23

23

23

28,30 31 30,31

28,30, 31,27 30,31

28,30 31 30,31

28,30,31

31'

30,31

31'

30,31 27,28, 30,31

30,31 27,28 30,31

30,31 28,30, 31

30,31 28,30, 31

31' 31

30

30

30

30

30

30

IR

UV

MS

30

C . [edg.

Cprtb.

Others Guettarda trimera (b) (23) G. trimera (b) (23) G. trimera (b) (23)

1 (27,28,30,

34,35,36) 1 (30,31,32, 36) I (30,31) I (27,28,30, 31,36)

s (32,33), I(33) s (35,33), 1 (33)

17,4',5',6'-Tetradehydro492

30

30

30

30

40

40

40

40

cinchophylline (6g) 18,19-Didehydro-3P,17aochrolifuanine (6h) 18,19-Didehydro-3P,17Pochrolifuanine (6i) Isocorynantheol (11) Aricine (5) Guettardine (10)

436 296 382 330

Quinamine (7a)

312

3-Epiquinamine (7b) Dihydroquinamine (7c)

312 314

30 39

30 37

30 37

30 37,39

Epidihydroquinamine (7d)

314

38,39

38,39

38,3Y

38,39

10-Methoxycinchonamine(9b)

326

Cinchonamine (9a)

296

30

436

Guettarda heterosepala (40)

37

35

35

C. ledg., Cinchona ledgeriana; C . pub., Cinchonapubescens (= C. succirubra); 1, leaves; b , bark; s, stem; r , roots. See also Section IV,A. See also Section IV,B. and Fig. 3.

lsertia hypoleuca (37,391 1. hypoleuca (38,391

338

ROBERT vEnPoonTE

et al.

A. QUINOLINE ALKALOIDS Ridsdale et al. (16) studied the alkaloid content of Hasskarl’s collections of bark of Cinchona and related species. By means of TLC, alkaloids could still be detected in the nearly 130-year-old plant material. Quinoline alkaloids were detected in almost all samples of Cinchona bark as well as in species of Ladenbergia (Rubiaceae) , Pimentelia (Rubiaceae), and Clethra (Cletheraceae). The indole alkaloid aricine (5) was found in some of the Cinchona samples.

5

Aricine

Quinoline alkaloids have also been isolated from the leaves of C. pubescens and C. ledgeriana (see Table I). Handa et al. (17)reported the isolation of cinchonine (1s)and dihydrocinchonine (lh) from Anthocephalus chinensis (Rubiaceae). Dihydrocupreine (lr) was isolated from Timonius kaniensis (Rubiaceae) (18). Outside the family Rubiaceae quinidine (lc) and dihydroquinidine (Id) have been found in the leaves of Aspidosperma marcgravianum (Apocynaceae). This plant also contained a number of related indole alkaloids (19). Schneider and Kleinert (20,21) reported the occurrence of cinchonine (lg), dihydrocinchonine (lh), and cinchonidine (le) in leaves of the olive tree (Olea europeae, family Oleaceae), which is surprising as this family is not otherwise known as a source of indole or quinoline alkaloids. The same authors reported the tentative identification of cinchonine (lg) and cinchonidine (le) by means of TLC in Ligustrum vulgare leaves. The first natural occurrence of quinidinone (2a) in Cinchona bark was reported by Crooks and Robinson (22). The quinicine derivatives Nmethyldihydroquinicine (3), N-methyldihydroquinicinol (4a), and 2-epiN-methyldihydroquinicinol (4b) have been isolated from Guettarda trimera (Rubiaceae). The structures of these alkaloids were confirmed by means of spectroscopy and comparison with a semisynthetic sample of N methyldihydroquinicinol (4a) (23). Marini-Bettolo and co-workers (24,25) studied the alkaloids of Strychnos pseudoquina, known in Brasil as false Cinchona bark. Several Strychnos-type alkaloids were identified, but a previous (microchemical) identification (58) of cinchonidine, quinine, and quinidine in this plant could not be confirmed.

6. CINCHONA ALKALOIDS

R

& \

2 2 R=OMe &2 R=H

3

Quinidinone Cinchoninone

N

quininone cinchonidinone

N-methyldihydroquinicine

4 a N-methyldihydroquinicinol

5

339

(B-OH)

2-Epi-N-methyldihydroquinicinol

(a-OH)

B. INDOLE ALKALOIDS The leaves of Cinchona species are a rich source of indole alkaloids. Major compounds are the cinchophylline-type of alkaloids. The first indoles isolated were cinchophylline (26), cinchophyllamine, and isocinchophyllamine (27,28). Guilhem (29) reported an X-ray structural analysis of isocinchophyllamine (3a,l7a-cinchophylline) (6a). Zkches et al. (30,31) reisolated these alkaloids from C. ledgerianu leaves and firmly established the structures of these alkaloids as stereoisomeric quasi-dimeric indole alkaloids. In addition, the fourth possible isomer (3/3,17a) was isolated. The stereochemistry was proved by 'H and 13C NMR as well as CD.

340

R o B m T VERPOORTE

et al.

The 3a,17a (isocinchophyllamine) (6a) and 3a,17/3 (cinchophylline) (6b) isomers have a trans-quinolizidine system, characterized by a chemical shift for H-3 upfield from 4 ppm in the 'H-NMR and by shifts of approximately 61.8, 22.6, and 60.5 ppm for C-21, C-6, and C-3, respectively, in the I3C-NMR spectra. The 3/3,17a (6c) and 3/3,17p (cinchophyllamine) (6d) isomers possess a cis-quinolizidine system, characterized by a chemical shift of around 4.2-4.5 ppm for H-3 in the 'H-NMR and shifts of approximately 54, 20, and 55 ppm for, respectively, C-21, C-6, and C-3 in the 13C-NMR spectra (all 13C-NMR data refer to spectra recorded in pyridine-d6; see Section IV for all spectral data). 3aU,17a-(6a) and 3a,l7/3-~inchophylline(6b) were also detected in C. pubescens leaves and stems (32,33). Keene et al. (34) isolated 18,19(6i), i.e., dehydro-3&17a- (6h) and 18,19-dehydro-3/3,17~-ochrolifuanine nonmethoxylated cinchophyllines, from leaf material of C. ledgeriana. Z2ches et al. (30) also isolated di- and tetradehydro derivatives of the 3a-cinchophylline series (6f and 6g) and 18,19-dihydro-3/3,17P-cincho-

6'

6a 3a,l7cc-Cinchophylline -

3a,l7B-Cinchophylline 3B,17a-Cinchophylline 3B,17B-Cinchophylline 18,19-dihydro-3B,17B-cinchophylline 17,4'-didehydro-3a-cinchophylline 1 7,4 ' ,5 ' ,6 '-tetradehydroc inchophy 11ine 6 h 18,19-didehydro-3B,I7a-ochrolifuanine 6i 18,19-didehydro-3B,17B-0chrolifuanine

6b 6c 6d he

R1 OMe OMe OMe OMe

R2 OMe OMe OMe OMe

OMe

OMe

Al8,19

OMe

OMe

A17,4'

OMe

OMe

A17,4' and A5',6'

H

H

H

H

saturated

6 . CINCHONA ALKALOIDS

341

phylline (6e) from C. ledgerianu leaves. Furthermore, the monomeres aricine (S), quinamine (7a), and 3-epiquinamine (7b) were isolated from this plant material. H 0

7a Quinarnine 7b 3-Epiquinamine -

7c 7d

3nH 3 BH Dihydroquinamine 3aH, 18,19-dihydro Epidihdyroquinamine 3BH, 18,19-dihydro

Quinamine (7a) has also been found in C. ledgeriana bark (22) and roots (35) and C. pubescens stems (32,33) and leaves (33,36).Epiquinamine (7b) was detected in leaves of C. pubescens and C. ledgeriana (36). Dihydroquinamine (7c) (37,39) and a stereoisomer 3-epidihydroquinamine (7d) (38,39) were isolated from Zsertia hypoleuca (Rubiaceae) leaves. Hydroxydihydrocinchonamine (8) was obtained from dihydroquinamine (7c) by means of lithium aluminum hydride reduction (39). H

8 Hydoxydihydrocinchonamine

Mulder-Krieger et al. (32,33) reported the presence of cinchonamine (9a) in stems of C. pubescens. By means of UV, MS, and 'H NMR they also identified a new alkaloid from C. ledgeriana leaves, 10methoxycinchonamine (9b) (35). This alkaloid was later isolated from C.

342

ROBERT VERPOORTE

et al.

R

9a Cinchonamine

1 H

9 b 10-Methoxycinchonamine Dihydrocinchonamine 9 d 3-Epicinchonamine

H

5

OMe H

3aH 3aH 3aH 3BH

10 Guettardine -

pubescens leaves (33,36) and stems as well (33). A related alkaloid is guettardine (lo), isolated from Guettarda heterosepalu. Its structure was determined by means of spectrometry and chemical correlation with dihydrocorynantheol (40). The occurrence of this alkaloid and quinicinetype alkaloids in Guettarda species are of interest from biosynthetic point of view, as they represent possible derivatives of intermediates in the biosynthesis of quinine-type alkaloids. Isocorynantheol (11) was isolated from the leaves of C. Zedgeriana (34).

1 1 3-Isocorynantheol

6 . CINCHONA ALKALOIDS

343

C. NONALKALOIDS Beside alkaloids a number of other compounds have also been isolated from Cinchona, and some of these may even interfere with the analysis of the alkaloids, as was reported by Wijnsma et al. (41) in case of the anthraquinones. These compounds were first identified with certainty in cell cultures of Cinchona, which produce these compounds in quite high amounts. A number of anthraquinones have been identified in both callus and suspension cultures (33,35,42-45,56). It was shown that these compounds act as phytoalexins in Cinchona (43);their biosynthesis could be induced by elicitors in cell cultures and by microbial infections in plantlets. Bark of Phytophthora cinnamomi-infected Cinchona trees were also found to contain anthraquinones, whereas healthy parts of the same tree were free of these compounds (46). This confirmed an earlier suggestion of Covello et al. (47),who tentatively identified some compounds observed in chromatograms of Cinchona bark extracts as anthraquinones. Nonaka and co-workers (48) reported on the isolation of tannins and related compounds from C. pubescens bark. They described structures of the cinchonains, a new class of phenylpropanoid-substituted flavan-3-01s (cinchonains Ia, Ib, Ic, and Id). The precursors of these compounds, caffeic acid and epicatechin, were also isolated. A subsequent study (49) described the isolation of some proanthocyanidins and two new phenylpropanoid-substituted proanthocyanidins (cinchonains IIa and IIb). Chialva et al. (50) analyzed the essential oil obtained from Cinchona bark (yield 0.005%). The major constituents were a-terpineol, linalool, terpinen-4-01, limonene, borneol, carvacrol, myrcene, p-cymene, cislinalyl oxide, hexan-1-01, benzaldehyde, C18H38,C21H44,and 2-hexyl-3methylmaleic anhydride. The triterpene quinovic acid and cincholic acid (51) were isolated from the heartwood of C. ledgeriana, together with the quinoline alkaloids (52). Raffauf et at. (53) found quinovic acid and its 3-rhamnoside in stems of C. pubescens. The quinovic acid was responsible for the weak cytotoxic activity of the bark. The flavonoids kaempferol and quercetin have been found in leaves of C. ledgeriana, C. robusta, and C. oficinalis (54,55). Avicularin (quercetin3-@a-~-arabinoside)was also found in these plants (55). From C. ledgeriana reynoutrin and delphinidin were isolated (54). Paris and Jacquemin (57) isolated a series of compounds from the leaves of C. pubescens and C. ledgeriana. They were identified as the phenolic acids protocatechinic acid, p-coumaric acid, caffeic acid, and chlorogenic acid; the anthocyanosides cyanidol-3-glucoside and cyanidol-3-rhamnoglucoside; and the flavonoids quercetol-3-galactoside, querceto1-3-rhamnog1ucoside7 a heteroside of

344

ROBERT VERPOORTE

et al.

quercetol with galactose and rhamnose, a rhamnoglucoside of isorhamnetol, and a heteroside of kaempferol with glucose and rhamnose. The flavonoids were studied in detail only in C. pubescens.

111. Synthesis

A. TOTAL SYNTHESIS

OF

Cinchona ALKALOIDS

In volume 14 of this treatise UskokoviC and Grethe (1) reviewed the various approaches to the total synthesis of quinoline alkaloids. Much of their own work, reported in a series of preliminary communications, was included in this review. These authors have since reported in more detail on the two different approaches they developed, and recently they reviewed the synthesis of Cinchona alkaloids (66). The two methods differ in that in one method the quinuclidine ring is constructed in the final steps whereas in the other method a quinuclidine-containing intermediate is used. In connection with the first approach the synthesis of meroquinene

-t

C6HS

A 15 -

13 SCHEME 1

345

6 . CINCHONA ALKALOIDS

0 17 -

19 -

la, lc,lt,lk

CH3O

19 SCHEME2

2 -

has been studied (67-70) as well as the subsequent step, condensation of N-benzoylmeroquinene methyl ester (12) or the meroquinene aldehyde (15) and 6-methoxylepidyllithium (13) (71,72) (Scheme 1). The compounds thus obtained (14 and 16) can, in a series of steps, be stereospecifically converted to quinine and quinidine. In the second approach meroquinene is first converted to a quinuclidine-type of compound (73). Quinuclidine aldehyde (17) or the more stable ester (18) is then coupled with 6-methoxy-4-quinolyllithium (19) (74,75) to yield quinidinone/ quininone (2) (Scheme 2). The quinuclidine intermediate could also be used for the synthesis of dihydrocinchonamine (9c) (see below). In another study this group reported a reinvestigation of the classic synthesis of quinine and quinidine as proposed by Rabe in the beginning of this century (76-79). In this synthesis homomeroquinene derivatives are employed. N-Benzoylhomomeroquinene ethyl ester (20) is coupled with ethylquinamate (21), yielding quinicine (quinotoxine) (23) after removal of the benzoyl group in (22) (Scheme 3). Grethe et al. (80) first developed a more efficient synthesis of homomeroquinene (Scheme 4). For obtaining N-benzoylhomomeroquinene, homocincholoipon ethyl ester (25) was employed as the starting material; this compound can be obtained from the readily available P-collidine (78).After N-chlorination of 26, photolysis in trifluoroacetic acid below 15°C yielded 27. Dehydrochlorination of 28 could be done most efficiently by saponification to give 29, followed by

346

ROBERT VERPOORTE

et al.

22 -

21 -

CsH5

I

20 -

cH30 \

SCHEME3

dehydrochlorination with potassium tert-butoxide in dimethyl sulfoxide (DMSO). Subsequent esterification yielded the desired intermediate, N-benzoylhomomeroquinene ethyl ester (20). The coupling of N-benzoylhomomeroquinene ethyl ester (20) with the quinoline moiety could be improved by using 6-methoxy-4-quinolyllithium (19) instead of ethylquininate (Scheme 3 ) . The final step, conversion of

347

6 . CINCHONA ALKALOIDS

COOCzHs

COOC2Hg

1

&H H 25 -

26 -

COOC2HS

I

COOH

27 -

COOCzHs I

A.0

C6HS

20 -

quinicine (23) to quinuclidine-containing alkaloids, can partially be performed stereospecifically, yielding either quinine and quinidine or their epimers (81). With diisobutylaluminum hydride reduction of the quinidinone/quininone mixture in anhydrous benzene, the erythro alkaloids (quinine and quinidine) were formed, whereas sodium borohydride reduction in ethanol gave stereospecifically high yields of the threo alkaloids (epiquinine and epiquinidine). The threo alkaloids could also be synthesized stereoselectively from quinotoxine directly via a C-8-C-9 erythro epoxide. The synthesis of dihydrocinchonamine (9c) (Scheme 5 ) could be performed by using the Madelung cyclization with compound 31. This intermediate is formed by the reaction of quinuclidine ester (18) with the lithium derivative of o-toluidine (30) (75,82). The stereoisomers obtained can be separated by chromatography. However, the mixture of stereoisomers can also be used for the cyclization, which is performed by heating in the presence of sodamide. After separation of the stereoisomers, the final step, introduction of the side chain at C-7 in 32, was performed by making the indolemagnesium iodide and reacting this with ethylene oxide. Both 3a and 3j3 isomers were synthesized. This approach could not be used for the synthesis of cinchonamine (9a)

348

ROBERT VERPOORTE

et al.

N

T

H

O

’-J H

32 -

9 -c

SCHEME5

SCHEME6

and its C-3 epimer as the vinyl group interferes with the Madelung cyclization. Therefore, Grethe et al. (83) developed another approach in which thermal dehydration of alcohol 36 results in the formation of the quinuclidine moiety via a conjugated iminium ion. Two approaches for the synthesis of 36 were described. In the first approach (Scheme 6) N benzoylmeroquinene was transformed to a bromoketone (33). This reacted

6 . CINCHONA ALKALOIDS

349

0

H.

39 -

+ I

9a cinchonamine 3aH 9 d 3-epicinchonamine 3pH

SCHEME7

with sulfonamide 34 to yield 35. Hydrolysis of the tosyl group was followed by sodium borohydride reduction. Subsequently the N-benzoyl group was removed. The second, more efficient approach (Scheme 7) to intermediate 36 was via reaction of the lithium derivative of l-benzenesulfonyl indole (37) with N-benzoylmeroquinene aldehyde (15). Differences in solubility of the two diastereoisomers obtained (38), allowed an easy separation at this stage. The next step in the synthesis of cinchonamine was the cyclization of 36 to form the quinuclidine ring system. This was done in o-dichlorobenzene at 155°C for 10-15 days. The reaction produced a 1: 1

350

ROBERT VERPOORTE

et al.

0

12 -

“I

L5 -

SCHEME 8

mixture of the two C-3 epimers of 40. The introduction of the side chain in 40 was done in the same manner as for dihydrocinchonamine (see above). Meroquinene is a key intermediate in the synthesis of Cinchona alkaloids; therefore, its synthesis has been the subject of several studies. Brown and Leonard (8#,85) employed secologanin (41) as the starting material for the stereoconservative synthesis of dihydromeroquinene (cincholoipon) (Scheme 8). In a sequence of reactions, this biosynthetic precursor of the quinoline alkaloids was converted to 3,4-dihydrosecoxyloganinonitrile tetraacetate (42). After deacetylation, a one-pot reaction with p-glucosidase and sodium cyanoborohydride at pH 6 in aqueous ammonium acetate afforded 43, with retention of the C-2 stereochemistry. This compound is readily converted to a dihydromeroquinenonitrile by removal of the carbomethoxy group and sodium borohydride reduction. Acid hydrolysis, benzoylation, and methylation finally yielded methyl N-benzoyldihydromeroquinenate (45).Use of methyldihydrosecoxyloganin tetraacetate (46) as an intermediate avoided the necessity of hydrolysis of the nitrile. Methylsecoxyloganin tetraacetate (47) was also studied as a starting material (Scheme 9) (86). In the one-pot reductive amination several reductive agents were tested, in the presence of p-glucosidase, over a range of p H values. At pH 6.5 with sodium cyanoborohydride the vinyl derivative (48) was obtained; at pH 5 the dihydro derivative (49) was a major product. In a similar fashion as described above the methyl N-benzoylmeroquinenate (12) was obtained.

351

6 . CtNCHONA ALKALOIDS

CH3OOC

CH3O0C& ___)

0

L7 -

+

COOCH3

COOCH3

COOCH3

'"O-GI~(OAC)~

pH 6.5

-

H

SCHEME9

vo 50 -

0

I1 H $

H&

c e c

o4

5L -

,

*o 53 -

SCHEME10

Takano et af. (87) reported an elaborate synthesis of (+)-meroquinene from ( &)-norcamphor (Scheme 10). Oxidation of norcamphor followed by alkylation yielded 50, which can be converted to 54 in a series of reactions via, among others, compounds 51, 52, and 53. The piperidine derivative

352

ROBERT VERPOORTE

et al.

(54) was obtained as a mixture of cis and trans forms, which were separated by means of column chromatography. A further series of reactions finally yielded N-benzoylmeroquinene aldehyde (15).

Stotter et al. (88) proposed a new concept for the synthesis of Cinchona alkaloids, the crux of which is a diastereoselective aldol condensation of an enolate like 55 with the appropriate aldehyde. A model study using the lithium enolate of 3-quinuclidine and benzaldehyde was performed to prove the feasibility of this new approach. A report on its application to the synthesis of Cinchona alkaloids has not yet, to our knowledge, been published. B. CONVERSION OF QUININE

TO QUINIDINE

As the production of quinidine from Cinchona bark does not meet actual demand, the conversion of quinine (la) to quinidine (lc) has considerable commercial interest. Several reports have been published on this two-step procedure, consisting of first oxidation to the keto compound quininone (2a), which readily epimerizes to quinidinone, followed by stereospecific reduction of quinidinone. Koenig et al. (89) described a modified Oppenauer oxidation that effectively converted quinine in quininone. By using fluorenone (2 equiv) and sodium hydride (4 equiv) in dimethylformamide, almost complete oxidation could be obtained. The reaction mechanism has previously been proposed by Woodward et al. (90) and is analogous to the Oppenauer oxidation. According to Pratap and Popli (91) this reaction did not go to completion when performed on a large scale. By performing the reaction in toluene, a 90% yield could be obtained. The stereospecific reduction of quinidinone has been performed with diisobutylaluminium hydride in anhydrous benzene (81). This Lewis acid is presumed to complex with N-1; the subsequent reduction is thought to occur stereospecifically. The yield of quinine and quinidine is determined by the ratio of quininone to quinidinone during the reaction. By using the crystalline keto compound, which is almost pure quinidinone, a 94% yield of quinidine could be obtained. Gignier and Bourrelly (92,93) patented a modification of this reaction. By performing the reduction in tetrahydrofuran containing 5% pyridine, a considerable decrease of the undesirable

6 . CINCHONA ALKALOIDS

353

by-products quinine (la), epiquinine (li), and epiquinidine (lk) was possible. Moreover, pyrrole and alkyl-substituted pyridine and pyrrole could act as stereospecific orienting agents in the reaction. For the largescale reduction of quinidinone, alane, generated from lithium aluminium hydride and sulfuric acid, has been proposed as a safer reagent than diisobutylaluminium hydride (91). Reduction with sodium borohydride of the keto compounds yielded epiquinine (8S,9S) (li) and epiquinidine (8R,9R) (lk), that is, epimerization occurs at C-8 before the hydride attacks from the presumably less hindered side (81). Small et al. (94) reported a method for the demethylation of quinidine using boron tribromide in dichloromethane at -75”C, affording cupreidine (1s) in 95% yield. C. MISCELLANEOUS Veeraraghavan and Popp (95) described the synthesis of a Reissert compound of quinine. This compound could be used to introduce substituents at the 2‘ position, adjacent to the quinoline nitrogen; a 2’-cyano compound, for example, was synthesized. Substitution of a nitro group at C-6’ could be achieved through a diacetyltetrahydroquinoline derivative (96,97). The nitration was most efficient for the 4’a isomer. Radical methylation or hydroxymethylation at the C-2’ position can be performed with high yields (98). Suszko and Thiel (99) studied the addition of hydrogen chloride and hydrogen bromide to the double bond in epiquinidine. Of the two compounds formed, the a-halogen product on hydrogen halide elimination, yielded the apoepiquinidine, an alkaloid having a 3,11 double bond of unknown stereochemistry ( E I Z ) . A cis/trans isomerization of the C-3 vinyl group could be induced by formaldehyde in quinicine ( 5 6 ) . This [ 3 . 3 ] sigmatropic rearrangement was used to obtain the epivinyl (3s) derivatives of Cinchona alkaloids (100).

56

quinicine

A two-step procedure for the synthesis of quinoline alkaloids labeled with I4C at C-11 was described by Gueremy et al. (101). The first step was

354

ROBERT VERPOORTE

et al.

selective oxidation of the vinyl group in quinidine (lc) with sodium metaperiodate in the presence of osmium tetroxide, yielding a C-10 aldehyde. The stereochemistry of the asymmetric carbon in quinidine was not affected by the oxidation. The labeled carbon was subsequently introduced by means of a Wittig condensation. The alkaloid obtained was in all other respects identical to quinidine. Labeled quinicine (56) was subsequently produced from quinidine by refluxing in aqueous acetic acid. Pinazzi et al. (102) reported on the reaction of polyalkadiene wchloroformates with Cinchona alkaloids to yield polyalkadiene ocarbonates at C-9. These compounds were of interest for further biological testing. In a series of publications Sawa and Matsumura (103-106) described the transformation of quinine (lc) to indole alkaloids, using a method previously reported by Ochiai and Ishikawa (107,108). The synthesis is summarized in Scheme 11. The normal and allo 9-benzoyl-2'oxyhexahydroquinines (58a and 58b) were converted by a von Braun reaction to four isomeric N-cyanobromides (59a, 59b, 60a, and 60b).

" 57

/

p

58a'b\ BrHzC

HOHZC

C Hcj6& O

N *O 59 -

60

4

1 1

Roy ../

CHzOH

63 a a Nb-methyl b

P

SCHEME 11

Nb-methyl

61 a -

R=CHI

b R.H.

Nb-methyl

6 . CINCHONA ALKALOIDS

355

Conversion to N-cyanoacetates with silver acetate in pyridine followed by acidic hydrolysis yielded the O-benzoylaminoalcohols (e.g., 61a and 61b). After protection of the primary alcohol groups with dihydropyran, the benzoyl groups were removed in boiling aqueous methanol, and subsequent cleavage of the lactam ring resulted in the quinolizidones (e.g., 62a and 62b). Reduction with lithium aluminum hydride gave the quinolizidines, of which the normal compound can be converted to an indole (63 and 64) by a modified Oppenauer oxidation and removal of the tetrahydropyranyl group. Using this synthesis, the dihydrohunterburnine methochlorides (63a and 63b) (103), 10-methoxydihydrocorynantheol (64a) (104), and ochrosandwine (64b) (104) were prepared. NMR and IR data of the reduced quinolizidine compounds 65 and 66 were used for determining the stereochemistry at C-4' in the stereoisomeric hexahydroquinine (67a) and 2'-oxohexahydroquinine (67b) (105).

67a

R=H2 b R-0

In another publication (106) the transformation of quinine to 10methoxydihydrocinchonamine (9d) was reported (Scheme 12). The normal and all0 2'-oxohexahydroquinines (67a and 67b) were converted by a modified Oppenauer oxidation to 2'-oxohexahydroquininone (68). The same product was obtained from the quinidine analogs. Treatment with ethanolic hydrochloric acid yielded two isomeric indole carboxylic acids (69). Lithium aluminum hydride reduction of the esters gave the alcohols, two C-3 epimeric 10-methoxydihydrocinchonamines (9d and 9e). By ring

356

ROBERT VERPOORTE

67 a.b -

c

H

3

0

et al.

68 -

*

q

--J

9 d 3aH e 3pH

70

SCHEME12

hv

CH3O

H

la -

/

\

H

71 -

I

H

73 -

72 -

SCHEME13

357

6 . C I N C H O N A ALKALOIDS H

cH30a LH3U

hv

~

I1 I1

7L

la -

H -C

0

75 -

SCHEME 14

closure (C-5-N-1) and concomitant quaternization, tetrahydrocarbolinium derivatives (70) were obtained in which the stereochemistry of C-3 could be determined by comparison with other indole alkaloids. The naturally occurring cinchonamine was thus confirmed to have a C-3a hydrogen. Travecedo (109) and Stenberg and co-workers (110,111) reported that the Cinchona alkaloids quinine, quinidine, cinchonine, cinchonidine, and dihydroquinine, when dissolved in 2 M hydrochloric acid, yielded the 9-deoxy compounds (e.g., 73), with retention of the C-8 stereochemistry, on irradiation with UV light (Scheme 13). Depending on conditions, the yield of this photochemical reduction was 1-74%. The reaction proceeds via the triplet state. Under neutral conditions photolysis of the C-4'-C-9 bond occurs in quinine, quinidine, cinchonine, and cinchonidine, yielding the 6-methoxyquinoline (e.g., 74) and a mixture of the two epimers of 5-vinylquinuclidine-2-carboxaldehyde (e.g., 75) (Scheme 14) (112). In aqueous citric acid solution, i.e., conditions as found in quinine-containing beverages, the 9-deoxy compounds (e.g., 73) were found to be formed as well as the 2'-( 1,3-dicarboxy-2-hydroxyprop-2-y1)derivatives of both quinine (76a) and its deoxyderivative (76b) (113). In dichloromethane solution in the light, a number of compounds were formed, quinidinone among others (2a) (114).

HOOC

7 6 0 R=OH b R=H

77 -

358

ROBERT VERPOORTE

et al.

IV. Spectroscopy A. ‘H-NMR SPECTROSCOPY Although the structures of quinoline alkaloids had been largely resolved before the advent of NMR spectroscopy, this diagnostic tool has been used in several studies of configuration and conformation of these alkaloids. In 1967 the remaining ambiguity regarding the configuration at C-9 was solved by Lyle and Keefer (115).They confirmed the erythro configuration of the natural occurring Cinchona alkaloids and the threo configuration for their epi analogs by stereospecifically converting several quinoline alkaloids to the conformationally rigid oxiranes (77). In these compounds the signal of H-9, well separated from the other signals, revealed whether a trans- or a cis-oxirane was formed, corresponding, respectively, to an erythro or threo compound. Yanuka et al. (116) used 100-MHz ‘H NMR to study the preferred conformations of quinidine and its dihydro derivative. Signals arising from some aromatic, the vinylic, the methoxy, and the C-9 protons were observed. The authors proposed a plausible conformation; however, the spectral evidence was very meager. In a subsequent study, the same group, using 300-MHz ‘H NMR (117), investigated some quinoline alkaloids and their monoprotonated salts. The same protons were observed. By comparing the protonation shifts in CDC13 and the difference between the shifts of the monoprotonated salt in D 2 0 and the free base in CDC13, they claimed the protonation site to be on N-1 in D 2 0 (as expected) and on N-1’ in CDC1,. The conformation should likewise differ in the two solvents. These conclusions are doubtful, however, as they were obtained by circular reasoning; in fact, they were disproved by a protonation study of quinidine using 15N NMR (118). Chazin and Colebrook (119,120) described ‘H-NMR experiments with some Cinchona alkaloids. In the first study, the configuration at C-8 was investigated by examining relaxation pathways in relation to ‘H spinlattice relaxation rates (R1).However, determination of the C-8 configuration solely from ‘H R1 values was not possible. NOE enhancements did allow determination of the C-8 configuration; in addition, insights into the relative orientation of the quinoline and quinuclidine rings were obtained. In the second study a 6’4s-methoxy conformation was determined for quinine and quinidine by examination of the spin-lattice relaxation of the adjacent protons H-5’, H-7’, and H-8’. Engler et al. (100) synthesized the epivinyl isomers (lu, l v , l w , and lx) of quinine, quinidine, epiquinine, and epiquinidine. In their study they assigned the ‘H NMR of the quinuclidine system of these alkaloids and of

6. C I N C H O N A ALKALOIDS

359

quinine and quinidine by selective decoupling. The exact value of all coupling constants could not, however, be determined because of overlapping signals. Williams et al. (121) found that mixtures of (+)- and (-)-dihydroquinine, when the ratio is not 1 : 1, gave two sets of signals for several protons, with peak areas proportional to the relative amount of each enantiomer. The racemic mixture showed signals at other chemical shifts than the optically pure enantiomers. This was illustrated by the shifts of the H-9, H-2', H-3', and H-8' protons. When deuterated methanol was used instead of CDC13 the differences were greatly reduced. Spectra of the acetates recorded in CDC13 also showed significantly smaller differences. The phenomenon was rationalized by assuming molecular interactions between the enantiomers. Similar phenomena with other enantiomeric compounds have been reported by Kabacknick et al. (122),who proposed a statistical theory to explain the anomalies. Quantitative analysis of quinidine in mixtures of quinidine and dihydroquinidine, using 'H NMR, has been reported by Huynh and Sirois (123,124). However, the signal they chose for the quantification of quinidine, a singlet at 5.16 ppm, was not a singlet, but a superposition of the left side of the doublets of H - l l a and H - l l b . Therefore, this signal was less suited for this purpose; in fact, the authors had to introduce a correction factor which they said was necessary because of interference from overlapping peaks. Persson et al. (125) applied 'H NMR to measure the diffusion rate of quinidine in polymeric membranes used for controlled drug release. Guentert et al. (126) used 'H NMR to identify quinidine N-oxide ( ~ S C )a, metabolite they isolated. A remarkable feature of the spectrum was the unusually high chemical shift of the methoxy protons (2.67 ppm). Robins and Rhodes (127) evaluated the tautomerism of quinidinone (2a) and cinchoninone (2b). Using 'H NMR they showed cinchoninone in CDCl, solution, as the crystalline compound, to have the (8R) configuration. In aqueous solution tautomerism occurs, resulting in a mixture of (8R) and (8s)isomers. By integration of the H-10 and H-11 peaks in the 'H-NMR spectrum the (8R/8S) ratio for cinchoninone was determined as 23 : 77, whereas for quinidinone the ratio was 33 : 67. In our laboratory we used the quinoline alkaloids quinine, quinidine, epiquinine, and epiquinidine as model compounds to study the effects of trifluoroacetic acid as a shift-inducing reagent in 'H NMR (128-130). Resolution of nearly all signals in the spectra could be obtained, enabling complete assignment of the spectra and determination of all coupling constants (Fig. 2). The acid-induced shifts for all signals were determined. These shifts clearly reflected the stereochemical differences. In fact, the

360

ROBERT VERPOORTE

et. al.

Quinine ddd

H18 ~ 9 d2 1951ti18 L 9 7 d 11701

1170.95.781

70 6 0 1

d d d d 1105 7 8 d d d d 170

H21 2 6 7 d d d 113 5 6 21H21 3 C 8 d d 1 1 3 5 , 1 0 5 1

158 HlL 171 h 3 3 15 H9 7 2 L

W l L

OCH,

d d d 1 1 3 5 1 0 5 21 ddd 1135 7 0 I! d d d (105 . 7 0 11 d

21 251

~

1116 ddd dddd (116

LH 192

I!

dddd 1 1 2 . 1 ! 6 . 2 5 d d d d I12 1 1 7 . 5 7

391 5

H ~ 1 . 7 3 3 dd

2 5 ,2

11 6

571

1 1 1

2 5 21

H- 2 5 19 d 1121

H - 6 7 1 9 d 1151 d 1151

271.

Quinidine H21 2 9 0

m

H19 5 9 9 d d d 1175

H I 8 502 d HI8503 d

11001 11751

H21 HlL HI1 H2

325 m 121 m

-

557 d

1101-

H9

720 d

1251

Ht--

-

200 m

OCH,

p

389 8

H I 1 7 3 1 d d 19 2 2 5 1 H12799 d 1921-

761

H16 1 5 2 m

H-Hl7 H

100

H2O 2 2 3 m

H

277m 1

7 290m b13 H6

310 d d d 1 8 5 8 5 LO1 7 5 3 d 1131

H5

869 d

1L31

Epiquinine

r-7

H19 5 7 6 a d d 1 1 7 1

103

H2O 2 3 5 m

H

3

313 C d d 1100 9 6 . 7 8 1 - - - - +

r

2

50L d 765 d

H9

O C H ~3 9 5 8

1'001------~----

---

1

12?!--

H I 1 7 3 9 dd

1 9 2 271---

H I 2 801 d

1921

-----

---

CH3O

H H-.

rN+i

H17 2 8 2 m

H7

H17 320m

OH

QfjZZ '\

H6 71261151 H5

876 d I L S 1

FIG. 2. 'H-NMR data of some Cinchona alkaloids (in CDC13 solution)

751

361

6 . CINCHONA ALKALOIDS Epl q u i n i d i n e

H 21 3

00 d d 1136 10

tJ.8 5 1 3 d H l 8 513 d

v

11

IlOl! HI9

117Ll -

H 2 1 3 0 0 d d 1136 8 2 1 H l L 1 3 5 d d d 112 2 9 0

101-

H l l 1 0 1 ddd I122 9 1 171

H2 H9 OCH,

510 d 758 d

H--

1981 1271-

H15

1 7 2 rn

H16

157

117L 1 0 1 6 2 1

m

H16 157 m H17 300 m

ti17 3 0 0

391 5

H11 7 3 7 d d 1 9 2 2 7 1 H12 8 0 3 d 1 9 2 1 -

5 9 2 ddd

H Z O 2 3 5 d d d d d I10 L 8 2 6 2 1 5

m

H3 H6

3 0 3 d d d 1 9 8 9 1 901 7 1 8 d 1151

H 5

876 d lL51

-PuNJ* --

FIG.2 (Confinued)

FIG. 3. Preferred conformations of quinine (la) and quinidine (lc).

preferred conformations could be deduced for all four alkaloids. These conformations are depicted in Fig. 3. The epi compounds possessed identical conformations, and only the stereochemistry at C-9 was different.

B . I3C-NMR SPECTROSCOPY The I3C-NMR data of a series of Cinchona alkaloids and metabolites are summarized in Fig. 4. The first I3C-NMR data on quinine were reported

362

ROBERT VERPOORTE

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21 1 59 8

Quinine ( 6 4 )

Dihydroquinine ( 6 4 )

110 5

Quinidine ( 6 4 )

Cinchonidine ( 6 4 )

Epiquinidine ( 6 4 )

Dihydroquinidine ( 6 4 )

Epiquinine ( 6 4 )

Dihydraepiquinidine ( 6 4 )

FIG.4. 13C-NMR data of some Cinchona alkaloids. Chemical shifts are given in ppm relative to TMS, and unless stated otherwise the solvent was CDCI3.

363

6 . CINCHONA ALKALOIDS

Quinine N -oxide

(63)

Quinidine 3 -oxide

(63)

L'-Quinidinone (in DMSO) ( 1 3 3 )

Cinchonamine (65)

Dihydroquinidine N -oxide (63)

3-tlydraxyqiiinidine

( I 56)

Hydroxydihydrocinchonarnine ( 3 9 )

FIG.4 (Continued)

364

ROBERT VERPOORTE

et al. H

260

Quinamine ( 6 5 )

268**

Dihydroquinamine (39)

3-Epidihydroquinnmine (39)

Guettardine ( 4 0 )

3a,i7a-Cinchophylline (in pyridine) (31)

3n,l?B-Cinchophylline

(in pyridine)(31)

FIG.4 (Continued)

by Crain et al. (131). Sequin and Scott (132) used quinine as a model compound for chemical suppression of long-range I3C-lH coupling, enabling assignment of quaternary carbons. Wenkert et al. (63) reported the complete assignment of the I3C-NMR spectra of quinine, quinidine,

365

6 . CINCHONA ALKALOIDS 10?8

1151

1515 235

36,17a-Cinchophylline ( i n pyridine) (31)

1008

558

3fi,17fi-Cinchophylline (in pyridine)(ji)

12 2

12

/

is 3

CH3O

CH3O

9- e p I- N - met hy Id I hy d r o q u i nic 01

N - methyldthydroquiniclnol

12 2

N-methyldihydroquinicine

FIG. 4 (Continued)

their dihydro derivatives, and the N-oxides (78a, 78b, 78c, and 78d) of all these alkaloids. Moreland and co-workers (64) described the I3C NMR of a series of quinoline alkaloids. The differences in chemical shifts of some carbons could be explained in terms of conformational differences of the stereoisomeric alkaloids. The shifts of C-2 and C-6, for example, can be used to distinguish between the quinine and quinidine derivatives. The shift of C-4’ can be used to distinguish between the normal and epi compounds. Intramolecular hydrogen bonding, as present in the epi series (C-9 OH with N-l), was found to have large effects on C-6 and C-7.

366

ROBERT VERPOORTE et

R

Quinine N -oxide I Dihydroquinine N -oxide Quinidine N - o x i b Dihydroquinldine N -oxide 1 3-Hydroxyquinine 3-Hydroxyquinidine 10,lI-Dihydroxydihydroquinine I0,ll-Dihydroxydihydroquinidine

al.

1

R2

OMe OMe OMe OMe OMe OMe

H H H H OH

OMe

OH H

OMe

H

R3 A10,II

H

H

A10,II H H A10,Il Al0,ll OH OH OH OH

C-8

C-9

S

R R

S R

S

R

S

S

R S R S

R S R

N-0 NI-0 N1-0

NI-0 1

DMSO-d6 can hydrogen bond to conformers not involved in intramolecular hydrogen bonding, and consequently these carbons in the normal series showed the largest changes in shift when the spectra were recorded in DMSO instead of CDC13. Two metabolites of quinidine were identified by 13C NMR (133). By chemical synthesis of one of these metabolites, 3-hydroxyquinidine (78f), and its C-3 epimer and subsequent *’C-NMR analysis, the stereochemistry at C-3 was determined. -The shifts of C-5 and C-7 (20.7 and 24.1 ppm, respectively) in the (3s) derivative versus the (3R) (23.9 and 22.4 ppm) compound were used for this purpose (134). Engler et al. (100) reported for the (3s) (epivinyl) derivatives (lu, l v , l w , and lx) of quinine, quinidine, epiquinine, and epiquinidine a shift of around 21 ppm for C-5; C-7 was observed at approximately 26 ppm in epivinylquinine and epivinyiquinidine. In the epi series of the epivinyl derivatives C-7 was found at about 30 ppm. The (3R) series (quinine and quinidine) had C-5 and C-7 at around 27 and 21 ppm, respectively. For the assignment of 13C-NMR spectra of complex molecules, Schneider and Agrawal (135) used ytterbium as a shift reagent. This method, among others was applied to quinine, and the results caused the authors to change the original assignment of C-3’ and C-7’. In the tautomerism studies of Robins and Rhodes (127) 13C NMR was also used. All six species which could potentially result from tautomerism were observed. In both quinidinone (2a) and cinchoninone (2b) the keto form was shown to be the dominant species; however, significant amounts

6. CINCHONA ALKALOIDS

367

of both the enol and the geminal diol were present. For the keto and diol forms the (8S)/(8R)ratio could also be determined. For the enol form assignment of the 2 or E configuration was not possible. C. "N-NMR SPECTROSCOPY

A study was performed by Schripsema et al. (118) in which naturalabundance I5N-NMR spectra of quinidine, quinine, and epiquinidine were recorded. The similarity of the spectra of quinidine and quinine was explained by mirror image conformations (Fig. 3). Epiquinidine gave different chemical shifts for both N-1 and N-1'. This was explained by different stereochemistry at C-9 and the allied possibility of intramolecular hydrogen bonding. The protonation of quinidine was also investigated using 15N NMR. The aliphatic N-1 was protonated first, causing a 10.9-ppm downfield shift, after which the aromatic N-1 was protonated. A 94.5-ppm upfield shift was observed for this signal. D . MASS SPECTROMETRY

1. Quinoline Alkaloids The first report on the mass spectrometry of quinoline alkaloids was by Spiteller and Spiteller-Friedmann (136). The major fragments observed were m/z 136 (quinuclidine moiety) and m / z 159 (quinoline moiety). Begue and Fetizon (137) studied the hydrogen rearrangement that takes place during rupture of the C-8-C-9 bond. Most of the supplemental hydrogen in the m / z 159 fragment was found to be derived from the tertiary C-8 ( 7 0 4 0 % ) . The authors were unable to determine what type of mechanism was involved. Based on reported data, Hesse and Bernhard (138) discussed several possible mechanisms for the fragmentation. Actual proof for the postulated mechanisms was not given. Differences in the intensities of peaks in the mass spectrum of quinine and quinidine can be used to differentiate these two alkaloids (Table II), although the tuning of the spectrometer and the configuration of the ion source do affect the absolute values of the abundancy of the various fragments (139). Fales and co-workers (140) reported the chemical ionization ((21)-MS of a series of alkaloids, including quinine. Methane was used as the reactant gas. The CI-MS results showed a very clear M f 1+ ion, whereas for electron ionization (EI) M+ has a low abundance. The m / z 136 fragment, the major one in EI-MS, is also observed in CI-MS. The spectra of quinine and quinidine differed quantitatively, in a temperature-dependent mode.

368

ROBERT VERPOORTE

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TABLE I1 MAJORFRAGMENTS OF QUININE AND QUINIDINE IN GC-MS" Quinidine (lc)

m/z

% base peak

Quinine (la)

m/z ~~

136 189 81 324 137 173 82 79 95 172

100 15 15 14 13 11 7 6 6 6

3'6 base peak ~~

136 137 81 189 79 95 82 117 158 172

100 13 7 5 4 3 3 3 2 2

a Electron ionization at 70 eV, 270°C. From Ref. 139.

The effect of stereochemistry on the fragmentation of quinoline alkaloids in CI-MS with methane as the reactant gas was studied by Madhusudanan et al. (141). In particular, the abundances of MH+ and the m/z 136 fragment were found to depend on the stereochemistry. The quinuclidine fragment in the dihydro series was found to be less abundant than the corresponding fragment in the vinyl-type alkaloids. The ratio 136 : MH+ in quinine and epiquinine is about 5 times higher than in the quinidine and epiquinidine series. This was explained in the quinine series by stabilization of the MH+ ion by hydrogen bonding between the C-9 hydroxyl group and the vinyl group. The higher MH+ abundance in the epi series compared to quinine and quinidine was explained by hydrogen bonding of the C-9 hydroxyl group with N-1. Further comparison of the data of the vinyl and the dihydro series confirmed this. The ratio 136: MH+ was also studied for a series of reactant gases (methane, water, diethyl ether, ammonia, methylamine). Increased proton affinity of the reactant gas resulted in a decrease of the ratio. For the analysis of Cinchona alkaloids in cell cultures, a linked scanning at constant B / E has been reported (142). Four modes of ionization were tested: fast atom bombardment (FAB), EI, CI with isobutane, and CI with ammonia as the reactant gas. Although FAB had a high yield of protonated molecules for quinine, quinidine, cinchonine, cinchonidine, dihydrocupreine (lr), and dihydrocupreidine (It), the spectra of the isomers were virtually identical. It was thus less suited for selective analysis. Comparing

6 . CINCHONA ALKALOIDS

369

El and CI it was observed that the latter (with isobutane) resulted in the highest yields of protonated molecules, with the major fragment ( m / z 136 for the vinyl and 138 for the dihydro alkaloids) also being clearly present. The sensitivity was about 7 times better than with ammonia as reactant gas. In addition, the yield of M+ with EI was considerably lower. The alkaloids tested (see above) all gave characteristic spectra that could be used to identify these compounds in cell culture extracts. For positive identification of all products and isomers present in an extract, however, combination with other (chromatographic) methods was deemed necessary. Palmer et al. (143) reported the MS of quinidine, 3-hydroxyquinidine (78f), 2'-quinidinone (79b), and their trimethylsilyl derivatives. The 2'-0xo compound showed the usual quinuclidine fragment at m / z 136; however, the typical quinoline fragment at m / z 189 was shifted to 205. In the spectrum of 20-hydroxyquinidine the quinuclidine fragment was shifted 16 mass units to m / z 152. References to the various mass spectroscopic methods applied to quinoline alkaloids are summarized in Table 111. 2. lndole Alkaloids The fragmentation patterns of quinamine (7a) and dihydroquinamine (7c) were extensively discussed by Bohrmann et al. (37). The most intense peaks in the quinamine spectrum were at m/z 108, 121, and 136. These were thought to be derived from the quinuclidine moiety. It has to be noted, however, that in contrast to the quinoline alkaloids the base peak is at 121 not 136. Apparently, different fragmentation processes occur in quinamine-type alkaloids, which lack the hydroxyl group adjacent to the quinuclidine ring system. More detailed studies are necessary in order to assign structures to the various fragments observed.

E. X-RAY CRYSTALLOGRAPHY X-Ray crystallographic data of quinidine have been reported in a study of the quinidine salt of an organometallic compound (152). Doherty et al. (153) reported on the crystal and molecular structure of quinidine base. The alkaloid does not exhibit intramolecular hydrogen bonding. Quinidine crystallizes from ethanolic solutions as the ethanolate: the alcoholic hydroxyl group is hydrogen bonded with N-1 of the quinuclidine moiety. On the other hand, strong intramolecular hydrogen bonding was found for the C-9 hydroxyl and N-1 in 10-bromodihydroepiquinidine (154,155);only intermolecular hydrogen bonding was reported for cinchonine. Guentert et al. (126) used X-ray crystallography to ascertain the structure of quinidine N-l-oxide, a quinidine metabolite. For more detailed discussions on

370

ROBERT VERPOORTE

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TABLE 111 REPORTSON MASSSPECTRAL DATAOF Cinchona Alkaloid Quinine

Quinidine

Cinchonine Cinchonidine

Di hydroquinine Dihydroquinidine Cupreine Dih ydrocupreine

QUINOLINE

MS

GC-MS

LC-MS

CI-MS

139 142 I36 137

139

I45 144 146

142 148 141 I49 170 I42 141 140 I47

143 139 147 136 142 142 I42 147 147

142

Dihydrocupreidine 142 Quinidinone 22 Epiquinine Epiquinidine Epidihydroquinine Epidihydroquinidine Quinine N-oxide 3-Hydroxyquinine 3-Hydroxyquinidine 143 2’-Quinidinone 143 10,ll-Dihydroxydihydroquinine

143 147

147 I50 147

146

I46

142 I42 147 141 147 142 150 142 150 I42

ALKALOIDS

CH4 NH,

x x

div

x

x

x

x X

x x x x

x x x

x x

x x

x x

x x

x

x

x

x

x

x

x

x

x x x x

x x x x

x x

141 141 141 I41 148 150

x x x x x x

150

x

the conformation of the Cinchona quinoline alkaloids, see Section IV, A. Guilhem (29) determined the structure of 3a,l7a-cinchophylline (6a) (isocinchophyllamine) by means of X-ray analysis.

F. MISCELLANEOUS TECHNIQUES Han and Purdie (62) reported the use of circular dichroism for the quantitative analysis of quinine and quinidine or cinchonine and cinchonidine in binary mixtures. Optical rotation dispersion data on Cinchona alkaloids were reported by Kashima and Kawamura (61). The fluorescence spectrum of quinine was studied by Heller et al. (151).

6 . C I N C H O N A ALKALOIDS

371

V. Chromatography A. THIN-LAYER CHROMATOGRAPHY

Thin-layer chromatography has been used extensively for analysis of Cinchona alkaloids. Verpoorte et al. (157) compared a number of TLC systems for separation of the major quinoline alkaloids. None of the systems reported in the literature was able to separate all the quinolines. For the separation of the vinyl and dihydro derivatives, solvents containing ammonia or TLC plates impregnated with a strong base should be used. Solvents containing diethylamine gave no separation of the dihydro and vinyl alkaloids. None of the more than 100 solvent systems tested gave a complete baseline separation of the four naturally occurring parent alkaloids quinine, quinidine, cinchonine, and cinchonidine. For a complete review of TLC analysis of Cinchona alkaloids, see Baerheim Svendsen and Verpoorte (158). B. GAS CHROMATOGRAPHY Gas chromatography in particular has been applied for the analysis of quinidine or quinine in biological material or of quinine in beverages. For review, see Verpoorte and Baerheim Svendsen (159). GC-MS has also been employed for identification purposes (see Section IV). Owing to the low volatility and lability of quinoline alkaloids and their metabolites, GLC is not the method of choice for analysis. C. HIGH-PERFORMANCE LIQUIDCHROMATOGRAPHY High-performance liquid chromatography has developed into a powerful tool in analysis of Cinchona alkaloids. The fluorescent properties of quinoline alkaloids allow sensitive and selective detection in analyses of complex biological materials. Some of the methods used in the analysis of quinine and quinidine and their respective metabolites are summarized in Table IV and briefly discussed in Section VII. Only a few HPLC systems suitable for the analysis of Cinchona alkaloids in plant material have been reported. Most of the previously reported systems did not give complete separation of all naturally occurring quinoline alkaloids (159). In Table V some of the recently reported systems capable of separating most of the quinoline alkaloids are summarized. Most of these systems have been applied primarily in analysis of alkaloids in plant cell cultures. Keene et al. (36) reported a straight-phase system for analysis of the alkaloids in leaves. This system was also suited

TABLE IV HPLC ANALYSIS OF QUINIDINE AND METABOLITES IN BIOLOGICAL MATERIALSO Stationary phase

W

N 4

Qd, HQd, Cd, 3-OH-Qd, Cud, 2'-quinidinone Qd, HQd, 3-OH-Qd, 2'-quinidinone, Qd N,-oxide, primaquine Qd, HQd, 2'-quinidinone, 3-OH-Qd Qd, HQd, Q, 3-OH-Qd, 2'quinidinone Qd, HQd, 3-OH-Qd, 2'-quinidinone, Qd A',-oxide Qd, HQd, 3-OH-Qd, 2'-quinidinone, Qd N-oxide Qd, HQd, 3-OH-Qd, 2'qinidinone Qd, HQd, 3-OH-Qd, 2'-quinidinone, Qd N,-oxide, Qd 10,ll-dihydrodiol

Ref

pBondapak C18300 x 4 mm

ACN-2.5% aq. AcOH, 12 : 88

200,203 204,189

Lichrosorb Si60 5 pm, 250 x 3 mm Micropak MCH 10, 250 x 2.1 mm pBondapak phenyl, 300 x 3.9 mrn

Hexane-EtOH-ethanolamine, 91.5 : 8.47 : 0.03 or 92.97 : 7.0 :0.03 CH,Cl,-hexane-MeOH-70% HC104, 60: 35 : 5.5 : 0.1 10 mM KH2P04in water, containing 0.85% H3P04and 10% MeOH ACN-THF-50 rnM Pi buffer (pH 4.5), 15 : 5 : 80

pBondapak Phenyl, 300 X 3.9 mrn

ACN-THF-50 mM Pi buffer (pH 4.75), 15: 5 :80

191,208

Partisil 10, 250 x 4.5 mm Ultrasphere CR, 150 x 4.6 mm

MeOH-1 M NH4N03-2 M NH,OH, 28:l:l ACN-MeOH-THF-10 mM TEA, 5 : 5 ;3 : 87 (pH 2.5 with H,PO,)

209

Mikropak Silo, 250

X

2.5 mm

205 206 207

185

Abbreviations: Qd. quinidine; HQd, dihydroquinidine; Cd, cinchonidine; 3-OH-Qd, 3-hydroxyquinidine; Cud, cupreidine; ACN, acetonitrile; MeOH, methanol; EtOH, ethanol; THF, tretrahydrofuran; TEA, triethylarnine.

HPLC SYSTEMS FOR Alkaloids separated C, Cd, HCd, Q, Qd, HQ, HQd C, Cd, HC, HCd, Q, Qd, HQ, HQd Q, Qd C, Cd, HC, HCd, Q, Qd, HQ, HQd C, Cd, HC, HCd, Q, Qd, HQ, HQd, Ca, Qa, 10-OMe-CA

c , Cd, Q, Qd C, Cd, Q, Qd C, Cd, HCd, Q, Qd, HQ, HQd

C, Cd, HC, HCd, Q, Qd, HQ, HQd

C, Cd, Q, Qd, HQ, HQd, QA, epi-QA CA, 10-OMe-CA, cinchophyllines: 3a, 17p, 3,/3,17p, 3ct,17a

THE

TABLE V SEPARATION AND QUANTIFICATION OF Cinchona ALKALOIDS'

Stationary phase Hypersil 5 pm, 250 X 4.5 mm Hypersil ODS 5 pm, 250 x 4.6 mm WBondapak phenyi, 300 x 3.9 mm pBondapak phenyl, 300 X 3.9 mm Lichrosorb RP-18, loaded with dodecylsulfonic acid and cetrimide, 300 x 4.6 mm Lichrosorb Si60 5pm, 250 x 4.6 mm Spherisorb CN 5 p m , 250 X 4.6 m m Ultrasphere Si 5 p m , 250 X 4.6 mm, plus Partisil PXS, 10/25, 250 X 4.6 mm Lichrosorb RP-8, select B 7 p m , 250 x 4 mm, or pBondapak C,, , 300 X 3.9 mm Lichrosorb Si60 5 pm, 250 X 4.5 mm

Mobile phase Hexane-CH,Cl,-MeOH-DEA, 66 : 31 :2 : 0.65 0.1 M KH2P04 (pH 3), containing 50 mM hexylamine with 4.0 or 5.6% ACN 50 mM NaH,P04-2-methyoxyethanoI-ACN, 60: 15 : 15 (pH 4.5) 50 mM NaH2PO,-2-methyoxyethanol-ACN, 80 : 5 : 15 (pH 4.5) 20 mM methanesulfonic acid in waterdioxane-sulfuric acid, 98.5 : 1.0: 0.5 (PH 3.5) CHCI3-i-PrOH-DEA-water, 940 : 57 : 2 : 1

Ref. 160-1 62 163.164 168 165,166 169

170

6.8 mM NaH,PO,-ACN-MeOH-THF, 50 : 17 : 28.7 : 3.3 THF-n-butylchloride-NH,OH, 60 : 40 : 0.25

I67

ACN-0.1 M KHZP04 (PH 3.0), 15: 85

164

CHC1,-MeOH-conc NH,OH, 500 : 7 : 1

36

171

a Abbreviations: C, cinchonine; HC, dihydrocinchonine; Cd, cinchonidine; HCd, dihydrocinchonidine; Q, quinine; HQ, dihydroquinine; Qd, quinidine; HQd, dihydroquinidine; CA, cinchonamine; QA, quinamine; 10-OME-CA, 10-methoxycinchonamine; ACN, acetonitrile; MeOH, methanol; DEA, diethylamine; i-PrOH, isopropanol; THF, tetrahydrofuran.

374

ROBERT VERPOORTE

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Qd I

Id done

I L I

I

I

30 t ( r n i n ) FIG.5. Separation of mixtures of Cinchona alkaloids. Conditions: 250 x 4.6 mm i.d. 5 p n Hypersil (silica) column; mobile phase, hexane-dichloromethane-methanol-diethylamine (66 :31 : 2.0 :0.65); flow rate, 1.0 ml/min; UV detection at 312 nrn (160). (See Table V for abbreviations of alkaloids; Qdone, quinidinone.) 15

for analysis of the indole alkaloids, the major compounds found in leaves. McCalley (160) also reported the use of a silica gel column for separation of the quinoline alkaloids (Fig. 5 ) . This system was among several employed by Harkes et al. (161) and Wijnsma et al. (162) for the analysis of quinoline alkaloids in Cinchona cell cultures. A reversed-phase separation was reported by McCalley (163). By adding hexylamine in a low concentration to the eluent an excellent separation of the quinoline alkaloids with rather good peak symmetry could be achieved. In a subsequent study a series of reversed-phase materials was studied with regard to their performance in separating basic compounds. Cinchona alkaloids were used as model compounds. With two columns separations could be achieved with a solvent system consisting of acetonitrile and a phosphate buffer (pH 3.0). For the other columns asymmetrical peaks were obtained, which, however, could be improved by the addition of hexylamine to the solvent (Fig. 6) (164). Smith (165) found an alkylphenyl-type column to be useful in the analysis of quinoline alkaloids (Fig. 7). This method was fruitfully employed by Wijnsma et al. (166) in the analysis of alkaloids in plant cell culture extracts. A CN-type column has also been used for the separation of the four parent quinoline alkaloids (167). A disadvantage of this method

375

6 . CINCHONA ALKALOIDS

I

I

10 t ( m i n )

FIG.6 . Separation of a mixture of Cinchona alkaloids. Conditions: 150 X 3.9 mm i.d. 4 pm Novapak Clo column; mobile phase, acetonitrile-0.5 M hexylarnine (7 : 93) (pH 3.0 with orthophosphoric acid); flow rate, 1.0 ml/min; UV detection at 220 nm (164).

as well as of the straight-phase systems operating with basic solvents is the poor fluorescent properties of the quinoline alkaloids under such conditions; for example, one loses the sensitivity as well as the selectivity of the fluorimetric detection relative to methods using acidic solvent systems.

I

I

I

1

I

I

I

20 LO 60 t iminl FIG.7. Separation of a mixture of Cinchona alkaloids. Conditions: 300 x 4.0 mm i.d. pBondapak phenyl column; mobile phase, acetonitrile-2-methoxyethanol-50 mM aqueous sodium dihydrogen phosphate (15 : 15 : 70); flow rate, 0.06 ml/min; UV detection at 254 nm (165). 0

376

ROBERT VERPOORTE

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VI. Biological Activities

Quinine as an antimalaria agent (3,4) and quinidine as an antiarrhytmia drug (9) have already for many years been used in pharmacotherapy. As a result, numerous papers have been published on the pharmacology and biopharmacy of these drugs. It would be beyond the scope of this chapter to deal with these aspects of Cinchona alkaloids. Only the metabolism of these alkaloids is reviewed, as far as the identification of new metabolites is concerned (see Section VII). In this Section we confine ourselves to newly reported biological activities of Cinchona alkaloids. A. ANTIMALARIA ACTIVITY Warhurst ( 4 ) reported quinidine to be a more active antimalaria drug than the diastereoisomeric quinine. Cinchonine also proved to be highly active. Several aspects of quinine and quinidine as antimalaria agents have been reviewed extensively (3-7).

B . AMEBICIDAL ACTIVITY

A number of Cinchona alkaloids have been studied for their amebicidal activity ( I 72).The quinoline alkaloids quinine, quinidine, and quinidinone showed some activity, similar to the indole alkaloids cinchonamine (9a), 10-methoxycinchonamine (9b), epiquinamine (7b), and aricine (5). However, these activities were noted at concentrations about 100-200 times higher than the model drug emetine. Extracts of Cinchona Eedgeriana leaves were more active than could be explained on the basis of the abovementioned alkaloids. In further studies (34,173,174)a series of quasi-dimeric indole alkaloids were tested in the same bioassay. These alkaloids, e.g., ochrolifuanine and cinchophylline types (e.g., 6a-6d), share structural similarities with emetine. The quasi-dimeric alkaloids showed amebicidal activity at levels about 5-30 times higher than emetine. From the results it was concluded that the stereochemistry at C-3 had little influence on activity. Methoxylation at C-10 did result in an increase of cytotoxicity. None of the alkaloids tested had activity better than that of emetine; as the quasi-dimeric indole alkaloids cannot form a planar molecule like emetine, it was suggested that the mode of activity differed for the indole alkaloids. Isocorynantheol (ll), a monomeric indole alkaloid, was much less active than the quasi-dimers. Thus, it can be concluded that the amebicidal activity is partly based on structural features other than those responsible for cytotoxicity: the ratios between the levels of activity in the two biological tests varied considerably for the various alkaloids.

6 . CINCHONA ALKALOIDS

377

C. CYTOTOXICITY Keene et al. (172) studied the cytotoxicity of a series of Cinchona alkaloids in connection with screening for amebicidal activity, with the aim to find compounds which have a more favorable ratio between effective amebicidal concentration and cytotoxic concentration than the model compound emetine. Furthermore, in this way a nonspecific mode of action could be excluded as well. Quinine had no cytotoxic activity at the levels > 100 pg/ml) in the in vitro guinea pig ear keratinocyte test tested 20.9 pg/ml), as did system. Quinidinone (2a) did show activity epiquinamine (7b) (1.0 pg/ml). Aricine (5) had no activity at the levels tested, and considering the amebicidal activity it had the most favorable ratio for these two activities of all Cinchona alkaloids tested. A similar study was made with a series of quasi-dimeric alkaloids (34). All cinchophylline-type alkaloids (e.g. 6a-6d, 6h, and 6i) and isocorynantheol (11) showed cytotoxic activity at levels of about 1-5 pg/ml, with 3a,17/3-cinchophylline (6b) being active at a level of 0.08 pg/ml. Although the ratios of cytotoxicity to amebicidal activity (1-2) are somewhat better than for emetine (0.29), they are very poor compared to an amebicidal drug such as metronidazole (ratio >455).

D. ANTIMICROBIAL ACTIVITY The antimicrobial activity of a series of quasi-dimeric alkaloids was studied by Massiot et al. (175). Among others the four stereoisomeric cinchophyllines (6a-6d), their 18,19-dihydro7 and 1S719-dihydro,19hydroxy derivatives were tested. All these alkaloids showed activity against gram-positive bacteria; no activity against gram-negative bacteria, a yeast, or several fungi could be observed at the concentrations tested. Concerning the stereochemistry, it was observed for the cinchophylline series that the 17p alkaloids were less active than the 17a alkaloids. Reduction of the vinyl side chain resulted in decreased activity. Aromatic substitution was found to reduce activity in the quasi-dimers. Of all alkaloids tested, 3a,l7a-cinchophylline (6a) was the most active; its minimum inhibitory concentration against Staphylococcus aureus was found to be 16 pg/ml, and bactericidal activity was shown at 64 pg/ml.

E. MISCELLANEOUS ACTIVITIES Quevauviller et al. (26) studied the pharmacology of 3a,17pcinchophylline (6b) (cinchophyllamine). A weak local anesthetic activity was observed, as well some analgesic activity. An adrenolytic activity was observed in dogs and cats. Antiviral activity against potato virus X has been reported for Cinchona quinoline alkaloids (176).

378

ROBERT VERPOORTE

et al.

VII. Metabolism Studies on the metabolism of quinine had been reported as early as 1869 (177),and further studies appeared in 1918-1919 and in 1944-1946. In the first extensive study of the metabolism of Cinchona alkaloids, Brodie et al. (178) reviewed this early work and investigated metabolites of the four parent alkaloids cinchonine, cinchonidine, quinine, and quinidine in human urine. Considerable variability in the ratios between the various metabolites of the parent alkaloids was found. The nonmethoxylated alkaloids cinchonine and cinchonidine were mainly converted to the 2'-oxygenated derivatives (79c and 79d). These were further oxidized in the quinuclidine ring, yielding a dihydroxy compound. The methoxylated alkaloids quinine and quinidine had as major metabolites compounds with hydroxyl groups in the quinuclidine moiety. The 2 position was thought to be the most likely site for the hydroxyl group. A quinine metabolite with two hydroxyl groups in the quinuclidine moiety was also found. The 2'-oxygenated compounds (79a and 79b) of quinine and quinidine were also found, however, not as major metabolites. The ratios of the various metabolites were different for quinine and quinidine.

R1

79a 79b 79c 79d 79e 79f

I

2'-Quininone 2'-Quinidinone 2'-Cinchonidinone 2'-Cinchoninone 3-Hydroxy-2'-quinidinone 2'-Cupreidinone

OMe OMe H H

OMe

o

R2 C-8

H H H H OH

n

S R S R R

n

C-9 R S R S

S

~s

Palmer et al. (143) reported a renewed study of the metabolites of quinidine using spectrometric methods not available earlier. They found that a minor but significant amount of the administered alkaloid was excreted as P-glucuronates. By using a combination of UV, IR, and mass spectrometry they showed one of the major metabolites to be the 2'-oxygenated derivative (79b), as previously found by Brodie et al. (178). Further metabolites were proved to have hydroxyl group(s) at undeter-

6. C I N C H O N A ALKALOIDS

379

mined sites in the quinuclidine moiety. The structure of one of these metabolites was later determined to be 3-hydroxyquinidine (78f), by using 13CNMR (see Fig. 3) (133).The same structure was proposed by Beerman et al. (179) based on mass and 'H-NMR spectral evidence. The stereochemistry at C-3 was finally ascertained as (S) by synthesis of this compound (134). (3s)-Hydroxyquinidine is probably identical to the hydroxy derivative of quinidine isolated by Brodie and co-workers (178).The structure of 2'-quinidinone (79b) was also confirmed by means of 13C NMR (133). A further minor metabolite in urine was identified as cupreidine (1s) (O-desmethylquinidine), it accounts only for 2-3% of the administered dose (180,181). Guentert and co-workers (126) reported quinidine N-1 oxide (78c) as a metabolite of quinidine in plasma. Its structure was determined by spectral data (UV, IR, MS, 'H NMR, 13CNMR) and finally confirmed by X-ray diffraction analysis. Repetitive administration of quinidine resulted in levels of 3-hydroxyquinidine of about 34-38% of that of quinidine; quinidine N-1-oxide reached levels of 8-33%. The level of 2'-quinidinone remained below 5% of the quinidine concentration in plasma (182). Two new quinidine metabolites, isolated from postmortem biological material, were thought to be C-9 lactic acid derivatives of quinidine and 3-hydroxyquinidine (183). According to D. McHale (personal communication) these metabolites are in fact ethylcarbonate derivatives ( S O ) , artifacts obtained with chloroform. Similar artifacts can be isolated from human urine using a method in which quinine is eluted from silica gel with chloroform.

The metabolism of quinine and quinidine in rats was studied by Barrow et al. (184). In addition to already known metabolites, diastereoisomeric 10,ll-dihydroxydihydroquinidines (78h) were identified. The 2'-oxygenated derivative could not be detected in rat urine. The metabolism of the two alkaloids in rats and humans were clearly different. Moreover, the two alkaloids are metabolized differently (see Fig. 8). The diol (78g) of

380

ROBERT VERPOORTE

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Cud

a

01

10 1

20 1

30 1

LO ,

50 1

I

1

I

I

1

60 1

7, 0 t ( m 1 n )

b

I

I

l

0 10 20 30 LO 50 60 70 t ( m i n ) FIG.8. Separation of (a) quinidine and (b) quinine metabolites from rat urine. Conditions: 300 X 3.9 mm i.d. pBondapak C,, column; mobile phase, gradient of A : B from 9 : 1 to 15 : 85 (A, water-acetic acid, 99 : 1; B, water-acetonitrile-acetic 0.9 ml/min; UV detection at 254 nm (184).

acid, 40 : 59 : 1); flow rate,

quinidine was also reported as a major metabolite in bile samples and as a minor metabolite in human urine (185). The metabolism of quinine in rabbits has been studied by Watabe and Kiyonaga (186). The three major metabolites in urine were isolated, and

6 . CINCHONA ALKALOIDS

381

their structures were determined as 2‘-quinidinone (79a) , 3-hydroxy-2’quinidinone (79e), and 2’-cupreidinone (79f). Apparently the metabolic route in rabbits differs from that in humans, where a preference for oxidation in the aliphatic part of the molecule is observed. Jovanovic et al. (148) reported the occurrence of a quinine N-oxide in human urine, following consumption of quinine-containing beverages. Based on MS, N-1‘ was suggested as the site of N-oxidation. According to McHale (personal communication) , however, the metabolite is, in fact, the aliphatic N-l-oxide (78a). Considering the synthesis reported by Jovanovic et al. (148) this is the most likely product. Liddle et al. (150) isolated a series of quinine metabolites from human urine using preparative TLC. Using CI GC-MS the metabolites were identified as 3-hydroxyquinine (78e), l0,ll-epoxydihydroquinine,10,lldihydroxydihydroquinine (78g), and cupreine (1s).According to McHale (personal communication) the epoxy derivative is the N-l-oxide (78i), the evidence being the abundant m / z 136 fragment in the mass spectrum (unsubstituted quinuclidine) and repetition of the synthesis of the product as reported by Liddle et al. (150), which yielded the N-1’-oxide and not an epoxide. McHale and co-workers (personal communication) found the major metabolites of quinine in human urine to be 2’-quininone (79a), 3-hydroxyquinine (78e), and cupreine (lq). 10,ll-Dihydroxydihydroquinine (%a) was found as a trace component. Quinine N-l-oxide (78h) could not be detected. As oxidation of quinine occurs readily under mild conditions, e.g., by peroxides in diethyl ether, it was suggested that this compound is, in fact, an artifact from the isolation procedure rather than a metabolite. The same metabolites could be detected in rats, however, with significant quantitative differences The 3-hydroxy compound 78e was produced in relatively greater amounts than 2’-quininone (79a). As quinidine has a narrow therapeutic index (187,188),determination of serum levels during its administration is necessary. For a review of the methods used, see, among others, Verpoorte and Baerheim Svendsen (159) and Guentert et al. (189-191). Quinidine is extensively metabolized in man (see above); less than 20% of the administered dose is found unchanged in urine (192,193), the major site of degradation being the liver (194). The metabolites also have pharmacological activity compatible with quinidine (180,181,194-198). Therefore, the metabolites should also preferably be determined in a quinidine assay. Such assays should thus be specific, because previously used nonspecific methods such as fluorimetry gave quinidine levels which are 30-91% higher than actually present as could be proved by means of selective HPLC analysis (189,190,199,200). Extraction and isolation methods can influence the quantitative analysis of quinidine and its metabolites (201,202). N-l-Oxides and ethylcarbonate

382

ROBERT VERPOORTE

et al.

derivatives are easily formed during extraction procedures (see above). A number of HPLC methods for the analysis of quinidine in biological materials have been reported, and some of these are summarized in Table IV. Serum levels of quinidine are normally in the range of 0.5-4 mg/liter; metabolites have levels of about 0.5-1.2 mg/liter for 3-hydroxyquinidine, 0.3-0.6 mg/liter for quinidine N-oxide, 0.01-1.2 mg/liter for 2‘quinidinone, and 0.3-0.6 mg/liter for 10,ll-dihydroxydihydroquinidine (182,185,189,190,199,200).

VIII. Biosynthesis The cooccurrence with the indole alkaloids quinamine (7a) and cinchonamine (9a) led to the idea that quinoline alkaloids are derived from indolic precursors (210). Based on Woodward’s biogenesis scheme for Strychnos alkaloids, a hypothetical biosynthesis scheme was proposed in which, through a number of intermediates, cinchonine and cinchonamine are formed after condensation tryptophan, 3,4-dioxyphenylalanine, and formaldehyde. A few years later, Woodward, too, proposed a similar scheme in which, however, cinchonine is synthesized from cinchonamine (211).In Scheme 15 the biosynthetic pathway as based on recent studies is summarized. Table VI gives results of the incorporation experiments leading to this proposal. Specific incorporations into quinine and other Cinchona alkaloids of ’H- and 14C-labeled tryptophan in C. ledgeriana plants (212) and some 15N- and 14C-labeled tryptophans t213-217) in C. piibescens plants proved convincingly that quinine and other Cinchona alkaloids are derived from indolic precursors. During the past decade much attention has been paid to the biosynthesis of Cinchona alkaloids in in vitro cell, tissue, and organ cultures of Cinchona spp. In these studies also, the incorporation of labeled tryptophan (168) and tryptamine (218)into quinine and quinidine, using a root organ suspension culture of C. ledgeriana, have been reported. Other experiments in which unlabeled tryptophan was fed to in vitro cultures of C. ledgerianu (168,218-221) resulted in increased alkaloid production, whereas Harkes et al. (222) reported the rapid death of cultured C. ledgeriana cells after transfer to a 1-tryptophan-containing medium. Under these conditions E-tryptophan was spontaneously partly converted to harman and norharman. There has been much speculation on the origin of the nontryptamine part of the indole alkaloids. Thomas (223) and Wenkert (224) independently proposed the “monoterpenoid hypothesis.” This theory was tested

'I CHiOOC

Canchonomine

Cinchonaminal

7 ,

Qui nomine

R = H Cinchonidinone R = OCH]

RLH R = OCHl Ouinidinone

1/12

R = H Cinchonidine R = OCHl Quinine

R Z H Cinchonine R - OCH] Ouinidine

SCHEME15. Biosynthesis of Cinchona alkaloids.

TABLE VI BIOSYNTHESIS OF Cinchona ALKALOIDS % incorporation"

Precursor

Q

Qd

C

Cd

Plant material

Cam

Cdon

Site of activity'

Feeding experiments with tryptophan/tryptamine (numbering system according to Le Men and Taylor) o~-Tryptophan-5-'~C 0.7 c-2'

+

~~-Tryptophan-l-'~N,Z-'~C 0.97

N-1', C-9

o~-Tryptophan-4-'~N,S-'~C 0.221

0.103

~-Tryptophan-6-'~C

0.1

0.2

TryptamineJ-t2,5-I4C Try~tarnine-6-'~C

0.33 0.258

0.320

N-1, C-2'

whole plant

cell/organ cultures

+ + +

% incorporation I4C in

cinchonamine not given % incorporation I4c identical with I5N % incorporation based on

+

w 00 P

0.20

0.12

0.47

+ I

Feeding experiments concerning the terpenoid moiety (numbering as depicted in Scheme 16) Gerani01-2-'~C 0.001 c -3 0.001 c-10 0.006 c-10 Loganin-5-t 0.015 0 c-8 0.2' c-11 0.6' c-11

+

Secologanin Secologanin-5-f

0.028

0.036

Comment

I

Ref.

213,214 216,217 215

1 4 c

80% of I4C label was taken up by root organ cultures 50% of 'H is retained 77% of l4C label was taken up by root organ cultures

Specific site of labeling not confirmed by degradation No increased alkaloid production 64% of 'H label was taken up by root organ cultures

168,218

212 218

229 214,230 216,217 231 233,234

232 218

Feeding experiments with condensation products of tryptamine and secologanin (numbering system according to Le Men and Taylor) Strictosidine-3-t No uptake by root organ cultures Stri~tosidine-6-'~C + % incorporation not given; alkaloids not identified + lJinco~ide-3-t,6-'~C No incorporation of labels; alkaloids not identified Vincoside-ar-t and its C-3 epimer 0.008 0.07 0.008 + No uptake by root organ cultures Feeding experiments with indole alkaloids (numbering system according to Le Men and Taylor) Corynantheal + + Major conversion to corynantheol 0.007 0.13 0.04 Corynantheal-ar-1 Corynantheal aldehyde-ur-t 0 0 0 Dihydrocorynantheine-18,19-f2 0 0 Cinchonamine No conversions observed Cinchonamine-ur-t 0.0001 0.001 0.0008 + 0 0 Feeding experiments with Quinoline alkaloids (numbering system according to Rabe) Cinchonidinone-11-t, 0.002 0.14 0.03 c-11 Cinchonidine-11-t, 0.06 Quinine-8-t + No conversions reported Quinidine-8-t No conversions reported

+

+

w

m WI

+ + +

+

+ + +

+

' Abbreviations: Q , quinine; Qd, quinidine; C, cinchonine; Cd, cinchonidine; Cam, cinchonamine; Cdon, cinchonidinone. Numbering system according to Rabe. Total incorporation in quinine and quinidine.

218 246

246

245 216

247 245 245 215 248 212, 215 212 212 218 218

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ROBERT VERPOORTE

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Sweroside

Geroniol

HOHzC

Logonin

Secologanin

COOH

2 Mevalonic acid

SCHEME16. Some intermediates in the terpenoid pathway.

by administration of labeled geraniols to Cathuranthus roseus plants and isolation of radioactive alkaloids (225-228). Levels of specific incorporation in agreement with hypothesis were found, demonstrating that a monoterpene was the actual precursor of the indole alkaloids in C. roseus. The C-9 unit of Cinchona alkaloids is also of monoterpenoid origin (Scheme 16). This was demonstrated by specific incorporation of 14Clabeled geraniol at C-2 (229) or C-3 (214,216,217,230) in quinine after feeding to C. ledgeriana and C. pubescens plants, respectively. Another intermediate in the biosynthesis of indole alkaloids is loganin (Scheme 16). After administration of 3H-labeled loganin to C. ledgeriana plants radioactive quinine and inactive cinchonine were isolated (232). Two explanations for the lack of label in cinchonine were given, namely, incorporation of loganin into cinchonine with loss of label as a consequence of an inversion step, which generates the (8R) configuration, and no synthesis of this alkaloid at all at the time of the experiment. The next step in biosynthesis of the monoterpenoid precursor is cleavage of the cyclopentane ring, yielding secologanin. The incorporation of this 3H-labeled compound into quinine and quinidine was recently proved by Hay et al. (218), using C. ledgerianu root organ suspension cultures. Using fine cell

6. CINCHONA ALKALOIDS

387

suspension cultures, Wijnsma et al. (232) could not find any alkaloid production on administration of secologanin. 14C-Labeled sweroside (Scheme 16), having a seco ring masked by lactol formation, was also specifically incorporated into quinine and quinidine after being fed to C. pubescens plants (233,234). There has been much confusion about the condensation product of secologanin and tryptamine, caused by, among other things, questions about the C-3 stereochemistry in vincoside (245). This matter has been reviewed extensively (235-241). Using cell-free enzyme preparations of C. roseus callus (236) and cell suspension cultures of C. roseus and other Apocynaceae plants (2#2,2#3), it has been shown that the exclusive condensation product of secologanin and tryptamine is strictosidine with a 3a (S) configuration. Strictosidine and not 3 p ( R ) vincoside is the key intermediate in the biosynthesis of indole alkaloids with either the 3a-H (S) or 3P-H ( R ) configuration (244). The precursor role of strictosidine in the biosynthesis of Cinchona alkaloids has been confirmed by feeding experiments with whole plants (246). In root organ suspension cultures of C. Zedgeriana feeding of either 3H-labeled strictosidine or vincoside did not result in the formation of radioactive alkaloids (218). The lack of incorporation of any label was explained by the fact that none of the precursors was taken up by the cultured cells. Some later stages in the biosynthesis of Cinchona alkaloids have also been investigated. No incorporation was found after administration of corynantheine aldehyde-ar-t @la) to C. ledgeriana (245) or dihydrocorynantheine-18,19-t2 (81b) to C. pubescens plants (215). Incorporation of

81a R=H 81b R=CH3,

18,19-dihydro

corynantheal-ar-t into Cinchona alkaloids in plants was reported by Battersby and Parry (245). Although 95% of corynantheal administered to in vitro cell cultures of C. ledgeriana was rapidly taken up, the feeding did not result in increased production of quinoline alkaloids (247). Corynantheal was, however, rapidly metabolized corynantheol, followed by

388

ROBERT VERPOORTE

et al.

H'. \CHO CORYNANTHEAL

CORYNAEJTHEOL

I

I I

-..P

-

H'

99 He'

CHO

3 , L - OEHYDROCORYNANTHEAL

-

-4

--.P

H,'

CHO 3.L.5.6-TETRAOEHYDROCORYNANTHEAL

cq$*oH -.3+

H"

CHzOH 3.2 - DEHYDROCORYNANTHEOL

3.L.5 6-TETRADEHYOROCORYNANTHEOL

SCHEME 17

subsequent oxidation of both corynantheal and corynantheol to the corresponding 3,4-dehydro and 3,4,5,6-tetradehydro derivatives (Scheme 17). Whether corynantheal is a true intermediate in the biosynthetic pathway of quinoline alkaloids needs further confirmation. The precursor role of the indole alkaloid cinchonamine was also tested. After administration of cinchonamine-ar-t to C. ledgerianu and C. pubescens, respectively, negligible (212) and zero (215) incorporation was found. Feeding of unlabeled cinchonamine to an in vitro cell culture of C. ledgerianu resulted in a rapid uptake of the alkaloid but not in increased quinoline alkaloid production (248). Cinchonamine is therefore presumably not an intermediate in the direct pathway to quinoline alkaloids. After feeding of cinchonidione-11-t2 to C. ledgerianu plants specific incorporation of this naturally occurring product was found in the Cinchona alkaloids, especially in non methoxylated ones. Labeled cinchonine was also incorporated into cinchonidinone (212). This result showed the reversibility of this reaction. Isaac et al. (249)have provided enzymological evidence supporting both the postulated intermediacy of cinchoninone in cinchonine and cinchonidine biosynthesis and the reversibility of the reduction of cinchoninone into cinchonine and cinchonidine. Moreover, one of the described NADP(H)-dependent isoenzymes also showed reversible activity with quinidinone, forming quinine and quinidine. The feeding of quinine-8-t and quinidine-8-t to root organ suspension cultures of C. ledgerianu could have confirmed the mechanisms described above; however, the fate of these labeled alkaloids was not described (218). Summarizing the results of the tracer experiments, it can be stated that the quinoline nucleus of Cinchona alkaloids is derived from tryptophan

6 . CINCHONA ALKALOIDS

389

and the quinuclidine nucleus is of monoterpenoid origin. The moment at which the aromatic methoxylation takes place is still unknown. MulderKrieger (35) has isolated 10-methoxycinchonamine (9b) from leaves of C. ledgeriaria. From the presence of this rnethoxylated indole alkaloid in combination with the (also methoxylated) cinchophyllines (6a-6d), Mulder-Krieger (250) suggested that the aromatic rnethoxylation occurs at an early stage of the biosynthesis, apparently at or before the stage of corynantheal (see Scheme 15), and that the formation of the non methoxylated quinoline alkaloids parallels that of the methoxylated ones, assuming involvement of the same enzymes in both pathways. Additional facts in support of this theory have recently been reported by Isaac et aZ. (249), who isolated two NADP(H)-dependent isoenzymes (cinchoninone:NADPH oxidoreductases I and 11) from suspension cultures of C. Zedgeriana, of which isoenzyme I reacts with only cinchoninone in the forward direction whereas isoenzyme I1 so reacts with either quinidinone or cinchoninone. It is the reduction of quinidinone by the latter isoenzyme which indicates that methoxylation could occur before the final step in the biosynthesis. As far as the enzymes involved in the biosynthetic pathway leading to Cinchona alkaloids are concerned, the two oxidoreductases mentioned above are the first enzymes with activities specific to the Cinchona quinoline pathway (249). As a result of the terpenoid-indole origin of Cinchona alkaloids, the enzymes involved in the early steps of the biosynthesis, e.g., tryptophan decarboxyiase (251),strictosidine synthase (251), and those from the secologanin pathway (252-254) are presumably the same as in other plants producing terpenoid-indole alkaloids.

XI. Biotechnology

An alternative to unsuccessful attempts to come to an economically feasible synthetic production is biotechnology. Particularly in recent years, research has been initiated to open the way for development of an economically feasible biotechnological production method. Two types of possibilities can be considered: de novo biosynthesis or biotransformation of readily available precursors. For the first option three systems can be considered: microorganisms, plant cell cultures, or plants. For biotransformation, microorganisms, plant cell cultures, or (immobilized) enzymes can be considered. De novo biosynthesis with microorganisms requires introduction of the genes responsible €or production of the alkaloids in the plant into a suitable

390

ROBERT VERPOORTE

et al.

Ouinoline alkaloid content :i mg/g dry weight

0.001

DIFFERENTIATION

flne cell suspensions

I

0.01

1.o

0.1

]

fine cell suspensions qrown on solid media

c a l l u s culture

root orqan suspension c u l t ore

',hoot culture

cornpact G_lobular Stuctures

-

-

L_____J

FIG 9 Alkaloid content of various types of Cinchona in vitro cultures as reported in the literature (11, 256, and references cited therein)

microorganism. As the biosynthesis of the alkaloids comprises a whole series of steps, involving different enzymes (see above), and consequently a series of genes, it will be necessary to transfer a series of genes to a microorganism. This task is too difficult given the present state of the art in genetic engineering, and it is likely to remain so for the near future. A prerequisite for such an approach will be the availability of all the enzymes involved in the biosynthesis, in order to isolate the genes. Such studies have only recently been initiated for Cinchona alkaloids (249,255). The use of plant cells for the production of the alkaloids has been studied extensively. This subject has been reviewed recently (11,12,256) (see also Section VIII). The productivity of the various types of cultures is summarized in Fig. 9. Fine cell suspensions, the preferred form for large-scale applications, do not produce alkaloids, and only cultures showing some form of differentiation produce alkaloids in reasonable amounts. The feasibility of growing Cinchona cells in bioreactors has been proven by Allan and Scragg (12,257), Robins and Rhodes (258), and Ten Hoopen et al. (our BDL laboratories, unpublished results). Under such conditions, however, the main products formed are anthraquinones. Why the alkaloids are not produced in cell suspensions is not understood, and this issue has been and still is the subject of further studies (11,256). Regarding biotransformation, two possibilities exist, namely, screening microorganisms for a certain type of conversion or introducing a gene responsible for one of the biosynthetic steps into a microorganism.

6. CINCHONA ALKALOIDS

391

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D . E. Drayer, B. Lorenzo, and M. M. Reidenberg, Clin. Ckem. 27, 308 (1981). T. W. Guentert and S. Riegelman, Clin. Ckem. 24, 2065 (1978). S. Sved, J. J. M. Gilveray, and N. Beaudoin, J . Chromatogr. 145, 437 (1978). N. Weidner, J. H. Ladenson, L. Larson, G. Kessler, and J. M. McDonald, Clin.Ckem. Acta 91, 7 (1979). 207. M. R. Bonora, T. W. Guentert, R. A. Upton, and S. Riegelman, Clin. Ckim. Acta 91, 277 (1979). 208. T. W. Guentert, A. Rakhit, R. A. Upton, and S. Riegelman, J . Ckromatogr. 183, 514 (1980). 209. L. K. Pershing. M. A . Peat, and B. S. Finkle, J . Anal. Toxicol. 6, 153 (1982). 210. R . Goutarel, M. M. Janot, V . Prelog, and W. I. Taylor, Helv. Ckim. Acra 33, 150 (1950). 211. R . B. Woodward, Angew. Ckem. 68, 13 (1956). 212. A. R. Battersby and R. J. Parry, Ckem. Commun., 31 (1971). 213. N. Kowanko and E. Leete, J . Am. Ckem. SOC. 84, 4919 (1962). 214. E. Leete, Acc. Ckem. Res. 2, 59 (1969). 215. D . McMinn, Ph.D. Thesis, University of Minnesota 17 (1970). 216. E. Leete and J. N. Wemple, J . Am. Ckem. Soc. 91, 2698 (1969). 217. J. N. Wemple, Diss. Abstr. B30, 144 (1969). 218. C. A. Hay, L. A. Anderson, J. D. Phillipson, D. Curless, and R. T. Brown, Plant Cell, Tissue Organ Cult. (in press) (1988). 219. C. S . Hunter, D. V. McCalley, and A . J. Barraclough, Proc. Znt. Congr. Plant Tissue Cell Cult, 5tk, Tokyo, 1982. 220. H. Koblitz, D. Koblitz, H. P. Schmauder, and D. Groger, Plant Cell Rep. 2,122 (1983). 221. H. P. Schmauder, D. Groger, H. Koblitz, and D. Koblitz, Plant Cell Rep. 4,233 (1985). 222. P. A. A. Harkes, P. J. de Jong, R. Wijnsma, R. Verpoorte, and T. van der Leer, Plant Sci. 47, 71 (1986). 223. R. Thomas, Tetrahedron Lett., 544 (1961). 224. E. Wenkert, J . Am. Ckem. SOC. 84, 98 (1962). 225. A . R . Battersby, R. T. Brown, J. A. Knight, J. A. Martin, and A. 0. Plunkett, Chem. Commun., 346 (1966). 226. E. S. Hall, F. McCapra, T. Money, K. Fukumoto, J. R. Hanson, B. S . Mootoo, G. T. Phillips, and A . I. Scott, Chem. Commun., 348 (1966). 227. E. Leete and S. Ueda, Tetrakedron Lett., 4915 (1966). 228. P. Loew, H. Goeggel, and D. Arigoni, Chem. Commun., 347 (1966). 229. A. R. Battersby, R. T. Brown, R . S. Kapil, J . A. Knight, J. A . Martin, and A . 0. Plunkett, Ckem. Commun., 888 (1966). 230. E. Leete and J. N . Wemple, J . A m . Ckenz. Soc. 88, 4743 (1966). 231. A. R. Battersby and E. S. Hall, Ckem. Commun., 194 (1970). 232. R . Wijnsma, unpublished results, 1986. 233. H. Inouye, S . Ueda, and Y . Takeda, Tetrahedron Lett., 407 (1969). 234. H. Inouye, S. Ueda, and Y . Takeda, Ckem. Pkarm. Bull. 19, 587 (1971). 235. G. A. Cordell, Lloydia 37, 219 (1974). 236. A. I. Scott, S. L. Lee, P. deCapite, M. G. Culver, and C. R. Hutchinson, Heterocycles 7, 979 (1977). 237. R. S. Kapil and R. T. Brown, in “The Alkaloids” (R. H. F. Manske and R. Rodrigo, eds.), Vol. 17, p. 552. Academic Press, New York, 1979. 238. R . S . Kapil and R. T. Brown, in “The Alkaloids” (R. H . F. Manske and R . Rodrigo, eds.), Vol. 17, p. 574. Academic Press, New York, 1979.

203. 204. 205. 206.

398

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239. E. Leete, in “A Specialist Periodical Report, Biosynthesis” (J. D. Bu’Lock, ed.), Vol. 6, p. 181. The Chemical Society, London, 1980. 240. M. H. Zenk, Lloydia 43, 438 (1980). 241. A. I. Scott, S. L. Lee, M. G. Culver, W. Wan, T. Hirata, F. Gueritte, R. L. Baxter, H. Nordlov, C. A. Dorschel, H. Mizukami, and N. E. Mackenzie, Heterocycles 15, 1257 (1981). 242. J . Stockigt and M. H. Zenk, Chem. Commun., 646 (1977A). 243. J. Stockigt and M. H. Zenk, FEBS Left. 79, 233 (1977). 244. M. Riiffer, N. Nagakura, and M. H. Zenk, Tetrahedron Left., 1593 (1978). 245. A . R. Battersby and R. J. Parry, Chem. Commun., 30 (1971). 246. N. Nagakura, M. Ruffer, and M. H. Zenk, J . Chem. Sac. Perkin I, 2308 (1979). 247. T. van der Leer, R. Wijnsma, H. J. van Groningen, C. A. Kievid, R. Verpoorte, and A. Baerheim Svendsen, in preparation. 248. Project group Plant Cell Biotechnology, Biotechnology Delfl Leiden. BDL, unpublished results. 249. J. E. Isaac, R. J. Robins, and M. J. C. Rhodes, Phyfochemistry 26, 393 (1987). 250. T. Mulder-Krieger, Ph.D. Thesis. Univ. of Leiden, 1984. 251. S. E. Skinner, N. J. Walton, R. J . Robins, and M. J . C. Rhodes, Phytochemistry 26,721 (1987). 252. K. M. Madyastha, R. Guarnaccia, C. Baxter, and C. J . Coscia, J . B i d . Chem. 248,2497 (1973). 253. K. M. Madyastha, T. D. Meehan, and C. J. Coscia, Biochemistry 15, 1097 (1979). 254. K. M. Madyastha and C. J. Coscia, Rec. Adv. Phytochem. 13, 85 (1979). 255. R. Verpoorte, E. J. M. Penning, T. van der Leer, R. van der Bosch, and J. H. C. Hoge, unpublished results. 256. R. Wijnsma and R. Verpoorte, in “Cell Culture and Somatic Genetics of Plants” (I. K. Vasil and F. Constabel, eds.), Vol. 5, in press. Academic Press, San Diego, 1988. 257. E. J. Allan and A. H. Scragg, BiotechnoL Lett. 8, 635 (1986). 258. R. J. Robins and M. J. C. Rhodes, Appl. Microbiol. Biotechnol. 24, 35 (1986). 259. L. Ray, C. Dai Gupta, and S. K. Majumdar, Appl. Environ. Microbiol. 45,1935 (1983).

CUMULATIVE INDEX OF TITLES Aconitum alkaloids, 4, 275 (1954) diterpenoid, 7, 473 (1960) C,9 diterpenes, 12, 2 (1970) Cz0diterpenes, 12, 136 (1970) Acridine alkaloids, 2, 353 (1952) Acridone alkaloids, experimental antitumor activity of acronycine, 21, 1 (1983) Actinomycetes, isoquinolinequinones, 21, 55 (1983) N-Acyliminium ions as intermediates in alkaloid synthesis, 32, 271 (1988) Ajmaline-Sarpagine alkaloids, 8, 789 (1965), 11, 41 (1968) Alkaloid structures spectral methods, study, 24, 287 (1985) unknown structure minor alkaloids, 5, 301 (1955), 7, 509 (1960) unclassified alkaloids, 10, 545 (1967), 12, 455 (1970), 13, 397 (19711, 14, 507 (1973), 15, 263 (1975), 16, 511 (1977) Alkaloids in the plant, 1, 15 (1950), 6, 1 (1960) Alkaloids from Ants and insects, 31, 193 (1987) Aspergillus,29, 185 (1986) Pauridiantha species, 30, 223 (1987) Tabernaemontana, 27, 1 (1986) Alstoniu alkaloids, 8, 159 (1965), 12, 207 (1970), 14, 157 (1973) Amaryllidaceae alkaloids, 2, 331 (1952), 6, 289 (1960), 11, 307 (1968), 15, 83 (1975), 30, 251 (1987)

Amphibian alkaloids, 21, 139 (1983) Analgesics, 5, 1 (1955) Anesthetics, local, 5, 211 (1955) Anthranilic acid, related to quinoline alkaloids, 17, 105 (1979), 32, 341 (1988) Antimalarials, 5, 141 (1955) Antitumor alkaloids, 25, 1 (1985) Apocynaceae alkaloids, steroids, 9, 305 (1967) Aporphine alkaloids, 4, 119 (1954), 9, 1 (1967), 24, 153 (1985) Aristolochia alkaloids, 31, 29 (1987) Aristotelia alkaloids, 24, 113 (1985) Aspidosperrna alkaloids, 8, 336 (1965), 11, 205 (1968), 17, 199 (1979) Azafluoranthene alkaloids, 23, 301 (1984) Bases simple, 8, 1 (1965) simple indole, 10, 491 (1967) 3 99

400

CUMULATIVE INDEX OF TITLES

Benzophenanthridine alkaloids, 26, 185 (1985) Benzylisoquinoline alkaloids, 4, 29 (1954), 10, 402 (1967) Bisbenzylisoquinoline alkaloids, 4, 199 (1954), 7, 439 (1960), 9, 133 (1967), 13, 303 (1971), 30, 1 (1987) occurrence, 16, 249 (1977) structure, 16, 249 (1977) pharmacology, 16, 249 (1977) synthesis, 16, 319 (1977) Bisindole alkaloids, 20, 1 (1981) Buxus alkaloids, steroids, 9, 305 (1967), 14, 1 (1973)

Cactus alkaloids, 4, 23 (1954) Calabar bean alkaloids, 8, 27 (1965), 10, 383 (1967), 13, 213 (1971) Calabash curare alkaloids, 8, 515 (1965), 11, 189 (1968) Calycanthaceae alkaloids, 8, 581 (1965) Camptothecin, 21, 101 (1983) Cancentrine alkaloids, 14, 407 (1973) Capsicum species, pungent principle of, 23, 227 (1984) Carbazole alkaloids, 13, 273 (1971), 26, 1 (1985) Carboline alkaloids, 8, 47 (1965), 26, 1 (1985) P-Carboline congeners and ipecac alkaloids, 22, 1 (1983) Cardioactive alkaloids, 5, 79 (1955) Celestraceae alkaloids, 16, 215 (1977) cephafotamtsalkaloids, 23, 157 (1984) Chemotaxonomy of papaveraceae and fumaridaceae, 29, 1 (1986) Chinese medicinal plants, alkaloids, 32, 241 (1988) Chromone alkaloids, 31, 67 (1987) Cinchona alkaloids, 14, 181 (1973) chemistry, 3, 1 (1953) Colchicine, 2, 261 (1952), 6, 247 (1960), 11, 407 (1968), 23, 1 (1984) Configuration and conformation, elucidation by X-ray diffraction, 22, 51 (1983) Corynantheine, yohimbine, and related alkaloids, 27, 131 (1986) Cularine alkaloids, 4, 249 (1954), 10, 463 (1967), 29, 287 (1986) Curare-like effects, 5, 259 (1955) Cyclopeptide alkaloids, 15, 165 (1975) Daphniphyllum alkaloids, 15,41 (1975), 29, 265 (1986) Delphinium alkaloids, 4, 275 (1954) diterpenoid, 7, 473 (1960) C,,-diterpenes, 12,2 (1970) C,-diterpenes, 12, 136 (1970) Dibenzopyrrocoline alkaloids, 31, 101 (1987) Diprorrhyncus alkaloids, 8, 336 (1965) C,,-Diterpene alkaloids Aconitum, 12, 2 (1970) Delphinium, 12, 2 (1970) Garrya, 12, 2 (1970) structure, 17, 1 (1970) synthesis, 17, 1 (1979) C,,-Diterpene alkaloids Aconitum, 12, 136 (1970) chemistry, 18, 99 (1981)

CUMULATIVE INDEX OF TITLES

Delphinium, 12, 136 (1970) Garrya, 12, 136 (1970) Distribution of alkaloids in traditional Chinese medicinal plants, 32, 241 (1988) Diterpenoid alkaloids Aconiturn, 7, 473 (1960), 12, 2 (1970) Delphinium, I, 473 (1960), 12, 2 (1970) Garrya, 7, 473 (1960), 12, 2 (1960) general introduction, 12, xv (1970) C,,-diterpenes, 12, 2 (1970) C,,-diterpenes, 12, 136 (1970) Eburnamine-Vincamine alkaloids, 8, 250 (1965), 11, 125 (1968), 20, 297 (1981) Elaeocarpus alkaloids, 6, 325 (1960) Elucidation, by X-ray diffraction structural formula, 22, 51 (1983) configuration, 22, 51 (1983) conformation, 22, 51 (1983) Enamide cyclizations, application in alkaloid synthesis, 22, 189 (1983) Enzymatic transformation of alkaloids, microbial and in vitro, 18, 323 (1981) Ephedra bases, 3, 339 (1953) Ergot alkaloids, 8, 726 (1965), 15, 1 (1975) Erythrina alkaloids, 2, 499 (1952), 7, 201 (1960), 9, 483 (1967), 18, 1 (1981) Erythrophleum alkaloids, 4, 265 (1954), 10, 287 (1967) Eupomatia alkaloids, 24, 1 (1985) Forensic chemistry, alkaloids, 12, 514 (1970) by chromatographic methods, 32, 1 (1988)

Galbulimima alkaloids, 9, 529 (1967), 13, 227 (1971) Gurrya alkaloids diterpenoid, 7, 473 (1960) ClgV-diterpenes, 12, 2 (1970) C,,-diterpenes, 12, 136 (1970) Geissospermum alkaloids, 8, 679 (1965), 33, 84 (1988) Gelsemium alkaloids, 8, 93 (1965), 33, 83 (1988) Glycosides, monoterpene alkaloids, 17, 545 (1979) Haplophyton cimicidum alkaloids, 8, 673 (1965) Hasubanan alkaloids, 16, 393 (1977), 33, 307 (1988) Holarrhena group, steroid alkaloids, 7, 319 (1960) Hunteria alkaloids, 8, 250 (1965) Zboga alkaloids, 8, 203 (1965), 11, 79 (1968) Imidazole alkaloids, 3, 201 (1953), 22, 281 (1983) Indole alkaloids, 2, 369 (1952). 7, 1 (1960), 26, 1 (1985) distribution in plants, 11, 1 (1968) simple, including 0-carbolines and Ocarbazoles, 26, 1 (1985) Indole bases, simple, 10, 491 (1967) Indolizidine, simple and quinolizidine alkaloids, 28, 183 (1986) 2.2’4ndolylquinuclidine alkaloids, chemistry; 8, 238 (1965), 11, 73 (1968) In vitro and microbial enzymatic transformation of alkaloids, 18, 323 (1981)

401

402

CUMULATIVE INDEX OF TITLES

Ipecac alkaloids, 3, 363 (1953), 7, 419 (1960), 13, 189 (1971), 22, 1 (1983) @-Carbolinealkaloids, 22, 1 (1983) Isolation of alkaloids, 1, 1 (1950) Isoquinoline alkaloids, 7, 423 (1960) biosynthesis 4, 1 (1954) "C-NMR spectra, 18, 217 (1981) simple isoquinoline alkaloids, 4, 7 (1954), 21, 255 (1983) Isoquinolinequinones, from actinomycetes and sponges, 21, 55 (1983)

Kopsia alkaloids, 8, 336 (1965) Local anesthetics, alkaloids, 5, 211 (1955) Localization of alkaloids in the plant, 1, 15 (1950), 6, 1 (1960) Lupine alkaloids, 3, 119 (1953), 7, 253 (1960), 9, 175 (1967), 31, 116 ( 1 W ) Lycopodium alkaloids, 5, 265 (1955), 7, 505 (1960), 10, 306 (1967), 14, 347 (1973), 26, 241 (1985) Lythracae alkaloids, 18, 263 (1981) Mammalian alkaloids, 21, 329 (1983) Marine alkaloids, 24, 25 (1985) Maytansinoids, 23, 71 (1984) Melodinus alkaloids, 11, 205 (1968) Mesembrine alkaloids, 9, 467 (1967) Metabolic transformation of alkaloids, 27, 323 (1986) Microbial and in vitro enzymatic transformation of alkaloids. 18, 323 (1981) Mitragyna alkaloids, 8, 59 (1965), 10, 521 (1967), 14, 123 (1973) Monoterpene alkaloids, 16, 431 (1977) glycosides, 17, 545 (1979) Morphine alkaloids, 2, 1 (part 1, 1952), 2, 161 (part 2, 1952), 6, 219 (1960), 13, 1 (1971) Muscarine alkaloids, 23, 327 (1984) Mydriatic alkaloids, 5, 243 (1955) a-Naphthaphenanthridine alkaloids, 4, 253 (1954). 10, 485 (1967) Naphthyl isoquinoline alkaloids, 29, 141 (1986) Narcotics, 5, 1 (1955) W-NMR spectra of isoquinoline alkaloids, 18, 217 (1981) Nuphar alkaloids, 9, 441 (1967), 16, 181 (1977)

Ochrosia alkaloids, 8, 336 (1965), 11, 205 (1968) Ournuparia alkaloids, 8, 59 (1965), 10, 521 (1967) Oxaporphine alklaloids, 14, 225 (1973) Oxindole alkaloids, 14, 83 (1973) Papaveraceae alkaloids, 10, 467 (1967), 12, 333 (1970), 17, 385 (1979) pharmacology, 15, 207 (1975) toxicology, 15, 207 (1975) Pavine and isopavine alkaloids, 31, 317 (1987) &ntaceras alkaloids, 8, 250 (1965) Peptide alkaloids, 26, 299 (1985) Phenanthroindolizidine alkaloids, 19, 193 (1981) Phenanthroquinolizidine alkaloids, 19, 193 (1981)

CZPMULATIVE INDEX OF TITLES

403

0-Phenethylamines, 3, 313 (1953) Phenethylisoquinolirre alkaloid4 14. 265 (1973) Phthalideisoquinoline alkaloids, 4, 167 (1954), 7, 433 (1960), 9, 117 (1967), 24, 253 (1985) Picralimu alkaloids. 14, 157 (1973) Picrulima nifiilo a[kalotds, 8, 119 (1965), 10, 501 (1967) Piperidhe alkaloids, 26, 89 (1985) Plant systematics, 16, 1 (1977) Pleiocarpa alkaloids, llv 326 (1965), 11, 205 (1968) Polyamine alkaloids, putrescine, spermidine, spermine, 22, 85 (1983) Pressor alkaloids, 5, 229 (1955) Protoberberine alkaloids, 4, 77 (1954), 9, 41 (1967), 28, 95 (1986), 33, 141 (1988) Protopine alkaloids, 4, 147 (1954) Pseudocinchona alkaloids, 8, 694 (1965) Putrescine and related polyamine alkaloids, 22, 85 (1983) Pyridine alkaloids, 1, 165 (1950), 6, 123 (1960), 11, 459 (1968), 26, 89 (1985) Pyrrolidine aIkaloids, 1, 91 (1950), 6, 31 (1960), 27, 270 (1986) Pyrrolizidine alkaloids, 1, 107 (1950), 6, 35 (1960), 12, 246 (1970), 26, 327 (1985) Quinazolidine alkdoids, see Indolizidine' Alkaloids Quinazoline alkaloids, 3, 101 (1953), 7, 247 (1960), 29, 99 (1986) Quinazolinocarbolines, 8, 55 (1965), 21, 29 (1983) Quinoline alkaloids other than Cinchnu, 3, 65 (1953), 7, 229 (1960) related to anthranilic acid, 17, 105 (19791, 32, 341 (1988)

Rauwo&a alkaloids, 8, 287 (1965) Reissert synthesis of isoquinoline and indole alkaloids, 31, 1 (1987) Reserpine, chemistry, 8, 287 (1965) Respiratory stimulants, 5, 109 (1955) Rhoeadine alkaloids, 28, 1 (1986)

Salumandra group, steroids, 9, 427 (1967) Sceletium alkaloids, 19, 1 (1981) Senecio alkaloids, see Pyrrolizidine alkaloids Secoisoquinoline alkaloids, 33, 231 (1988)

Securinega alkaloids, 14, 425 (1973) Sinomenine, 2, 219 (1952)

Solanum alkaloids chemistry, 3, 247 (1953) steroids, 7, 343 (1960), 10, 1 (1967), 19, 81 (1981) Sources of alkaloids, 1, 1 (1950) Spectral methods, alkaloid structures, 24, 287 (1985) Spermidine and related polyamine alkaloids, 22, 85 (1983) Spermine and related polyamine alkaloids, 22, 85 (1983) Spirobenzylisoquinoline alkaloids, 13, 165 (1971) Sponges, isoquinolinequinones, 21, 55 (1983) Stemona alkaloids, 9, 545 (1967) Steroid alkaloids Apocynaceae, 9, 305 (1967), 32, 79 (1988) Bums group, 9, 305 (1967), 14, 1 (1973), 32, 79 (1988) Holarrhena group, 7, 319 (1960)

404

CUMULATIVE INDEX OF TITLES

Salamandra group, 9, 427 (1967) Sofanum group, 7, 343 (1960), 10, 1 (1967), 19, 81 (1981) Eratiurn group, 7, 363 (1960), 10, 193 (1967), 14, 1 (1973) Stimulants respiratory, 5, 109 (1955) uterine, 5, 163 (1955) Structural formula, elucidation by X-ray diffraction, 22, 51 (1983) Strychnos alkaloids, 1, 375 (part 1-1950), 2, 513 (part 2-1952), 6, 179 (1960), 8, 515, 592 (1965), 11, 189 (1968) Sulfur-containing alkaloids, 26, 53 (1985) Taxus alkaloids, 10, 597 (1967) Toxicology, Papaveraceae alkaloids, 15, 207 (1975) Transformation of alkaloids, enzymatic, microbial and in vifro, 18, 323 (1981) Tropane alkaloids, 1, 271 (1950), 6, 145 (1960), 9, 269 (1967), 13, 351 (1971), 16, 83 (1977), 33, 1 (1988) Tropoloisoquinoline alkaloids, 23, 301 (1984) Tropolonic Colchicum alkaloids, 23, 1 (1984) Tyrophora alkaloids, 9, 517 (1967)

Uterine stimulants, 5, 163 (1955)

Wratrum alkaloids chemistry, ‘3, 247 (1952) steroids, 7, 363 (1960), 10, 193 (1967), 14, 1 (1973) “Vinca” alkaloids, 8, 272 (1965), 11, 99 (1968) Wacanga alkaloids, 8, 203 (1965), 11, 79 (1968) X-Ray diffraction, elucidation of structural formula, configuration, and conformation, 22, 51 (1983) Yohimbe alkaloids, 8, 694 (1965) Yohimbine alkaloids, 11, 145 (1968), 27, 131 (1986), see also Coryantheine

INDEX

B

A 3-Acetylaconitine toxicity of, 128, 134 1-Acetylluciculine, 166 12-AcetylnapellineN-oxide, 166 Acetylsongoramine, 167 14-Acetyltalatizamine, 134 Aconifine, 135 Aconine, 104 toxicity of, 128 Aconitine, 95, 97 Aconitum alkaloids, 95 analytical methods, 132 pharmacology of, 126 toxicity of, 127 Aconosine, 103 Afrocurarine, 231 Ajmalicinial, 231 Akagerine, 232 biosynthesis of, 291 synthesis of, 316, 318 Akagerine lactone, 233 Alipinone, 188 Aljesaconitine, 135, 136 toxicity of, 128 2-Alkylthiotryptamines, 40 Allocryptopine, 187 Alstonine, 233 Amauromine, 5 5 , 61 Angustidine, 234 Angustine, 234 Angustoline, 235 Anhydrocannabisativine, 81 synthesis of, 83, 89 Anhydrodiacetyldelcosine, 104 Anisezochasmaconitine, 136 Antirhine, 235 Antirhine lactone, 236 Aricine, 338 Atisine, 96, 106 Atisinium chloride, 107 Atisinone, 106 Azonalenine, 54

Barterine, 236 Beiwutine, 137 Benzoylheteratisine, 138 1-Benzoylkarasmine, 138 14-Benzoylneoline, 138 2,2’-Bisindoles, 40 Bisnor-C alkaloid D, 237 Bisnor-C alkaloid H, 237 Brafouedine, 238 Brevianamide E, 25 synthesis of, 60 6-Bromotryptamine, 15 Bullatine C, 135

c C-Calebassine, 239 c-Curarine, 241 Calycanthidine, 41, 42 Camptoneurine, 239 Cannabis alkaloids, 77 pharmacology of, 91 Cannabisativine, 81 synthesis of, 90 Cantleyine, 285 Caracurine V, 240 Chaetocin, 46 Chasmanine, 98 total synthesis, 109 Chetomin, 48 Chetracin A, 47 Chimonanthine, 41, 42 synthesis of, 43 Cinchona alkaloids, biological activities of, 377 biosynthesis of, 382 chromatography of, 371 ‘V-NMR spectra, 361 isolation of, 333, 335 metabolism of, 378 numbering of, 333 405

406

INDEX

spectroscopy of, 358 synthesis of, 344 X-ray analysis of, 369 Cinchonamine, 342 synthesis of, 347 Cinchonidine, 334 2-Cinchonidinone, 378 Cinchonine, 334 Cinchoninone, 339 2-Cinchoninone, 378 Cinchophylline, 339, 340 Columbianine, 139 Columbidine, 139 Commaconine, 103 Condensamine, 240 Condylocarpine, 241 Corycavamine, 187 Corycavidine, 187 Corycavine, 187, 203 Corydinine, 187 Coulteropine, 187 Crassicaulidine, 140 Crassicauline A, 139 Crassicauline B, 167 Cryptocavine, 187 Cryptopalmatine, 187 Cryptopin, 182 synthesis of, 194 Cupreidine, 334 2-Cupreidinone, 378 Cupreine, 334 Cycloalanyltroptophan, 18

D 8-Deacetylyunaconitine, 142 Debromodihydroflustramine C, synthesis of, 61 Debromoflustramine, 57 Dehydrolucidusculine, 168 Dehydropreakuammicine, formation of, 294 1-Dehydrosongorine, 168 14-Dehydrotalatisamine, 142 Delavaconitine, 142 Delcosine, 104 Delphinifoline, 143 Delphisine, 103 Demethoxycarbonyl-3,14-dihydrogambirtannine, 243 Denudatine, 129

18-Deoxy Wieland-Gumlich aldehyde, 243 Deoxybrevianamide E, 26, 59 Deoxydelsoline, 143 Deoxyjesaconitine, 143 Desbenzoylpyroaconitine, 104 Descussine, 242 13-Desocydelphonine, 116 Dethiotetramethylthiochetomin,49 Diaboline, 244 Dihydro-cycb-akagerine, 246 Dihydropalustrine, 83 synthesis of, 87 Dihydroatisine, 106 Dihydroatisinone, 106 Dihydrocinchonamine, 342 synthesis of, 347 Dihydrocinchonine, 334 Dihydrocorynantheol, 246 Dihydrocupreidine, 334 Dihydrocupreine, 334 Dihydrodecussine, synthesis of, 317 Dihydroflustramine, 52 Dihydroflustramine C N-oxide, 53 18,18 '-Dihydromatopensine, 261 Dihydromonticamine, 144 Dihydroprotopine, 188 Dihydroquinamine, 341 Dihydroquinidine, 334 Dihydroquinidine N-oxide, 366 Dihydroquinine, 334 Dihydroquinine N-oxide, 366 Dihydrotoxiferine, 247 10,ll-Dihydroxydihydroquinidine,366 10,ll-Dihydroxydihydroquinine,366 Dimethylallylindole, 61 Dimethylallyltryptophan, 65 Dinklageine, 286 Diploceline, 248 Ditryptophenaline, 48 Dolichantoside, 248 Dolichocurine, 249 Dolichothyrine, 249 Duclouxine, 144

E Echinuline, 49 Ellipticine, 250 Ellipticine, biosynthesis of, 297 3-Epicinchonamine, 342

407

INDEX

Epicinchonidine, 334 Epicinchonine, 334 Epidihydrocinchonidine, 334 Epidihydrocinchonine, 334 Epidihydroquinamine, 341 Epidihydroquinidine, 334 Epidihydroquinine, 334 Epidithiodiketopiperazines, 22 Epingouniensine, 262

2-Epi-N-Methyldihydroquinicinol,339 3-Epiquinamine, 341 Epiquinidine, 334 Epiquinine, 334 Episopalidjne, 169 Episcopalisine, 144 Episcopalisinine, 145 Episcopalitine, 145 Epivinylepiquinidine, 334 Epivinylepiquinine, 334 Epivinylquinidine, 334 Epivinylquinine, 334 Epoxuvomicine, 284 Ezochasmaconitine, 146 Ezochasmanine, 146

F Fagarines, 187 Finaconitine, 146 Finetianine, 168 Flavaconitine, 147 Flavopeirerine, 250 Fluorocurarine, 251 Fluorocurarine, synthesis of, ??? Flustrabromine, 52 Flustramide A, 52 Flustramine D N-oxide, 53 Flustramines, 50 Flustraminols, 51, 52 Folicanthine, 41, 42 synthesis of, 43 Foresaconitine, 147 Foresticine, 147 Forestine, 148 Formylkynurenine, from tryptophan, 35 Franchetine, 148 Fumarine, 187 Fumitremorgins, 55 Fuziline, 151

G Geissoschizine, biosynthesis of, 293 Geissoschizoline, synthesis of, 306 Geniconitine. 149 Gentianine, 286 Gigaconitine, 149 Glucoalkaloid, 252 Guan-fu base A, G. 169 Guayewuanine B, 149 Guettardine, 342 Gymnaconitine, 150

H Hanamisine, 170 Harmane, 252 Henningsamine, 253 Henningsoline, 253 Hetisine, 108 Hexadecamide, 79 Hexahydroquinine, 355 Higenamine, pharmacological actions, 130 Hodgkinsine, 44 Hokbusine A, 150 Hokbusine B, 151 Holstiine, 254 Holstiline, 254 Homochelidonine, 187 Homomeroquinene, 345 Hordenine, 80 Hunnemanine, 187 3-Hydro~y-2~quinidinone, 378 17-Hydroxyalloibogamine,254 Hydroxydihydrocinchonamine, 341 9-Hydroxynominine, 170 Hydroxyperoxyindolenines,27, 33 13-Hydroxyprotopine, 187 3-Hydroxypyrroloindoles,reactions of, 38 3-Hydroxyquinidine, 366 3-Hydroxyquinine, 366 12-Hydroxystrychnine, 269 5-Hydroxytryptophan from tryptamine, 12 10-Hydroxyusambarine, 282

I Ibukinamine, 151 Icajine, 255

408

INDEX

Ignavine, 170 Indole alkaloids, from Cinchona alkaloids, 354 Indole-Indolenine tautomerism, 1 Indoline, oxidation with fremy salt, 11 Isoaconitine, 152 Isoatkine, 107 Isobrafouedine, 238 Isocinchophylline, 339 3-Isocorynantheol, 342 Isodelphinine, 98 Isodelphonine, 99 Isoflustramine D, 53 Isomalindine, 260 Isopsychotridine, 45 Isoretulinal, 264 Isoretuline, 266 Isositsirikine, 256 Isosplendine, 256 Isostrychnobiline, 270 Isostrychnofoline, 272 Isostrychnopentamine, 273 Isostrychnophylline, 274 Isostrychnosplendine, 275 Izmirine, 182, 187

J Janussine, 257 Jesaconitione, toxicity of, 128 Jynosine, 171

K Karasamine, 152 Kobusine, 109 synthesis of, 125 Koshlands reagent, reactions of, 65 Kribine. 257

L Lappaconitine, 129 Lipoaconitine, 153 toxicity of, 128 Lipodeoxyaconitine, 153 Lipohypaconitine, 154 Lipomesaconitine, 154 Liwaconitine, 155

LL 490 p, 54 Longicaudatine, 258 formation from monomers, 300 Longicaudatine Y, F, 2,259 Luciculine, 129 Lycoctonamic acid, 104 Lycoctonine, 95, 104

M Macleyine, 187 Macusine, 259 Malindine, 260 Matopensine, 261 Matopensine N-oxide, 261 Mavacurine, 262 Measaconitine, 97 Melatonin, photooxidation of, 32 Melinacidin, 47 Melinonine F, 262 Meroquinene, 345, 350 1-Methoxyallocryptopine, 187 10-Methoxycinchonamine, 342 1-Methoxycryptopine, 187 11-Metholxyretdine, 265 10-Methoxytsilamine, 278 8-0-Methylaconine, 104 Methylgymanaconitine, 155 N4-Methylstrychnine, 269 Monticamine, 156 Monticoline, 156 Muramine, 187

N N-Deacetlranaconitine, 141 N-Deacetylfinaconitine, 140 N-Deacetylscaconitine, 141

N-Deethyldehydrolucidusculine,167 N-Methoxycarbonyltr yptamine, photooxidation of, 29 N-Methyldihydroquinicine,339 N-Methyldihydroquinicinol,339 N-Methyltryptamine, photooxidation of, 28

N Nagarine, 103, 156 Napelline, synthesis of, 122

409

INDEX Nevadenine, 157 Nevadensine, 157 Ngouniensine, 262 biosynthesis of, 298 synthesis of, 318 Nigritanine, 282 Nominine, 171 Nonalkaloids, in Cinchona, 343 Norisornalindine, 261 Normacusine B, 260 Normalindine, 261 Novacine, 263

Pseudokobusine, 109 Pseudoprotopine, 187 synthesis of, 195 Pseudostrychnine, 263 Psychotridine, 44 Puberaconidine, 159 Puberaconitine, 137 Puberanidine, 141 Puberanine, 160 Pyrochasmanine, 160 Pyrodelphine, 103 Pyroneoline, 103

0 0-Demethyltsilamine, 278

8-O-Ethylbenzoylmesaconitine,145 8-0-Methyltalatizamine, 155 Ochrobirine, 187 Oreonone, 188 Oreophiline, 187, 189 Oxaline, 54 13-Oxoallocryptopine, 188 13-Oxocryptopine, 188 2-Oxohexahydroquinine, 355 13-Oxomuramine, 188 Oxonitine, 97 13-Oxoprotopine, 188

Q Quadrigemines, 44 Quinamine, 341 Quinicine, 353 Quinidine, 334 Quinidine N-oxide, 366 Quinidinone, 339 2-Quinidinone, 378 Quinine, 334 conversion into quinidine, 352 Quinine N-oxide, 366 2-Quininone. 378 Quinoline alkaloids, in Cinchona species, 338

P Palustridine, 81 Palustrine, 81 Penduline, 101, 157 Polyschistine A, B, 158 Polyschistine C, 159 Prenylated indoles inverted analogs, 58 synthesis of, 55 Protopine alkaloids, 181 biosynthesis of, 201 conformation of, 190 occurrence of, 183 pharmacology of, 203 transformations of, 198 Protopine methosulfate, 188 Protopine N-oxide, 188, 190 Protopine, 187 synthesis of, 196 Protothalipine, 187, 188

R Ranaconitine, 161 Retulinal, 264 Retuline, 265 Retuline N-oxide, 265 Rhazidine, 26 from quebrachamine, 27 Rindline, 267 Roquefortine, 54 Rosibiline, 267 Ryosenamine, 171 Ryosenaminol, 172

S Sadosine, 172 Sanyonamine, 173 Scaconine, 161 Scaconitine, 162

410

INDEX

Senbusine A, 162 Senbusine E, 163 Sepentriodine, 137 Septentrionine, 163 Serotonine, synthesis of, 12 Serpentine, 268 Spermidhe alkaloids, in Cannubis, 80 Spermostrychnine, 268 Splendoline, 268 Sporidesmins, 19, 20 absolute configuration, 21 interconversions, 21 synthesis of 22 Strellidimine, 288 Strychnine, 269 biosynthesis of, 296 Strychnobaridine, 269 Strychnobiline, 270 formation from monomers, 300 Strychnobrasiline, 271 Strychnocarpine, 271 Strychnofendlerine, 271 Strychnofluorine, 272 Strychnofoline, 272 Strychnofuranine, 273 Strychnopentamine, 273 Strychnopentamines, formation of, 299 Strychnophylline, 274 Strychnopivotine, 274 Strychnos alkaloids alkaloid content of Strychnos species, 218 biosynthesis of, 288 ethnobotany of, 215 pharmacology of, 319 spectroscopy of, 301 structures of, 231 synthesis of, 305 tabulation of alkaloids, 219 Strychnosplendine, 275 Strychnovoline, 287 Strychnoxanthine, 276 Strychnozairine, 276 Sungucine, 277 Sweroside, 386

Talatisine, 173 Tanwushe, 173 Tchibangensine, 280 Tetrahydroakagerine, 277 Tetrahydroalstonial, 278 Thalictncine, 187, 189 Thalisopyrine, 187 'Ifyptamine, cyclic tautomers, polymeric forms, 41 Tryptathionine, 39 Tryptophan, cyclic tautomers, 4 Tryptophans, photooxidation of, 27, 34 Tsilamine, 278 'hbotaiwinal, 279 'hbotaiwine, 279 synthesis of, 306, 315

U Usambarensine, 280 Usambarine, 282

V Vaillantine, 187 Vallesciachotamine, 284

Vallesiachotamine-antirhine types, biosynthesis of, 291 Veatchine, 96 Venoterpine, 287 Verticillines, 47 Vilmorrianine A, 165 Vilmorrianine D, 165 Vomicine, 284

W Wieland-Gumlich aldehyde, 285 transformation of, 295

T Takaonine, 164 Takaosamine, 164

Y Yunaconitine, 165