THE ALKALOIDS Chemistry and Biology 65 VOLUME Edited by Geoffrey A. Cordell Evanston, Illinois Amsterdam(cid:2)Boston(cid:2)Heidelberg(cid:2)London(cid:2)NewYork(cid:2)Oxford Paris(cid:2)SanDiego(cid:2)SanFrancisco(cid:2)Singapore(cid:2)Sydney(cid:2)Tokyo ACADEMIC AcademicPressisanimprintofElsevier PRESS Academic Press isanimprint ofElsevier 84Theobald’sRoad,London WC1X 8RR,UK Radarweg 29,PO Box 211,1000AEAmsterdam,The Netherlands Linacre House,JordanHill,OxfordOX2 8DP,UK 30CorporateDrive, Suite400, Burlington, MA01803, USA 525BStreet, Suite1900, SanDiego, CA92101-4495, USA Firstedition 2008 Copyrightr2008Elsevier Inc.Allrights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher PermissionsmaybesoughtdirectlyfromElsevier’sScience&TechnologyRights DepartmentinOxford,UK:phone(+44)(0)1865843830;fax(+44)(0)1865853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://www.elsevier.com/locate/ permissions, andselectingObtaining permission to useElsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made ISBN:978-0-12-374296-4 ISSN:1099-4831 ForinformationonallAcademicPresspublications visitourwebsiteatbooks.elsevier.com Printed andbound inUSA 0809101112 109 8 76 5 4 32 1 CONTRIBUTORS Hans-Joachim Kno¨lker, Department Chemie, Technische Universita¨t Dresden, Dresden, Germany Kethiri R. Reddy, Department Chemie, Technische Universita¨t Dresden, Dresden, Germany vii PREFACE This is the latest volume in the series ‘‘The Alkaloids: Chemistry and Biology’’ and covers a group of alkaloids comprising the carbazole nucleus. Single-topic volumes inthisserieshavebeenrare,andthelastonediscussedantitumoralkaloidsandwas publishedasVolume25in1985.Thisisthefirstvolumededicatedtoasinglealkaloid structure type since Volume 8, which dealt with the monoterpene indole alkaloids over 40 years ago. Thisbackgroundplacesincontexthowsignificantthisremarkablyextensiveand exceptionally well-organized single volume review of the ‘‘Chemistry and Biology of CarbazoleAlkaloids’’by Kno¨lker and Reddy actuallyis.Itbrings together for thefirst time all of the alkaloids that have been isolated from plant, microbial, and marine sources, which possess, in any aspect of their structure, a carbazole nucleus. The alkaloids are reviewed from the detailed aspects of their structure elucidation, and for the first time an approach is offered with respect to an overall view of their biogenetic pathways, which differ substantially based on the further structural elementsofwhichthecarbazoleisacorefeature.Thesediscussionsarefollowedbya detailed exposition of how the significance of these alkaloids has been enhanced through the determination of several important, clinically relevant, biological properties. Finally, the authors offer a comprehensive presentation of the many syntheticapproachestothevariouscarbazolealkaloids,fromthesimplealkaloidsto the more complex alkaloids, including the bis-alkaloids. Geoffrey A. Cordell Evanston, Illinois, USA ix 1 CHAPTER Introduction The initial discovery of carbazole in the anthracene fraction of coal tar followed by the first isolation of the antimicrobial murrayanine (3-formyl-1-methoxycarbazole) from the plant Murraya koenigii Spreng., started the enormous development of carbazole chemistry. Since then, there has been a strong interest in this area by chemists and biologists due to the intriguing structural features and promising biological activities associated with many carbazole alkaloids. The progress in the chemistryofcarbazolealkaloidsisemphasizedbythepublicationofseveralreviews and book chapters covering the field (1–10). In this series, three earlier reviews on carbazole alkaloids were published by Kapil(1),Husson(2),andChakraborty(3)intheVolumes13,26,and44,respectively. The present chapter introduces a new classification of carbazole alkaloids and summarizes the recent synthetic efforts. The nomenclature of carbazole alkaloids used in this review is that of Chemical Abstracts. As shown in Scheme 1.1, the conventional tricyclic ring system of carbazole 1 is denoted by A, B, and C, and the numberingstartsfromringA.Thetermcarbazolegenerallyreferstoa9H-carbazole. Several hypotheseshavebeen proposed for thebiogenesisof carbazolealkaloids (seeChapter3ofthisreview).However,thereisnodeepexperimentalknowledgeof the biosynthesis of this class of alkaloids. Acomparison of the structural features of carbazole alkaloids isolated from higher plants suggests that 3-methylcarbazole (2) may represent the key intermediate in their biosynthesis (Scheme 1.1). In the present review, we summarize the occurrence, biogenesis, biological activity, and the chemistry of carbazole alkaloids, which have been classified based on their natural sources, ring system, and substitution pattern. While a comprehen- sive overview is given on all carbazole alkaloids isolated from natural sources,only their total syntheses published since 1990 are discussed. 5 4 Me 6 4b4a 3 C A 7 B 2 8 8aN99a 1 N H H 1 2 3-Methylcarbazole Carbazole Numbering Scheme 1.1 1 2 CHAPTER Occurrence, Isolation, and Structure Elucidation A large number of carbazole alkaloids has been isolated from higher plants of the genera Murraya, Glycosmis, and Clausena, all belonging to the family Rutaceae (subtribe Clauseninae, tribe Clauseneae, subfamily Aurantioideae). The occurrence of carbazole alkaloids in these three genera of the Rutaceae is of chemotaxonomic importance and justifies their classification as an independent subtribe. Carbazoles have also been reported from other genera, such as Micromelum of the family Rutaceae (subtribe Micromelinae), Ekebergia of the Meliaceae, and Cimicifuga of the Ranunculaceae. The genus Murraya, especially the species Murraya euchrestifolia Hayata, represents the richest source of carbazole alkaloids among all terrestrial plants. Various monomeric and also bis-carbazole alkaloids, formed by the combination of two of the monomeric units, have been reported from the genus Murraya. It is noteworthy that, while Murraya koenigii grown in Indian soil did not affordanybis-carbazolealkaloids,thesamespeciesgrowninagreenhouseinJapan fromseedscollected inTaiwangeneratedbis-carbazolealkaloids.Dependingonthe seasonal and geographical variation, the genus Murraya is known to provide different alkaloids. Moreover, carbazole alkaloids have been isolated from several different Streptomyces species. The indolocarbazole alkaloids have been obtained frommicroorganisms, slime molds, and marine sources. Further natural sources for carbazole alkaloids are for example the blue-green alga Hyella caespitosa, species of the genera Aspergillus, Actinomadura, Didemnum, and Iotrochota, and the human pathogenic yeast Malassezia furfur. The natural sources of carbazole alkaloids are listed in Table 2.1. Carbazole and variousalkylcarbazoleshavealsobeenobtainedfromothersources,suchascoaltar, petroleum oil, soil humus, the polluted atmosphere of industrial areas, as well as cigarette smoke. I. CARBAZOLE ALKALOIDS FROM HIGHER PLANTS Theisolationofcarbazole(1),3-methylcarbazole(2),andseveraloxidizedderivatives of 3-methylcarbazole from taxonomically related higher plants of the genera Glycosmis, Clausena, and Murraya (family Rutaceae) indicates that the aromatic methyl group of the biogenetic key intermediate 3-methylcarbazole can be eliminated oxidatively (5,6). Most of the carbazole alkaloids isolated from the 3 4 ChemistryandBiologyofCarbazoleAlkaloids Table2.1 Biological sources ofcarbazole alkaloids A. Higher plants Murraya euchrestifolia Hayata M. koenigii Spreng. M. siamensis Craib. Clausena anisata Hook. C. heptaphylla Wight & Arn. C. excavata Burm f. C. indica Oliver C. lansium (Lour.) Skeels C.harmandianaPierreex.Guillaumin[syn. C. wampi (Blanco) Oliv.] G. arborea (Roxb.) DC. G. parviflora (Sims) Little G. mauritiana Tanaka G. montana Pierre Glycosmis pentaphylla (Retz.) DC. (syn. G. arborea (Roxb.) DC. Micromelum ceylanicum Wight M. hirsutum Oliver Ekebergia senegalensis Fuss. Cimicifuga simplex Wormsk. ex DC. B. Other sources Streptoverticillium ehimense (i) Microbial sources S. mobaraense (schizomycophyta) Streptomyces chromofuscus DC 118 S. exfoliates 2419-SVT2 Streptomyces sp. S. staurosporeus Anaya Actinomadura madurae A. melliaura Arcyria denudata Aspergillus flavus A. tubingensis Nocardia aerocoligenes Nocardiopsis dassonvillei Nocardiopsis sp. (ii) Marine sources (cyanophyta) Hyella caespitosa Bron et Flah Tedania ignis Nostoc sphaericum Tolypothrix tjipanasensis New Zealand Ascidian Australian Ascidian Didemnum sp. Marine Sponge Iotrochota sp. Marine Sponge Dictyodendrilla verongiformis (iii) Diverse sources Bovine urine Malasseziafurfur(humanpathogenicyeast) Occurrence,Isolation,andStructureElucidation 5 Rutaceae family (11) have a one-carbon-substituent at position 3 of the carbazole nucleusandanoxygensubstituentatposition1or2.Thestructureofthesealkaloids can vary from simple substituted carbazoles to molecules containing complex terpene moieties. A. Tricyclic Carbazole Alkaloids 1 Carbazole Carbazole (1) was isolated first from the anthracene fraction of coal tar in 1872 by GraebeandGlaser(12).In1987,almostacenturylater,Bhattacharyyaetal.reported for the first time the isolation of carbazole (1) from a plant source, Glycomis pentaphylla (13) (Scheme 2.1). Structural assignment of carbazole (1) was based on UV, IR, and mass spectral data, as well as direct comparison with an authentic sample. 2 3-Methylcarbazole and Non-Oxygenated Congeners Chakraborty (14), Joshi (15), and Connolly et al. (16) reported independently the isolationof3-methylcarbazole(2)fromtherootsofdifferentClausenaspecies,suchas Clausenaheptaphylla,Clausenaindica,andClausenaanisata.In1987,Bhattacharyyaetal. isolated 3-methylcarbazole (2), along with carbazole (1), from the root bark of Glycosmis pentaphylla (13). A decade later, 3-methylcarbazole (2) was obtained by Chakrabartyetal.fromthechloroformextractoftherootsofM.koenigii(17).In1998, Wuetal.isolated3-methylcarbazole(2)duringastudyoftheantiplateletaggregation activity of the acetone extract of the root bark of M. euchrestifolia (18). In the IR spectrum (n 3450, 1600, 1493, 1390, 885, 808 cm(cid:2)1), the band at 808 cm(cid:2)1, which max is characteristic of 3-methylcarbazole, confirms the position of the methyl group (19). The UV spectrum (l 230, 236, 243, 260, 296, 330nm) was max superimposible with that of 3-methylcarbazole and confirmed the structural assignment (20,21). In 1988, Furukawa et al. reported the isolation of 3-formylcarbazole (3) from the rootbarkofM.euchrestifolia(22).Threeyearslater,McChesneyandEl-Feralyisolated 3-formylcarbazole (3), along with methyl carbazole-3-carboxylate (4), from the roots of Clausena lansium (23). The roots of this ornamental tree are used in traditional medicineinTaiwantotreatbronchitisandmalaria(23).In1992,Bhattacharyyaetal. described the isolation and structural elucidation of 3-formylcarbazole (3) from G. pentaphylla (24). 3-Formylcarbazole has UV spectral characteristics as reported previously in the literature (25–27). The IR spectrum showed the band of an NH group (3460 cm(cid:2)1) and of a conjugated carbonyl function (1685 cm(cid:2)1). The 1H-NMR and 13C-NMR N H 1 Carbazole Scheme 2.1 6 ChemistryandBiologyofCarbazoleAlkaloids Me CHO COOMe N N N H H H 2 3-Methylcarbazole 3 3-Formylcarbazole 4 Methyl carbazole-3-carboxylate Scheme 2.2 Me Me N N COOC H CHO 2 5 5 9-Carbethoxy-3-methylcarbazole 6 9-Formyl-3-methylcarbazole Scheme 2.3 spectrumconfirmedthepresenceofanaldehydegroupatposition3.Inthearomatic region,the1H-NMRspectrumofmethylcarbazole-3-carboxylate(4)isverysimilarto thatof3-formylcarbazole(3).Themethylesterwasindicatedbyasingletatd3.91for themethoxygroupand,inthe13C-NMRspectrum,byasignalatd168.0fortheester carbonyl group. In addition to the spectroscopic proof, chemical support was derived from transformation into the corresponding acid and remethylation with diazomethane to afford methyl carbazole-3-carboxylate (4) (Scheme 2.2). In 1997, Chakrabarty et al. reported the isolation of 9-carbethoxy-3-methylcarba- zole (5) and 9-formyl-3-methylcarbazole (6) from the roots of M. koenigii (17). These metabolites are the first 9-formyl and 9-carbethoxy carbazole derivatives obtained from plant sources. 9-Formyl-3-methylcarbazole (6) showed weak cytotoxicity against both mouse melanoma B16 and adriamycin-resistant P388 mouse leukemia celllines.ThestructuralassignmentofthesetwoalkaloidswasbasedontheIR-and 1H-NMR spectra which were lacking any signal of an NH group. Additional structural support for 9-carbethoxy-3-methylcarbazole (5) was provided by the similarityoftheUVabsorptionspectrumwiththatofasyntheticsample,obtainedby reaction of 3-methylcarbazole with ethyl chloroformate in the presence of base. Further structural support for 9-formyl-3-methylcarbazole (6) was derived from a comparison of the UV spectrum and the IR carbonyl absorption (1696 cm(cid:2)1) with those of an authentic sample of 9-formyl-3-methylcarbazole (1700 cm(cid:2)1), prepared by the treatment of 3-methylcarbazole (2) with 98% formic acid (17) (Scheme 2.3). 3 1-Oxygenated Tricyclic Carbazole Alkaloids ThehigherplantsofthegenusMurraya(familyRutaceae),whicharetreesgrowingin SouthernAsia, arethemajorsourceof 1-oxygenated carbazole alkaloids.Extracts of theleaves andbarkof thistreehavebeen usedintraditionalmedicine for analgesia and local anesthesia, and for the treatment of eczema, rheumatism, and dropsy. Murrayafoline A (7) was isolated from the ethanolic extract of the root bark of M. euchrestifolia collected in Taiwan (28,29). Recently, Cuong et al. isolated the same alkaloid from Glycosmis stenocarpa Guillaumin collected in Northern Vietnam (30). Occurrence,Isolation,andStructureElucidation 7 R R2 Me R3 R1 N N H OMe R4 H OMe 7 Murrayafoline A 7 Murrayafoline A R = Me R1,R2,R3,R4 = H 8 Koenoline 12 Murrayastine R = CH2OH R1,R2 = H; R3,R4 = OMe 9 Murrayanine 13 Clausenapin R = CHO R1 = prenyl; R2,R3,R4 = H 10 Mukoeic acid 14 Clausenine R = COOH R1,R3,R4 = H; R2 = OMe 11 Mukonine R = COOMe Scheme 2.4 The UV spectrum [l 225, 243, 251 (sh), 292, 330, and 334 (sh)nm] of max murrayafoline-A (7) was similar to that of other 1-oxygenated carbazoles. The 1H- NMRspectrumof(7),withathree-protonsignalatd6.9(cid:2)7.3andasignalatd7.87for H-5, indicated an unsubstituted C-ring. The signals for a methoxy substituent at d3.76andanaromaticmethylgroupatd2.40,alongwiththesignalsforH-4andH-2 at d 7.33 and 6.55, respectively, suggested a C-1 methoxy and a C-3 methyl group. ThisregiochemistrywasconfirmedbyNOEexperiments.Thus,murrayafoline-A(7) was assigned as 1-methoxy-3-methylcarbazole, previously described as an inter- mediate in the synthesis of murrayanine (9) (Scheme 2.4) (31). The cytotoxic carbazolealkaloid koenoline (8) wasisolated from the rootbark of M. koenigii (32). Koenoline exhibited the characteristic UV spectrum (l 241, 251, max 289, and 323nm) of a 1-methoxycarbazole derivative. In addition, the IR spectrum indicated an NH (3445 cm(cid:2)1) and an OH group (3235 cm(cid:2)1), as well as an aromatic system (1585 and 1500 cm(cid:2)1). The 1H-NMR spectrum showed the signals of an aromatic methoxy (d 4.01), a benzylic methylene group (d 4.5), H-2 and H-4 as two broad singlets at d 6.95 and 7.66, and an unsubstituted C-ring. Based on these spectroscopic data, koenoline was structurally assigned as 8. Further confirmation wasderivedfromthe13C-NMRdataandapartialsynthesisbysodiumborohydride reduction of murrayanine (9) (31). Chakraborty et al. reported the isolation of murrayanine (9) independently from two different genera of the family Rutaceae, M. koenigii (33) and C. heptaphylla (34). More recently, Cuong et al. isolated murrayanine (9) from G. stenocarpa Guillaumin from Northern Vietnam (30). Murrayanine (9) showed antimicrobial properties againsthumanpathogenicfungi(35).TheUVspectrumofmurrayanine(9)(l 238, max 244,273,288,and327nm)wasstrikinglysimilartothatof3-formylcarbazole(3).The IR spectrum showed the presence of an NH group (3450 cm(cid:2)1), an aromatic aldehyde (2800, 1681 cm(cid:2)1), and an aromatic ring (1631, 1613, 1585 cm(cid:2)1). In addition, peaks at 850 and 725 cm(cid:2)1 suggested tetra- and di-substituted aromatic rings. The 1H-NMR spectrum showed an aldehyde proton (d 9.98), an aromatic