O O MeO OMe O O O OMe OAc OAc O O OH OH HO OH 1 2 3 Pigments of fungi (Macromycetes) Melvyn Gill School of Chemistry, The University of Melbourne, Parkville, Victoria 3052, Australia Received (in Cambridge) 21st October 1998 Covering: March 1996 to August 1998 Previous review: 1996, 11, 513 1 Introduction 2 Pigments from the shikimate–chorismate pathway 2.1 Compounds derived from arylpyruvic acids 2.1.1 Terphenylquinones 2.1.2 Pulvinic acids and related butenolides 2.2 Compounds derived from phenylalanine and tyrosine 2.3 Compounds derived from cinnamic acids 2.4 Compounds derived from 4-hydroxybenzoic acid 3 Pigments from the acetate–malonate pathway 3.1 Pentaketides 3.2 Hexaketides 3.3 Octaketides 3.3.1 Anthraquinones and anthraquinone carboxylic acids 3.3.2 Pre-anthraquinones 3.3.3 Coupled pre-anthraquinones 3.3.4 Pyranonaphthoquinones 3.4 Nonaketides 3.5 Higher polyketides and compounds of fatty acid origin 3.5.1 Non-isoprenoid polyene pigments 3.5.2 Quinones with extended unbranched side chains 4 Pigments from the mevalonate pathway 5 Nitrogen heterocycles 5.1 Phenoxazin-3-ones 5.2 Indole pigments 5.2.1 Simple indoles 5.2.2 Bisindolylmaleimides 5.3 Miscellaneous N-heterocyclic pigments 6 Other N-containing compounds 7 Acknowledgements 8 References 1 Introduction This review, like its predecessors, 1–3 surveys the chemical, biological and mycological literature dealing with the isolation, characterisation and chemistry of colouring matters manu- factured by those fungi that produce conspicuous fruit bodies (Macromycetes). Also included, as before, are pigments from slime moulds (Myxomycetes) and, in certain circumstances, pigments produced by macromycetes grown in mycelial culture and some colourless metabolites where these are of current significance. The Report covers work that has appeared in the literature between March 1996 and the present time (August 1998). As usual, compounds are classified according to their perceived biosynthesis. Although experimental evidence in support of this classification is in many cases lacking there has been significant progress in this area. 2 Pigments from the shikimate–chorismate pathway 2.1 Compounds derived from arylpyruvic acids 2.1.1 Terphenylquinones. Betulinans A 1 and B 2 are two simple derivatives of polyporic acid that have been isolated from the fruiting bodies of Lenzites betulina (Polyporaceae). 4 The betulinans 1 and 2 were not known as natural products previously and they are potent inhibitors (IC 50 0.46 and 2.88 mg ml 21 , respectively) of lipid peroxidation in rat liver microsomes. In this regard, betulinan A is four times as active as vitamin E (1.68 mg ml 21 ). Another terphenylquinone with inhibitory properties is polyozellin 3, which is produced by the Korean toadstool Polyozellus multiplex. 5 Polyozellin 3 is the leuco diacetate of the well known fungal metabolite thelephoric acid 1–3 and is an inhibitor of prolyl endoperoxidase, a serine protease. 2.1.2 Pulvinic acids and related butenolides. The orange–red caps of Tricholoma aurantium contain the unique red pigment aurantricholone 4. 6 Aurantricholone 4 is rather unstable but its structure could be established unequivocally after careful chromatography by a series of NMR experiments. Pigment 4 is closely related to the naphthalenoid pulvinic acids such as badione A 5 and norbadione A 6, which are responsible for the intense brown and golden yellow colours, respectively, of the cap skins of Suillus badius and the gleba of the taxonomically related gasteromycete Pisolithus tinctorius. 1 Aurantricholone 4 is the first naphthalenoid pulvinic acid to be found so far that contains side chains of the pulvinone type, and perhaps more significantly, incorporates a benzotropolone nucleus. Like the pigments 5 and 6, aurantricholone 4 occurs naturally as a metal chelate; in this case the predominant counter ion is calcium. Two cometabolites of aurantricholone 4 in Tricholoma aurantium are the intensely fluorescent auran- tricholides A 7 and B 8, which are biogenetically closely related to aurantricholone. The structures of the aurantricholides have been confirmed by total synthesis. 6 Interestingly, aurantricho- lide B 8 was found some years ago along with 3,4A,4- trihydroxypulvinone in Suillus grevillei. 1,7 The role of norba- dione A 6, as its potassium salt, in the mycorrhizal relationship between P. tinctorius and higher plants has been discussed previously, 1 but recently it has been suggested that triterpenoid metabolites found in the mycelium and in the ectomycorrhizae are involved in the mycorrhizal exchange. 8 The fruit bodies of the hypogeous basidiomycete Melanoga- ster broomeianus contain the well known cyclopentanoids Nat. Prod. Rep., 1999, 16, 301–317 301 Published on 01 January 1999. Downloaded on 25/12/2013 23:54:39. View Article Online / Journal Homepage / Table of Contents for this issue
Transcript
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Pigments of fungi (Macromycetes)
Melvyn Gill
School of Chemistry, The University of Melbourne, Parkville, Victoria 3052, Australia
Received (in Cambridge) 21st October 1998Covering: March 1996 to August 1998Previous review: 1996, 11, 513
1 Introduction2 Pigments from the shikimate–chorismate pathway2.1 Compounds derived from arylpyruvic acids2.1.1 Terphenylquinones2.1.2 Pulvinic acids and related butenolides2.2 Compounds derived from phenylalanine and
tyrosine2.3 Compounds derived from cinnamic acids2.4 Compounds derived from 4-hydroxybenzoic acid3 Pigments from the acetate–malonate pathway3.1 Pentaketides3.2 Hexaketides3.3 Octaketides3.3.1 Anthraquinones and anthraquinone carboxylic
acids3.3.2 Pre-anthraquinones3.3.3 Coupled pre-anthraquinones3.3.4 Pyranonaphthoquinones3.4 Nonaketides3.5 Higher polyketides and compounds of fatty acid
origin3.5.1 Non-isoprenoid polyene pigments3.5.2 Quinones with extended unbranched side chains4 Pigments from the mevalonate pathway5 Nitrogen heterocycles5.1 Phenoxazin-3-ones5.2 Indole pigments5.2.1 Simple indoles5.2.2 Bisindolylmaleimides5.3 Miscellaneous N-heterocyclic pigments6 Other N-containing compounds7 Acknowledgements8 References
1 Introduction
This review, like its predecessors,1–3 surveys the chemical,biological and mycological literature dealing with the isolation,characterisation and chemistry of colouring matters manu-factured by those fungi that produce conspicuous fruit bodies(Macromycetes). Also included, as before, are pigments fromslime moulds (Myxomycetes) and, in certain circumstances,pigments produced by macromycetes grown in mycelial cultureand some colourless metabolites where these are of currentsignificance. The Report covers work that has appeared in theliterature between March 1996 and the present time (August1998). As usual, compounds are classified according to theirperceived biosynthesis. Although experimental evidence insupport of this classification is in many cases lacking there hasbeen significant progress in this area.
2 Pigments from the shikimate–chorismate pathway
2.1 Compounds derived from arylpyruvic acids
2.1.1 Terphenylquinones. Betulinans A 1 and B 2 are twosimple derivatives of polyporic acid that have been isolated
from the fruiting bodies of Lenzites betulina (Polyporaceae).4The betulinans 1 and 2 were not known as natural productspreviously and they are potent inhibitors (IC50 0.46 and 2.88mg ml21, respectively) of lipid peroxidation in rat livermicrosomes. In this regard, betulinan A is four times as activeas vitamin E (1.68 mg ml21). Another terphenylquinone withinhibitory properties is polyozellin 3, which is produced by theKorean toadstool Polyozellus multiplex.5 Polyozellin 3 is theleuco diacetate of the well known fungal metabolite thelephoricacid1–3 and is an inhibitor of prolyl endoperoxidase, a serineprotease.
2.1.2 Pulvinic acids and related butenolides. Theorange–red caps of Tricholoma aurantium contain the uniquered pigment aurantricholone 4.6 Aurantricholone 4 is ratherunstable but its structure could be established unequivocallyafter careful chromatography by a series of NMR experiments.Pigment 4 is closely related to the naphthalenoid pulvinic acidssuch as badione A 5 and norbadione A 6, which are responsiblefor the intense brown and golden yellow colours, respectively,of the cap skins of Suillus badius and the gleba of thetaxonomically related gasteromycete Pisolithus tinctorius.1Aurantricholone 4 is the first naphthalenoid pulvinic acid to befound so far that contains side chains of the pulvinone type, andperhaps more significantly, incorporates a benzotropolonenucleus. Like the pigments 5 and 6, aurantricholone 4 occursnaturally as a metal chelate; in this case the predominant counterion is calcium. Two cometabolites of aurantricholone 4 inTricholoma aurantium are the intensely fluorescent auran-tricholides A 7 and B 8, which are biogenetically closely relatedto aurantricholone. The structures of the aurantricholides havebeen confirmed by total synthesis.6 Interestingly, aurantricho-lide B 8 was found some years ago along with 3,4A,4-trihydroxypulvinone in Suillus grevillei.1,7 The role of norba-dione A 6, as its potassium salt, in the mycorrhizal relationshipbetween P. tinctorius and higher plants has been discussedpreviously,1 but recently it has been suggested that triterpenoidmetabolites found in the mycelium and in the ectomycorrhizaeare involved in the mycorrhizal exchange.8
The fruit bodies of the hypogeous basidiomycete Melanoga-ster broomeianus contain the well known cyclopentanoids
Nat. Prod. Rep., 1999, 16, 301–317 301
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chamonixin 9 and involutin 10.9 This finding, together with theobservation that M. broomeianus can host the yellow parasiticmould Sepedonium chrysospermum points to a close relation-ship between the Melanogastraceae and Boletales. In particular,it seems to indicate that Melanogaster and Paxillus are closelyrelated.9 Taxonomic links beween the Sclerodermataceae andBoletales based on the presence of pulvinic acids and infectionby S. chrysospermum have been made before.1,3
2.2 Compounds derived from phenylalanine andtyrosine
The strikingly deep violet colour of the fruiting bodies ofCortinarius violaceus has intrigued organic chemists for manyyears. The species is found in both the northern and southernhemispheres and, despite much effort, there has hitherto been no
work published on the nature of the pigment(s) responsible forthe unmistakable colour. Recently, Steglich and coworkers10
have overcome the many problems associated with the isolationand purification of the labile blue–purple pigments present in C.violaceus and have shown that the principal coloured con-stituent is the unique iron(iii) complex 11 of (3R)-b-DOPA.
Cortinarius violaceus is the first example of an organism thatowes its colour to a charge transfer complex between a phenoland a transition metal.10 The isolation of 11 began with freshlyfreeze-dried fungal material and depended for its success onrapid, careful gel permeation under an inert atmosphere. Thestructure of the pigment 11 was supported by comparison of thepH dependent electronic spectrum of the natural product withthat of other iron(iii) catechol complexes. C. violaceus containsabout 100 times more iron than is found in most otherbasidiomycetes.10
2.3 Compounds derived from cinnamic acids
The race between Zeneca and BASF to introduce into themarket the first broad spectrum antifungal agents based on thestrobilurins1 reached a significant stage in 1996 when Zeneca’sazoxystrobin 12 and BASF’s kresoxime methyl 13 were granted
regulatory approval for commercial products.11 Over twentycompanies have now published patents on strobilurin analoguesand a production investment of $28M (Zeneca) and $20M(BASF) reflects expected sales, e.g. of azoxystrobin 12, of$500M per annum.
One of the structurally most complex strobilurins found todate is strobilurin E 14, which was isolated some years ago fromCrepidotus futomentosus.12 It has now been synthesised in
racemic form by a sequence of transformations that includesthree consecutive Wittig reactions carried out after the acidlabile spiroacetal system had been assembled (Scheme 1).13
The presence of a compound of the strobilurin-oudemansintype in the orange fruiting bodies of Flavolaschia caloceracollected in the North Island of New Zealand was mentionedbriefly in the last report.3 The structure 15 for this compoundwas deduced from the results of detailed NMR experiments onthe natural product and comparison with data from modelcatechol derivatives.14 Interestingly, the spectroscopic data for15 were recognised by the New Zealand chemists as being
identical with those reported previously for 9-methoxystrobi-lurin K (earlier assigned structure 16) and 9-methoxystrobilurinL (earlier assigned structure 17) and, as a consequence, the
structures of the last two named compounds have been revisedto 15. It also became apparent during this re-examination of thepublished data that the epoxyprenyl structures previouslyassigned to strobilurin D and hydroxystrobilurin D should alsobe revised.14 Other constituents of F. calocera include thecarboxylic acid 18, its methyl ester 19, and the relatedsecondary alcohol 20.15 The S absolute stereochemistry of 18and 19 was determined by methanolysis of 19 and subsequentapplication of the Mosher ester method to the correspondingsecondary alcohol which, incidentally, is also a minor con-stituent of F. calocera.15 A short review on the styrylpyronesfrom Inonotus hispidus and their biological properties hasappeared.16
2.4 Compounds derived from 4-hydroxybenzoic acid
The biosynthesis of the meroterpenoids boviquinone 4 28 andboviquinone 3 29 has been studied by feeding 4-hydroxy- and3,4-dihydroxy[1-13C]benzoic acid at low concentrations (0.04mmol per sporophore) to young fruiting bodies of Suillusbovinus and Chroogomphus rutilus, respectively (Scheme 2).17
The labelling pattern in 28, determined after methylation of thenatural product, indicates that boviquinone 4 is formed in S.bovinus by geranygeranylation of 3,4-dihydroxybenzoic acid 22at C-2 to give 21, which is then oxidatively decarboxylated toafford the phenol 24. Further hydroxylation and oxidation of 24then gives boviquinone 4 28. 4-Hydroxybenzoic acid acts as aprecursor of 3,4-dihydroxybenzoic acid 22. Interestingly, whenhigher doses of the labelled precursor 22 ( ~ 0.2 mmol perfruiting body) were administered to S. bovinus a change inmetabolism was induced and, instead of boviquinone 4 28, alarge amount of bovilactone-4,4 30 was obtained.17 Thelabelling pattern observed in 30 is in accord with a biosynthesisof bovilactone-4,4 involving coupling of the phenol 24 with the
Scheme 1 Reagents: i, K2CO3, acetone, reflux, 57% yield; ii, 3-methylbut-2-enal, PPTS, benzene, reflux, 57% yield; iii, Ph3PNCHCHO,benzene 30 h, reflux; iv, MeC(NPPh3)COCO2Me, 170–175 °C; v,Ph3PNCHOMe, THF, 15 h, rt; vi, hn (l > 300 nm), acetone–benzene(10 : 1), 30 min.
Scheme 2 Incorporation of 3,4-dihydroxy[1-13C]benzoic acid 22 into theboviquinones 4 28 and 3 29 by fruiting bodies of Suillus bovinus andChroogomphus rutilus, respectively.
quinone 28, as has been proposed earlier.1 Bovilactone-4,4 30with the labelling pattern shown was also obtained fromcultures of S. bovinus grown in the presence of [1A-13C]tyr-osine.17 The acetate derivative 25 of phenol 24 occurs in S.variegatus and incorporation of label from 3,4-dihydroxy[1-13C]benzoic acid 22 into 25 by S. variegatus is in accord withScheme 2.17
In contrast to the events leading to boviquinone 4 28 in S.bovinus the biosynthesis of boviquinone 3 29 in Chroogomphusrutilus involves farnesylation of 3,4-dihydroxybenzoic acid 22at C-5 to yield 23 and thence the phenol 26 and the quinone 29(Scheme 2). The acetate derivative 27 of phenol 26 is aconstituent of C. rutilus.17 The prenyl side chains in thequinones 28 and 29 arise via mevalonate in the usual way.
An X-ray crystallographic analysis of the (2)-camphenateester of tridentoquinone 31 has established the absoluteconfiguration of the natural product as R,18 not S as waspreviously suggested.1 By feeding 3,4-dihydroxy[1-13C]ben-zoic acid 22 to fruiting bodies of Suillus tridentinus it became
clear that tridentoquinone 31 is formed, like boviquinone 3 29,by prenylation of 3,4-dihydroxybenzoic acid at C-5 followed byoxidative decarboxylation to the trihydric phenol 32 and thence,by a pathway yet to be defined, to tridentoquinone 31. Theisolation of boligrevilol 33, labelled as shown, from S.tridentinus adds further support to this proposal.18 Tridentor-ubin 34, a new, minor constituent of S. tridentinus, appears to beformed by coupling between tridentoquinone 31 and the phenol
32 (or the corresponding quinone).18 This suggestion wassupported by the formation of the model system 35 whentridentoquinone 31 was stirred at room temperature with1,4-benzoquinone.18 Incorporation of label into tridentorubin31 from 4-hydroxy[1-13C]benzoic acid by S. tridentinus, asshown, is in accord with this mode of biosynthesis.
Rhizopogone 36 was first isolated from the very rare Alpinetruffle Rhizopogon pumlionus several years ago.1 The fungushas recently been found again, the pigment re-isolated and itsstructure confirmed by modern NMR methods.19 The absolute
configuration of rhizopogone 36 was determined by CDcomparison with the seco derivative 37 of tridentoquinone.Other prenylated phenolics isolated recently from fungi include
the fatty acid esters 38 and 39 from Flavolascia calocera15 andthe asiaticusins A 40 and B 41 from Boletinus asiaticus and B.paluster.20
3 Pigments from the acetate–malonate pathway
3.1 Pentaketides
Two new benzoquinones, the anserinones A 42 and B 43, withantifungal, antibacterial and cytotoxic acivities, have beenisolated from liquid cultures of the coprophilous (dung-
colonising) fungus Podospora anserina.21 The absolute config-uration of anserinone B 43 was determined by NMR spectros-
copy after formation of esters with (±)- and with(R)-(2)-3-phenylbutanoyl chloride. Naphthalene-1,3,8-triol 44,an inhibitor of malate synthase has been isolated fromsubmerged cultures of a Lachnelula species along with thesimple pentaketides 45–47. The absolute configuration of thetrans-diol 46 has not been determined.22
3.2 Hexaketides
The hexaketide fragments 48 and 49 are among the productsidentified by GCMS after pyrolysis (250 °C) of the black solidsobtained after extraction of the tuber cuticle (peridium) and thepulp (gleba) of freshly collected truffles Tuber melano-sporum.23 It is suggested that the black pigments are allomela-nins of polyketide origin.
3.3 Octaketides
3.3.1 Anthraquinones and anthraquinone carboxylicacids. The isolation and structural elucidation of the anthraqui-none carboxylic acids cardinalic acid 50, from Dermocybecardinalis, and clavorubin 51, from Cortinarius sp. WAT
24723, were discussed in the last issue of this series and detailsneed not be repeated here.3 Suffice it to say that full details ofboth projects have now been published. In the case of cardinalicacid 50, the structure was secured by total synthesis and an X-ray crystallographic analysis of methyl 1,7,8-tri-O-methylcardi-nalate.24 In the case of clavorubin 51 the structure wasconfirmed by an X-ray analysis of the 6-O-methyl ether 52 ofclavorubin methyl ester.25 Clavorubin 51 was known previouslyonly from the rye fungus Claviceps purpurea (‘ergot’).1
Austrocorticinic acid 53 occurs together with (R)-aus-trocorticin 54 and noraustrocorticin 55 in the bright orangefruiting bodies of Dermocybe sp. WAT 19352.2 Austrocorti-
cinic acid 53 has been synthesised from 6-ethyl-4-methoxy-2-pyrone 56 and the anion generated from the orsellinate ether57 as shown in Scheme 3.26 The anthraquinone 53 has beensynthesised before using Friedel–Crafts and Diels–Aldermethods.2,3
The new xanthone derivatives dermoxanthone 58 and itsmethyl ester 59 are responsible for the bright yellow fluores-
cence under UV light of the stipe and mycelium of Dermocybesemisanguinea.27 The biosynthesis of dermoxanthone 58 proba-bly involves oxidative cleavage of a precursor anthraquinonesuch as noraustrocorticin 55. The dermoxanthones 58 and 59have been detected chromatographically in several otherDermocybe species and it has been proposed that their presenceis characteristic for European members of this genus.27
Several chlorinated anthraquinones are known from Corti-narius.1 The halogenation of emodin (1,6,8-trihydroxy-3-me-thylanthraquinone) and 7-chloroemodin using a commerciallyavailable fungal chloroperoxidase (EC 1.11.1.10) has beenstudied.28 Whereas emodin is converted to a mixture of5-chloro, 7-chloro and 5,7-dichloroemodin the 7-chloro deriva-tive gives 5,7-dichloroemodin in nearly quantitative yield.Hypericin 60, a purple quinone found in the extracts of several
Dermocybe species from the southern hemisphere1,2 showsactivity against herpes simplex virus type 1 that is greatlyincreased by light.29
3.3.2 Pre-anthraquinones. An elegant new synthesis ofboth enantiomers 66 and 72 of atrochrysone using tandemMichael–Dieckmann condensation of the orsellinate derivative
Scheme 3 Reagents: i, LDA, THF, 278 °C; ii, Me2SO4, K2CO3, acetone;iii, LDA, ethyl acetate, THF, 278 °C; iv, Ac2O, pyridine; v, K2Cr2O7,AcOH; vi, K2CO3, MeOH; vii, KOH, MeOH.
61 with each of the chiral 3-methoxycyclohexenones 65 and 71,respectively, has been developed by Steglich and coworkers(Schemes 4 and 5).30,31 The (R)-cyclohexenone 65 required for
the synthesis of (R)-atrochrysone 66 was prepared by thestereoselective hydrolysis of dimethyl 3-hydroxy-3-methylglu-tarate 62 catalysed by pig liver esterase (PLE). The resultinghalf ester 63 (99% ee) was esterified with tert-butyldimethylsi-lyl triflate and the least hindered ester carbonyl group in theproduct was methylenated regioselectively with Tebbe’s rea-gent to afford the enol ether 64. Cyclisation of 64 to the opticallypure (R)-cyclohexenone 65 was induced after ester hydrolysiswith N-triflyl-(N,N-dimethylamino)pyridinium triflate. Sub-sequent deprotonation of the orsellinate 61 and conjugateaddition of the corresponding anion to the cyclohexenone 65followed by hydrogenolysis of the benzyloxymethyl groups and
cleavage of the silyl ether by using fluoride ion gave (R)-atrochrysone 66 in 15% yield from 65.
Stereoselective ring opening of the prochiral 3-hydroxy-3-methylglutaric anhydride 67 with the (R)-naphthylethanol 68(Scheme 5) was the starting point for the synthesis of (S)-atrochrysone 72. Silylation of the alcohol 69 (86% de) followedby methylenation to 70, ester hydrolysis and cyclisation to 71proceeded essentially as before. Reaction of 71 with the anionformed from 61 then gave (S)-atrochrysone 72.
It has long been presumed that the dimeric dihydroan-thracenones that are so prevalent in Cortinarius and Dermocybeare formed biosynthetically by phenolic coupling betweenmonomeric precursors such as atrochrysone and/or torosachry-sone.1–3 This has now been firmly established independently bytwo groups. Thus, 13C-labelled (R)-torososachrysone 75 wasprepared by methylation of methyl orsellinate 73 with [Me-13C]methyl iodide and condensation of the orsellinate derivative74 with the cyclohexenone 65 (Scheme 6). Feeding 75 to young
fruiting bodies of Cortinarius odorifer resulted in a 10%incorporation of label into the phlegmacins A1 76 and B1
77.30,31 On the other hand, (S)-torosachrysone containing a 13C-label in the 6-O-methyl group (10% enrichment) has beenobtained, as the 8-O-b-d-gentiobioside 78, by feeding [Me-13C]methionine to fruiting bodies of the Australian Cortinariussp. WAT 20880.2 When the gentiobioside 78 was fed to fruitingbodies of Cortinarius sinapicolor it was incorporated efficiently(2% 13C) into the C-6 and C-6Amethoxy groups (but not into theC-8A substituent) in the phlegmacin derivative 79.32 More willbe said about these experiments in Section 3.3.3.
All four stereoisomers 89, 93, 94 and 95 of the fungaltetrahydroanthraquinone austrocortilutein1–3 and the two dia-stereoisomers 98 and 99 of the closely related austrocortir-ubin1,3 have been synthesised for the first time in optically pureform.33 The source of chirality was dimethyl citramalate, whichis commercially available in both enantiomeric forms. For thesynthesis of (1S,3S)-austrocortilutein 89 (Scheme 7) theGrignard reagent prepared from 2-bromo-1,4-dimethoxyben-
Scheme 4 Reagents: i, PLE, 92% yield; ii, ButMe2Si-Tf, 2,6-lutidine,86% yield; iii, Tebbe’s reagent, 61% yield; iv, K2CO3; v, DMAP, Tf2O,41% yield from 64; vi, anion generated from 61 with LDA in THF; vii, H2,Pd/C, MeOH; viii, HF, MeCN.
Scheme 5 Reagents: i, DMAP; ii, DCC, MeOH, 75% yield; iii, ButMe2Si-Tf, 2,6-lutidine; iv, Tebbe’s reagent, 53% yield over two steps; v, KOH,EtOH; vi, DMAP, Tf2O, 26% yield from 70; vii, as for vi, vii, and viii inScheme 4.
Scheme 6 Reagents: i, [13C]MeI, K2CO3, acetone; ii, benzyloxymethylchloride, NaOMe, THF; iii, LDA, THF, 65; iv, H2, Pd/C, MeOH; v, HF,MeCN.
zene 80 was exposed to the (R)-epoxide 81 (the synthesis of ent-81 from dimethyl (S)-citramalate has been described before).34
The resulting (S)-benzyl ether 82 was hydrogenolysed and the
primary alcohol 83 was oxidised with the Dess–Martinperiodinane to afford the (S)-aldehyde 84. Stereoselectivecyclisation of 84 catalysed by stannic chloride gave exclusivelythe cis-diol 85, which was oxidatively demethylated to affordthe pivotal quinone 86. Regioselective Diels–Alder cycloaddi-tion between the new quinone 86 and the diene 87 gave (1S,3S)-austrocortilutein 89 that was identical in all respects with themajor yellow pigment from Dermocybe splendida.33 For thesynthesis of (1R,3S)-austrocortilutein 93 (Scheme 7) thesecondary hydroxy group in the cis-diol 85 was epimerised viathe ketone 90. The trans-quinone 92, obtained by oxidativedemethylation of 91, when treated with the diene 87 gave 93.Similar chemistry involving ent-81 led to the (1R,3R)-diol 96,which gave both (1R, 3R)- and (1S, 3R)-austrocortilutein 94 and95, respectively. Diels–Alder reaction between the chiralquinone 86 and the tetraoxygenated diene 88 (Scheme 8) led to
(1S,3S)-austrocortirubin 98 while ent-91 with the same dienegave the natural product 99.
When 13C-labelled (S)-torosachrysone 8-O-b-d-gentiobio-side 78 (see above) was administered to young fruiting bodies ofD. splendida the label was efficiently incorporated into the C-6methoxy group of the pigments 89 and 98, thus proving thattorosachrysone is a biogenetic precursor of the tetrahydroan-thraquinones present in this fungus.35
3.3.3 Coupled pre-anthraquinones. Details of the isola-tion and structural elucidation of the new cis-4,4A-dihydroxy-flavomannin dimethyl ether 100, together with the correspond-ing 10,10A-dihydroxy-4,4A-diketone 101 and the quinonoiddimers 102 and 103 from the purple fruiting bodies ofDermocybe sp. WAT 21566 have been reported.36 The axial
Scheme 7 Reagents: i, Mg, I2, THF, 65 °C, 2 h; ii, 81, Li2CuCl4 (0.1 M), THF, 278 °C; iii, H2, Pd/C, MeOH; iv, Dess–Martin periodinane, H2O, CH2Cl2,6 h; vi, aq. CAN, MeCN, 5 min; vii, 87, benzene, rt, 12 h; viii, O2, H2O, 12 h; ix, TMAP, CH2Cl2, 6 h; x, Me4NBH4 (prepared in situ from NaBH4 andMe4NOH), MeCN, AcOH, 0 °C.
Scheme 8 Reagents: i, 88, benzene, rt, 18 h; ii, O2, H2O, 12 h; iii, BCl3,CH2Cl2, 278 °C, 4 h.
stereochemistry of the flavomannins 100–103 can be assignedas M from the strong negative first and strong positive secondCotton effects in the CD spectrum. Emodin and citreorosein (w-hydroxyemodin) are minor constituents of this fungus.2
The absolute configurations at C-3 and C-3A in atrovirin B2
104 from D. icterinoides and at C-3A in the icterinoidins A 105and B 106 from the same fungus2 have been assigned as R by
comparison of the chemical shifts of the C-4Amethylene protonsin the 1H NMR spectrum with those of related compounds.37
Thus, the pseudo-axial and pseudo-equatorial protons at C-4 inthe dihydroaromatic ring of torosachrysone and several of its
simple derivatives, e.g. 107, resonate coincidentally between d3.02 and 3.10 in the 1H NMR spectrum.1 This is also the case indimers in which the biaryl bond is far removed from the C-4methylene protons and consequently these protons are un-perturbed by the anisotropic influence of the adjacent aromaticrings.1 In the case of the atrovirins and icterinoidins, however,where the biaryl axis links C-5 and C-5A, the C-4A methyleneprotons suffer anisotropic shielding to a greater or lesser extentdepending on their stereochemical relationship to the C-5Aaromatic residue. For example, the 4A-Hax and 4A-Heq in thespectrum of atrovirin B2 104 appear as a broad two protonsinglet at d 2.87, both protons being shielded relative to theircounterparts in torosachrysone, but not differentially so. Incontrast, in the 1H NMR spectrum of atrovirin B1 108, a
compound isolated from Cortinarius atrovirens that, like 104,has P axial chirality, 4-Heq and 4-Hax appear as an AB quartetwith components, well separated, at d 2.73 and 2.95, re-spectively. This trend, which can only be due to differences inthe absolute configuration at C-3 (and C-3A) in the case of 104and 108, was first documented in pre-anthraquinone systems byOertel.38 It has been rationalised in the case of the atrovirins 104and 108 and in related systems (vide infra) by the type ofshielding effects that are shown, for example, in Fig. 1.37,38 Inthe case of atrovirin B1 108 [Fig. 1(a)] the strong shielding of4-Heq compared to 4-Hax is well accommodated by the modelshown that has P axial stereochemistry and S chirality at the C-3
and C-3A stereogenic centres. In such a situation, which Oerteltermed a ‘syn’ relative configuration based on the disposition ofthe C-3 hydroxy group and the C-5 naphthalene nucleus,38 theequatorial proton is more vulnerable to the shielding effects ofthe aromatic rings than is its axial neighbour. Fig. 1(b) shows
Fig. 1 Computer simulation of the relative disposition between thenaphthalene ring system (simplified for the sake of clarity) and thesubstituents in the dihydroanthracenone ring in (a) atrovirin B1 108 and (b)atrovirin B2 104. The terminology is explained in the text.
the alternative situation that results from the combination of a Paxial configuration with R stereochemistry at C-3A, as is foundin 104. In this case (refered to as ‘anti’ for the reasonsmentioned above) both of the C-4A protons are exposed to somedegree to the shielding of the C-5A substituent but notdifferentially so.37 Coupled with knowledge of the axialconfiguration, gleaned from the CD spectrum, this phenomenonhas proved useful in the determination of stereochemical detailin several other coupled pre-anthraquinone systems (see below).For the icterinoidins A 105 and B 106, which from the CDspectrum are atropisomers, the former with M and the latter withP chirality, the 1H NMR data are consistent with the ‘syn’ and‘anti’ models, respectively, which leads to the (3AR)-absoluteconfiguration for both compounds.37
The incorporation of 13C-labelled (R)-torosachrysone 75 intothe phlegmacins A1 76 and B1 77 by fruiting bodies ofCortinarius odorifer was mentioned earlier.30,31 This experi-ment not only demonstrates the biogenetic relationship betweentorosachrysone and the dimeric pigments but also proves thatthe absolute configuration at the chiral centres in 76 and 77 mustbe (3R,3AR). The absolute axial configuration of phlegmacin A1
76 is P, as shown, while that of phlegmacin B1 77 is M fromtheir respective CD spectra.30,31 Importantly, these assignmentsare in agreement with the ‘anti’ relationship between the C-3Ahydroxy group and the C-10A b-naphthyl substituent in 76 andtheir ‘syn’ relationship in 77 as determined by 1H NMRspectroscopy.30–32,38
The isolation and structural elucidation of the phlegma-cinquinones 79 and 109 from the fruiting bodies of Cortinariussinapicolor was mentioned in an earlier report.2 When the 13C-
labelled (S)-torosachrysone derivative 78 was administered toC. sinapicolor the label was incorporated into the C-6 and C-6Amethoxy groups of 79, thus establishing the involvement oftorosachrysone in the biogenesis of the pigments 79 and109.32,35 Furthermore, it indicates that the absolute configura-tion of 79 must be (2AS,3AS,P),† which agrees with the ‘syn’configurational model (d4A-H2
2.93 and 2.73 for 79; 2.83 and2.66 for 109).32 Final proof of the configuration at C-3A in theanhydrophlegmacin-9,10-quinone 8A-O-methyl ether 79 wasobtained by cleavage of the biaryl bond in 79 with alkalinedithionite and isolation of the resulting (S)-torosachrysone 8-O-methyl ether 107 by using chiral HPLC.32
Minor pigments from C. sinapicolor are the red ortho-quinone 110 and the tetramer 111, both of which are closelyrelated biogenetically to the 2A-hydroxyphlegmacin 79.39
Details of the structural elucidation and biosynthesis ofdermocanarin 4 113, one of a new group of coupled pre-anthraquinone dimers found among Australasian Cortinariusand Dermocybe species,2,3 have recently appeared.40 Thelactone 113 occurs in the fruiting bodies of Cortinariussinapicolor3 and some details of its biosynthesis have been
determined by feeding sodium [1,2-13C2]acetate to the mush-rooms in their natural habitat. It was subsequently shown by anINADEQUATE NMR experiment that the labelling pattern in113 is as depicted in Scheme 9 and therefore that the
biosynthetic pathway shown must be operating.40 The(3S,3AR,M) absolute configuration of dermocanarin 4 followedfrom a combination of CD, 1H NMR and additional feedingexperiments.35,39 Thus, when the 13C-labelled (S)-torosach-trysone derivative 78 was fed to C. sinapicolor, dermocanarin 4113 was obtained with double the natural abundance level of13C in the C-6 methoxy group. Interestingly, there was nomeasurable increase in the 13C content of the 6A-O-methyl groupin 113, which is in accord both with the R stereochemistry at C-3A and with the derivation of the naphthalene ring and lactonebridge in 113 from the carboxylic acid precursor 112 (or itsequivalent) of torosachrysone, rather than from torosachrysoneitself (Scheme 9).35
Dermocanarin 10 114, from Dermocybe sp. WAT 26640, isunusual in that, unlike all of the other dermocanarins,2,3 thebiaryl bond and the lactone bridge span C-5–C-9aA and C-6–C4aA, respectively. The structure of 114, which was deducedoriginally from the NMR data,3 has now been confirmed by asingle crystal X-ray analysis of the natural product (Fig. 2) thatestablished the (3R*,M*) relative configuration.41 The absoluteconfiguration of dermocanarin 10 114 can be assigned as(3R,M) from the CD spectrum in which the longer wavelengthCotton effect is positive and the shorter wavelength Cotton
† The assignment of axial chirality as P was made, as usual, by theapplication of the exciton chirality method to the CD spectrum.
Scheme 9 Incorporation of label from sodium [1,2-13C2]acetate intodermocanarin 4 113 by fruiting bodies of Dermocybe cardinalis. Boldbonds represent intact acetate units, filled circles signify disconnectedacetate carbons.
effect is negative.41 Other pigments from D. sp. WAT 26640include the pseudophlegmacinquinones 115 and 116 for whichthe (3AR,P) absolute configuration can be deduced from the CDspectrum and reductive cleavage to (R)-torosachrysone 8-O-methyl ether.42
3.3.4 Pyranonaphthoquinones. The discovery of a seriesof novel, dimeric pyranonaphthoquinones, the cardinalins, inthe biologically active extracts of the purple New Zealandtoadstool Dermocybe cardinalis was foreshadowed in theprevious report.3 Full details of the isolation and structuralelucidation of the cardinalins 1 117 to 6 122 and 8 123 to 12 127have now been published.43,44 The most potent activity againstmurine leukaemia P388 is exhibited by the deep red cardinalins4 120 and 5 121, both of which show an IC50 of less than 0.5mg ml21.
The structures and relative configuration of the cardinalinsfollowed from detailed analysis of spectroscopic data, inparticular the chemical shifts and coupling constants in the 1H
NMR spectrum. The CD spectra43,44 show Cotton effects thatare consistant with the P axial configuration shown. Theabsolute configuration at the numerous chiral centres remainsunknown at this time. Four additional quinones, the cardinalins13 128 to 16 131, are believed to be artefacts formed during theisolation and purification process.44
Although other antibiotic pyranonaphthoquinones are wellknown as plant and microbial metabolites, the cardinalins arethe first and, to date, the only pigments of this type to be found
Fig. 2 Molecular structure of dermocanarin 10 114 as determined by asingle crystal X-ray analysis.
in the higher fungi. In particular, it is most unusual for a memberof Dermocybe to manufacture octaketides that clearly do notarise via the torosachrysone–endocrocin–emodin pathway.45
The biosynthesis of the cardinalins in D. cardinalis has beenstudied by feeding sodium [1,2-13C2]acetate to the toad-stools.35,46 This led to unequivocal incorporation of label intocardinalin 2 118 at the positions shown by bold bonds in Fig. 3.
Incorporation of label at the other expected sites in 118 was alsoevident but 13C–13C coupling constants could not be measuredin some instances because of signal overlap. The observedlabelling pattern shows that the C-1 and C-1A methyl groups in118 originate from the acetate starter units that propagateoctaketide formation in each half of the molecule, while the C-3and C-3Amethyls are the sites of decarboxylation. This contrastswith the mode of biosynthesis of microbial pyranonaphthoqui-nones such as the nanaomycins and actinorhodins in which theoctaketide is folded from the C-1 methyl towards C-3. In thesecases too, the terminal carboxylic acid group in the octaketidechain is retained in the natural products. To our knowledge,nothing was known previously about the biosynthesis of1,3-dimethylpyranoquinone systems.
A new approach to the synthesis of monomeric and dimericpyranoquinones with defined absolute stereochemistry is beingdeveloped.47 The method relies for its success on the availabil-ity of the individual enantiomers of mellein, e.g. 133 (R = H),
which are readily available from chiral acetylenic esters of thetype 132 (Scheme 10).48 Regiospecific Diels–Alder reaction ofthe chiral benzoquinone 134 with oxygenated dienes then leadsto pyranonaphthoquinones such as 135, which corresponds toone half of the cardinalin 3 molecule 119.
3.4 Nonaketides
The isolation of the 8-O-b-d-gentiobioside 136 of dermochry-sone from the water soluble fraction of the extractives fromDermocybe sanguinea (sensu Cleland) was reported in the lastreview.3 Full details have now been published.49
3.5 Higher polyketides and compounds of fatty acidorigin
3.5.1 Non-isoprenoid polyene pigments. The absoluteconfiguration of 3-acetoxy-2,3-dihydropiptoporic acid 137from Piptoporus australiensis has been confirmed as R by thesynthesis of both optical antipodes 140 and 141 of methyl
3,5-diacetoxypentanoate and correlation of the R enantiomer140 with a degradation product obtained by ozonolysis of thenatural product.50 Methyl 3-acetoxy-2,3-dihydropiptoporate138 had previously been correlated with a somewhat obscurecrystalline derivative of methyl (R)-3-acetoxyglutarate.1 Thesynthesis of the enantiomeric esters 140 and 141 is stereodivergent in that both compounds originate from the samestereoisomer of the differentially protected pentane-1,3,5-triol139, as is shown in Scheme 11.
Fig. 3 Pattern of unequivocal incorporation of sodium [1,2-13C2]acetateinto cardinalin 2 118 by fruiting bodies of Dermocybe cardinalis.
Scheme 10 Reagents: i, 1-methoxycyclohexa-1,3-diene, b-naphthyl-amine, 2,3-dimethylmaleic anhydride, sealed tube, 185 °C, 26 h, 79% yield;ii, PTSA, CH2Cl2, rt, 4 h, 84% yield; iii, 45% HBr, AcOH, reflux, 4 h, 97%yield; iv, MeLi (3.5 equiv.), THF, 278 °C; v, TFAA (3.5 equiv.), Et3SiH(3.5 equiv.), CH2Cl2, 270 °C, 89% yield over two steps ; vi, Br2, NBS (2equiv.), DMF, 24 h; vii, aq. CAN, MeCN; viii, 87, benzene, 60 °C, 20 h,then silica gel, 75% yield.
3.5.2 Quinones with extended unbranched side chains.The flesh of the resupinate fungus Phlebia chrysocrea, whichforms extended yellow patches on the bark of hardwood andconiferous trees in Japan and the eastern United States, turns redwhen touched with aqueous alkali. The pigments responsiblefor this colour reaction have been isolated and identified as agroup of biologically active benzoquinone derivatives.51 Thesequinones, the phlebiachrysoic acids A 142-E 146, have C15-C17
side chains that are interspersed with cis and/or trans doublebonds and terminate with a carboxylic acid group. The length ofthe side chain and the position and geometry of the doublebond(s) was deduced in each case from the NMR data andconfirmed by Lemieux–von Rudloff oxidation, which gave adicarboxylic acid (or two, in the case of 142 and 144) that wasmethylated and identified by GCMS. The phlebiachrysoic acidsB 143, C 144 and D 145 inhibit leukotriene C4 biosynthesis inRBL-1 cells.51
The phlebiachrysoic acids C 144, D 145 and E 146 can belinked biogenetically to merulinic acid A 147, an antibioticallyactive benzoic acid derivative previously isolated from Phlebiaradiata and Merulius tremellosus.1 Merulinic acid 147 occurstogether with several closely related metabolites, including theresorcyclic acid derivatives 148–150, in another wood-rottingresupinate fungus, the North American Hapalopilus mutans,which produces cream coloured fruiting bodies that can coverlarge areas of the tree trunk.52 The flesh of this fungus turns dullred when exposed to alkali. Also present in H. mutans is a seriesof benzoquinones, the mutaquinones A 151, B 152 and C 153,and the quinone 154, which was known previously from somehigher plants belonging to Myrsinaceae.52 The third and mostinteresting group of new compounds found in H. mutansconsists of the novel spirodiones 155–158, the mutadionesA–D, respectively.52 The mutadiones are probably formed
by coupling together of two mutaquinone molecules followedby an intramolecular rearrangement as has been suggested forbovilactone-4,4 biosynthesis.1 The complex mixture of phenols147–150, benzoquinones 151–154 and spirodiones 155–158from H. mutans was separated by preparative HPLC. Thearrangement of the two different side chains in the spirodionesB 156 and C 157 is still to be clarified.52
4 Pigments from the mevalonate pathway
Extracts of the beautiful blue fruiting bodies of the toadstoolEntoloma hochstetteri, which are found in the rain forests on thewest coast of the South Island of New Zealand, contain theazulene 159 and two other azulenes represented here by formula
Scheme 11 Reagents: i, NaH, PhCH2Br, Bun4NI; ii, BuLi, CH2O; iii,
160 that are incompletely characterised at this time.15 Thestructure 159 followed from the spectroscopic data but thelability of the extractives and distinctive colour changes notedduring chromatography suggest that the azulene 159 is, at leastin part, an artefact of the isolation and purification procedure.15
Of course, there is a long and well documented history ofunstable azulene metabolites, especially from members of thegenus Lactarius.1
Subvellerolactone 161 is a pale yellow metabolite isolatedfrom the ethanolic extracts of the fruiting bodies of Lactariussubvellereus.53 The structure 161, which is based on the unusualand characteristic lactarane skeleton, was determined fromspectroscopic analysis.53
Sesquiterpenes belonging to the malabaricane class haverecently been found for the first time in Basidiomycetes. Thefruiting bodies of the rare toadstools Cortinarius sodagnitus, C.fulvoincarnatus and C. arcuatorum show a remarkable ink-redcolour reaction when treated with aqueous base. The com-pounds responsible for this colour change are a series ofchromogenic malabaricanes, the sodagnitins A 162–F 167.54
Small quantities (0.5–5 mg) of each of the six members of thegroup were isolated from the extracts of C. sodagnitus. Largeramounts of the sodagnitins A 162 and C 164 (32 and 102 mg,respectively) were available from C. fulvoincarnatus, whileonly sodagnitin C 164 was found in C. arcuatorum. Thesodagnitins were separated by using a combination of HPLCand gel permeation through Sephadex LH-20 and the structureand relative stereochemistry of each compound was deducedfrom accumulated spectroscopic data, especially the results ofNOESY and 2D heteronuclear NMR experiments.54 It has beenproposed that the ink-red colour reaction of the sodagnitins withalkali is due to the generation of the delocalised anion 168(Scheme 12), which should exhibit a very similar longwavelength UV–VIS absorption to that shown by the tetra-enolate anion 169.54
Two other sesquiterpenoids that possess a malabaricaneskeleton, the rotundisines A and B, have been isolated recentlyfrom the cytotoxic extracts of Cortinarius rotundisporus.15,55
The structures 170 and 171 for these compounds, which aregeometrical isomers about the C-14–C-15 double bond, werededuced from spectroscopic data including the results ofHMBC, HSQC–TOCSY and NOESY experiments. The relativestereochemistry in the tricyclic nucleus and around thetetrahydropyran ring in 170 and 171 was determined fromNOESY correlations but a lack of such connectivity between
the two parts of the molecule has left open the question ofrelative stereochemistry overall. As is the case with thesodagnitins 162–167, the absolute configuration of the rotundi-sines 170 and 171 is not yet known.
Zhankuic acid F 172, the C-17 hydroxy derivative ofzhankuic acid A 173, occurs in the fruiting bodies of theparasitic Formosan fungus Antrodia cinnamomea, which hasbeen used in traditional Chinese medicine for the treatment offood and drug intoxication, diarrhoea, abdominal pain, hyper-tension, skin itches and liver cancer.56
5 Nitrogen heterocycles
5.1 Phenoxazin-3-ones
Culture fluids of the wood-rotting basidiomycete Pycnoporuscinnabarinus produce cinnabarinic acid 174 by laccase-induced
Scheme 12 Proposed mechanism for the formation of the delocalisedanion 168 from the sodagnitins A–F.
oxidation of 3-hydroxyanthranilic acid.57 Cinnabarinic acid 174is active against several Gram-positive bacteria of the Strepto-coccus group.
5.2 Indole pigments
5.2.1 Simple indoles. The synthesis of haematopodin 175from 6,7-bis(benzyloxy)indole, details of which were disclosedin the previous report,3 have now been published in full.58
Haematopodin 175, a red pigment isolated from fruiting bodiesof Mycena haematopus, was the first pyrroloquinone alkaloidfound in a fungus.
In the course of screening for free radical scavengingsubstances from microorganisms two simple indoles, 176 and177, have been found in the fruiting bodies of Agrocybecylindracea.59 Compounds 176 and 177 inhibit lipid peroxida-tion in rat liver microsomes, with IC50 values of 4.1 and 3.9 mgml21, respectively. Monomeric and dimeric indoles are found inthe bitter-tasting fruiting bodies of Tricholoma sciodes.60 Oneof these, 5-methoxy-2,4-dimethylindole 178, is known as adegradation product formed on acid treatment of the bitterprinciple, lascivol, from Tricholoma lascivum. The dimers179–181 can be envisaged as arising by oxidative coupling ofthe corresponding monomers, which are also present in thefungus.
5.2.2 Bisindolylmaleimides. Bisindolylmaleimide alka-loids continue to attract attention because of their antitumouractivity and their ability to inhibit protein kinase C. Thestructures and several syntheses of bisindolylmaleimides iso-lated from Myxomycetes (slime moulds) have been discussed inearlier reviews.1–3 More recently, syntheses of arcyroxocin A18261 and arcyriacyanin A 18362 have appeared in the literature.
Arcyroxocin A 182 was originally isolated from the redsporangia of the slime mould Arcyria denudata; arcyriacyaninA 183 is a constituent of the yellow sporangia of A. obvelata ( =A. nutans). The synthesis of arcyroxocin A 182 follows thegeneral method developed by Steglich for the synthesis ofunsymmetrically substituted bisindolylmaleimides.2 Thus, re-action of the indolyl magnesium reagent 184 (Scheme 13) withbromomaleimide 185 gave the tetrahydropyranyl ether 186 andfrom it the phenol 187. Ring closure of 187 to introduce theoxocin ring in 190 involved intramolecular addition of thephenolic hydroxy group to the acyliminium ion 188 generatedby protonation of the imide carbonyl group in the cross-conjugated merocyanine system, followed by dehydrogenationof the resulting dihydro derivative 189 by using DDQ. Finally,the N-methylimide 190 was deprotected to the free indole 191,which was transformed to the free imide 182 via the anhydride192.61
Three related routes to arcyriacyanin A 183 have beenpublished.62 In the method depicted in Scheme 14 the 2,4A-biindole 195 was obtained by Stille coupling of the stanny-lindole 193 with 1-tosyl-4-bromoindole followed by removal ofthe protecting groups from 194. The biindole 195 was convertedto its bisbromomagnesium salt and this was reacted with3,4-dibromomaleimide in refluxing toluene in the usual way.Arcyriacyanin A 183 was obtained in 41% yield from 195 andproved indistinguishable from the natural product. The secondmethod used a ‘biomimetic’ intramolecular Heck reaction forthe cyclisation of the triflate derivative of the phenol 187(Scheme 13) to afford N-methylarcyriacyanine A 197 (Scheme15) in high yield. N-Methylarcyriacyanine A 197 was convertedto the natural product 183 by alkaline hydrolysis, acid workupand reaction of the resulting anhydride with hexamethyldisila-zane. The key step in the third approach to arcyriacyanine A 183is a domino Heck reaction between bromo(indolyl)maleimide185 and 4-bromoindole 196 (Scheme15). The reaction takesplace in acetonitrile under standard conditions and gave the N-methyl derivative 197 of arcyriacyanine A 183 in 33% yield inone step.62
5.3 Miscellaneous N-heterocyclic pigments
Physarium polycephalum is one of the few slime moulds thatcan be grown in the laboratory. The polyenoyltetramic acidphysarorubinic acid 198 has been isolated from the micro-plasmodia of P. polycephalum and the structure determined by2D NMR, IR and UV spectroscopy and mass spectrometry.63
The S absolute configuration of physarorubinic acid 198 wasdetermined by comparison of the CD spectrum of the naturalproduct with that of the synthetic analogue 200. The (S)-model200 was prepared from N-methyl-(S)-serine 199 by the methodsummarised in Scheme 16. The isolation of the yellow polyene
pigment physarochrome A from P. polycephalum was reportedearlier.2
A general synthesis of optically active 2H-azepines of thetype 201 starting from a-amino acids was detailed in the lastreport and need not be repeated here.3 Full details are now
available.64 Mention has been made of the method having beenapplied to the synthesis of chalciporone 202, the pungentprinciple of the common mushroom Chalciporus piperatus.64
Thus, by beginning from (S)-alanine the method led tochalciporone 202 identical with the natural product and therebyestablished the absolute configuration of chalciporone as (S).
The cytotoxic and antimicrobial extracts of the fruit bodies ofa Cortinarius sp. collected in Otago Province, New Zealand, has
Scheme 13 Reagents: i, 40 °C then 65 °C, 57% yield; ii, Amberlist-15,MeOH, 65 °C, 30 min, 83% yield; iii, PPTS (cat.), benzene, 80 °C, 6 h; iv,DDQ (1.3 equiv.), 78% yield from 188; v, 180 °C, 10 min, 89% yield, or drysilica gel, 50 °C, 12 mbar, 24 h, 90% yield; vi, 10% KOH, 30 min, 100 °Cthen 2 M HCl, EtOAc, 80% yield; vii, hexamethyldisilazane (15 equiv.),MeOH (7.5 equiv.), DMF, rt, 72 h, 78% yield.
Scheme 14 Reagents: i, toluene, 80 °C, Pd(PPh)4, 20 h, 46–75% yield; ii,EtOH, 80 °C, 20% NaOH, 3 h, 50–68%; iii, THF, rt, EtMgBr (2 equiv.) thentoluene, 110 °C, 3,4-dibromomaleimide, 2 h, 41% yield.
Scheme 15 Reagents: i, Pd(OAc)2, PPh3, NEt3, MeCN, 80 °C, 3 h,10–30% yield.
Scheme 16 Reagents: i, SOCl2, MeOH; ii, ButMe2Si-Tf, 2,6-lutidine; iii,acetonide derivative of acetoacetic acid, o-xylene; iv, 40% HF in MeCN; v,ButOK, ButOH.
afforded a series of dithiopyridine N-oxide metabolites.15
Cortamidine oxide 203 was isolated by bioassay-directedisolation procedures and the structure was determined by NMRand model studies. The closely related dithiols 204 and 205have also been isolated and characterised from the sameextracts.
6 Other N-containing compounds
2-Aminobenzaldehyde 206 is the source of the sweet odour ofthe fruiting bodies of Hebeloma sacchariolens. The bio-synthetic origin of the aldehyde 206 from anthranilic acid hasbeen established by feeding [carboxy-13C]- and 15N-labelledanthranilic acid to fruiting bodies of H. sacchariolens where-upon a very high incorporation of label into 206 was observed.65
The enzymes responsible for the reduction of anthranilic acid to206 are not specific and will accept a range of substitutedanthranilic acids as the substrate. These results make aninteresting contrast to those discussed in Section 5.1, in whicha substituted anthranilic acid suffers oxidation by fungalenzymes.
The polyene pigments scaurin A 207 and scaurin B 208 havebeen isolated from the fruiting bodies of Cortinarius scaurus, afungus that grows abundantly in Sphagnum bogs in theEuropean Alps. The structures were determined by spectro-scopic analysis and the S absolute configuration of 207 and 208was determined by hydrolysis to l-glutamic acid.66 Thescaurins A 207 and B 208 are the first polyene pigments to befound in Cortinarius.
Several hydroxyphenylazoformamide derivatives have beenisolated from the fruiting bodies of the gasteromycete Calvatiacraniformis.67 4-Hydroxyphenyl-1-azoformamide 209 and itsN-oxide 210 were characterised from the spectroscopic data
from the phenols themselves and from the correspondingmethyl ethers. Both 209 and 210 are new natural productsalthough the methyl ether of 209 has been isolated before fromLycoperdon pyriforme, another gasteromycete. Structure 211represents the enol tautomer of rubroflavin 212, a benzoquinonesemicarbazone pigment reported previously from the NorthAmerican puff ball Calvatia rubro-flava.1 There are differencesin the 1H and 13C NMR spectra of 211 and 212 in line with thestructures proposed and the methyl ether of 211 shows theexpected 2D and NOE correlations for a benzenoid system. Thelast member of the group is the sulfoxide 213 (craniformin), thestructure of which was confirmed by a single crystal X-rayanalysis.67
The characteristic violet colour reaction with aqueous ferricchloride exhibited by fruiting bodies of Lyophyllum connatum isdue to the hydroxamic acid derivatives connatin 214 and N-hydroxy-NA,NA-dimethylurea 215.1 A third metabolite of L.connatum is the unusual hydroxycarboxamide lyophyllin 216.That the biosynthesis of lyophyllin 216 in the fruiting bodies isdue to an oxidative condensation between the urea derivative215 and N-methylhydroxylamine 217 was established bylabelling studies with the mushroom growing in its naturalenvironment (Scheme 17).68 Thus, when [carbonyl-13C]-215
was fed, a 50% incorporation of label into lyophyllin 216 wasobserved. Still higher levels of incorporation (95%) wereobserved by feeding 13C-labelled 217. On the other hand,neither methylamine nor N,N,NB-trimethylsemicarbazide wasincorporated into 216 by L. connatum. It has been shown thatthe condensing enzyme is remarkably non specific in that it cantake a variety of hydroxyureas (e.g. the diethyl analogue of 215)and various alkylhydroxylamines (e.g. ethyl, isopropyl, tert-butyl analogues of 217) and transform them into a series oflyophyllin homologues.68
7 Acknowledgements
The author is grateful to Professor Wolfgang Steglich, Mün-chen, for the disclosure of much unpublished work and detailsfrom manuscripts submitted and/or in preparation. Thanks arealso extended to many coworkers in Melbourne, named in thecitations, for making their skillful contributions to the workdiscussed herein. Catherine Elsworth is thanked for some helpwith literature retrieval. The Research School of Chemistry,Australian National University is thanked for a VisitingFellowship during the tenure of which this review was firstdrafted.
8 References
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Koshino and I.-D. Yoo, J. Nat. Prod., 1996, 59, 1090.5 J.-S. Hwang, K.-S. Song, W.-G. Kim, T.-H. Lee, H. Koshino and I.-D.
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Scheme 17 Incorporation of 13C-labels in connatin 216 by fruiting bodiesof Lyophyllum connatum.
11 A. Miller, Chem. Ind. (London), 1997, 7.12 W. Weber, T. Anke, B. Steffan and W. Steglich, J. Antibiot., 1990, 43,
207.13 G. Bertram, A. Scherer, W. Steglich, W. Weber and T. Anke,
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