THE BIOSYNTHESIS OF PHENALENONES ROBERT THOMAS Department of Chemistry, University of Surrey, Guildford. Surrey, Eng land. ABSTRACT Over twenty phenalenones and related compounds have been isolated from fungi and from higher plants. The route of biosynthesis of the compounds isolated from fungi is however quite different from that of the phenalenones produced by green plants. The fungal phenalenones atrovenetin, herqueinone and norherqueinone are typical acetate-derived polyketides, the biosynthetic interrelationships of which have been clarified by the recent isolation of deoxyherqueinone from Penicillium herquei. In the monocot, Haemodorum corymbosum, on the other hand, the phenalenone nucleus is synthesised from two C 9 residues derived from shikimate, and a single acetate unit, possibly via a diarylheptanoid intermediate. INTRODUCTION The phenalene nucleus (I) was reported as a constituent ofnatural products in 1955 and 1956, by two groups concerned with structural investigations of 1 2 apparently unrelated pigments produced by fungi imperfecti • and by an 3 Australian monocotyledonous plant . These initial sturlies described the occurrence of the phenalenone (II) nucleus in three fungal pigments, atro­ venetin (lila), norherqueinone (IVa) and herqueinone (IVb) from Penicillium herquei and P. atrovenetum, the isolation and interrelationships of which had 4 6 been previously established - , and also the higher plant cellobioside haemocorin (Va) from the rhizome of Haemodorum corymbosum (Haemo­ doraceae). 3 2 9 4 I 8 60 5 (I) (II) Comparatively few new natural products containing an intact phenalenone nucleus have been reported since these early publications, although a number of related naphthalene derivatives have been isolated from both fungi and higher plants, including several oxaphenalenes and one aza­ phenalene. The known products of this group are listed in Table 1, which 515 ROBERT THOMAS OR OR 0 OH HO '-': OH HO OH 0 _......Me ........ Me 0 Me Me 01-I ,:';'H (Va) R1 = cellobiosc, R2 = OMe 1 2 (lila) R = H (IVa) R = H (Vb) R = R R = OMe 1 2 (lllb) R = Me (IVb) R = Me (Vc) R ",.. R = H 2 (Vd) R1 = sugar, R = OH includes possible isolation artefacts such as the fungal naphthalic anhydride 7 15 10 (VI) · and isoherqueinone (an epimer of herqueinone). Precursor incorporation studies have shown that the structurally distinct 12 20 21 fungal and plant phenalenones are also biosynthetically unrelated · · . Table 1. Naturally occurring phenalcnoncs Name Source Structure Reference - ---···- ·--·----- ~ - ----- -- -- ------------·- -~- -- 1. Atrovenetin Penicillium herquei lila L2 P. atrovenetum 2. Deoxyherqueinone P. herquei Illb II 3. Norherqueinone P. herquei IVa 2, 5 4. Herqucinone P. herquei IVb 2,4, 5, 6 5. Isoherqueinonc P. herquei Epimer 5, 10, 13 ofiVb 6. P. herquei naphthalic P. herquei VI 7, 15 anhvdridc Fusicoccurn putrefaciens 7. Du;lauxin P. duclauxi VII 8 8. Xenoclauxin P. duclauxi VIII 8 9. Cryptoclauxin P. duclauxi IX 8 10. 2,7-Dihydroxy-5 -methylnaphthaquinone Verticillium larnellicola X 9 11. V. larnellicola chloro- naphthalic ester V. lamel/icola XI 9 12. Lamellicolic anhydride V. larnellicola XII 9 13. 0-Carbomethoxy- V. lamellicola XIII 9 lamellicolic anhydride 14. Resistomycin Streptornyces resistornycijlcus XXVI 16 15. Haemocorin Haernodorum coryrnbosurn Va 3, 14 16. Haemocorin aglycone H. corymbosurn Vb 12 17. Lachnanthocarpone Lachnanthes tinctoria Vc 17 18. Lachnanthoside L. tinctoria Vd 17, 18 19. N-(2-hydroxyethyl) L. tinctoria XVa 18 lachnanthopyridone 20. Lachnanthopyrone L. tinctoria XVb 16 21. L tinctoria naphthalide L. tinctoria XVI 16 22. L tinctoria L. tinctoria XIV 19 naphthalic anhydride 23. Lachnanthofluorone L. tinctoria XXV 17 516 BIOSYNTHESIS OF PHENALENONES NATURALLY OCCURRING PHENALENONES AND RELATED PRODUCTS The present review is primarily concerned with the biosynthesis of 14 22 23 phenalenones, and since previous articles • · have detailed the chemistry of phenalenes and the structural elucidation of naturally-occurring phenale­ nones, only chemical aspects of biosynthetic relevance will be considered. 0 OH HO Me (VI) 0 (VII) 0 OH 0 OH 0 Mc 0 HO o/'''''''' 0 0 0 0 ~ HO Me (VIII) (IX) 0 CO R1 CO R2 HOWOH HO~OH Yf'ct \1e 0 \1e OH (X) (XI) R1 = Me, R2 = H or R1 = H, R2 = Me 517 ROBERT THOMAS OMe HOWOH HO ~ I ~ HsC6 0 Mc OR (XII) R = H 0 (XIII) R = COOMe (XIV) OH OMe HO HsC6 H5C6 X ~ 0 0 0 (XVa) X = N --CH 2 -CH 2 0H (XVI) (XVb)X = 0 An important consequence of the presence of a carbonyl group in the phenalenone nucleus, is that it allows the carbon skeleton which contains an odd number of atoms (C 13), to assume a fully conjugated structure. When suitably hydroxylated, this nucleus can exist in different tautomeric forms, for example (XVIIa) and (XVIIb). This behaviour is also exhibited by the tropolone nucleus (C 7 ), which is present in another small group of natural products, e.g. stipitatic acid (XVIIIa ~ XVIIIb) from Penicillium stipitatum. Whereas atrovenetin and deoxyherqueinone can be represented as fully conjugated structures, the conjugation is interrupted in herqueinone and norherqueinone by the presence ofa tertiary Hnhydroxyl group. This substituent, R OH wR 0 (XVIIa) (XVIIb) 00H0 rAfoOH HO~ HO~ C0 H 2 C0 2 H (XVIJfal (XVIIIb) 518 BIOSYNTHESIS OF PHENALENONES which appears to be associated with the susceptibility of the C 5 moiety to acid cleavage in herqueinone and norherqueinone, is removed on reduction with zinc in acetic acid at room temperature, with the formation of deoxy­ herqueinone or atrovenetin respectively. The phenalene nucleus is unusual in that it can give rise to a relatively stable anion (XIX), cation (XX) or radical (XXI) by the respective loss of a proton, a hydride ion, or a radical. Such considerations led to the successful prediction that suitable hydrocarbon derivatives, e.g. the indeno-[2,1-a] 4 25 phenalene (XXIIf · would possess aromatic character. Phenalenes undergo facile oxidation to phenalenones even on exposure to air. The phenalenone nucleus, which incidentally exhibits considerable basicity (pKb = 0.4f6 is comparatively stable, although oxidative cleavage leading to naphthalic anhydrides takes place under fairly mild conditions. Thus atrovenetin (lila) and deoxyherqueinone (lllb) are readily converted to the naphthalic anhydride (VI), either with alkaline hydrogen peroxide2 or 27 27 alternatively by photochemical oxidation • In view of this latter report , and the observation by Kriegler and Thomas (unpublished results) that in crude fungal extracts containing atrovenetin and deoxyherqueinone, the concentration of the anhydride is initially very low and increases with time 7 15 of storage, it is possible that the reported natural anhydride • may arise, at least in part, as an artefact of the isolation procedure. (XIX) (XX) (XXII) (XXI) The plant phenalenones, in contrast to their fungal counterparts, all possess a phenyl substituent. The orientation of the hydroxyl substituents allows the existence of two tautomeric forms of haemocorin aglycone, the corresponding dimethyl ethers of which (XXIIIa) and (XXIIIb) have been 3 prepared . Subsequent structure assignments ofhaemocorin-related products were facilitated by the mass spectral observation that a phenalenone with a phenyl substituent peri to the carbonyl group as in (Va) exhibits a pre­ dominant (M - 1)+ ion,2° the provisional structure of which (XXIV) was 28 suggested by Shannon . Thus it was observed that the methyl ethers (XXIIIa) and (XXIIIb) yielded spectra characterized by major (M - 1) + 20 29 and M+ peaks respectively · . A product related to structure (XXIV), 519 ROBERT THOMAS namely lachnanthofluorone (XXV), has been prepared by Weiss and Edwards17 by photochemical oxidation of the aglycone of its co-metabolite, the Lachnanthes tinctoria pigment lachnanthoside (Vd). Mass spectrometric studies by Waight29 have demonstrated that a prominent (M - lt ion is a characteristic of other compounds with peri-orientated phenyl and carbonyl groups, such as 2-phenylanthraquinone. OMe OMe 0 MeO ~ ~ ~ ~'üMe ~ OMc OMe 0 (XXII Ia) (XXIllb) OH OH + 0 ~ 0 "'::::::: ~ ~ OMe ~ OH OH (XXIV) (XXV) The phenalenone pigments have been the subject of synthetic studies. A partial synthesis of atrovenetin was described by Bycroft and Eglington30 and recently a complete synthesis of the racematewas reported by Frost and 31 Morrison . Morrison and co-workers have demonstrated the absolute R-configuration of atrovenetin (lila) and have also effected a synthesis of the aglycone of the plant phenalenone haemocorin32 (Vb). The configura­ tional relationship of herqueinone and its epimer isoherqueinone, both of which contain two chiral centres, has been investigated by the groups of 13 10 Morrison and Cason , who however arrived at different conclusions. The position of the methoxyl substituent has also been investigated by these two 10 33 groups, · both of which favour the orientation shown in structure (IVb). An additional microbial product containing the phenalenone nucleus is resistomycin (XXVI)16 although the structural relationship can only be regarded as a superficial one, since the phenalenone moiety is part of a pentacyclic structure based on a cl9 hydrocarbon. A new group of what appear to be phenalenone-derived metabolites of Verticillium lamellicola, structures (X) to (XIII), has recently been discovcrcd 9 by McCorkindale and co-workers . In addition to novel features such as the chloro substituent (XI) and an 0-carbomethoxy group (XIII), these com­ pounds are notably distinct from the P.