Biosynthesis of Xenovulene Areg;: formation of a cyclopentenoneviaa unique ring expansionndash;ring contraction mechanism

摘要

O O OH H H H Me H O Me Me Me O O OH OH O R O OH OH O H H 1 4 7 12 13 15 16 19 18 14 10 2 3 R = H 4 R = OH 20 21 22 1 Me CO2Na SAM l n XS O O Me O O Me HO2C OH Me OH n n 5 O O OH CHO n O OH CHO n OH OH O O O OH O O O n O n l O O OH OH O n l O O OH OH O OH 4 3 7 6 O O O O O O n l H l O OH C H O OH2 O O n l O O * O O n l OH OH OH O O n l OH OH 2 ndash;HCO2H O O O C HO H O O OH2 n l O O l OH OH O O O OH n 1 ndash;HCO2H O O O O H C H O l O H l n n l * * * * * * * * * * * * * * * * Biosynthesis of Xenovulene Areg;: formation of a cyclopentenone via a unique ring expansionndash;ring contraction mechanism Mairi E.Raggatt,a Thomas J. Simpson*a and M. In�es Chicarelli-Robinsonb a School of Chemistry, University of Bristol, Cantockrsquo;s Close, Bristol, UK BS8 1TS b Natural Products Chemistry Department, Xenova Discovery Ltd, 545 Ipswich Road, Slough, Berkshire, UK SL1 4EQ Incorporation studies with 13C-labelled acetates and methionine in Acremonium strictum indicate that the cyclopentenone moiety in Xenovulene Areg; is formed via a unique pathway in which a C-methylated phenolic precursor undergoes ring expansion to a tropolone followed by two successive ring contractions resulting in incorporation of the C-methyl carbon into the five-membered ring.Xenovulene Areg; 1, a novel inhibitor of benzodiazepine binding to the GABA-benzodiazepine receptor,1 was isolated from cultures of Acremonium strictum.2 The structure of 1 contains an unusual furocyclopentenone moiety fused to a humulenederived 11-membered ring.A number of minor co-metabolites of 1 have been isolated3 in which the cyclopentenone moiety is replaced by a trioxygenated benzene 2 or highly oxygenated tropolone rings, 3 and 4. While fungal metabolites containing all of these types of rings are known, the presence of all three in a single family is unique. Recently, two very similar fungal metabolites, pycnidione4 and eupenifeldin5 were isolated. These both contain a similar fusion of a tropolone to humulene.The higher plant metabolite, lucidene, also contains a humulene ring fused to an o-hydrobenzyl moiety.6 The fungal tropolone, stipitatonic acid is derived from 3-methylorsellinic acid 5 with incorporation of the methionine-derived 3-methyl group into the polyketidederived aromatic ring.7 In addition, the fungal cyclopentenone metabolites, terrein8 and cryptosporiopsonol9 have been shown to be derived by ring contraction of the aromatic ring of dihydroisocoumarin intermediates.Thus a plausible pathway to the Xenovulene metabolites 1ndash;4 appeared to be via the polyketide-derived 3-methylorsellinic acid 5 (Scheme 1) which would be converted by standard biosynthetic modifications to the lactol 6. This would react with the 6,7 double bond of humulene, either via a carbocation mediated addition or cycloaddition of the readily derived quinomethane to give the key tetracyclic intermediate 7. Ring expansion of the phenolic ring in 7 followed by hydroxylation would give the tropolones 3 and 4, whereas oxidative decarboxylation of 7 would give 2, ring contraction and decarboxylation of which would lead to Xenovulene Areg; 1.We now report incorporation studies with 13C-labelled acetates and methionine which render this branched pathway untenable and indicate a linear relationship among the metabolites. The results of incorporationsdagger; of 1-13C-, 2-13C- and 1,2-13C2-acetates and methyl-13Cmethionine into Xenovulene Areg; are summarised in Fig. 1. Surprisingly, no intact acetate unit was present in the cyclopentenone ring and C-15 was not Scheme 1 Chem. Commun., 1997 2245O O O OH Me CO2Na SAM l n l l l l l n l l l l * * * * * * * * * * * * derived from either the methyl or carboxy carbons of acetate. This was accounted for on feeding 13C-labelled methionine which gave an approximately 8 enrichment at C-15. To accommodate these results, the remarkable sequence outlined in Scheme 1 is proposed.Hydroxylation of 7 allows ring expansion via an a-ketol rearrangement to give 3 which, after further tautomeric adjustment, can ring contract and lose formic acid to form the trioxygenated benzenoid metabolite 2 with incorporation of the methionine-derived carbon into the aromatic ring. Further oxidation, a-ketol mediated ring contraction and loss of formate generates Xenovulene Areg; with the observed labelling pattern. Work is in progress to obtain further information on the detailed mechanisms and the exact oxidation levels of the intermediates on the pathway.For example, analogous pathways in which the formyl group in 7 is at the hydroxymethyl or carboxy level can be proposed, but these necessitate more extensive reductive and oxidative adjustments to obtain the final structures. Overall, this represents a remarkable and unprecedented pathway in which a cyclopentenone is formed from a phenolic precursor via one ring expansion and two successive ring contractions.Footnotes and References * E-mail: tom.simpson@bristol.ac.uk dagger; Precursors were fed to shake flask cultures of A. strictum at levels of 1.0 g l21 (1-13C- and 2-13C-acetates) or 0.5 g l21 (1,2-13C2acetate and methyl-13Cmethionine). Xenovulene Areg; (approximately 10 mg l21) was purified by flash chromatography followed by reversed phase HPLC. Incorporation of singly 13C-labelled acetates resulted in enrichments of 1ndash;3 atom.No significant differences in level of labelling were observed between the polyketide and terpenoid moieties. The minor metabolites 2ndash;4 were not isolated. 1 P. Thomas, H. Sundaram, B. J. Krishek, P. Chazot, X. Xie, P. Bevan, S. J. Brocchini, C. J. Latham, P. Charlton, M. Moore, S. J. Lewis, D. M. Thorton, F. A. Stephenson and T. G. Smart, J. Pharmacol. Exp. Ther., 1997, 282, 513. 2 A. M. Ainsworth, M. I. Chicarelli-Robinson, B. R. Copp, U. Fauth, P. J. Hylands, J. A. Holloway, M. Latif, G. B. Orsquo;Beirne, N. Porter, D. V. Renno, M. Richards and N. Robinson, J. Antibiot., 1995, 48, 568. 3 Xenova Discovery Ltd., manuscript in preparation. 4 G. H. Harris, K. Hoogsteen, K. C. Silverman, S. L. Raghoobar, G. F. Bills, R. B. Lingham, J. L. Smith, H. W. Dougherty, C. Cascales and F. Pelaez, Tetrahedron, 1993, 49, 2139. 5 F. Mayerl, Q. Gao, S. Huang, S. E. Klohr, J. A. Matson, D. R. Gustavson, D. M. Pirnik, R. L. Berry, C. Fairchild and W. C. Rose, J. Antibiot., 1993, 46, 1082. 6 H. Weenen, M. H. H. Nkunya, A. A. El-Fadl, S. Harkema and B. Zwanenburg, J. Org. Chem., 1990, 55, 5107. 7 A. I. Scott, H. Guildford and E. Lee, J. Am. Chem. Soc., 1971, 93, 3534. 8 R. A. Hill, R. H. Carter and J. Staunton, J. Chem. Soc., Chem. Commun., 1975, 380. 9 J. S. E. Holker and K. Young, J. Chem. Soc., Chem. Commun., 1975, 525. Received in Glasgow, UK, 8th July 1997; 7/04864E Fig. 1 2246 Chem. Commun., 19