BIOSYNTHESIS OF FUNGAL METABOLITES - ASPERLACTONE AND ITS RELATIONSHIP TO OTHER METABOLITES OF ASPERGILLUS-MELLEUS

The biosynthesis of asperlactone (5) from [1,2-13C2]-, [2-13C, 2-2H3]- and [3H3]-acetate was investigated. The pattern of incorporation of intact acetate units was checked with [1,2-13C2]acetate. The presence in the 13C NMR spectrum of a 2-bond coupling (J7.5 Hz) between C-2 and C-8 was verified by sine bell resolution enhancement of the Free Induction Decay. Therefore these 2 C, originally joined in a C2 unit derived from acetate, become separated by a rearrangement, and so finish in a 1,3 relationship. The C skeleton is therefore built up in identical fashion to that of the co-metabolite aspyrone (1). The retention of 2 acetate-derived hydrogens from [2-13C, 2-2H3]acetate at C-7 rules out the intermediacy of structures in which this C forms part of an aromatic ring. This experiment also confirms that C-10 can retain 3 deuterium atoms and so is a chain starter methyl group. The overall retention of acetate H was determined by a 2H NMR study of [2-2H3]acetate-enriched asperlactone. To account for these results a biosynthetic model involving partially reduced polyketone intermediates is suggested (Scheme 4). Mechanistically feasible decarboxylation and rearrangement steps are followed by stereospecific opening of an epoxide ring to generate 1 or 5. The possible derivation of the aromatic co-metabolites, mellein and hydroxymellein, from the same intermediate, generated on the same polyketide synthase, is discussed.