Metabolites of Aspergillus ustus. Part 4. Stable-isotope labelling studies on the biosynthesis of the austalides
Incorporationof [1-13C]-, [1,2-13C2]-and [1-13C,2,2,2-2H3]-acetate, and (3RS)-[2-13C]mevalonolactone into austalide D(2), a metabolite of Aspergillus ustus, shows that the austalides are derived from 6[(2E,6E)farnesyl]-5,7-dihydroxy-4-methylphthalide. The proposed mechanism for the subsequent cyclisation and oxidative modifications of the farnesyl moiety, consistent with the relative stereochemistry of the austalides and the structures of the cometabolites, austalides J (4), K (5), and L (6), is supported by the incorporation of austalide K into austalide D. The addition of ethanol to growing cultures of A. ustus severely inhibits normal metabolite production and induces the formation of a new metabolite, ethyl 6-ethyl-2,4-dihydroxy-3-methylbenzoate (11). The biosynthetic origin of this metabolite was studied using [1-13C]- and [1-13C,2,2,2-2H3]acetate and (2S)-[methyl-13C]methionine as precursors.