New polyketide metabolites from Aspergillus melleus: structural and stereochemical studies

Abstract

The structures of two new fungal products asperlactone (5) and isoasperlactone (6) have been deduced by comparison of their spectroscopic data with that of aspyrone (3), a known co-metabolite. All three compounds have been degraded by ozonolysis to produce crystalline derivatives of 2,3-epoxybutyric acid (9) and 1-methylglycerol (11). From measurements of optical activity it has been established that aspyrone and isopasperlactone have the (5S,6R,8S,9S) configuration, whereas that of asperlactone is (5R,6S,8S,9S). The relative stereochemistry of aspyrone has been checked by an X-ray crystallographic study: [P2₁2₁2₁, a= 4.427(3), b= 14.272(3), c= 14.968(3)Å, Z= 4, R= 0.033 for 1 131 unique observed reflections]. The stereochemical relationships are discussed in terms of a biosynthetic proposal by which (3) and (5) would be biosynthesised from a common intermediate via alternative cyclisations of a carboxy group onto an epoxide residue. It is possible that (6) is a chemical artefact derived from (3).