Metabolites of Aspergillus ustus. Part 1. Application of the heteronuclear selective population inversion (SPI) n.m.r. technique to the structure elucidation of the austalides A–F, novel ortho ester meroterpenoids

The isolation and characteristics of 12 biosynthetically related metabolites, austalides A–L from cultures of Aspergillus ustus are reported. The structure elucidation of the austalides A–F is based on a detailed study of their high-field 1H and 13C n.m.r. spectra and especially on the three- and two-bond (C,H) connectivity pattern as determined by heteronuclear 13C-{1H} selective population inversion (SPI) experiments. The conformation and relative configuration of austalides A and D were deduced from the observed proton–proton nuclear Overhauser effects (n.O.e.s) and the magnitude of the proton–proton coupling constants. Base-catalysed hydrogen–deuterium exchange on the diketone derivative (18), obtained by Jones oxidation of austalide F, leads to the regio- and stereo-specific incorporation of deuterium atoms at C-12, C-18, and C-21. A mechanism is proposed to explain the incorporation of a deuterium atom at C-21, a position γ to the C-13 carbonyl group.