Carbon-13 nuclear magnetic resonance assignments of some fungal C20 anthraquinones; their biosynthesis in relation to that of aflatoxin B1

The 13C n.m.r. spectra of four related secondary fungal metabolites, averufin, averufanin, averantin, and 1′-methoxyaverantin, have been assigned. The distributions of 13C label and of the intact acetate units have been determined from the 13C n.m.r. spectra of averufin derived from single and doubly labelled [13C]acetate, These data enabled a prediction of the mode of folding of the single chain acetate–polymalonate-derived polyketide precursor of these C20 anthraquinones. A modified pathway which involves an epoxide intermediate for the conversion of averufin into the versicolorins, and, therefore, into the aflatoxins is proposed.