Austocystins. Six novel dihydrofuro[3′,2′:4,5]furo[3,2-b]xanthenones from Aspergillus ustus
Extraction of maize meal cultures of Aspergillus ustus gave six novel metabolites, designated austocystins, to which were assigned the substituted 3a, 12a-dihydrofuro[3′,2′:4,5]furo[3,2-b]xanthen-5-one structures (1), (20), (21), (23), (26), and (29). The substitution pattern of the austocystins was confirmed by the results of chemical degradation as well as by use of n.m.r. techniques such as acetylation shifts, benzene-induced solvent shifts, and nuclear Overhauser effect experiments.