Structures of (-)-mycousnine, (+)-isomycousnine and (+)-Oxymycousnine, new usnic acid derivatives from phytopathogenic Mycosphaerella nawae.

Abstract

The structures of three novel phytotoxins named (-)-mycousnine (II), (+)-isomycousnine (III) and (+)-oxymycousnine (IV), from the phytopathogenic fungus Mycosphaerella nawae, were elucidated on the hasis of spectroscopic and chemical evidence. 1H- and13C-NMR signals of II, III and IV were assigned by 1H-13C COSY, 13C-{1H} LSPD, HMBC and NOESY. II and III were obtained directly from (+)-usnic acid (I) by heating with 15% hydrogen chloride in dry MeOH, and IV from II by H2O2 oxidation in pyridine. Methanolysis of IV gave a novel enonediol (V). II and III showed potent antimicrobial properties. These new metabolites are the first usnic acid derivatives isolated from fungi.