Total synthesis of O-methylsterigmatocystin using N-alkylnitrilium salts and carbonyl-alkene interconversion in a new xanthone synthesis

A general strategy is described for the preparation of substituted xanthones and embodied in the preparation of (+/-)-O-methylsterigmatocystin (OMST), the most advanced, known intermediate in the biosynthesis of aflatoxin. The essential features of this approach are the reaction of N-alkylnitrilium salts with activated aromatic rings, and protection of the derived xanthones as their corresponding alkenyl xanthenes. The latter are readily synthesized by reaction with n-butyllithium and dehydration. The resulting stabilization of the xanthone nucleus enables a wide range of chemical modification reactions to be carried out, and a facile and unusual cleavage of the alkene with peracid restores the desired xanthone. Compatibility of these operations with the preparation of the sensitive dihydrobisfuran is exemplified in the synthesis of OMST.