Regioselective Tandem [4+1]-[4+2] Synthesis of Amino-Substituted Dihydroxanthones and Xanthones

A highly convergent and operationally simple approach to mycotoxin-related 4-amino-substituted 1-hydroxy-dihydroxanthones is described. The target compounds are obtained in one pot by the multicomponent reaction of 3-carbonylchromones, isocyanides, and nonsymmetric dienophiles. The reaction, which involves a tandem [4 + 1]-[4 + 2] cydoaddition, efficiently affords a variety of both monomeric and dimeric polysubstituted dihydroxanthones structurally similar to bioactive ergochromes. Further aromatization to the corresponding xanthones is readily achieved by treatment with DBU under microwave irradiation.