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-hydroxy­dihydro­xanthones 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] cycloaddition, 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.