Efficient synthesis of highly functionalized indazoles and 2,3-dihydro-1,2-benzisoxazoles by reaction of stable Fischer dienyl carbenes and isocyanides.

A range of stable chromium and tungsten Fischer dienyl carbenes have been prepared by [3+2] cycloaddition of alkenylethynyl carbene complexes with nitrones or diazoalkanes. Treatment of these systems with isocyanides gives entry to highly functionalized 2,3-dihydro-1,2-benzisoxazoles and indazoles in a completely regioselective fashion, under mild conditions, and with high yields. This methodology can be also applied to the preparation of analogous naphthoisoxazoles starting from arylethynyl Fischer complexes. Reductive cleavage of the isoxazole moiety in the prepared heterocycles also enables the efficient synthesis of highly substituted p-aminophenols.