Azafulvenium methides: new extended dipolar systems

Abstract

The transient 1-azafulvenium methides 24, 26 and 28 generated by thermal extrusion of sulfur dioxide from pyrrolo[1,2-c][1,3]thiazole 2,2-dioxide 20 (R = Me), 22 and 23 undergo sigmatropic [1,8]H shifts and the 1-acyl derivatives 30 electrocyclise to give novel pyrrolo[1,2-c][1,3]oxazines 32. The analogous 1,2-diazafulvenium methide 36 has been intercepted in [8π + 2π] cycloadditions with electron-rich silylated acetylenes to give adducts 37–40. This behaviour is partially explained by Frontier MO theory.