Ring contraction of dihydro-1,3-thiazines to pyrroles on cyclisation

Abstract

Dihydrothiazines such as 8 and 12, which are disubstituted at C-7, undergo an interesting cyclisation–ring contraction reaction with substituted acrylic acids yielding the pyrroles 9 and 13 respectively. This contrasts with the corresponding reaction of thiazines 2 which are monosubstituted at C-7 which undergo a simple cyclisation reaction. A mechanism is suggested for the cyclisation–ring contraction process.