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

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