Photochemical rearrangement of fused 1λ4,2-thiazines (2-azathiabenzenes); rapid migration of methylthio and phenylthio groups

Abstract

Irradiation of fused 1λ⁴,2-thiazines (1) gives the isomeric fused pyrroles (2). The structures of the thienopyrroles (2a,b) are assigned by decoupling and n.O.e. experiments on the thieno[3,2-b]pyrroles (4a,b) formed from them by hydrolysis and decarboxylation. A mechanism is proposed for conversion of thiazines (1) into pyrroles (2)(Scheme): nitrenes (8) are formed and cyclise to the 2H-pyrroles (9) which aromatise by [1,5] SR and H shifts. Surprisingly, the SR groups migrate faster than ester, acetyl, or formyl.