Thermal rearrangements of fused 1λ4,2-thiazines (2-azathiabenzenes); [1,4] shifts in cyclic sulphur–nitrogen ylides
Abstract
When heated in xylene the fused 1λ4,2-thiazines (3), with the exception of the benzo compound (3c) which is thermally stable, rearrange to the 4aH-isomers (4). S-Phenyl groups migrate exclusively 1,4 to carbon with consequent disruption of the thiophene and furan aromaticity. This thermal [1,4] shift occurs more readily when aromaticity is not disrupted; thus thienothiazine (7) isomerises to (8) in toluene, and benzothiazine (11) isomerises to (12) spontaneously on formation in xylene. With the S- methyl compound (3g) this rearrangement is accompanied by proton tautomerism and [1,2] rearrangement to give the thienothiazepine (5) and, at higher temperature (156 °C), the thienopyrrole (6) is formed (Scheme 2). The 4aH-thienothiazine (4a) rearranges more extensively at higher temperatures to give, sequentially, the pyrrolo[1,2-b]isothiazole (13) at 156 °C (Scheme 5) and the thiophene (17) at 180 °C (Scheme 6).