Studies related to dihydro-1,4-thiazines. Part 9. Thermal equilibration of thiazine and thiazoline S-oxides

Abstract

In boiling toluene, methyl (1RS)-3-(1-methoxycarbonylvinyl)-2,2-dimethyl-Δ⁴-thiazoline-5-carboxylate 1-oxide (7)undergoes equilibration by way of the sulphenic acid (3b) with the racemate of dimethyl (1R, 2R)-3-isopropenyl-2-methyl-Δ⁴-thiazoline-2,5-dicarboxylate 1-oxide (8b); there is no detectable concentration of the racemate of dimethyl (1S, 3R)-3,4-dihydro-4-isopropenyl-2H-1,4-thiazine-3,6-dicarboxylate 1-oxide (2b) in the mixture. Under corresponding conditions, interconversion occurs between the racemates of the dimethyl (1S,3R)-3,4-dihydro-2H-1,4-thiazine-3,6-dicarboxylate 1-oxides (2a and d–h) and of the dimethyl (1R,2R)-2-methyl-Δ⁴-thiazoline-2,5-dicarboxylate 1-oxides (8a and d–h). The position of the equilibrium is dramatically dependent upon the nature of the N-substituent. For example, with a hydrogen atom the thiazine S-oxide (2a) is overwhelmingly preferred whereas with an acetyl group the thiazoline S-oxide (8h) is favoured.