Conversion of 1,3-thiazolidine and its sulfoxide to dihydro-1,4-thiazine

Two methods for constructing dihydro-1,4-thiazine 4 were described. 1,3-Thiazolidines 5 were converted to dihydro-1,4-thiazines 4 by chlorinolysis through the unisolable chlorosulfonium salt 10 and sulfenyl chloride 11. Oxidation of the sulfides 5 gave a mixture of pairs of diasteromers 1,3-thiazolidine 1-oxyde 6. In the presence of acid catalyst, both sulfoxides were converted to dihydrothiazine 4 through sulfenic acid 22. In this reaction the stepwise ring opening involving carbocation 23 seems more probable. The structures of 4 were proven by the independent synthesis involving 3-bromo-2-oxobutanoic acid derivatives