The synthesis of A2-dihydrothiapyran 1, l-dioxide by an indirect route involving the Dieckmann cyclization of ethyl 7-(carbethoxymethy1mercapto)-butyrate is described. Rearrangement of the a, p-unsaturated sulfone to its p,?-unsaturated isomer, A3- dihydrothiapyran 1, l-dioxide, is observed to occur when A'-dihydrothiapyran l, l-dioxide is treated with triethylamine or dilute alkali. Addition of bromine to the a,@-unsaturated ...