Chemo- and diastereoselectivity in the dimethyldioxirane oxidation of 2,3-dihydro-4H-1-benzothiopyran-4-ones and 4H-1-benzothiopyran-4-ones. Unusual reactivity of 4H-1-benzothiopyran-4-one 1-oxides

The oxidation of the 1-thiochromanones 1-3 by dimethyldioxirane (DMD) produ ced the corresponding sulfoxides 4-6 or sulfones 7-9; their relative amount s depended on the amount of oxidant used. A low diastereoselectivity was ob served in the sulfoxidation of the 8-substituted 1-thiochromanones 2 and 3, due to the small steric differentiation during the DMD attack. An unusual reactivity pattern was found in the DMD oxidation of the 1-thiochromones 10 -12, in that the sulfoxides 13-15 were more reactive toward the electrophil ic oxidizing agent than the corresponding sulfides. The observed anomaly ma y be explained in terms of transannular stabilization of the transition str ucture (TS) for the sulfone formation, promoted through favorable conformat ional effects in the sulfoxide. Higher sulfoxide/sulfone ratios were found in solvents of greater hydrogen bond donor capacity, which is in accordance with the postulated stabilizing effect.