α-Oxothioaldehyde S-oxides 4a-f can be generated under very mild conditions from the corresponding sulfinyl compounds 3a-f, which in turn are obtained by m-chloroperoxybenzoic acid (MCPBA) oxidation of α-oxothiophthalimides 2a-f. The reactive sulfine intermediates can be trapped as electron-poor dienophiles as well as electron-poor dienes with formation of dihydrothiopyran S-oxides 5a-e or 1,4-oxathiin