Keten dithioacetals. Part 22. Reaction of polarised ketoketen dithioacetals with dimethyl acetylenedicarboxylate

Ketoketen dithioacetals (1a-e) react with dimethyl acetylenedicarboxylate (2) to give open-chain dienes (4a-e) respectively. The formation of these dienes is explained through ring opening of the cyclobutene formed by [2 + 2]cycloadditions of (1) and (2). The cyclic ketoketen dithioacetal (9) reacted with (2) to give an unusual compound, which was characterized as thiapyran-2-one (10). A plausible mechanism for the formation of (10) has been suggested