alpha-thioxothioamides: [4+2] cycloaddition reactions with dimethyl acetylenedicarboxylate and methyl propiolate

The title compounds are prepared from methyl ketones through a three-step reaction: chloration with thionyl chloride; amination of the resulting alpha-chloro-beta-oxosulfenyl chlorides; and thionation of alpha-oxothioamides with Lawesson's reagent. Two competitive hetero-Diels-Alder processes were observed in the presence of acetylenic dienophiles according to the involved system, 1,4-dithiabutadiene or possible 1-thiabutadiene arrangement. Cycloaddition on dithiabutadiene gives unstable 1,4-dithiines which contract to thiophenes via sulfur extrusion. Cycloaddition on thiabutadiene yields the corresponding fused thiopyrans. The regiochemical preferences in these [4 + 2] cycloaddition and ring contraction reactions have been demonstrated.