Addition reactions of acetylenic esters with substituted thioureas

Thiourea and monosubstituted and sym-disubstituted alkyl and aryl thioureas react with dimethyl acetylenedicarboxylate to give, in a good yield, 2,3-dihydro-1,3-thiazin-4-ones, and not thiazolidinones as has been claimed previously. Monosubstituted thioureas react to give 2-alkylimino compounds exclusively. The adducts can be hydrogenated to give the perhydro-thiazinones. High-resolution mass spectrometry of the dihydro- and perhydro-thiazinones shows many common fragmentations typical of the ring system and often characteristic of the type and position of substituents, which may be used to identify such compounds readily.