2-cyano-3-mercapto-3-(methyl-thio)acrylamide (1) with benzoic acid in the presence of polyphosphate ester gave 5-carbomoyl-4-methylthio-2-phenyl-1,3-thiazin-6-one (5); the structures of (5) and its 2,3-dihydro-derivative (6) were determined by single-crystal X-ray diffraction.