anhydro-2,3-Diphenyl-4-hydroxythiazolium hydroxide readily underwent cycloaddition with dimethyl acetylenedicarboxylate to dimethyl 1,6-diphenyl-2-pyridone-4,5-dicarboxylate with extrusion of sulphur, whereas anhydro-4-hydroxy-2,3,5-triphenylthiazolium hydroxide under the same conditions gave dimethyl 2,5-diphenylthiophen-3,4-dicarboxylate with elimination of phenyl isocyanate.