Reaction of 2,6-disubstitutedthiazolo5,4-dpyrimidin-7(6H)-ones 2 with ethyl chloroformate/ethanol mixture afforded 5-(ethoxycarbonylamino) -2-(substitutedthio) thiazole-4-(N-substituted)-carboxamides 4a-c. Thermal fusion of 4 followed by treatment with dimethyl sulfate gave the corresponding trisubstituted thiazolo[5,4-d]-pyrimidinediones 6. When the thiazole esters 7a,b were reacted with ethyl chloroformate, the corresponding ethyl 5-(ethoxycarbonyl)-aminothiazole-4-carboxylates 8 were Oblained. Reaction of 8b with benzylamine gave the 2-benzylthio-5-(3-benzylureido)thiazole-4-(N-benzyl)carboxamide 9.