Ethyl chloroformate/DMF mixture is used as a reagent for the facile ring closure of 5-amino-2- (ethylthio) thiazole-4-carboxamide (1a) to afford an excellent yield of 2-ethylthiothiazolo [5, 4- d] pyrimidin-7 (6H)-one (3). The action of phosphoryl chloride on the latter product gave its 7- chloro derivative (4). The corresponding 7-(dicyanomethyl)(5a), 7-[cyano (ethoxycarbonyl) methyl](5b), and 7-(substituted amino) derivatives (6a–j) are synthesized from reactions of ...