Synthesis of [1,3,4]thiadiazolo[3,2-a]pyrimidines in the presence of formic acid

Formic acid-phosphorus pentoxide was effective for the preparation of 5,7-dimethyl1,3,4]thiadiazolo- and -1,3]thiazolo3,2-a]pyrimidin-4-ium salts. Further, the pyrimidine ring transformation and the isocyanation of 5imino-6H-1,3,4]thiadiazolo- and -1,3]thiazolo3,2-a]pyrimidin-7-ones were carried out in the presence of formic acid and triethyl orthoformate, respectively.