Syntheses and reactions of [1,3,4]thiadiazolo[3,2-a]pyrimidinone derivatives

5-Imino-6H-7-one, 7-amino-5-one and 5-isocyano-7-one derivatives of 1,3,4]thiadiazolo- and -1,3]thiazolo-3,2-a]pyrimidines were synthesized. 5-Isocyano-7-one derivatives were obtained by the reaction of the corresponding 5-imino-6H-7-ones with the Vilsmeier reagent in one step.