Preparation of 5-methyl-2-sulfanyl-7h-1,3,4-thiadiazolo[3,2-a]-pyrimidin-7-ones

7H-1,3,4-Thiadiazolo[3,2-a]pyrimidin-7-ones can be prepared by the acylation of 5-amino-1,3,4-thiadiazoles with diketene and subsequent ring closure (dehydration). Whereas arylthio substituents (SC6H5) can be introduced in 2-position by the replacement of Br, alkylthio groups (SC2H5) have to be already presentin the starting 5-amino-1,3,4-thiadiazole. The ambident nucleophile 2-thiazolidinethione reacts in the Br substitution reaction on the N atom.