Regiospecific alkylation of 5-substituted 2-acylamino1,3,4-thiadiazoles

5-Substituted 2-acylamino-1,3,4-thiadiazoles (2 and 3b) are quantitatively and regiospecifically alkylated with alkyl bromide at the 3-position nitrog en (endo-products) rather than at the 2-position amide-nitrogen (exo-produc ts) under basic conditions. This alkylation required more than 1.5 equivale nts of base and more than four equivalents of alkyl bromide. The structures of the products, 5-substituted 2-acylimino-3-alkyl-1,3,4-thiadiazolines (4 and 5b(2)), were clearly established by spectroscopic data. Especially, th e structure of 3-ethyl-5-ethylthio-2-p-toluoylimino-1,3,4-thiadiazoline (4d (1)) was proven by X-Ray crystallography.