Abstract Ethyl (3-substituted 5-thioxo-1, 2, 4-triazolin-4-yl) acetates were prepared by addition-cyclization reaction of ethyl isothiocyanatoacetate with carboxylic acid hydrazides in the presence of sodium ethoxide. Thermal cyclization of the adduct in dimethylformamide afforded 1-acetamido-2-thiohydantoin. The effect of substituents on the cyclization course and the thione-thiol tautomerism are discussed.
