Cyclisation reactions giving 5,5-dimethyl-2-phenylimidazolin-4-thiones and 4,4-dimethyl-2-phenylthiazolin-5-ones

Five substituted 2-benzoylamino-2-methyl-thiopropanamides have been prepared and their ring-closure reaction studied in basic and strongly acid media. In bases the ring closure takes place at the terminal nitrogen atom of thioamide group to give the corresponding 5,5-dimethyl-5-phenylimidazoline-4-thiones, while in strongly acid medium the reaction involves the sulphur atom to give the substituted 4,4-dimethyl-2-phenylthiazolin-5-ones. The 5,5-dimethvl5-phenylimidazoline-4-thiones are stable in both acid and base media. The 4,4-dimethyl-2-phenylthiazolin-5-ones undergo hydrolysis to 2-methyl-2-thiobenzoylamino-propionic acids. The mechanism of ring closure in basic medium has been studied in detail: its rate-limiting step is the decomposition of tetrahedral intermediate.