Efficient Formation of a Triazole Ring via Novel Ring-Opening Reaction of (Z)-2-Methyl-4-arylmethylene-5(4H)-oxazolones with Hydrazides

Abstract

The ring-opening mode of the title oxazolones with hydrazide nucleophiles was investigated from both synthetic and mechanistic points of view. It was found that the novel ring-opening reaction proceeded to give (Z)-2-(3-methyl-5-substituted 1,2,4-triazol-4-yl)-3-aryl-2-propenoic acids (1,2,4-triazole-substituted (Z)-α-dehydroarylalanines| as major products, along with minor or negligible amounts of their isomers. Substituent and solvent effects on the relative composition of these two isomers confirmed that this composition was dependent on steric and electronic factors of the aryl (in the oxazolones) and acyl (in the hydrazide nucleophiles) substituents as well as on solvent polarity. MM2 and PM5 calculations provided a strong piece of evidence in support of the mechanism in which the triazole ring is constructed via the preferential nucleophilic addition of the hydrazino nitrogen to the C=N double bond in the oxazolone ring.