Polarised ketene dithioacetals. Part 50. Reactions of α-aroyl-α-bromoketene dithioacetals with hydrazine hydrate: formation of rearranged pyrazoles

Abstract

The reactions of α-aroyl-α-bromoketene dithioacetals (2a–c) with hydrazine hydrate yields unexpected pyrazoles, i.e. 3(5)-aryl-5(3)-4-bis(alkylthio)pyrazoles (7), 3(5)-arylpyrazoles (8), and 4-amino-5(3)alkylthiopyrazoles (9) in varying yields. The corresponding S-ethyl-α-bromoketene dithioacetal (2d) gave only the bis(ethylthio)pyrazole (7d) and 4-amino-4(3)-ethylthiopyrazole (9d) under similar conditions. The reactions of cyclic α-bromo ketene dithioacetals (2e–g) with hydrazine hydrate on the other hand afforded only the [1,4]dithiinopyrazole derivatives (7e–g) in high yields. The probable mechanisms for the formation of the pyrazoles (7)–(9) have been suggested.