Cyclization of Isothiosemicarbazones. 6. The Formation and Structures of N-Alkenyl-1,2,4-triazoles and Related Compounds

Article abstract of DOI:10.1021/jo00350a015

Alkanophenone 4-<2,2-bis(ethoxycarbonyl)vinyl-3-methylisothiosemicarbazones and ortho-substituted acetophenone 4-<2-cyano-2-(ethoxycarbonyl)vinyl-3-methylisothiosemicarbazones cyclize to 1-aryl-1-<3-methylthio)-1H-1,2,4-triazol-1-yl>alkenes in acetic acid in moderate to good yields with elimination of diethyl malonate and ethyl cyanoacetate, respectively.When geometrical isomerism is possible, the triazolylalkenes have the E configuration.Aldehyde 4-<2,2-bis(ethoxycarbonyl)vinyl>-or 4-<2-cyano-2-(ethoxycarbonyl)vinyl>-3-methylisothiosemicarbazones undergo cleavage in acid to alkyl- or aryl<3-(methylthio)-1H-1,2,4-triazol-1-yl>methanol O-acylates and the corresponding malonate or cyanoacetate ester.The yields of O-acylate range from good in aromatic to poor in aliphatic compounds.The proposed machanism for this acid-catalyzed cleavage of 4-(substituted vinyl)-3-methylisothiosemicarbazones involves a resonance-stabilized iminium cation as a common intermediate.