Synthesis of 2-Alkenylpyrazol-3(2H)-one Derivatives Under Mild Conditions

The title compounds have been synthesized by a one-pot two-stage reaction of PCl3, ketone methyl-hydrazones and methyl acetoacetate. Both stages were carried out at room temperature and the corresponding 2-alkenyl-1,5-dimethylpyrazol-3(2H)-ones 5, which are an unknown series of pyrazolones, were obtained in good yields. Use of a variety of enolizable ketones has shown that the reaction always occurs with the predominance of the (E)-isomer. X-Ray structure determinations of an (E)- and a (Z)-derivative [(E)-5c·HCl and (Z)-5b] permitted the assignment of their structures. The aromaticity of these derivatives is discussed.