Synthesis of Biologically Active 6-(Tolylhydrazinylidene)Pyrazolo[1,5-a]Pyrimidinones

New pyrazolo[1,5- a ]pyrimidinones functionalized with the tolylhydrazone fragment were obtained as a result of cyclization of 3-oxo-2-(tolylhydrazinylidene) esters with 3-aminopyrazoles. In this case, methyl- and trifluoromethyl-containing derivatives regioselectively form pyrazolo[1,5- a ]pyrimidin-7-ones, while cyclization of polyfluoroalkyl-substituted analogs can take place competitively with the formation of dihydropyrazolo[1,5- a ]pyrimidin-5-ones, for which ring-chain isomerism is characteristic in solution. It was found that 6-(tolylhydrazinylidene)pyrazolo[1,5- a ]pyrimidin-7-ones have a pronounced analgesic activity. The trifluoromethyl-containing analog selectively inhibits carboxylesterase in micromolar concentration.