Copper-Catalyzed Synthesis of 5-Aryl-6-(Trifluoromethyl)-2,3-Dihydropyrazolo[1,2-a]Pyrazol-1(5H)-One.

An efficient protocol for the synthesis of 5-aryl-6-(trifluoromethyl)-2,3-dihydropyrazolo[1,2-a]pyrazol-1(5H)-one derivatives through a copper-catalyzed [3+2]-cycloaddition of azomethine imines with 3,3,3-trifluoropropyne (generated in situ from dehydrobromination of 2-bromo-3,3,3-trifluoropropene under base conditions) is developed. The advantages of this transformation are the broad substrate scope and the good functional group compatibility. The subsequent oxidation and nucleophilic substitution/aromatization provide a new approach to 4-trifluoromethylated pyrazol-1-yl propanoic acids.