Regioselective Cycloaddition of Trifluorodiazoethane with Electron-Deficient Allenic Esters and Ketones: Access to CF3-Substituted Pyrazolines and Pyrazoles

A highly regioselective cycloaddition procedure of electron-deficient allenes with trifluorodiazoethane (CF3CHN2) is described. In absence of bases, the reaction proceeded smoothly to give 5-(trifluoromethyl)pyrazolines, whereas the utility of Et3N led to the formation of 3-(trifluoromethyl)pyrazoles.