1,3-Dipolar Cycloaddition of Nitrile Imines with Functionalized Acetylenes: Regiocontrolled Sc(OTf)(3)-Catalyzed Synthesis of 4-and 5-Substituted Pyrazoles

1,3-Dipolar cycloaddition of C-aryl-N-aryl- and C-carboxymethyl-N-aryl-nitrile imines with functionalized acetylenes have been studied. Regioisomeric mixtures have been obtained with the 5-substituted pyrazole as the major cycloadduct. Under scandium triflate catalysis a reversal in the regiochemistry was observed, especially in the case of C-carboxymethyl-N-aryl-nitrile imines.