From 2000 to Mid-2010: A Fruitful Decade for the Synthesis of Pyrazoles

Progress made in the preparation of substituted pyrazoles from 2000 to mid-2010 both in solution and on solid phase is presented. The search for new and efficient procedures for the synthesis of pyrazole derivatives has experienced an unprecedented growth during this period. One-pot procedures and the use of new reaction conditions have made it possible to overcome limitations such as the preparation of the starting materials or the lack of regioselectivity in the synthesis of substituted pyrazoles. The Michael-type addition of N-monosubstituted hydrazones to nitroolefins, reported for the first time in 2006, has emerged as a versatile method for the regioselective synthesis of a plethora of alkyl-, aryl-, and heteroaryl-substituted pyrazoles. Stille, Sonogashira, Negishi, and Heck C-C cross-coupling reactions have been applied to the regioselective introduction of alkenyl, alkynyl, acyl, and (het)aryl substituents at the C-3, C-4, and C-5 positions of prepared pyrazoles bearing adequate heteroatoms containing functionalities.