Construction of C5-Tetrasubstituted Pyrazolines and Isoxazolines: An Update

Abstract

Heterocyclic pyrazolines and isoxazolines serve as valuable precursors in various pharmaceutically, biologically active organic molecules and natural products. In recent years, considerable research interest has been directed toward the synthesis of pyrazolines and isoxazolines. In this review, we are presenting the recent advances (2020-present) in the synthesis of pyrazolines and isoxazolines containing a C5-tetrasubstituted carbon center as a key structural motif in contemporary organic synthesis. Further, we also describe the synthetic applications, reaction pathways, reactive species, substrate scope, and limitations involved in these processes. Reaction mechanisms and future challenges are also discussed wherever appropriate. Pyrazolines and isoxazolines can be produced using a variety of methods, including 1,3-dipolar cycloaddition reactions, metal-free and metal-catalyzed conditions. More importantly, intramolecular cyclization of hydrazones and oximes is among the predominant strategies for the construction of C─C and C−heteroatom bond forming reactions in such heterocycle synthesis.