Copper-Catalyzed 2H-Azirine Ring Expansion: An Efficient Route to N-Substituted Pyrazoles.

An efficient approach has been devised for the synthesis of N-substituted pyrazoles by employing 2H-azirine-2-carbaldehyde as a key synthon. The methodology involves a copper-catalyzed intramolecular azirine ring-opening followed by N─N bond formation in a cascade sequence. The reaction proceeds under mild conditions without the use of hazardous reagents, offering a sustainable and practical alternative to conventional methods used for pyrazole synthesis. The protocol exhibits broad functional group tolerance, accommodating a variety of amines including aryl amines, sulfonamides, and tosylamine, affording the corresponding pyrazole derivatives in good to excellent yields. The synthetic utility of this method is further demonstrated by the rapid assembly of drug-like scaffolds, including celecoxib analogs, underscoring its potential application in medicinal chemistry.