Regioselective Synthesis of Pyranone-Fused Indazoles Via Reductive Cyclization and Alkyne Insertion

A novel and efficient method for the one-pot synthesis of 2 H-indazole from readily available building blocks is reported. The reaction of 2-nitrobenzylamines with zinc and ammonium formate underwent partial reduction to nitroso-benzylamine followed by an intramolecular cyclization to afford 2 H-indazole via N-N bond formation. The carboxylic acid moiety of indazole was proceeded to regioselective alkyne insertion under ruthenium catalysis to form pyranone-fused indazoles. The regioselectivity is influenced by the weak coordination of indazole ring nitrogen to the metal center.