PIDA-mediated C-N coupling cyclization for the synthesis of 3-aryl-1H-indazoles

Synthesis of 6,7-dihydro-2H-indazoles by iodobenzene diacetate mediated intramolecular C-N coupling cyclization of acylhydrazones derived from (E)-2-methylidenecyclohex-3-enones was reported for the first time. Oxidative aromatization of these dihydroindazole intermediates with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone followed by in situ deprotection of the indazole nitrogen produced a series of 3-aryl-1H-indazoles in moderate to high yields. The cyclization method has the advantages of mild reaction conditions, broad substrate scope, and operational simplicity, which provides a novel synthetic route toward indazole derivatives and enriches the application of hypervalent iodine reagents in the synthesis of N-heterocyclic compounds.