Synthesis of Indazoles and Indazolones

Like their benzimidazole counterparts, indazoles have also been a popular target for the development of synthetic routes incorporating intramolecular and intermolecular C—N bond formations as well as tandem catalytic processes. Indeed, many of the 

A multitude of similar routes have been reported employing such intramolecular C—N bond formation. A-Tosyl hydrazones have also been established as effective indazole precursors [78] and were utilized in the synthesis of the natural product nigel-licine [79]. 3-Amino- l/f-indazoles were also prepared by similar palladium-catalyzed cyclizations [80]. Generating the appropriate halo-substituted arylhydrazone or aryl-hydrazine in situ has proved a popular tactic and has led to the development of effective one-pot processes [81-85]. 

Zhu et al. showed that N2-C3-fused examples can also be prepared using similar chemistry [87]. 

A strategy involving intermolecular C—N bond formation has also been described by Lefebvre et al. They demonstrated that arylation of benzophenone hydrazone with a 2-bromobenzonitrile derivative and subsequent acid-catalyzed cyclization is also an effective route toward the synthesis of 1/f-indazoles [91]. A route to 2H-indazoles incorporating an intermolecular C—N bond formation has also been demonstrated by Kumar et al. They reported a novel one-pot three-component approach utilizing 2-bromobenzaldehydes, primary amines, and sodium azide under the action of a copper 

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