1H- and 2H-indazoles by thermal and photolytic decomposition of o-azidobenzoic acid and o-azidobenzaldehyde derivatives

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Abstract
Ethyl o-azidobenzimidate (6) and o-azidobenzamidine (7) on thermolysis yield 3-ethoxy-(9; X = OEt) and 3-amino-1H-indazole (9; X = NH2), respectively. The former product is also obtained on irradiation of the imidate in methanol–tetrahydrofuran solution. 2-Aryl- and 2-heteroaryl-3-chloro-2H-indazoles have been prepared by the action of hot thionyl chloride on o-azidoanilides. The trichloro-2H-indazoles obtained by the action of phosphorus pentachloride on o-azidobenzanilide (12) and by the action of thionyl chloride on N-(o-azidobenzoyl)-2-aminopyridine have been identified as the 3,5,7-trichloro derivatives. The reductive dehalogenation, nitration, and reactivity of the halogen towards nucleophiles, of 3-chloro-2-phenyl-2H-indazole (19) are reported. A new practicable synthesis of o-azidobenzaldehyde (31; X = O) is described. Thermolysis of its phenylhydrazone and semicarbazone derivatives yields 2H-indazoles.

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