A simple procedure for the preparation of indazole derivatives

A simple method for the preparation of 1-arylindazole derivatives 4a-e is described. Oxidation of the arylhydrazones 1a-e, derived from the corresponding aromatic ketones, with tert-butylhypochlorite gave the corresponding chloro-azo derivatives 2a-b. Subsequent treatment of 2a-b with antimony pentachloride afforded 1-arylindazolium hexachloroantimonate 3a-e in good yields. Treatment of 3a-e with Na2CO3 gave 4a-e in good yield. An application of this methodology to synthesize thieno{[}2,3-d]pyrazole is described.