General procedure for the synthesis of 3-chloro-6-nitro-1H-indazole from 6-nitro-1H-indazole: 6-nitro-1H-indazole (5.0 g, 31 mmol) was added to an aqueous (150 mL) solution of sodium hydroxide (5.0 g), and the mixture was heated until a red color solution formed. The reaction mixture was cooled in an ice water bath for 15 minutes, followed by the slow addition of sodium hypochlorite solution (60 mL, 5.25%, 45 mmol). The reaction mixture was stirred continuously at 0 °C for 5 h. Upon completion of the reaction, the pH was adjusted with dilute hydrochloric acid to 7. The reaction mixture was extracted with ethyl acetate, the organic layers were combined and washed with distilled water, followed by concentration of the organic phase under reduced pressure. The residue was purified by fast column chromatography to afford the target product 3-chloro-6-nitro-1H-indazole (5.5 g, 92% yield). The structure of the product was confirmed by 1H NMR (300 MHz, DMSO-d6): δ 7.93 (m, 1H), 8.02 (m, 1H), 8.50 (m, 1H), 14.01 (s, 1H); the mass spectrum (DCI/NH3) showed m/z 197 [M+H]+.
