General procedure for the synthesis of 1H-indazol-5-ol using 5-methoxy-1H-indazole as starting material: a dichloromethane solution of boron tribromide (18.5 mL, 18.5 mmol) was slowly added dropwise to a solution of 5-methoxy-1H-indazole (1.24 g, 8.40 mmol) in dichloromethane (84 mL) at 0 °C. After the dropwise addition was completed, the reaction mixture was warmed to room temperature and stirred continuously for 10 hours. Upon completion of the reaction, the reaction solution was poured into an ice water bath and subsequently extracted with ethyl acetate. The organic phases were combined and washed sequentially with saturated aqueous sodium bicarbonate solution and saturated aqueous sodium chloride solution. The organic layer was dried over anhydrous magnesium sulfate and concentrated under reduced pressure to remove the solvent. The resulting crude product was purified by silica gel column chromatography (eluent: chloroform/methanol=96/4, v/v) to afford 1H-indazol-5-ol (877 mg, 71% yield) as a white solid. The structure of the product was confirmed by 1H-NMR (DMSO-d6): δ 6.88 (1H, dd, J = 8.8, 2.2 Hz), 6.96 (1H, d, J = 2.2 Hz), 7.34 (1H, d, J = 8.8 Hz), 7.84 (1H, s).
