Step 2: Concentrated sulfuric acid (1 mL) was slowly added to a suspension of 5-bromo-1H-indazole-3-carboxylic acid (1.30 g, 5.39 mmol) in anhydrous methanol (50 mL) under argon protection. The reaction mixture was heated to reflux for 4 hours. Upon completion of the reaction, the solution was cooled to room temperature and subsequently evaporated under reduced pressure to remove methanol. The residue was dissolved in ethyl acetate and washed with deionized water. The organic phase was dried over anhydrous sodium sulfate, filtered and concentrated to afford methyl 5-bromo-1H-indazole-3-carboxylate (1.35 g, 5.29 mmol, 98% yield) as a white solid. The product was confirmed by 1H NMR (DMSO-d6): δ 14.13 (s, 1H), 8.21 (d, J = 1.6 Hz, 1H), 7.67 (d, J = 7.2 Hz, 1H), 7.59 (dd, J = 7.2, 1.2 Hz, 1H), 3.92 (s, 3H); ESIMS analysis result: C9H7BrN2O2 m/z 256.0 (M + H).
