At room temperature, 4-bromo-1H-indazole (1.0 g, 5.07 mmol) was dissolved in dichloromethane (DCM, 20 mL), followed by sequential addition of triethylamine (TEA, 0.85 mL, 6.1 mmol) and 4-dimethylaminopyridine (DMAP, 0.062 g, 0.50 mmol). Di-tert-butyl dicarbonate (Boc anhydride, 1.28 mL, 5.58 mmol) was added slowly and dropwise to the reaction mixture under continuous stirring. The reaction mixture was continued to be stirred at room temperature for 2.5 hours. Upon completion of the reaction, the mixture was poured into water (30 mL) and extracted with dichloromethane (2 x 30 mL). The organic phases were combined, dried over anhydrous sodium sulfate (Na2SO4), filtered and concentrated under reduced pressure. The crude product was purified by fast column chromatography (eluent: hexane solution of 15-20% ethyl acetate) to afford the target compound tert-butyl 4-bromo-1H-indazole-1-carboxylate (1.5 g, 5.06 mmol). The product was analyzed by LCMS (Method C) showing a retention time of 2.601 min and a mass spectrum (MS: ES+) m/z of 297.19; NMR hydrogen spectrum (400 MHz, DMSO-d6) δ ppm: 8.20 (s, 1H), 8.15 (d, J = 8.0 Hz, 1H), 7.47 (dd, J = 0.8 Hz, 7.6 Hz. 1H), 7.37-7.41 (m, 1H), 1.74 (s, 9H).
