General procedure for the synthesis of 4-amino-6-bromo-1H-indazole from 4-nitro-6-bromo-1H-indazole: 4-nitro-1H-indazole (commercially available, CAS: 2942-40-7, 1.4 g, 8.26 mmol) or 6-bromo-4-nitro-1H-indazole (commercially available, CAS: 885518-46-7, 2.0 g, 8.26 mmol) in a round-bottomed flask containing ethanol (20 mL) and water (10 mL) as the solvent mixture. Ammonium chloride (221.5 mg, 4.13 mmol) was added and stirred for 5 min at room temperature. Subsequently, iron powder (Part I: 1.3 g, 23.46 mmol; Part II: 1.0 g, 17.86 mmol) was added in batches and stirred at 80 °C for 5 min and 20 min, respectively. Immediately after completion of the reaction, the reaction mixture was filtered and the filtrate was washed with ethanol (10 mL). The filtrate was concentrated under reduced pressure and then dissolved in ethyl acetate (8 mL). The organic phase was washed with brine, dried over anhydrous Na2SO4 and concentrated again. The resulting solid was purified by silica gel column chromatography using petroleum ether with ethyl acetate (8:1) as eluent to afford the target product 4-amino-6-bromo-1H-indazole (10a: 379.0 mg, 93% yield; or 10b: 1.62 g, 92% yield) as a yellow solid.
