General procedure for the synthesis of 6-bromo-5-nitro-1H-indazole from 4-bromo-2-fluoro-5-nitrobenzaldehyde: First, potassium nitrate (0.55 g, 5.4 mmol) was dissolved in concentrated sulfuric acid (8 mL) and the solution was cooled to 0 °C. Subsequently, 4-bromo-2-fluorobenzaldehyde (1 g, 4.9 mmol) was added to this cooled solution. The reaction mixture was stirred at room temperature for 1 hour. After completion of the reaction, the mixture was slowly poured into ice water and the precipitate was collected by filtration. The resulting solid was washed sequentially with water and saturated aqueous sodium bicarbonate, followed by air drying to afford 4-bromo-2-fluoro-5-nitrobenzaldehyde (1.1 g, 90% yield). Next, the above product (0.1 g, 0.4 mmol) was dissolved in ethanol (5 mL) and hydrazine monohydrate (0.02 mL, 0.4 mmol) was added slowly and dropwise. The reaction mixture was heated to reflux at 80 °C for 8 hours. At the end of the reaction, it was cooled to room temperature and the solution was concentrated by evaporation under reduced pressure. The residue was dissolved in ethyl acetate and washed sequentially with water and saturated saline. The organic layer was dried with anhydrous sodium sulfate, filtered and evaporated under reduced pressure to give 6-bromo-5-nitro-1H-indazole (0.09 g, 95% yield). The product was characterized by NMR (300 MHz, DMSO): δ 13.71 (s, 1H), 8.61 (d, J = 3.1 Hz, 1H), 8.33 (s, 1H), 8.05 (s, 1H).
