6-Fluoro-1H-indazole (1 g, 6.74 mmol) was used as a raw material and mixed with concentrated sulfuric acid. Concentrated sulfuric acid (22 mL) and potassium nitrate (0.74 g, 7.34 mmol) were added in batches at 0°C. After addition, stirring was continued for 10 minutes at room temperature. Upon completion of the reaction, the reaction mixture was cooled to 0 °C, alkalized to pH neutral with saturated sodium bicarbonate solution and subsequently extracted with ethyl acetate. The organic phase was washed with saturated brine and dried over anhydrous sodium sulfate. After concentration under reduced pressure to remove the solvent, the crude product was purified by fast column chromatography (eluent: dichloromethane/methanol=9.8:0.2) to afford 6-fluoro-5-nitro-1H-indazole (0.4 g, 30% yield) as a yellow solid.1H NMR (400 MHz, DMSO-d6) δ: 13.7 (broad peak, 1H), 8.78 (d, J=7.4 Hz, 1H ), 8.34 (s, 1H), 7.68 (d, J=11.8Hz, 1H). Mass spectrum (ESI) m/z: 180 ([M-H]-).
