Synthesis
5-Fluoro-2-methylaniline (3 g, 0.024 mol) was used as a raw material, mixed with potassium acetate (2.8 g, 0.028 mol) and acetic anhydride (6.8 mL, 0.072 mol) in chloroform (30 mL), and heated for 0.5 hours at 40 °C. Subsequently, isovaleric acid nitrate (3.8 mL, 0.028 mol) was added at the same temperature and the reaction mixture was warmed to 80 °C and left to react for 12 hours. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure and the residue was alkalized with aqueous sodium carbonate and extracted with ethyl acetate. The organic layer was washed with water and brine sequentially and concentrated under reduced pressure to obtain the crude product. The crude product was purified by silica gel column chromatography (eluent: 30% ethyl acetate/hexane) to obtain the pure compound. The pure compound was treated with methanolic hydrochloric acid solution (60 mL) for 30 min, and the reaction mixture was concentrated under reduced pressure, alkalized with aqueous sodium carbonate, and extracted again with ethyl acetate. The organic layer was washed with water and brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to give the crude product. Finally, the crude product was purified by column chromatography (eluent: n-hexane/ethyl acetate=7:3) to give 6-fluoroindazole (2.0 g, 62% yield) as a light yellow solid.
References
[1] Patent: WO2015/104662, 2015, A1. Location in patent: Page/Page column 37; 38
[2] Patent: US2016/326151, 2016, A1. Location in patent: Paragraph 0206
[3] Bioorganic and Medicinal Chemistry, 2008, vol. 16, # 4, p. 1966 - 1982
[4] Bioorganic and Medicinal Chemistry Letters, 2007, vol. 17, # 11, p. 3177 - 3180
[5] Bioorganic and Medicinal Chemistry, 2004, vol. 12, # 9, p. 2115 - 2137
