6-fluoro-1H-indazol-5-ol

Synthesis

In a pressure vessel intermediate 3 (0.659g, 4.67 mmol) and potassium acetate (11.39 mmol, 1.12 g) were dissolved in chloroform (30 mL) at 0⁰C. Then acetic anhydride (21.01 mmol, 1.98 mL) was added drop wise to the reaction. The reaction was then warmed to room temperature and stirred for 30 min. The reaction was then heated to 80 ⁰C and isopentyl nitrite (5.14 mmol, 0.69 mL) was added drop wise.The reaction was further stirred at 80 ⁰C overnight turning a dark brown color. The reaction was neutralized with NaHCO3 to pH 7. Dichloromethane was then added to dilute the reaction and the layers were separated. The organic layer was then washed 2x with NaCl, dried over MgSO4, and concentrated. Dissolved resulting residue in MeOH (15 mL) and 6N HCl (15 mL) and refluxed overnight at 40 °C. Neutralized the reaction with NaOH and then concentrated off the MeOH. Extracted the aqueous layer with ethyl acetate 2x, then washed the ethyl acetate 2x with NaCl, dried over MgSO4, and concentrated to give a brown residue. Purified the resulting residue using a gradient of 40% - 100% EtOAc/Hexanes to give 6-fluoro-1H-indazol-5-ol (4b) (1.571 g, 60%).1H NMR (400 MHz, DMSO-d6) δ 12.83 (s, 1H), 9.55 (s, 1H), 7.88 (s, 1H), 7.30 (dd, J = 10.9, 1.1 Hz, 1H), 7.17 (d, J = 8.5 Hz, 1H).