General procedure for the synthesis of 6-fluoro-1H-indazol-3-ol from 2-amino-4-fluorobenzoic acid: 2-amino-4-fluorobenzoic acid (25 g, 161.2 mmol) was suspended in 250 ml of deionized water and 39 ml of concentrated hydrochloric acid. A 30 ml aqueous solution of sodium nitrite (11.2 g, 161.2 mmol) was slowly added dropwise under ice bath conditions, keeping the temperature below 10 °C. After the dropwise addition, the reaction was stirred at room temperature for 30 min. Subsequently, 250 ml of aqueous solution of sodium sulfite (69 g, 400 mmol) was added and the reaction was continued with stirring for 2 hours. After completion of the reaction, 30 ml of concentrated hydrochloric acid was added and the reaction mixture was left to stand overnight. On the following day, the reaction mixture was heated and refluxed for 9 h. At the end of the reaction, it was cooled to room temperature. The pH of the reaction solution was adjusted to 5.5 with sodium bicarbonate solution, the precipitate was collected by filtration and washed with deionized water and dried to obtain the target product 6-fluoro-1H-indazol-3-ol. Yield: 19.8 g, 81% yield, mass spectrometry detection [M+H]+: 152.94.
