7-Fluoro-1H-indole-2,3-dione (13.0 g, 78.7 mmol) was used as raw material, which was dissolved in 10N sodium hydroxide solution (125 mL), heated to 70°C and stirred for 1 hour. At the same temperature, 30% hydrogen peroxide solution (25 mL) was added slowly and dropwise over 20 minutes and the reaction continued to be stirred at 70 °C for 1 hour. Upon completion of the reaction, the reaction solution was cooled in an ice bath and concentrated. Subsequently, hydrochloric acid was slowly added to adjust the pH of the solution to 4. The organic phase was extracted with ethyl acetate, and the extract was washed with saturated brine and dried with anhydrous magnesium sulfate. The solvent was removed by distillation under reduced pressure to give the crude product. The crystals were collected by filtration to finally obtain 2-amino-3-fluorobenzoic acid (7.15 g, 59% yield). The NMR hydrogen spectrum (CDCl3) data were as follows: δ 6.00 (2H, broad single peak), 6.60 (1H, double-triple peak, J = 5.2 Hz, 8.0 Hz), 7.16 (1H, multiple peaks, J = 1.4 Hz, 8.0 Hz, 11.2 Hz), 7.72 (1H, triple-double peaks, J = 1.4 Hz, 8.0 Hz), and one additional hydrogen signal was masked.
