Trifluoroacetic anhydride (9.009 mL, 64.8 mmol, 2.2 eq.) was added slowly and dropwise to a stirred solution of 6-fluoroindole (4.000 g, 29.6 mmol, 1.0 eq.) dissolved in DMF (25 mL) at 0 °C (temperature controlled using an external ice bath). After the reaction mixture was kept in the ice bath for 3 h, the reaction was quenched with 40 mL of 2N sodium carbonate solution. The precipitate was collected by filtration and washed with distilled water. The resulting solid was dissolved in 4 M NaOH solution (50 mL) and refluxed for 4 hours. After completion of the reaction, the mixture was cooled to room temperature and poured into 50 mL of distilled water. The mixture was cooled again in an ice bath and the pH was adjusted slowly by adding 6N HCl to 3. The precipitate precipitated was collected by filtration, washed with distilled water and dried under vacuum to give the off-white solid product 6-fluoro-1H-indole-3-carboxylic acid (3.5 g, 66% yield).
