1-(5-nitro-3-indolyl)-2,2,2-trifluoroethanone (7 g, 24.7 mmol) was used as the raw material, which was dissolved in 40% aqueous NaOH solution (40 mL), and the reaction was stirred for 3 hours at 60 °C. After completion of the reaction, the reaction mixture was cooled to 0 °C and acidified by slowly adding 1N HCl aqueous solution to adjust the pH to ~3. Subsequently, the precipitate was collected by filtration, the filter cake was washed with water, and finally the target product 5-nitroindole-3-carboxylic acid (5.9 g, 100% yield) was obtained by concentration as a brown solid. The product was detected by LC/MS (ESI), m/z: 207 [M + 1]+.
