The general procedure for the synthesis of 2-indolone-4-carboxylic acid from 4-carboxylic acid methyl ester dihydroindol-2-one was as follows: to a 50 mL round-bottomed flask was added 2-oxo-2,3-dihydro-1H-indole-4-carboxylic acid methyl ester (150 mg, 0.78 mmol, 1.00 equiv.), methanol (2 mL) and tetrahydrofuran (8 mL). An aqueous solution (5 mL) of LiOH (150.8 mg, 6.30 mmol, 8.00 equiv) was subsequently added. The reaction mixture was stirred at 25 °C overnight. After completion of the reaction, the solvent was concentrated under reduced pressure. The residue was diluted with distilled water (20 mL) and washed with ethyl acetate (30 mL). The pH of the aqueous phase was adjusted to 6 with 1 mol/L HCl. The aqueous phase was extracted with ethyl acetate (2 x 40 mL), the organic phases were combined and dried over anhydrous sodium sulfate. The dried organic phase was concentrated under reduced pressure to afford 2-oxo-2,3-dihydro-1H-indole-4-carboxylic acid (85 mg, 61% yield) as a light brown solid. Mass spectrum (electrospray, m/z) [M + H]+: 178.
