General procedure for the synthesis of 2-oxoindoline-6-carboxylic acid from methyl 2-oxoindoline-6-carboxylate: Sodium hydroxide solution (1N, 20 mL) was added to a solution of 2-oxoindoline-6-carboxylic acid methyl ester (2 g, 10.46 mmol, 1.00 equiv) in methanol (20 mL). The reaction mixture was stirred at 80 °C for 2 hours. After completion of the reaction, the mixture was cooled to 30 °C, diluted with 50 mL of water and extracted with dichloromethane (2 x 30 mL). The aqueous phases were combined and the pH was adjusted with aqueous hydrochloric acid (6N) to 2. The precipitated solid was collected by filtration and dried to give the title compound 1.28 g (69% yield) as a brown solid.1H NMR (400 MHz, CDCl3) δ: 12.86 (s, 1H), 10.50 (s, 1H), 7.55 (m, 1H), 7.32-7.30 (m. 2H), 3.56 (s, 2H).
