The general procedure for the synthesis of 5-hydroxyindol-2-one from 2-indolone was as follows: the target compound 1 was prepared according to the method described by Itoh et al [22]. The procedure was as follows: 799 mg (6.0 mmol) of hydroxyindole was dissolved in 40 mL of aqueous dichloromethane at room temperature, followed by the addition of 3.1 g (7.2 mmol) of phenyl iodine (III)-bis (trifluoroacetate) (PIFA) in batches and 4.6 mL of trifluoroacetic acid. The reaction mixture was stirred overnight at room temperature and then evaporated to remove the dichloromethane. To the residue, 60 mL of 5% sodium bicarbonate solution and 50 mL of ethyl acetate were added, and the organic layer was separated through a partition funnel. The aqueous layer was extracted with 50 mL of ethyl acetate, and the combined organic layers were dried over anhydrous sodium sulfate, filtered, and the solvent evaporated under reduced pressure. The crude product was purified by column chromatography (silica gel, dichloromethane/methanol=9:1 (v/v)) to afford 627 mg (70% yield) of target compound 1 as light brown crystals. Melting point: 266-268 °C (literature value: 260-263 °C).1H NMR (DMSO-d6, δ ppm): 3.36 (s, 2H, H-3); 6.55 (d, 1H, J=8.6 Hz, H-6); 6.60 (d, 1H, J=8.7 Hz, H-7); 6.66 (s, 1H, H-4); 8.93 (s, 1H, OH); 10.47 (s, 1H, NH).ESI-MS (ES-): m/z=148 [M-H]-.
