Synthesis
The general procedure for the synthesis of 4-chloro-2-oxoindole (4-chloroindolone) from 4-chloroindoline-2,3-dione was as follows: titanium tetrachloride (TiCl4) (0.7 mL, 6 mmol) was slowly added to a stirring suspension of zinc powder (0.78 g, 12 mmol) in freshly distilled anhydrous THF (15 mL) at room temperature and under dry nitrogen atmosphere. After addition, the mixture was refluxed for 2 hours. After the formed suspension of low-valent titanium reagent was cooled, a THF (10 mL) solution of indigo red or its derivative (2 mmol) was added dropwise. The reaction mixture was stirred at room temperature for about 5 min under nitrogen protection. After confirming the completion of the reaction by thin layer chromatography (TLC) analysis, the reaction was quenched with 3% hydrochloric acid (15 mL) and subsequently extracted with chloroform (3 x 50 mL). The organic phases were combined, washed with water (3 x 50 mL) and dried over anhydrous sodium sulfate. After concentration of the solvent under reduced pressure, the crude product was purified by column chromatography (petroleum ether/ethyl acetate = 5:1) to afford the target compound 4-chloro-2-oxindole (4-chloroindolone).
References
[1] Tetrahedron Letters, 2014, vol. 55, # 14, p. 2238 - 2242
