1-(1′-Acetyl-5-(cyclopentylthio)spiro[indoline-3,3′-pyrrolidin]-1-yl)-2,2,2-trifluoroethanone (13 mg, 0.032 mmol) was dissolved in methylene chloride (1 mL). Thereafter, sodium hydrogen carbonate (8 mg, 0.096 mmol) was added to the above obtained solution, and then, under cooling on ice, m-chloroperbenzoic acid (11 mg, 0.064 mmol) was further added thereto. The thus obtained mixture was stirred at room temperature for 1 hour. Thereafter, the reaction solution was diluted with water, and was then extracted with ethyl acetate. The organic layer was washed with a saturated aqueous solution of sodium hydrogen carbonate, water, and brine, and was then dried over anhydrous magnesium sulfate, followed by concentration in vacuo. The obtained residue was purified by preparative thin-layer chromatography (chloroform:methanol=10:1). The obtained colorless oily product was dissolved in methanol (1 mL). A saturated solution of ammonium-methanol (0.5 mL) was added to the above obtained solution, and the thus obtained mixture was then stirred at room temperature for 15 minutes. Thereafter, the reaction solution was concentrated in vacuo, and the obtained residue was then purified by preparative thin-layer chromatography (chloroform:methanol=10:1) to obtain 1-(5-(cyclopentylsulfonyl)spiro[indoline-3,3′-pyrrolidin]-1′-yl)ethanone (2 mg, 17%) in the form of a colorless oily product.
