m-Chloroperbenzoic acid → methyl 8-(1-(3,5-difluorophenylamino)ethyl)-2-(ethylsulfinyl)-4-oxo-4H-chromene-6-carboxylate

3-chlorobenzoperoxoic acid (326 mg, 1.13 mmol) was added in one portion to a stirred suspension of methyl 8-(1-(3,5-difluorophenylamino)ethyl)-2-(ethylthio)-4-oxo-4H-chromene-6-carboxylate (475 mg, 1.13 mmol) in DCM (5 mL) cooled with an water/ice bath. The resulting mixture was stirred at RT for 1 h. The suspension was cooled to −15° C. and filtered, the solid was washed with cold DCM (5 mL). The filtrate was then washed with an aq. sol. of sodium thiosulfate pentahydrate in water (10 mL), and with a mixture of a sat. sol. of NaHCO3 and water (1:1, 15 mL). The organic layer was decanted, dried over MgSO4 and evaporated to afford the crude product methyl 8-(1-(3,5-difluorophenylamino)ethyl)-2-(ethylsulfinyl)-4-oxo-4H-chromene-6-carboxylate (500 mg, 100%) as a reddish foam. Mass Spectrum: M+H+ 436.