m-Chloroperbenzoic acid → 6-(4-bromo-2-chlorophenyl)-8-ethyl-2-(methylsulfinyl)pyrido[2,3-d]pyrimidin-7(8H)-one

To a solution of 6-(4-bromo-2-chlorophenyl)-8-ethyl-2-(methylthio)-pyrido[2,3-d]pyrimidin-7(8H)-one (10, 1.30 g, 3.16 mmol) in dichloromethane (20 mL) was added dropwise a solution of 3-chloroperbenzoic acid (77%, 0.57 g, 2.54 mmol) in dichloromethane (5 mL) at 0-5° C. and the mixture was stirred for 5 h. The reaction mixture was washed with saturated sodium bicarbonate solution (2×20 mL) and water (10 mL), and the organic layer was dried over sodium sulfate, filtered and evaporated. The crude product was purified by silica gel column chromatography using dichloromethane:ethyl acetate (5:1→5:2→2:1→1:1) to give the title compound as an off-white solid (0.96 g, 2.25 mmol, 71%). ESMS m/z 426 (M+H)+;