m-Chloroperbenzoic acid → 6-Bromo-8-ethyl-2-methylsulfinylpyrido[2,3-d]pyrimidin-7-one

To a solution of 6-bromo-8-ethyl-2-methylsulfanyl-8H-pyrido[2,3-d]pyrimidin-7-one (3, 0.96 g, 3.19 mmol) in dichloromethane (40 mL) was added 3-chloroperbenzoic acid (77%, 0.68 g, 3.04 mmol) in dichloromethane (10 mL) at 0-5° C. and the mixture was stirred at 0-5° C. for 1 h. The reaction mixture was washed with 10% sodium bicarbonate solution (1×20 mL) and in water (1×20 mL). The organic layer was dried over sodium sulfate, filtered and evaporated. The title compound was obtained as a pale yellow solid (0.98 g, 3.10 mmol, 97%). ESMS m/z 316 (M+H)+.