A 50-mL single-neck round-bottomed flask equipped with a magnetic stirrer and a reflux condenser was charged with (S)-(6-aminopyridin-3-yl)(3-methylmorpholino)methanone (222 mg, 1.0 mmol), 3,5-dibromo-1-methylpyridin-2(1H)-one (320 mg, 1.2 mmol), cesium carbonate (652 mg, 2 mmol), and 1,4-dioxane (10 mL). After bubbling nitrogen through the suspension for 10 minutes, Xantphos (58 mg, 0.10 mmol) and tris(dibenzylideneacetone)dipalladium(0) (46 mg, 0.050 mmol) were added. The system was subject to three cycles of vacuum/argon flush and heated at reflux for 2.5 h. It was then cooled to room temperature and filtered. The solid was washed with dichloromethane (2×30 mL) and the combined filtrate was concentrated under reduced pressure. The residue was purified by silica-gel column chromatography eluting with petroleum ether/ethyl acetate (2:1 to 1:2) to afford 257a (280 mg, 69%) as a yellow solid. MS-ESI: [M+H]+ 407.3
