A 25-mL round-bottomed flask equipped with a magnetic stirrer and a reflux condenser was charged with (6-aminopyridin-3-yl)((3R,5S)-3,5-dimethylmorpholino)methanone 320a (235 mg, 1.0 mmol), 4-bromo-6-chloro-2-methylpyridazin-3(2H)-one (232 mg, 1.05 mmol), cesium carbonate (652 mg, 2.0 mmol), and 1,4-dioxane (6.0 mL). After bubbling nitrogen through the suspension for 10 minutes, Xantphos (116 mg, 0.20 mmol) and tris(dibenzylideneacetone)dipalladium(0) (70 mg, 0.10 mmol) were added. The system was subjected to three cycles of vacuum/nitrogen flush and heated at reflux for 2.5 h. It was then cooled to room temperature and filtered. The solid was washed with dichloromethane (3×10 ml). The combined organic layer was concentrated under reduced pressure. The residue was purified by silica-gel column chromatography eluting with petroleum ether/ethyl acetate (2:1 to 1:10) to afford 321a (140 mg, 37%) as a yellow solid. MS-ESI: [M+H]+ 378.3
