Br[C:2]1[C:3]([Cl:13])=[CH:4][C:5]2[O:6][CH2:7][C:8](=[O:12])[NH:9][C:10]=2[N:11]=1.[C:14]1(/[CH:20]=[CH:21]/B(O)O)[CH:19]=[CH:18][CH:17]=[CH:16][CH:15]=1.C(=O)([O-])O.[K+]>O1CCOCC1.O.C(OCC)(=O)C>[Cl:13][C:3]1[C:2](/[CH:21]=[CH:20]/[C:14]2[CH:19]=[CH:18][CH:17]=[CH:16][CH:15]=2)=[N:11][C:10]2[NH:9][C:8](=[O:12])[CH2:7][O:6][C:5]=2[CH:4]=1
6-Bromo-7-chloro-4H-pyrido[3,2-b][1,4]oxazin-3-one (15a) (22 g, 83.7 mmole) and trans-2-phenylvinylboronic acid (17.33 g, 117 mmole) were dissolved in 1,4-dioxane (300 mL) and the solution was degassed with argon. (Ph3P)4Pd (1.9 g, 2 mole %) was added, followed by a solution of potassium hydrogen carbonate (21 g, 210 mmole) in H2O (100 mL). The reaction was heated at reflux under argon overnight, then was cooled to room temperature and diluted with ethyl acetate (1 L). The solution was washed sequentially with H2O and brine, dried (Na2SO4), and concentrated in vacuo. The residue was slurried with chloroform (120 mL), then diluted with diethyl ether (100 mL). The precipitated product was collected by filtration and washed with ether to provide the product (16.4 g, 68%) as an off-white solid.
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