[Br:1][C:2]1[C:3]([F:13])=[CH:4][C:5]2[O:10][CH2:9][C:8](=[O:11])[NH:7][C:6]=2[CH:12]=1.C([O-])([O-])=O.[K+].[K+].Br[CH:21]([CH3:27])[C:22]([O:24][CH2:25][CH3:26])=[O:23]>CC(C)=O>[CH2:25]([O:24][C:22](=[O:23])[CH:21]([N:7]1[C:6]2[CH:12]=[C:2]([Br:1])[C:3]([F:13])=[CH:4][C:5]=2[O:10][CH2:9][C:8]1=[O:11])[CH3:27])[CH3:26]To a mixture of 6-bromo-7-fluoro-4H-benzo[1,4]oxazin-3-one (41.0 g, 167 mmol) and K2CO3 (23.0 g, 167 mmol) in acetone (300 mL) was added ethyl 2-bromopropanoate (75.0 g, 53.8 mL, 417 mmol) drop-wise at ambient temperature and the mixture was heated at reflux overnight. The mixture was cooled to ambient temperature and filtered. The filtrate was evaporated in vacuo and the residue was purified by column chromatography on silica gel (eluting with 10% EtOAc in petroleum ether) to give 2-(6-bromo-7-fluoro-3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)-propionic acid ethyl ester (43.0 g, 75%). 1H NMR (CDCl3, 400 MHz): δ 6.97 (d, J=6.4 Hz, 1H), 6.85 (d, J=8.4 Hz, 1H), 5.25 (q, J=7.2 Hz, 1H), 4.59 (dd, J=15.2 Hz, 2H), 4.27 (m, 2H), 1.63 (d, J=7.2 Hz, 3H), 1.23 (t, J=7.2 Hz, 3H).
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