[H-].[Na+].[NH:3]1[C:8]2[S:9][CH:10]=[CH:11][C:7]=2[C:6](=[O:12])[O:5][C:4]1=[O:13].[CH2:14](Br)[C:15]1[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=1>CN(C=O)C>[CH2:14]([N:3]1[C:8]2[S:9][CH:10]=[CH:11][C:7]=2[C:6](=[O:12])[O:5][C:4]1=[O:13])[C:15]1[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=1NaH (60% dispersion in mineral oil, 1.0 g, 0.025 mol) was added to a solution of 1H-thieno[2,3-d][1,3]oxazine-2,4-dione (56) (3.53 g, 0.021 mol) in anhydrous DMF stirred at 0° C. under argon. The solution was stirred for 15 minutes before adding benzyl bromide (2.97 mL, 0.025 mol). The solution was allowed to come to room temperature and further stirred for 12 hours. The solution was poured into ice water and the precipitated solid was collected by vacuum filtration to yield 4.0 g (74%) of 1-benzyl-1H-thieno[2,3-d][1,3]oxazine-2,4-dione. 1H-NMR (DMSO-d6) δ 5.13 (s, 2H), 7.22 (d, J=5.6 Hz, 1H), 7.28 (d, J=5.6 Hz, 1H), 7.30-7.45 (m, 5H) ppm; EIMS m/z 260 (M+1).
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