[NH2:1][C:2]1[S:3][CH:4]=[CH:5][C:6]=1[C:7]([O:9][CH3:10])=[O:8].[OH-].[K+].ClC(OC(Cl)(Cl)Cl)=[O:15]>O>[NH:1]1[C:2]2[S:3][CH:4]=[CH:5][C:6]=2[C:7](=[O:8])[O:9][C:10]1=[O:15]Methyl 2-amino-thiophene-3-carboxylate (5 g, 31.72 mmol) was added to a solution of potassium hydroxide (3.55 g, 63.45 mmol) in 10 mL water. The solution was heated at 90° C. until to get a clear solution. The solution was then cooled to 0° C. and trichloromethyl chloroformate (5.74 mL, 47.57 mmol) was added slowly. The solution was allowed to come to room temperature and further stirred for 30 min. The precipitated solid was collected by vacuum filtration to yield 4.7 g (88%) of 1H-thieno[2,3-d][1,3]oxazine-2,4-dione. MP 233° C. 1H-NMR (DMSO-d6) δ 7.15 (d, J=6.0 Hz, 1H), 7.19 (d, J=6.0 Hz, 1H) ppm.
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