[NH2:1][C:2]1[S:3][C:4]([CH3:11])=[CH:5][C:6]=1[C:7]([O:9][CH3:10])=[O:8].[OH-].[K+].ClC(OC(Cl)(Cl)Cl)=[O:16]>O>[CH3:11][C:4]1[S:3][C:2]2[NH:1][C:10](=[O:16])[O:9][C:7](=[O:8])[C:6]=2[CH:5]=1Methyl 2-amino-5-methyl-thiophene-3-carboxylate (16.8 g, 299 mmol) was added to a solution of potassium hydroxide (25 g, 146 mmol) in 300 mL water. The solution was heated at 90° C. until to yield a clear solution. The solution was heated at same temperature for another 30 min and then cooled to 0° C. Trichloromethyl chloroformate (26.42 mL, 219 mmol) was added slowly without allowing the temperature to rise beyond 10° C. The solution was further stirred for 2 h. The precipitated solid was collected by vacuum filtration to yield 21.6 g (83%) of 6-methyl-1H-thieno[2,3-d][1,3]oxazine-2,4-dione. 1H-NMR (DMSO-d6) δ 2.38 (d, J=1.2 Hz, 3H), 6.90 (q, J=1.2 Hz, 1H), 12.44 (b, 1H) ppm; EIMS m/z 184 (M+1).
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