[C:1]([OH:10])(=O)[C:2]1[C:3](=[CH:5][CH:6]=[CH:7][CH:8]=1)[SH:4].C([N:13]([CH2:16]C)CC)C.ClC(OCC)=[O:20].N>O1CCCC1>[S:4]1[C:3]2[CH:5]=[CH:6][CH:7]=[CH:8][C:2]=2[C:1](=[O:10])[NH:13][C:16]1=[O:20]30 g of thiosalicylic acid and 39 g of triethylamine were dissolved in 125 ml of tetrahydrofuran. The mixed solution was maintained at a temperature of 0 - 5° C., to which 42 g of ethyl chloroformate was added dropwise. The system was allowed to stand for 30 minutes and then, 50 ml of aqueous ammonia (ammonia concentration: 25 - 30%) was added thereto. The resulting solution was allowed to stand over night at room temperatue (about 20 - 30° C.). Then, the solvent used was removed by distillation under reduced pressure, while the residue was filtered off, and recrystallized from ethanol. 21 g of 2H-1,3-benzothiazin-2,4(3H)-dione was obtained. The melting point of the product was 210 - 211° C. (In addition, this product can be also prepared by using as a starting material the ester derivatives of thiosalicylic acid, which are obtained using usual esterification reactions, in a similar manner as above.)
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