[S:1]1(=[O:13])(=[O:12])[C:6]2[CH:7]=[CH:8][CH:9]=[CH:10][C:5]=2[C:4](=[O:11])[CH2:3][NH:2]1.O.[C:15]1(C)C=CC(S(O)(=O)=O)=CC=1.C([O-])([O-])OC>CO>[CH3:15][O:11][C:4]1[C:5]2[CH:10]=[CH:9][CH:8]=[CH:7][C:6]=2[S:1](=[O:12])(=[O:13])[NH:2][CH:3]=1A mixture of 1.97 g (10 mmol) of 3,4-dihydro-2H-1,2-benzothiazin-4-one 1,1-dioxide, 500 mg (2.6 mmol) of p-toluenesulfonic acid monohydrate, 20 ml of methyl orthoformate and 20 ml of methanol was refluxed for 3 hours. The solvent was distilled off and the residue was purified by chromatography on a silica gel column (eluent: methylene chloride:ethyl acetate=40:1), whereby 2.06 g of the title compound were obtained (yield: 97%).
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