[Cl:1][C:2]1[CH:7]=[CH:6][C:5]([C:8]2[C:9](=[O:22])[NH:10][CH:11]=[C:12]3[C:17]=2[NH:16][C:15]2[CH:18]=[CH:19][CH:20]=[CH:21][C:14]=2[S:13]3)=[CH:4][CH:3]=1.C(O)(=[O:25])C.C(OO)(=O)C>O>[Cl:1][C:2]1[CH:3]=[CH:4][C:5]([C:8]2[C:9](=[O:22])[NH:10][CH:11]=[C:12]3[C:17]=2[NH:16][C:15]2[CH:18]=[CH:19][CH:20]=[CH:21][C:14]=2[S:13]3=[O:25])=[CH:6][CH:7]=1To 3 parts of the product of the process described in Example 14, 4-(4-chlorophenyl)-5H-pyrido[3,4-b][1,4]benzothiazin-3(2H)-one, suspended in 5 parts acetic acid is added and 1 part 40% peracetic acid. After ten minutes reaction time, water is added to the now homogeneous reaction mixture and the precipitate which forms is collected and recrystallized from aqueous DMF to yield white needles of 4-(4-chlorophenyl)-5H-pyrido[3,4-b][1,4]benzothiazin-3(2H)-one 10 oxide, melting above 310°.
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