FC(F)(F)C(O)=O.C(OC([N:15]1[CH2:20][CH2:19][CH:18]([NH:21][C:22]([C:24]2[CH:25]=[CH:26][C:27]3[S:32][CH2:31][C:30](=[O:33])[NH:29][C:28]=3[CH:34]=2)=[O:23])[CH2:17][CH2:16]1)=O)(C)(C)C>ClCCl>[NH:15]1[CH2:20][CH2:19][CH:18]([NH:21][C:22]([C:24]2[CH:25]=[CH:26][C:27]3[S:32][CH2:31][C:30](=[O:33])[NH:29][C:28]=3[CH:34]=2)=[O:23])[CH2:17][CH2:16]1Trifluoroacetic acid (1.73 mL, 22.20 mmol, 15.0 eq) is added at 0° C. to a stirred solution of 4-[(3-oxo-3,4-dihydro-2H-benzo[1,4]thiazine-6-carbonyl)-amino]-piperidine-1-carboxylic acid tert-butyl ester (610 mg, 1.48 mmol, 1.0 eq) in dichloromethane (20 mL). After 2 hours stirring at room temperature, the reaction mixture is extracted with dichloromethane (3×20 mL) and water (20 mL) and the pH is adjusted to 12 by the addition of a 1N sodium hydroxide aqueous solution. The combined organic layers are dried over sodium sulfate, filtered and concentrated to afford 3-oxo-3,4-dihydro-2H-benzo[1,4]thiazine-6-carboxylic acid piperidin-4-ylamide as a light brown solid (244 mg, 51% yield).
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