[F:1][C:2]1[CH:3]=[C:4]([CH:20]=[CH:21][C:22]=1[C:23]([F:26])([F:25])[F:24])[CH2:5][CH:6]1[CH2:11][CH:10]([C:12]([O:14]C)=[O:13])[CH2:9][CH2:8][N:7]1[C:16]([O:18][CH3:19])=[O:17].[Br-].[Li+].C(N(CC)CC)C>C(#N)C.O>[F:1][C:2]1[CH:3]=[C:4]([CH:20]=[CH:21][C:22]=1[C:23]([F:26])([F:24])[F:25])[CH2:5][CH:6]1[CH2:11][CH:10]([C:12]([OH:14])=[O:13])[CH2:9][CH2:8][N:7]1[C:16]([O:18][CH3:19])=[O:17]
To a solution of dimethyl 2-(3-fluoro-4-(trifluoromethyl)benzyl)piperidine-1,4-dicarboxylate (7.17 g, 19.00 mmol) in acetonitrile (60 mL) and water (1.2 mL) was added lithium bromide (13.20 g, 152.02 mmol) and triethylamine (10.54 mL, 76.01 mmol). The resulting mixture was heated under reflux for 2 h, then cooled to room temperature. Water was added and acetonitrile removed by evaporation. The aqueous layer was further diluted with water, then washed with MTBE. The organic layer was extracted with water. The combined aqueous layers were acidified with 3.8 M HCl, a colorless precipitate formed. The solids were extracted with DCM 6 times. The aqueous layer was extracted twice again with DCM. The combined organic layers were dried over MgSO4 and evaporated. 2-(3-Fluoro-4-(trifluoromethyl)benzyl)-1-(methoxycarbonyl)piperidine-4-carboxylic acid (6.56 g, 95%) was isolated as an off-white solid. MS m/z 364 (M+H)+
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