F[C:2](F)(F)[C:3]([OH:5])=[O:4].C(OC([CH:15]1[CH2:20][CH2:19][N:18]([C:21]([O:23][CH2:24][C:25]2[CH:30]=[CH:29][CH:28]=[CH:27][CH:26]=2)=[O:22])[C:17](=C)[CH2:16]1)=O)(C)(C)C>C(Cl)Cl>[C:3]([CH:2]=[C:15]1[CH2:20][CH2:19][N:18]([C:21]([O:23][CH2:24][C:25]2[CH:26]=[CH:27][CH:28]=[CH:29][CH:30]=2)=[O:22])[CH2:17][CH2:16]1)([OH:5])=[O:4]Trifluoroacetic acid (10 ml) was added to 4-tert-butoxycarbonyl-methylene-N-CBZ-piperidine (20 g, 60.3 mmol) in methylene chloride (30 ml) and the solution was stirred at room temperature for 1.5 hours. After evaporation of the solvent, the residue was triturated with diethyl ether to give 4-carboxymethylene-N-CBZ-piperidine as a crystalline white solid.
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