To a solution of 4-benzyloxycarbonylamino-piperidine-1,4-dicarboxylic acid mono-tert-butyl ester (2.09 g, 5.52 mmol) in DMF (10 mL) was added 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (1.27 g, 6.63 mmol), 1-hydroxy-7-azabenzotriazole (0.373 g, 2.76 mmol) and triethylamine (0.924 mL, 6.63 mmol), followed by ammonia (0.5 M in MeOH, 13.3 mL, 6.63 mmol). After 18 h the mixture was concentrated and the residue was partitioned between saturated aqueous NaHCO3 and ethyl acetate, the organic layer dried over MgSO4, and concentrated. The crude product was purified by silica gel chromatography, eluting with a gradient of 0 to 6% methanol:dichloromethane to give the title compound (0.43 g). MS: m/z=378.2 (M+1).
