A mixture of (2S,5R)-5-(N-(benzyloxy)-2-nitrophenylsulfonamido)-1-(tert-butoxycarbonyl)piperidine-2-carboxylic acid (8.56 g, 16 mmol), EDCI (3.2 g, 16 mmol), HOBt (2.2 g, 16 mmol), NH4Cl (1.7 g, 32 mmol), and DIPEA (4.0 g, 32 mmol) in dry DMF (40 mL) was stirred at it overnight. The mixture was diluted with EtOAc (300 mL), and washed with water (3×). The organic layer was dried over Na2SO4 and concentrated to give (2S,5R)-tert-butyl 5-(N-(benzyloxy)-2-nitrophenylsulfonamido)-2-carbamoyl-piperidine-1-carboxylate as yellow solid (7.4 g, 86%). ESI-MS (EI+, m/z): 557 [M+Na]+.
