NH4Cl (1.39 g, 26.18 mmol), HOBt (3.53 g, 26.18 mmol), EDCI (5 g, 26.18 mmol), and DIPEA (8.44 g, 65.45 mmol) were added to a solution of (2S,5R)-5-(N-(benzyloxy)-2-nitrophenylsulfonamido)-1-(tert-butoxycarbonyl)piperidine-2-carboxylic acid (10 g, 18.7 mmol) in DMF (90 mL) The mixture was stirred at rt for 17 hrs, then, EtOAc (500 mL) was added. he mixture was washed with saturated sodium chloride (6×), dried over Na2SO4, and concentrated. The residue was washed with petroleum ether/EtOAc (95:5, 200 mL) to give (2S,5R)-tert-butyl 5-(N-(benzyloxy)-2-nitrophenylsulfonamido)-2-carbamoylpiperidine-1-carboxylate (9.9 g, 99%) which was used in the next step. ESI-MS (EI+, m/z): 535.0 [M+H]+.
