To a solution benzyl N2,N2-bis(tert-butoxycarbonyl)-5-(2,3-dichlorophenyl)-D-lysinate (0.45 g, 0.77 mmol) in MeOH (4 mL) and water (2 mL) was added 1N NaOH (3.1 mL, 3.1 mmol). This solution was heated to 40° C. for 6 h, then neutralized with 1N HCl and concentrated. To a solution of this residue in DCM (10 mL) were added EDC (0.19 g, 0.97 mmol) and HOAT (53 mg, 0.39 mmol) followed by triethylamine (0.32 mL, 0.73 mmol). After 2 h, NaHCO3 was added, the layers separated and the aqueous phase backwashed with DCM. The combined organics were dried over magnesium sulfate, filtered and concentrated. This residue was purified by silica gel chromatography (0.5%→10% MeOH/DCM) to give di-tert-butyl [(3R)-6-(2,3-dichlorophenyl)-2-oxoazepan-3-yl]imidodicarbonate (300 mg). MS 473 (M+1). Standard deprotection of a portion of this material gave the title product. MS 273 (M+1).
