To a room temperature CH2Cl2 solution (50 mL) of 2-benzyloxy-4-nitro-benzenesulfonyl chloride from Route J (2.62 g, 8.0 mmol) was added (S)-3-amino-succinamic acid tert-butyl ester (1.50 g, 8.0 mmol) and pyridine (2.60 mL). The reaction mixture was allowed to stir for 12 h at room temperature. The reaction was then diluted with ethyl acetate (200 mL) and washed with 5% citric acid and then saturated NaHCO3. The organic phase was dried (MgSO4), and then the solvent was removed under vacuum. The resulting crude product was purified using silica gel flash chromatography (40% hexanes in EtOAc). (S)-3-(2-Benzyloxy-4-nitro-benzenesulfonylamino)-succinamic acid tert-butyl ester was obtained in 43% yield (1.66 g) as an off white foam. MS (APCI) m/z 478.3 (M−1).
