[CH2:1]([O:8][C:9](=[O:22])[CH:10]([NH:14][C:15]([O:17][C:18]([CH3:21])([CH3:20])[CH3:19])=[O:16])[CH2:11][CH2:12][OH:13])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.C(N(CC)CC)C.S(=O)(=O)=O.N1C=CC=CC=1>CS(C)=O>[CH2:1]([O:8][C:9](=[O:22])[CH:10]([NH:14][C:15]([O:17][C:18]([CH3:20])([CH3:19])[CH3:21])=[O:16])[CH2:11][CH:12]=[O:13])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1A dimethyl sulfoxide (10 mL) solution of 2-tert-butoxycarbonylamino-4-hydroxy-butyric acid benzyl ester (13.58 g, 43.9 mmol) and triethyl amine (18.35 mL, 131.7 mmol) was cooled to 0° C. Pyridine sulfur trioxide (21.0 g. 131.7 mmol) in dimethyl sulfoxide (80 mL) was added in a steady stream. The cooling bath was removed and the reaction stirred 1.5 hours. The reaction was poured into 130 mL ice water and extracted with diethyl ether (2×180 mL and 100 mL). The combined organic portion was washed sequentially with saturated sodium bicarbonate, water, brine and dried over magnesium sulfate. The solvent was removed in vacuo and the crude product was chromatographed on silica gel (900 mL) eluting with ethyl acetate/hexanes (1:4), to give the title compound as a clear, colorless oil (9.20 g, 70%). MS (CI) m/z 208.0 (M−99).
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