[C:1]([O:5][C:6]([NH:8][CH:9]([CH3:13])[C:10]([OH:12])=[O:11])=[O:7])([CH3:4])([CH3:3])[CH3:2].C(=O)(O)[O-].[Na+].S(Cl)(O[CH2:23][Cl:24])(=O)=O>S([O-])(O)(=O)=O.C([N+](CCCC)(CCCC)CCCC)CCC.O.ClCCl.C(OCC)(=O)C>[Cl:24][CH2:23][O:11][C:10](=[O:12])[CH:9]([NH:8][C:6]([O:5][C:1]([CH3:4])([CH3:2])[CH3:3])=[O:7])[CH3:13]To a solution of 2-tert-butoxycarbonylamino-propionic acid (0.507 g, 2.68 mol), sodium bicarbonate (1.13 g, 13.4 mmol) and tetrabutylammonium hydrogen sulfate (0.090 g, 0.268 mmol) in H2O (21 mL) at 0° C. was added dichloromethane (12 mL) and the mixture was stirred vigorously. Chloromethyl chlorosulfate (0.326 mL, 3.22 mmol) in dichloromethane (7 mL) was added and the mixture was warmed to rt overnight. The reaction mixture was diluted with ethyl acetate, washed with brine, dried (MgSO4) and concentrated. The resulting oil was purified by flash column chromatography (silica gel, 0 to 30% ethyl acetate/hexane) to give a colorless oil (4) (0.6207 g, 97%). 1H NMR (300 MHz, CDCl3): 5.75 (d, J=5.4 Hz, 1H), 5.57 (d, J=5.4 Hz, 1H), 5.04 (br s, 1H), 4.28 (m, 1H), 1.37 (s, 9H), 1.32 (s, 3H).
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