[CH2:1]([O:3][CH2:4][CH2:5][CH2:6][N:7]1[CH2:12][CH:11]([OH:13])[C:10]2[CH:14]=[CH:15][S:16][C:9]=2[S:8]1(=[O:18])=[O:17])[CH3:2].[CH:19]([O:21][CH2:22][CH3:23])=[CH2:20].C(=O)(O)[O-].[Na+]>C1COCC1.C1(C)C=CC(S(O)(=O)=O)=CC=1>[CH2:19]([O:21][CH:22]([O:13][CH:11]1[C:10]2[CH:14]=[CH:15][S:16][C:9]=2[S:8](=[O:17])(=[O:18])[N:7]([CH2:6][CH2:5][CH2:4][O:3][CH2:1][CH3:2])[CH2:12]1)[CH3:23])[CH3:20]A solution of the product from Step B (13.0 g, 44.77 mmol) and p-toluenesulfonic acid (0.20 g) in THF (250 mL) was cooled in an ice bath and ethyl vinyl ether (4.7 mL, 49.24 mmol) was added slowly; this mixture was stirred at room temperature for 18 hr. Sodium bicarbonate (2.0 g) was added and the mixture stirred for 30 min, washed with water (3×75 mL), saturated aqueous sodium chloride (100 mL) and dried (MgSO4). The mixture was evaporated to a residue which was filtered through silica gel with 3:1 hexane/ethyl acetate to give a pale yellow oil (7.53 g, 46%) which was used in the next reaction.
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