[CH2:1]([N:4]1[CH2:9][CH:8]([OH:10])[C:7]2[CH:11]=[CH:12][S:13][C:6]=2[S:5]1(=[O:15])=[O:14])[CH:2]=[CH2:3].[CH2:16]([O:18][CH:19]=[CH2:20])[CH3:17]>O1CCCC1.C(OCC)(=O)C.C1(C)C=CC(S(O)(=O)=O)=CC=1>[CH2:1]([N:4]1[CH2:9][CH:8]([O:10][CH2:17][CH2:16][O:18][CH2:19][CH3:20])[C:7]2[CH:11]=[CH:12][S:13][C:6]=2[S:5]1(=[O:15])=[O:14])[CH:2]=[CH2:3]The product from Step A (4.2 g, 17.1 mmol) was dissolved in tetrahydrofuran (75 mL) and cooled to 0° C. at which point p-toluenesulfonic acid (163 mg, 0.6 mmol) was added followed by ethylvinyl ether (3.3 mL, 34.3 mmol). This mixture was stirred at 0° C. for 2 h, diluted with cold ethyl acetate (100 mL) and washed with saturated sodium bicarbonate (70 mL) and brine (70 mL). The organic layer was dried (MgSO4) and evaporated to provide 5.2 g of crude product which was used in the next step without further purification.
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