C(O)(=O)C.[CH3:5][O:6][C:7]([C@@H:9]1[CH2:14][CH2:13][C@@H:12]([NH2:15])[C@H:11]([OH:16])[CH2:10]1)=[O:8].[C:17](=O)([O:23]C(C)(C)C)[O:18][C:19]([CH3:22])([CH3:21])[CH3:20].C(N(C(C)C)CC)(C)C>O1CCCC1>[CH3:5][O:6][C:7]([C@@H:9]1[CH2:14][CH2:13][C@@H:12]([NH:15][C:17]([O:18][C:19]([CH3:22])([CH3:21])[CH3:20])=[O:23])[C@H:11]([OH:16])[CH2:10]1)=[O:8]The acetate salt (c) (15.6 g, 66 mmol) was suspended in tetrahydrofuran (200 ml) and treated with di-tert-butyl carbonate (16 g, 73 mmol) and diisopropylethylamine (14 ml, 10.4 g, 80 mmol). The mixture was heated at 50° C. for 18 hours then evaporated to dryness. The residue was partitioned between water and ethyl acetate. The aqueous phase was further extracted with ethyl acetate and the combined extracts washed with brine, dried and evaporated to give a solid (18.5 g, 100%).
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