N[C@@H:2]1[CH2:7][CH2:6][N:5]([C:8]([O:10][C:11]([CH3:14])([CH3:13])[CH3:12])=[O:9])[CH2:4][C@H:3]1[C:15]([O:17][CH2:18][CH3:19])=[O:16].C(N([CH2:25][CH3:26])CC)C>C(Cl)Cl>[CH2:11]([O:10][C:8]([C@@H:2]1[CH2:7][CH2:6][N:5]([C:8]([O:10][C:11]([CH3:14])([CH3:13])[CH3:12])=[O:9])[CH2:4][C@@H:3]1[C:15]([O:17][CH2:18][CH3:19])=[O:16])=[O:9])[C:26]1[CH:25]=[CH:4][CH:3]=[CH:2][CH:7]=1A solution of 252E (180 mg, 0.66 mmol) in CH2Cl2 (5 mL) was treated with benzyloxychloroformate (0.1 mL, 0.73 mmol) and triethylamine (0.12 mL, 0.86 mmol). The reaction was stirred at room temperature for 18 hours, diluted with CH2Cl2 (10 mL) and washed with water (2×10 mL), 0.1 N HCl (2×10 mL), saturated aqueous NaHCO3 (2×10 mL) and brine (1×10 mL). The organic layer was dried (Na2SO4), filtered and concentrated to give 243 mg (yield: 91%) of Compound 254A as an oil.
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