[C:1]([O:5][C:6](=[O:23])[NH:7][C@H:8]([CH2:20][CH:21]=[CH2:22])[CH2:9][O:10][CH2:11][C:12]1[CH:17]=[CH:16][CH:15]=[CH:14][C:13]=1[CH2:18]O)([CH3:4])([CH3:3])[CH3:2].P(Br)(Br)[Br:25]>C(Cl)(Cl)(Cl)Cl>[C:1]([O:5][C:6](=[O:23])[NH:7][C@H:8]([CH2:20][CH:21]=[CH2:22])[CH2:9][O:10][CH2:11][C:12]1[CH:17]=[CH:16][CH:15]=[CH:14][C:13]=1[CH2:18][Br:25])([CH3:4])([CH3:3])[CH3:2]A solution of compound 18i (3.0 g, 9 mmol) in CCl4 (10 mL) was cooled in an ice bath. To this mixture was added PBr3 (1.0 g, 4.5 mmol) dropwise and the mixture was stirred at 0° C. for 2 hr. The mixture was quenched with water and extracted with DCM (3×50 mL). The combined organic layers were washed with brine and concentrated. The crude compound 18j (2.8 g, 80% yield) was used in the next step without purification. LCMS (m/z): 385.1 (MH+).
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