[CH2:1]([NH:5][C:6]([C:8]1[C@:9]2([CH2:25][CH2:24][C@H:23]3[C@@H:14]([CH2:15][CH2:16][C:17]4[CH:18]=[C:19]([O:26]C)[CH:20]=[CH:21][C:22]=43)[C@@H:11]2[CH2:12][CH:13]=1)[CH3:10])=[O:7])[CH2:2][CH2:3][CH3:4].B(Br)(Br)Br>C(Cl)Cl>[CH2:1]([NH:5][C:6]([C:8]1[C@:9]2([CH2:25][CH2:24][C@H:23]3[C@@H:14]([CH2:15][CH2:16][C:17]4[CH:18]=[C:19]([OH:26])[CH:20]=[CH:21][C:22]=43)[C@@H:11]2[CH2:12][CH:13]=1)[CH3:10])=[O:7])[CH2:2][CH2:3][CH3:4]To a solution comprising 119 mg of compound 9a in 2 ml of anhydrous CH2Cl2 was added 0.64 ml of BBr3 (1M solution in CH2Cl2) at −15° C. under nitrogen atmosphere. The reaction solution was stirred at room temperature for 1.3 hours and quenched by adding methanol and water at 0° C. The organic layer was separated and the aqueous layer was extracted with chloroform (CHCl3). The combined organic layer was dried with Na2SO4 and concentrated in vacuo. The residue was triturated with 2.5 ml of EtOAc and 2.5 ml of hexane, yielding compound 10a (89 mg, 78%).
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