F[B-](F)(F)F.[CH2:6]([O+](CC)CC)[CH3:7].[CH3:13][O:14][C:15]1[CH:24]=[C:23]2[C:18]([C:19]([O:26][C@H:27]3[CH2:31][N:30]([C:32]([O:34][C:35]([CH3:38])([CH3:37])[CH3:36])=[O:33])[C@H:29]([C:39]([O:41][CH3:42])=[O:40])[CH2:28]3)=[CH:20][C:21](=[O:25])[NH:22]2)=[CH:17][C:16]=1[CH:43]=[CH2:44].CCN(C(C)C)C(C)C.C([O-])(O)=O.[Na+]>C(Cl)Cl>[CH2:6]([O:25][C:21]1[CH:20]=[C:19]([O:26][C@H:27]2[CH2:31][N:30]([C:32]([O:34][C:35]([CH3:38])([CH3:37])[CH3:36])=[O:33])[C@H:29]([C:39]([O:41][CH3:42])=[O:40])[CH2:28]2)[C:18]2[C:23](=[CH:24][C:15]([O:14][CH3:13])=[C:16]([CH:43]=[CH2:44])[CH:17]=2)[N:22]=1)[CH3:7]Triethyloxonium tetrafluoroborate (1.28 g, 6.75 mmol) was added to a solution of the product from step 3 (2.0 g, 4.5 mmol) in DCM (41 mL). DIEA (0.236 mL, 1.35 mmol) was then added after 15 min. After 45 additional min, the reaction mixture was worked up with NaHCO3 and DCM. The organic layer was then dried over MgSO4, and the solvent was removed in vacuo. The crude material was then purified on silica (gradient elution, 10-60% EtOAc/hexanes) to yield the title compound as an oil. LRMS (M+H)+=473.4.
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