[O:1]=[C:2]1[NH:6][C@H:5]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH2:4][CH2:3]1.[Li+].C[Si]([N-][Si](C)(C)C)(C)C.[CH:24]1[CH:29]=[CH:28][C:27]([CH2:30][O:31][C:32](Cl)=[O:33])=[CH:26][CH:25]=1>C1COCC1>[O:1]=[C:2]1[N:6]([C:32]([O:31][CH2:30][C:27]2[CH:28]=[CH:29][CH:24]=[CH:25][CH:26]=2)=[O:33])[C@H:5]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH2:4][CH2:3]1To a solution of 1a (2.00 g, 10.8 mmol) in 50 mL THF at −78° C., was added LiHMDS (1M THF, 11.9 mL, 11.9 mmol). The mixture was stirred at −78° C. for 20 min, then Cbz-Cl was added, dropwise over 5 min. The mixture was stirred at −78° for 20 min, then quenched with sat. NH4Cl. The mixture was diluted with H2O and extracted with EtOAc (3×). The organic phase was washed with brine, dried (Na2SO4), and concentrated. The crude product was purified by flash chromatography (30 to 35 to 40% EtOAc/hexanes) to afford 3.12 g (90%) of 8a as a colorless solid.
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