CN(C(ON1N=NC2C=CC=NC1=2)=[N+](C)C)C.F[P-](F)(F)(F)(F)F.CCN(C(C)C)C(C)C.[NH2:34][C@@H:35]([C@H:46]([OH:55])[C:47]1[CH:52]=[CH:51][C:50]([O:53][CH3:54])=[CH:49][CH:48]=1)[C:36]([O:38][CH2:39][C:40]1[CH:45]=[CH:44][CH:43]=[CH:42][CH:41]=1)=[O:37].[NH:56]([C:63]([O:65][C:66]([CH3:69])([CH3:68])[CH3:67])=[O:64])[C@H:57]([C:60](O)=[O:61])[CH2:58][OH:59]>CN(C=O)C>[C:66]([O:65][C:63]([NH:56][C@@H:57]([CH2:60][OH:61])[C:58]([NH:34][C@@H:35]([C@H:46]([OH:55])[C:47]1[CH:52]=[CH:51][C:50]([O:53][CH3:54])=[CH:49][CH:48]=1)[C:36]([O:38][CH2:39][C:40]1[CH:41]=[CH:42][CH:43]=[CH:44][CH:45]=1)=[O:37])=[O:59])=[O:64])([CH3:69])([CH3:68])[CH3:67]Sequentially HATU (645 mg, 1.70 mmol) and DIEA (0.99 mL, 5.7 mmol) were added to a 0° C. solution of (2S,3R)-benzyl 2-amino-3-hydroxy-3-(4-methoxyphenyl)propanoate (HCl salt, 477 mg, 1.41 mmol) and Boc-Ser-OH (290 mg, 1.41 mmol) in DMF (8 mL). The reaction mixture was allowed to warm to ambient temperature and stirred for 30 min. The mixture was concentrated and the residue was purified by flash column chromatography on silica gel (petroleum ether/EtOAc=2:1) twice to afford (2S,3R)-benzyl 2-((S)-2-((tert-butoxycarbonyl)amino)-3-hydroxypropanamido)-3-hydroxy-3-(4-methoxyphenyl)propanoate (646 mg, 93% yield) as a colorless solid.
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