[Br:1][C:2]1[S:6][C:5]([CH2:7][C@H:8]([NH:12][C:13]([O:15][C:16]([CH3:19])([CH3:18])[CH3:17])=[O:14])[C:9]([OH:11])=[O:10])=[CH:4][CH:3]=1.[CH3:20]CN(C(C)C)C(C)C.CI.O>CN(C=O)C>[CH3:20][O:10][C:9](=[O:11])[C@@H:8]([NH:12][C:13]([O:15][C:16]([CH3:19])([CH3:18])[CH3:17])=[O:14])[CH2:7][C:5]1[S:6][C:2]([Br:1])=[CH:3][CH:4]=1Esterification. To a solution of (S)-3-(5-bromo-thiophen-2-yl)-2-ter t-butoxycarbonylamino-propionic acid in DMF (50 mL) was added DIEA and methyl iodide. The reaction mixture stirred at rt for 2.5 h and was poured onto EtOAc and water. The organic layer was washed with 1 N HCl and 10% sodium carbonate, dried over sodium sulfate and concentrated to furnish (S)-3-(5-bromo-thiophen-2-yl)-2-tert-butoxycarbonylamino-propionic acid methyl ester. This product was used without further purification.
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