Benzyl bromide (4.40 g, 25.7 mmol) was added dropwise to a mixture of (2S,3R)-2-(tert-butoxycarbonylamino)-3-hydroxy-3-(4-methoxyphenyl)propanoic acid (4.00 g, 12.9 mmol) and Cs2CO3 (4.20 g, 12.9 mmol) in DMF (80 mL) at 0° C. The reaction mixture was allowed to warm to ambient temperature and stirred for 1 h. Water (80 mL) was added and the resulting mixture was extracted with EtOAc (100 mL×2). The combined extracts were washed with water (100 mL) and brine (100 mL), dried over anhydrous sodium sulfate, and concentrated. The residue was purified by flash column chromatography on silica gel (petroleum ether/EtOAc=10:1 to 4:1) to afford (2S,3R)-benzyl 2-((tert-butoxycarbonyl)amino)-3-hydroxy-3-(4-methoxyphenyl)propanoate (3.7 g, 66% yield) as a colorless solid.
