Methyl (S)-2-((tert-butoxycarbonyl)(methyl)amino)-3-(4-((tert-butyldimethylsilyl)oxy)phenyl)propionate (57) (4.0 g, 9.44 mmol) was used as a raw material, which was dissolved in a mixed solvent of THF:MeOH:H2O (3:1:1, 14.4 mL:4.8 mL:4.8 mL). LiOH-H2O (0.714 g, 17.02 mmol) was added to this solution at 0 °C. The reaction mixture was stirred at the same temperature for 1 h. LCMS monitoring showed complete consumption of the feedstock. After completion of the reaction, the reaction mixture was diluted with water (40 mL) and ethyl acetate (50 mL). The aqueous layer was acidified to pH 4-5 with 5% citric acid solution and subsequently extracted with ethyl acetate (2 x 100 mL). The organic layers were combined, dried over sodium sulfate, and concentrated under reduced pressure to afford (S)-2-((tert-butoxycarbonyl)(methyl)amino)-3-(4-hydroxyphenyl)propanoic acid (58) (2.78 g, 9.4 mmol, 100% yield) as a slightly reddish oil, which could be used for the next step of the reaction without further purification.MS (ESI): 294.2, [M-1]-.
