30.5 g (207 mmol) (S)-2-Amino-4-methoxy-4-oxobutanoic acid was dissolved in 726 mL of distilled water and 260 mL of ether was added. Subsequently, 40.15 g (290 mmol) of potassium carbonate was added and stirred until completely dissolved, then 41.5 mL (290 mmol) of benzyl chloroformate was slowly added. The reaction mixture was stirred continuously for 4 hours at room temperature. After completion of the reaction, phase separation was carried out and the organic layer was discarded. The aqueous layer was washed several times with ether and the pH was subsequently adjusted with 1 N hydrochloric acid to 1. The aqueous layer was again extracted several times with ether, the organic layers were combined and dried over magnesium sulfate. Finally, 55 g of (2S)-4-methoxy-4-oxo-2-(phenylmethoxycarbonylamino)butanoic acid was obtained by vacuum concentration in 94% yield. The product was characterized by 1H NMR (300 MHz, CDCl3): δ 8.23 (s, 1H), 7.35 (m, 5H), 5.87 (d, 1H), 5.12 (s, 2H), 4.69 (m, 1H), 3.68 (s, 3H), 2.97 (dd, 2H).
