Acetyl chloride (54.6 mL, 0.75 mol) was slowly added dropwise to ethanol (316 mL) at 0-5 °C. After the dropwise addition was completed, the ice bath was removed and the reaction solution was continued to be stirred at room temperature for 30 min. Subsequently, D-aspartic acid (19.1, 25 g, 0.188 mol) was added to the reaction system. The reaction mixture was heated to reflux for 2 hours. Upon completion of the reaction, the reaction solution was concentrated in vacuum and dried under high vacuum (0.4 mmHg) overnight to afford compound 19.2 (42 g, 99% yield) as a white solid, which can be used in subsequent reactions without further purification.
