GENERAL STEPS: To a mixed solution of 1,4-dioxane (15 mL) and water (90 mL) containing L-serine methyl ester hydrochloride (10.0 g, 64.3 mmol), sodium bicarbonate (10.8 g, 129 mmol) was added. The reaction mixture was stirred at room temperature for 15 minutes. Subsequently, a solution of 1,4-dioxane (60 mL) of 2,5-dioxopyrrolidin-1-yl 9H-fluoren-9-ylmethyl carbonate (21.7 g, 64.3 mmol) was slowly added to the reaction system, and stirring was continued at room temperature for 14 hours. Upon completion of the reaction, water was added to the reaction mixture and extracted three times with ethyl acetate. The organic layers were combined and washed sequentially with water and saturated brine. The organic layer was dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. Ether and heptane were added to the residue and the precipitate was collected by filtration to give fmoc-L-serine methyl ester (22.3 g, yield: quantitative). The product was characterized by 1H-NMR (400 MHz, CDCl3): δ 2.00-2.15 (1H, m), 3.81 (3H, s), 3.89-4.07 (2H, m), 4.20-4.28 (1H, m), 4.39-4.53 (3H, m), 5.63-5.74 (1H, m), 7.29-7.37 (2H , m), 7.38-7.46 (2H, m), 7.55-7.65 (2H, m), 7.74-7.82 (2H, m).
