N-(Tert-butoxy carbonyl)-D-serine methyl ester (10 g, 45.6 mmol) was dissolved in dichloromethane (200 mL). Methane sulfonyl chloride (Ms-Cl, 6 mL, 77.5 mmol) was then added to this solution under vigorous stirring. The reaction mixture was cooled to 0 °C before triethylamine (TEA, 23 mL, 165.9 mmol) was added dropwise. The solution was stirred at 0 °C for 1 h, and then for a further 2 h at room temperature. The reaction mixture was then washed with a potassium bisulfate solution (1 %) to neutrality. The organic phase was dried over Na2SO4, filtered, and the solvent removed under reduced pressure. The product was further purified via column chromatography with silica gel (n-hexane:ethyl acetate = 8/2 (v/v)). The product was then dried under reduced pressure to obtain a colorless oil with a yield of 87 % (8 g, 39 mmol).