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Sodium hydroxide (8.4 g, 0.110 mol) was dissolved in water (53 mL) and stirred at room temperature until completely dissolved, followed by cooling to 5-10 °C. D-serine (10.5 g, 0.100 mol) and di-tert-butyl dicarbonate (26.2 g, 0.120 mol) were slowly added while the temperature was maintained at ≤10 °C. The reaction mixture was gradually heated to 30-35 °C and the reaction was stirred at this temperature for 20 hours. Upon completion of the reaction, an aqueous solution of BOC-L-serine was obtained (yield based on 100%, HPLC purity 99.1%, chiral purity 99.4%). This aqueous solution can be used directly in the subsequent synthesis steps without further purification.