GENERAL STEPS: In a 500 mL single-necked round-bottomed flask, L-aspartic acid (4 g, 30 mmol), 1,4-dioxane (120 mL) and deionized water (60 mL) were mixed to form a heterogeneous solution. Under continuous stirring, 1 M aqueous sodium hydroxide solution was added dropwise until the solution became homogeneous and clarified, indicating complete dissolution of L-aspartic acid. Subsequently, the reaction mixture was cooled in an ice bath and di-tert-butyl dicarbonate (7.2 g, 33 mmol) dissolved in 1,4-dioxane (20 mL) was added slowly and dropwise. The reaction mixture was stirred overnight at room temperature. Upon completion of the reaction, the solvent was partially evaporated using a rotary evaporator until the final volume was about 30 mL. ethyl acetate (20 mL) was then added and the aqueous layer was acidified to pH 2 with aqueous potassium bisulfate under ice-bath cooling conditions. the mixture was transferred to a partitioning funnel, and the product was extracted with ethyl acetate (3 aliquots at a time). The organic layers were combined and dried with anhydrous sodium sulfate. Subsequently, the solvent was evaporated using a rotary evaporator and further dried under vacuum overnight to give BOC-L-aspartic acid as a white solid (6 g, 85% yield).1H NMR (600 MHz, DMSO-d6) data: δ 1.38 (s, 9H), 2.51-2.55 (m, 1H), 2.65-2.69 (m, 1H). 4.24-4.28 (m, 1H), 7.05-7.07 (d, 1H, J = 12 Hz), 12.5 (s, 2H).
