Cbz-L-aspartic acid di-tert-butyl ester (8.25 g, 21.8 mmol) was added to an ethanol solution (20 mL) of Pd/C (10%, 830 mg, 10% w/w) and ammonium formate (34.3 g, 0.54 mol). The suspension was stirred overnight at room temperature. Upon completion of the reaction, the suspension was filtered through diatomaceous earth and concentrated under reduced pressure to remove the solvent. The resulting residue was dissolved in ethyl acetate, cooled to 0 °C, and a solution of ethyl acetate saturated with HCl was added slowly dropwise and stirred for 15 min. The precipitated solid was collected by filtration, washed with cold ethyl acetate and dried under vacuum to give (S)-di-tert-butyl 2-aminosuccinate hydrochloride (4.28 g, 17.4 mmol, 80% yield) as a white solid, which can be used in subsequent steps without further purification.
