Step 2: Synthesis of 2,5-dioxopyrrolidin-1-yl 2-(tert-butoxycarbonylamino) acetate. N-Boc-glycine (7.31 g, 41.7 mmol) was dissolved in 100 mL of dichloromethane (DCM) and the solution was cooled to 15 °C. Subsequently, N-hydroxysuccinimide (5.28 g, 45.9 mmol) was added to the cooled solution. Under vigorous stirring, N,N'-dicyclohexylcarbodiimide (9.47 g, 45.9 mmol) was added to the formed suspension. The reaction mixture transformed into a cloudy white suspension within seconds. The reaction mixture was gradually warmed to room temperature and stirring was continued for 1 hour. Upon completion of the reaction, the insoluble by-products were removed by filtration through diatomaceous earth, the filtrate was washed with 50 mL of saturated sodium bicarbonate solution, dried over anhydrous sodium sulfate, and finally concentrated under vacuum to give the product as a white crystalline powder. Yield: 7.02 g, yield 61.8%. The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 1.38 (s, 9H), 2.80 (s, 4H), 4.07 (d, J = 6 Hz, 2H), 7.43 (br s, 1H). Mass spectrum (ESI) m/z 567.2 [2M + Na]+.
