Dicyclohexylcarbodiimide (1.55 g, 7.52 mmol) and N-hydroxysuccinimide (762 mg, 6.63 mmol) were sequentially added to a stirred solution of Fmoc-L-valine (1.5 g, 4.42 mmol) in dichloromethane (25 mL) at room temperature. The reaction mixture was stirred continuously for 3 hours at room temperature. Upon completion of the reaction, the white solid dicyclohexylurea produced in the reaction was removed by filtration. The organic phase was washed sequentially with 0.1 N HCl solution and deionized water, followed by drying with anhydrous sodium sulfate. The solvent was removed by distillation under reduced pressure to give the crude product. The crude product was purified by fast column chromatography using 1% dichloromethane solution in methanol as eluent to give the final target product (S)-2,5-dioxopyrrolidin-1-yl 2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-methylbutyrate as a white solid (1.7 g, 89% yield). Mass spectrum (MS): m/z 459 [M + Na]+. 1H NMR (400 MHz, CDCl3) δ 7.80 (d, J = 7.5 Hz, 2H), 7.68-7.55 (m, 2H), 7.43 (t, J = 7.4 Hz, 2H), 7.34 (dd, J = 15.9, 8.5 Hz, 2H), 4.70 (d, J = 4.6 Hz, 1H), 4.56-4.40 (m, 3H), 4.28 (t, J = 6.6 Hz, 1H), 2.85 (s, 4H), 2.37 (dd, J = 12.3, 6.5 Hz, 1H), 1.08 (dd, J = 11.0, 6.9 Hz, 6H).
