To a stirred solution of L-glutamine (1.75 g, 12 mmol) and sodium carbonate (1.27 g, 12 mmol) in 30 mL of water, a solution of 9-fluorenylmethyl-N-succinimidyl carbonate (Fmoc-OSu, 3.0 g, 8.9 mmol) dissolved in 60 mL of tetrahydrofuran (THF) was slowly added. The reaction mixture was stirred overnight at room temperature. Subsequently, THF was removed by distillation under reduced pressure. the residue was diluted with 100 mL of 1N hydrochloric acid and the precipitated white solid was collected by filtration. The solid was further recrystallized in a mixed aqueous solution of DMF with 0.1 N hydrochloric acid to afford the target product (S)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-5-amino-5-oxopentanoic acid as a white powder (2.76 g, 84% yield). The product was characterized by 1H-NMR (90 MHz, DMSO-d6) with chemical shifts of 12.47 (1H, s), 7.89-7.20 (10H, m), 6.68 (1H, s, br), 4.22-3.90 (4H, m), and 2.25-1.86 (4H, m).
