[Synthesis]
General procedure for the synthesis of (R)-4-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-5-amino-5-oxoglutarate from (R)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-5-amino-5-oxopentanedioic acid: Fmoc-D-Glu-OH (59.8 g, 1.0 equiv) was dissolved in anhydrous tetrahydrofuran (THF, 324 mL). N,N'-dicyclohexylcarbodiimide (DCC, 40.1 g, 1.2 eq.) was added slowly with stirring under cooling in an ice-water bath. The reaction mixture was gradually warmed to room temperature and stirring was continued for 8 hours to produce a 1,3-dicyclohexylurea (DCU) precipitate. The precipitate was removed by filtration and washed with a small amount of THF. Subsequently, dry ammonia was passed into the filtrate under NaCl-ice bath cooling and stirring was continued for 1.5 h until no white solid precipitated. After standing for 30 min, a small amount of methanol (MeOH) was added to dissolve the solid. The mixture was again cooled in an ice water bath and the pH was adjusted to 2-3 by slow dropwise addition of 2.0 N HCl solution. the solvent was removed by concentration under reduced pressure. The resulting solid was dissolved in ethyl acetate (AcOEt) and washed sequentially with dilute HCl, saturated aqueous NaHCO3 solution and deionized water. The organic layers were separated, combined and dried with anhydrous magnesium sulfate (MgSO4) overnight. After filtration, the solvent was evaporated under reduced pressure and the residue was recrystallized by ethyl acetate-cyclohexane system. After filtration, 46.5 g of the target product was obtained in 78% yield. The product had a melting point of 204-205 °C and a specific optical rotation [α] = -4.2° (c = 10 mg/mL, DMF).1H-NMR (500 MHz, DMSO-d6) δ: 7.88 (2H, d, J = 8.0 Hz), 7.72 (2H, m), 7.42 (2H, m), 7.40 (1H, m), 7.40 (1H, br. s), 7.32 (2H, m), 7.02 (1H, br. s), 4.27 (2H, m), 4.20 (1H, m), 3.93 (1H, dd, J = 13.5, 8.5 Hz), 2.25 (2H, m), 1.89 (1H, m), 1.73 (1H, m).13C-NMR (125 MHz, DMSO-d6 ) δ: 173.9, 173.4, 155.9, 143.8, 140.7, 127.6, 127.0, 125.3, 120.0, 65.6, 53.8, 46.6, 30.4, 27.2. ESI-MS: m/z 369.03 [M + H]+, 759.98 [2M + Na]+. HR-MS (TOF) : m/z 369.1448 [M + H]+ (calculated value: 369.1449), 759.2623 [2M + Na]+ (calculated value: 759.2625), molecular formula C20H20N2O5. |