The (S)-2-((tert-butoxycarbonyl)amino)-3-(3-chloro-4-fluorophenyl)propanoic acid (183.0 mg, 0.576 mmol) was suspended in a 4M hydrochloric acid (1440 μl, 5.76 mmol) solution in dioxane at room temperature for 2 h. After solvent evaporation, the crude product was cooled to 0° C. using an ice bath before being dissolved in MeCN (288 μl) and a 2.0M aqueous solution of Na2CO3 (288 μl, 0.576 mmol). (9H-fluoren-9-yl)methyl (2,5-dioxopyrrolidin-1-yl) carbonate (194 mg, 0.576 mmol) was added to the solution and the reaction mixture was allowed to stir for 30 min. The mixture was diluted in 5.0 mL of EtOAc and the aqueous phase was neutralized with a few drops of an aqueous 1N HCl solution. The organic solvents were evaporated and the crude product was then partitioned between a saturated aqueous NH4Cl solution and EtOAc. The crude product was extracted with EtOAc. The organic fractions were combined and dried over anhydrous sodium sulfate. After filtration and concentration, a yellow crystalline solid was obtained. Purification by silica gel chromatography using 100% hexanes to 30% EtOAc/Hexanes yielded the desired product as a white crystalline solid. A second purification on silica gel was performed using 100% DCM to 20% MeOH/DCM as eluant. The desired product, (S)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-(3-chloro-4-fluorophenyl)propanoic acid, was obtained as a white solid (0.111 g, 44%). 1H NMR (400 MHz, methanol-d4) δ 7.79 (d, J=7.5 Hz, 2H), 7.60 (t, J=6.8 Hz, 2H), 7.47-7.23 (m, 5H), 7.25-7.14 (m, 1H), 7.13 (d, J=9.0 Hz, 1H), 4.52-4.11 (m, 4H), 3.19 (dd, J=14.3, 4.6 Hz, 1H), 2.92 (dd, J=13.9, 9.7 Hz, 1H).
