L-citrulline (1.1 equiv.) and sodium bicarbonate (2.2 equiv.) were stirred in water (0.2 M) at room temperature for 1.0h, then ethylene glycol dimethyl ether was added to the above (dimethyl ether:water= 1:1, total concentration of 0.1 M) reaction mixture, and then Fmoc-Cl (1.0 equiv.) was added, and the reaction was stirred at room temperature for 24.0h, the solvent dimethyl ether was removed under reduced pressure . The aqueous solution was extracted three times with ethyl acetate. The aqueous phase was acidified with 2M HCl (pH=1) to form a white precipitate. 10% iPrOH-EtOAc was added to the precipitate in water and stirred well to dissolve all the white precipitate in the organic solvent. The organic phase was collected and the aqueous layer was further extracted 2 times with the same 10% iPrOH-EtOAc solvent. The collected organic phase was dried over sodium sulfate. The solvent was evaporated under reduced pressure to give a clear viscous liquid. Ultrasonication with ether gave the white solid powder product Fmoc-L-citrulline, the white solid was further dried under vacuum in 96% yield.
