The general procedure for the synthesis of Fmoc-L-cystine from L-cystine and 9-fluorenylmethyl chloroformate was as follows: sodium carbonate (4.6 g, 43.6 mmol) and L-cystine (5.0 g, 20.8 mmol) were dissolved in deionized water (200 mL). The reaction system was cooled to 10 °C. Methyl 9-fluorenyl chloroformate (Fmoc-Cl, 11.85 g, 45.8 mmol) was dissolved in 1,4-dioxane (80 mL) and this solution was slowly added dropwise to the above aqueous L-cystine solution. The reaction mixture was stirred continuously at 10 °C for 2 h, followed by natural warming to room temperature. Upon completion of the reaction, a thick white precipitate was produced, which was drained through a sintered glass funnel. The resulting solid was washed with ether (50 mL) and ground, followed by drying under vacuum for 48 hours. The final N,N'-bis(Fmoc)-L-cystine (14.0 g, 98% yield) was obtained as a white powdery product.
