The general procedure for the synthesis of N2-(((9H-fluoren-9-yl)methoxy)carbonyl)-N6-(tert-butoxycarbonyl)-N6-methyl-L-lysine from di-tert-butyl dicarbonate and (S)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-6-(methylamino)hexanoic acid was performed as follows: the light yellow solid crude product was dissolved in water (500 mL) and THF (500 mL) in a 1:1 solvent mixture, sodium bicarbonate (63 g, 0.75 mol) was added, and the solution was stirred until clarified. Subsequently, Boc2O (142 g, 0.65 mol) was dissolved in tetrahydrofuran (200 mL), added to the reaction system and reacted for 4.0 h at room temperature. Complete reaction of the feedstock was confirmed by HPLC monitoring. After completion of the reaction, the reaction solution was neutralized with dilute hydrochloric acid to pH 5-6. The organic solvent was removed by concentration under reduced pressure and the aqueous phase was extracted with ethyl acetate. The organic phase was washed with saturated brine and concentrated to give a white solid product (235 g). This white solid was further purified by recrystallization from ethyl acetate and n-hexane, resulting in 224 g of white solid powder in 93% yield and 99.8% purity.
