(S)-2-(((9H-fluoren-9-yl)methoxy)carbonyl)amino)-6-aminohexanoic acid (Fmoc-lysine, 2.03 g, 5.51 mmol) was used as a starting material, which was co-dissolved in methanol (70 mL) with CuSO4 (88 mg, 0.35 mmol) and K2CO3 (2.46 g, 17.83 mmol). Subsequently, imidazole-1-sulfonyl azide hydrochloride (3,1.40 g, 6.70 mmol) was added in batches and the resulting suspension was stirred overnight at room temperature. Upon completion of the reaction, the solvent was removed by vacuum evaporation and the residue was redissolved in dichloromethane (50 mL). The resulting solution was washed with 1 M aqueous hydrochloric acid (50 mL) and the aqueous phase was re-extracted with dichloromethane (3 x 50 mL). The organic phases were combined and the crude product was purified by gradient column chromatography (eluent ratio: dichloromethane/methanol/acetic acid, 96.5%/3%/0.5% → 93.5%/6%/0.5%). The purified product was lyophilized from dioxane to give a colorless oil (1.94 g, 89% yield). The structure of the product was confirmed by 1H-NMR (400 MHz, CDCl3) and HRMS (ESI+): 1H-NMR δ 7.76 (d, J = 7.5 Hz, 2H), 7.59 (d, J = 6.9 Hz, 2H), 7.40 (t, J = 7.5 Hz, 2H), 7.31 (t, J = 7.5 Hz, 2H), 5.34 (d, J = 7.8 Hz, 1H), 4.54 (br s, 1H), 4.43 (d, J = 6.6 Hz, 2H), 4.22 (t, J = 6.6 Hz, 1H), 3.27 (t, J = 6.6 Hz, 2H), 1.3-2.0 (m, 2H); HRMS (ESI+) m/z calcd for C21H23N4O4 [M + H ]+ 395.1719, found 395.1718.
