4.2. Synthesis of Fmoc-l-Lys(pentynoyl)-OtBu (2)

Fmoc-l-Lys(pentynoyl)-OH (1) (2.05 g, 4.6 mmol) in CH₂Cl₂ (20 mL) was added t-butyl 2,2,2-trichloroacetimidate (2.18 g, 10 mmol). The resulting mixture was stirred at RT for 22 h, and purified by flash column chromatography eluted with 1:1 ethyl acetate/hexane to 100% ethyl acetate. The desired product 2 was obtained as a white solid (1.82 g, 79%). ¹H-NMR (400 MHz, CDCl₃) δ ppm: 1.39–2.08 (m, 7H), 1.47 (s, 9H), 2.37 (t, J = 7.0 Hz, 2H), 2.52 (dt, J = 2.2, 7.0 Hz, 2H). 3.28 (m, 2H), 4.22 (t, J = 7.0 Hz, 2H), 4.39 (m, 2H), 5.42 (d, J = 8.0 Hz, 1H), 5.90 (bs, 1H), 7.31 (dt, J = 1.0, 7.4 Hz, 2H), 7.40 (t, J = 7.5 Hz, 2H), 7.60 (d, J = 7.4 Hz, 2H), 7.77 (d, J = 7.5 Hz, 2H). ESI-MS: calculated [M + H]⁺ for Fmoc-l-Lys(pentynoyl)-OtBu C₃₀H₃₆N₂O₅ 505.3; found 506.0.