To a solution of (S)-2-(tert-butoxycarbonylamino)pent-4-enoic acid (400 mg, 1.86 mmol), N-benzylbut-3-en-1-amine (449 mg, 2.79 mmol) in DMF (5 mL) was added EDC (410 mg, 2.14 mmol), HOBt (370 mg, 2.42 mmol) and followed by TEA (0.259 mL, 1.858 mmol). The reaction was stirred at rt for 3 days. The reaction was diluted with EtOAc and washed with water (2×), 1 M aqueous KHSO4 (2×), saturated aqueous NaHCO3 (2×) and saturated aqueous NaCl. The organic layer was separated, dried over Na2SO4, filtered and concentrated. The crude product was purified using silica gel chromatography (ISCO system) eluting with a gradient of 0-60% EtOAc/Hex to give (S)-tert-butyl 1-(benzyl(but-3-enyl)amino)-1-oxopent-4-en-2-ylcarbamate (270 mg, 0.753 mmol, 40.5% yield). Anal. Calcd. for C21H30N2O3 m/z 358.4. found: 359.1 (M+H)+; 1H NMR (400 MHz, CDCl3) δ ppm 7.43-7.13 (m, 5H), 5.85-5.62 (m, 2H), 5.40-5.27 (m, 1H), 5.19-4.97 (m, 3H), 4.87 and 4.37 (d, J=14.9 Hz, 1H), 4.76-4.51 (m, 2H), 3.59-3.22 (m, 3H), 2.39 (ddq, J=25.6, 21.3, 7.1 Hz, 4H), 1.44 and 1.42 (s, 9H).
