To a solution of Fmoc-glycine (297 mg, 1.00 mmol) in DMF (5 mL) at 0° C. was added DIEA (348 μL, 2.00 mmol) and HBTU (379 mg, 1.00 mmol). After stirring for 10 min at 0° C., the resulting activated ester solution was added to crude dipeptide 73 (1.00 mmol) and the reaction mixture was stirred for 2 h at 0° C. It was diluted with ethyl acetate, washed with H2O, 1N HCl solution, saturated NaHCO3 solution and saturated NaCl solution. The organic layer was dried over MgSO4, filtered and concentrated to dryness. The crude product was purified by silica gel chromatography on a Biotage™ flash chromatography system using a gradient of 0-75% ethyl acetate in CH2Cl2. Tripeptide 74 was obtained as a white solid (546 mg, 98%). 1H NMR (400 MHz, CDCl3) δ 1.44 (s, 9H), 3.10 (d, J=6.8 Hz, 2H), 3.74-3.95 (m, 4H), 4.21 (t, J=7.0 Hz, 1H), 4.40 (d, J=7.2 Hz, 2H), 4.70 (q, J=7.5 Hz, 1H), 5.36 (bs, 1H), 6.31 (bs, 1H), 6.53 (d, J=7.4 Hz, 1H), 7.17-7.22 (m, 3H), 7.24-7.28 (m, 2H), 7.30-7.34 (m, 2H), 7.41 (t, J=7.4 Hz, 2H), 7.59 (d, J=7.3 Hz, 2H), 7.77 (d, J=7.3 Hz, 2H).
