tert-Butyl [(1S)-2-[((1S)-3-(Benzyloxy)-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)amino]-1-(cyclohexylmethyl)-2-oxoethyl]carbamate
To a solution of tert-butyl ((1S)-3-(benzyloxy)-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)carbamate (200 mg, 0.53 mmol) in dioxane (5 mL) was added 4 M hydrochloric acid/dioxane (5 mL), and the solution was stirred at an ambient temperature for 4 h before removing the solvents in vacuo, azeotroping the residue with toluene (2 × 10 mL), and drying in vacuo for 1 h. The crude hydrochloride salt was taken into DMF (3 mL), and the solution was cooled to 0 °C before adding N-Boc-cyclohexylalanine-OH (139 mg, 0.53 mmol), collidine (156 μL, 1.22 mmol), and HATU (194 mg, 0.53 mmol) in order, and the resulting suspension was stirred at 0 °C for 5 h. The reaction was quenched by the addition of water (30 mL), and the mixture was extracted with diethyl ether (3 × 75 mL). The combined organics were dried over MgSO4, filtered, and the solvents were removed in vacuo to yield the crude product, which was purified by flash chromatography, eluting with 1–3% methanol/dichloromethane to afford the title compound as a pale brown gum, 205 mg, 76%. The product was contaminated with ∼20% of another diastereoisomer. 1H NMR (400 MHz, CDCl3) δ 7.69 (d, J = 5.6 Hz, 1H), 7.26–7.42 (m, 5H), 5.93 (s, 1H), 4.94 (d, J = 7.6 Hz, 1H), 4.72 (m, 1H), 4.60 (d, J = 11.6 Hz, 1H), 4.55 (d, J = 11.6 Hz, 1H), 4.14–4.34 (m, 3H), 3.27 (m, 2H), 2.23–2.51 (m, 3H), 1.56–2.05 (m, 9H), 1.33–1.48 (m, 9H), 1.05–1.29 (m, 4H), 0.80–1.03 (m, 2H).