N2-(tert-Butoxycarbonyl)-N1-((1S)-3-chloro-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)-l-leucinamide
A solution of (3S)-3-[(2S)-2-amino-4-chloro-3-oxobutyl]pyrrolidin-2-one hydrochloride (391 mg, 1.6 mmol) and Boc-Leu-OH (412 mg, 1.8 mmol) in DMF (9 mL) was placed under an atmosphere of N2 and cooled to 0 °C. This clear pale yellow solution was successively treated with HATU (678 mg, 1.8 mmol) followed by N-methylmorpholine (0.41 mL, 3.7 mmol). The reaction mixture gradually became opaque and after 1 h was quenched with 1:1 ice/sat aqueous NaHCO3 (40 mL) and extracted three times with ethyl acetate (40 mL). The combined organics were washed once with brine (30 mL), dried over MgSO4, filtered, and concentrated to give a yellow syrup. This material was purified by LC (50 g 230–400 SiO2, 3–5% methanol/chloroform) to afford 636 mg (40%) of N2-(tert-butoxycarbonyl)-N1-((1S)-3-chloro-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)-l-leucinamide as a white foam. 1H NMR (DMSO-d6) δ 8.47 (d, J = 8 Hz, 1H), 7.64 (s, 1H), 7.04 (d, J = 8 Hz, 1H), 4.60 (d, J = 16 Hz, 1H), 4.53 (d, J = 16 Hz, 1H), 4.40 (m, 1H), 3.90 (m, 1H), 3.16 (m, 1H), 3.08 (m, 1H), 2.24 (m, 1H), 2.10 (m, 1H), 1.98 (m, 1H), 1.63 (m, 2H), 1.45–1.37 (m, 11 H), 0.89 (d, J = 4 Hz, 3H), 0.85 (d, J = 4 Hz, 3H); MS (ESI+) for C19H32ClN3O5m/z 418.1 (M + H)+. Anal. calcd for C19H32ClN3O5·0.6 H2O: C, 53.22; H, 7.81; N, 9.80. Found: C, 53.00; H, 7.65; N, 9.54. HRMS (ESI+) calcd for C19H32ClN3O5+H1 418.2103, found 418.2091.