N1-((1S)-3-(Benzyloxy)-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)-N2-(tert-butoxycarbonyl)-4-methyl-l-leucinamide (33)
Following the procedure described for the preparation of tert-butyl [(1S)-2-[((1S)-3-(benzyloxy)-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)amino]-1-(cyclohexylmethyl)-2-oxoethyl]carbamate but substituting Boc-(β t-butyl)-Ala-OH and making noncritical variations provided a brown syrup. This material was purified by LC (33 g 230–400 SiO2, 1–2.5% methanol/chloroform) to afford 258 mg (55%) of the title compound as a light yellow foam. 1H NMR (DMSO-d6) δ 8.23 (d, J = 8 Hz, 1H), 7.62 (s, 1H), 7.32 (m, 5H), 6.96 (d, J = 8 Hz, 1H), 4.46 (s, 2H), 4.35–4.33 (m, 2H), 4.21 (d, J = 16 Hz, 1H), 3.95 (m, 1H), 3.09 (m, 2H), 2.24 (m, 1H), 2.07 (m, 1H), 1.92 (m, 1H), 1.58 (m, 2H), 1.46 (m, 2H), 1.35 (s, 9H), 0.87 (m, 9H); MS (ESI+) for C27H41N3O6m/z 504.2 (M + H)+.