CI.[NH:3]([C:18]([O:20][C:21]([CH3:24])([CH3:23])[CH3:22])=[O:19])[C@@H:4]([C:9]([NH:11][C@H:12]([C:14]([O:16][CH3:17])=[O:15])[CH3:13])=[O:10])[CH2:5][CH2:6]SC>CN(C=O)C.ClCCl>[C:21]([O:20][C:18]([NH:3][C@@H:4]1[CH2:5][CH2:6][N:11]([C@@H:12]([CH3:13])[C:14]([O:16][CH3:17])=[O:15])[C:9]1=[O:10])=[O:19])([CH3:24])([CH3:23])[CH3:22]Note: This sequence must be conducted under dry conditions with dry solvents otherwise epimerisation will occur. Methyl iodide (10 ml) was added to Boc-(D)-Met-(L)-Ala-OMe (8 g) in a mixture of DMF (20 ml) and dichloromethane (20 ml) and the mixture was allowed to stand for 16 hours and then evaporated to dryness. Further dichloromethane (2×50 ml) was added and evaporated to remove residual methyl iodide and the residue was dissolved in a mixture of DMF (300 ml) and dichloromethane (300 ml). The mixture was cooled to ˜5° C. and sodium hydride (0.76 g of an 80% dispersion in mineral oil) was added in one portion and the mixture was stirred at this temperature for 2 hours. Saturated aqueous ammonium chloride (50 ml) was added and the mixture was evaporated to dryness and then partitioned between water and ether. The ether extract was washed with brine and dried and evaporated to give a gum which was purified by flash chromatography on a sinter funnel (25% ethyl acetate:hexane to 100% ethylacetate) to give methyl (2S)-2-[(3R)-3-(N-[tert-butyloxycarbonyl]amino)-2-oxo-pyrrolidin-1-yl]propionate as a gum (4.2 g) which crystallised on standing: NMR (CDCl3): 1.4 (s, 9H), 1.4 (d, 3H), 1.8 (m, 1H), 2.6 (m, 1H), 3.4 (m, 2H), 3.7 (m, 3H), 4.2 (m, 1H), 4.9 (q, 1H), 5.2 (bs, 1H).
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