C(=O)([O-])[O-].[Cs+].[Cs+].[CH2:7]([O:9][C:10](=[O:15])[CH2:11][CH2:12][CH2:13][NH2:14])[CH3:8].[C:16]([C@H:19](OS(C)(=O)=O)[CH2:20][CH3:21])(=[O:18])[NH2:17]>C(#N)C>[CH2:7]([O:9][C:10](=[O:15])[CH2:11][CH2:12][CH2:13][NH:14][C@@H:19]([C:16](=[O:18])[NH2:17])[CH2:20][CH3:21])[CH3:8]A mixture of powdered caesium carbonate (9.0 g, 27.4 mmol, 5 equiv.), ethyl-4-aminobutyrate (IVb) (1.85 g, 11.0 mmol, 2 equiv.) and methanesulfonic acid (R)-1-carbamoyl-propyl ester (IIi) (1.0 g, 5.5 mmol, 1 equiv.) in acetonitrile (20 ml, 20 vol.) is heated to 60° C. for 16 hours. The mixture is then filtered and the cake rinsed with acetonitrile (50 ml). Combined filtrates are concentrated to dryness under reducd pressure. The crude methyl-(R)-4-(1-carbamoylpropylamino)butyrate is used directly in the next step.
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