C(=O)([O-])[O-].[K+].[K+].[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 potassium carbonate (14.0 g, 96.2 mmol, 5 equiv.), ethyl-4-aminobutyrate (IVb) (6.5 g, 38.6 mmol, 2 equiv.) and methanesulfonic acid (R)-1-carbamoyl-propyl ester (IIi) (3.5 g, 19.3 mmol, 1 equiv.) in acetonitrile (40 ml, 11 vol.) is heated to 80° C. for 17 hours. The mixture is then filtered and the cake rinsed with acetonitrile (30 ml). Combined filtrates are concentrated to dryness under reduced pressure. The crude methyl-(S)-4-(1-carbamoylpropylamino)butyrate (VIg) obtained as a yellow oil is used directly in the next step.
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