Cl.[CH3:2][C@H:3]1[CH2:7][CH2:6][CH2:5][NH:4]1.C(=O)([O-])[O-].[K+].[K+].Br[CH2:15][CH2:16][CH2:17][C:18]([O:20][CH2:21][CH3:22])=[O:19]>CC(=O)CC>[CH2:21]([O:20][C:18](=[O:19])[CH2:17][CH2:16][CH2:15][N:4]1[CH2:5][CH2:6][CH2:7][C@@H:3]1[CH3:2])[CH3:22](S)-2-methyl-pyrrolidine hydrochloride (0.8 g, 6.6 mmol, 1.1 eq) was dissolved in 2-butanone (20 mL) and potassium carbonate (1.7 g, 12.6 mmol, 2.1 eq) was added. Ethyl 4-bromobutyrate (0.86 mL, 6.0 mmol, 1.0 eq) was added and the reaction mixture was refluxed for 2 days. The mixture was allowed to cool to room temperature and any solid present was filtered off and washed with ether. The filtrate was concentrated under reduced pressure to give 1.20 g of the title compound (yield 99%) which was used in the next step without further purification.
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