[Br:1][C:2]1[CH:19]=[CH:18][C:5]([N:6]([CH2:11][CH2:12][CH2:13][CH2:14][C:15]([OH:17])=[O:16])[CH2:7][CH:8]([CH3:10])[CH3:9])=[C:4]([CH:20]=[O:21])[CH:3]=1.[C:22](=O)([O-])[O-].[K+].[K+].IC.O>CN(C=O)C>[Br:1][C:2]1[CH:19]=[CH:18][C:5]([N:6]([CH2:11][CH2:12][CH2:13][CH2:14][C:15]([O:17][CH3:22])=[O:16])[CH2:7][CH:8]([CH3:10])[CH3:9])=[C:4]([CH:20]=[O:21])[CH:3]=1To a suspension of 5-(4-bromo-2-formyl-N-isobutylanilino)pentanoic acid (15.0 g), potassium carbonate (7.0 g) in DMF (50 ml) was added dropwise a solution of iodomethane (8.4 g) in DMF (10 ml) at 0° C. under a nitrogen atmosphere. The mixture was warmed to room temperature, and stirred for 2 hours under a nitrogen atmosphere. After addition of water, the solution was extracted with ethyl acetate. The organic layer was washed with water and saturated saline, and dried over magnesium sulfate. After distilling off the solvent under reduced pressure, the resulting residue was purified by silica gel column chromatography (hexane:ethyl acetate=9:1 to hexane:ethyl acetate=1:1) to obtain brown oil of methyl 5-(4-bromo-2-formyl-N-isobutylanilino)pentanoate (13.1 g).
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