[Cl:1][C:2]1[CH:3]=[C:4]2[C:8](=[CH:9][CH:10]=1)[NH:7][C:6](=[O:11])[C:5]2([CH:16]([C:21]#[N:22])[C:17]([O:19]C)=[O:18])[CH2:12][N+:13]([O-:15])=[O:14].[OH-].[K+].O>CO>[Cl:1][C:2]1[CH:3]=[C:4]2[C:8](=[CH:9][CH:10]=1)[NH:7][C:6](=[O:11])[C:5]2([CH:16]([C:21]#[N:22])[C:17]([OH:19])=[O:18])[CH2:12][N+:13]([O-:15])=[O:14]To a solution of methyl [5-chloro-3-(nitromethyl)-2-oxo-2,3-dihydro-1H-indol-3-yl](cyano)acetate, intermediate 27 (5.00 g; 15.45 mmol) in MeOH (150 ml) was added a potassium hydroxide solution in water (9.27 ml; 5.00 M; 46.34 mmol). After stirring for 2.5 l the solvent was evaporated. The residue was taken up with ethyl acetate and the organic phase extracted with 1N HCl, dried (MgSO4) and the solvent removed in vacuo to give the crude [5-chloro-3-(nitromethyl)-2-oxo-2,3-dihydro-1H-indol-3-yl](cyano)acetic acid (4.89 g, quant.). This was a mixture of diastereomers, which was sufficiently pure for subsequent reaction (HPLC purity 93%).
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