[CH2:1]([O:3][C:4]([C:6]1[C:7]([CH3:25])=[C:8]([C:18]([O:20][C:21]([CH3:24])([CH3:23])[CH3:22])=[O:19])[NH:9][C:10]=1[CH2:11][CH2:12][C:13]([O:15]CC)=[O:14])=[O:5])[CH3:2].CO.[OH-].[Li+]>O1CCCC1>[CH2:1]([O:3][C:4]([C:6]1[C:7]([CH3:25])=[C:8]([C:18]([O:20][C:21]([CH3:24])([CH3:23])[CH3:22])=[O:19])[NH:9][C:10]=1[CH2:11][CH2:12][C:13]([OH:15])=[O:14])=[O:5])[CH3:2]5-(2-Ethoxycarbonyl-ethyl)-3-methyl-1H-pyrrole-2,4-dicarboxylic acid 2-tert-butyl ester 4-ethyl ester 1d (23.6 g, 66.8 mmol) was dissolved in 190 ml of tetrahydrofuran and 90 ml of methanol under stirring, and added with aqueous lithium hydroxide solution (80 ml, 10 mol/L, 0.8 mol) at room temperature. The color of the reaction solution gradually turned from light yellow to cyan, and stirred for another 1 hour. After thin lay chromatography showed the disappearance of starting materials, the resulting mixture was concentrated under reduced pressure to evaporate organic solvent. The residue was adjusted pH to 2 with hydrochloric acid solution (2 mol/L) in an ice-water bath under stirring. White precipitates were formed. The mixture was filtered, and the filter cake was dried to obtain the title compound 5-(2-carboxy-ethyl)-3-methyl-1H-pyrrole-2,4-dicarboxylic acid 2-tert-butyl ester 4-ethyl ester 1e (24 g, yield 98%) as a white solid.
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