[CH2:1]([O:3][C:4]([C:6]1[NH:7][C:8]([CH3:17])=[C:9]([CH2:12][CH2:13][C:14]([OH:16])=[O:15])[C:10]=1[CH3:11])=[O:5])[CH3:2].[O:18]1CCCC1>C(O)(=O)C.O.[Cl-].[Na+].O>[CH2:1]([O:3][C:4]([C:6]1[NH:7][C:8]([CH:17]=[O:18])=[C:9]([CH2:12][CH2:13][C:14]([OH:16])=[O:15])[C:10]=1[CH3:11])=[O:5])[CH3:2]To a mixture of 7 g 4-(2-carboxyethyl)-3,5-dimethyl-1H-pyrrole-2-carboxylic acid ethyl ester in 35 mL each of tetrahydrofuran (THF), acetic acid (AcOH) and water at −10° C. was added 70 g of ceric ammonium nitrate in portion over 20 minutes, the reaction temperature being maintained at about 5° C. The resulting mixture was them cooled to 0° C., stirred for another 2 hr and then diluted with 250 mL of brine. The mixture was then extracted with 2×300 mL 10% MeOH in dichloromethane (DCM). The organic extracts were combined and dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated. The residue was purified on silica gel column with hexanes(hex)/ethyl acetate (EtOAc)/AcOH (6:4:0.05) followed by crystallization in EtOAc to give 1.5 g of 4-(2-carboxyethyl)-5-formyl-3-methyl-1H-pyrrole-2-carboxylic acid ethyl ester as a white solid.
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