P(Cl)(Cl)(Cl)=O.[CH2:6]([O:8][C:9]([C:11]1[NH:12][CH:13]=[C:14]([CH3:16])[CH:15]=1)=[O:10])[CH3:7].[OH-].[Na+].CN(C)[CH:21]=[O:22]>O>[CH2:6]([O:8][C:9]([C:11]1[NH:12][C:13]([CH:21]=[O:22])=[C:14]([CH3:16])[CH:15]=1)=[O:10])[CH3:7]To the ice-cold 3 mL (39.2 mmol) of dimethylformamide (DMF) was added phosphorus oxychloride (0.67 mL, 7.18 mmol) drop wise and the resultant mixture was stirred for 30 minutes. A solution of 1 g (6.53 mmol) of 4-methyl-1H-pyrrole-2-carboxylic acid ethyl ester in 3 mL of DMF was added to the reaction. After 1 h, the reaction was warmed to room temperature for another 2.5 h. The reaction mixture was diluted with water (100 mL) and basified to pH=11 with 1N sodium hydroxide solution. The precipitate was removed by filtration, rinsing with water and dried to afford 0.8 g (68%) of 5-formyl-4-methyl-1H-pyrrole-2-carboxylic acid ethyl ester as a white solid.
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