Ammonium sulfate ((NH4)2SO4, 0.09 g, 0.67 mmol) was added to hexamethyldisilazane (HMDS, 20 mL) as 2,4-dimethyl-5-formyl-1H-pyrrole-3-carboxylic acid (8, 1.0 g, 6.61 mmol) and 5-fluoroindol-2-one (14, 1.1 g, 3.77 mmol) at room temperature in a stirred mixture. Subsequently, the reaction mixture was heated to reflux and maintained at this temperature for at least 5 h. The reaction process was monitored by gas chromatography (GC). Upon completion of the reaction, 5-fluoroindol-2-one (14) and trimethylsilyl trifluoromethanesulfonate (TMSOTf, 0.29 g, 1.32 mmol) were added. Stirring of the mixture was continued until high performance liquid chromatography (HPLC) analysis showed that the reaction was complete. The reaction mixture was quenched with water (6 mL) and acetonitrile (MeCN, 30 mL), filtered and the filter cake was washed sequentially with acetonitrile (20 mL) and ethanol (EtOH, 5 mL). Finally, the product was dried under vacuum at 40 °C overnight to afford the target compound 5-((Z)-(5-fluoro-2-oxoindolidine-3-ylidene)methyl)-2,4-dimethyl-1H-pyrrole-3-carboxylic acid (15, 1.92 g, 97% yield) as a yellow to brown powder with an HPLC purity of about 82.1%.
