Synthesis of 8,9‐difluoro‐2‐methyl‐6‐oxo‐1,2‐dihydropyrrolo[3,2,1‐ij]quinoline‐5‐carboxylic acid

A new and efficient seven‐step synthesis of 8,9‐difluoro‐2‐methyl‐6‐oxo‐1,2‐dihydropyrrolo[3,2,1‐ij]‐quinoline‐5‐carboxylic acid (9), an important intermediate used in the synthesis of quinolone antibacterials, has been developed beginning with commercially available 2,3,4‐trifluoronitrobenzene. Selective displacement of the 2‐fluorine of the starting material with the anion of ethyl acetoacetate and subsequent hydrolysis and decarboxylation affords the arylacetone derivative 11. Reduction of the ketone and nitro groups of 11 followed by condensation with diethyl ethoxymethylenemalonate gives 14, which is cyclized to the indole derivative 15 by the Mitsunobu procedure. Friedel‐Crafts cyclization of 15 and acid hydrolysis gives the title compound 9.