Quinoline- and 1,8-naphthyridine-3-carboxylic acids using a self-catalyzed Friedlنnder approach

One-step syntheses of 2-alkyl- and 2,4-dialkyl-substituted quinoline-3-carboxylic acids and 1,8-naphthyridine-3-carboxylic acids are reported using a catalyst-free Friedländer reaction. The reaction is carried out in one step by simple heating of 2-aminobenzaldehyde, 2-amino-5-chlorobenzaldehyde, 2-aminonicotinaldehyde, or 2-aminoacetophenone with a β-ketoester in toluene or xylene for 24 h. Under these conditions, the carboxylic acid product is isolated directly from the reaction mixture without need for further purification. The observation that the reaction starts slowly and accelerates as it proceeds suggests that the transformation is self-catalyzed. This hypothesis is also supported by the finding that attempts to extend the current reaction to diketones, which cannot hydrolyze to an acid, were generally unsuccessful.