Facile Synthesis of Alkyl and Aryl Substituted Dibenzo[b,g][1,8]naphthyridin-5-ones

The reaction of 2,4-dichloroquinolines with o-aminoacetophenone and o-aminobenzophenone under neat conditions yielded 2'-acetyl and 2'-benzoyl substituted-4-chloro-2-(N-phenylamino)quinolines, respectively, which on treatment with sodium methoxide afforded the 2'-substituted-4-methoxy-2-(N-phenylamino)quinolines. These potential intermediates, on polyphosphoric acid-catalyzed cyclization at two different temperatures, gave the respective 6-methyl and 6-phenyl substituted dibenzo[b,g][1,8]naphthyridin-5-ones. These temperature differences for the formation of the final products were due to the in situ formation of the respective 2'-substituted-2-(N-phenylamino)quinolin-4-ones from the chloro and methoxy intermediates. The naphthyridin-5-ones were subjected to N-methylation, where the methyl group in the 1-position was found to hinder the reaction sterically, consequently increasing the reaction time to more than that of the other derivatives.