A simple and efficient approach has been developed for the synthesis of previously inaccessible 6-arylbenzo[c][1,7]naphthyridin-4(3H)-ones, which is based on the interaction of 3-amino-6-methyl-4-arylpyridin-2(1H)-ones and aromatic aldehydes in a strong acidic media (trifluoroacetic, phosphorous or polyphosphorous acids) under heating. The initial imine derivatives undergo Pictet–Spengler condensation to form intermediate 6-aryl-5,6-dihydrobenzo[c]-1,7-naphthyridin-4(3H)-ones, which are oxidized by atmospheric oxygen to give 3-amino-6-methyl-4-arylpyridin-2(1H)-ones as the final products.
A one-pot method for obtaining novel thieno[3,2-c][1,7]naphthyridine derivatives based on the reaction of 3-amino-4-(thien-2-yl)pyridin-2(1H)-one with aromatic aldehydes in 80% phosphoric acid at 130 °C has been developed. The formation of the thieno[3,2-c][1,7]naphthyridine ring was due to the intermediate generation of the corresponding azomethine, which underwent intramolecular cyclization with electrophilic attack of the β-carbon atom of the thiophene core under Pictet–Spengler conditions. The isolated 5,7-dihydrothieno[3,2-c][1,7]naphthyridin-4(3H)-ones underwent oxidative aromatization in air to give thieno[3,2-c][1,7]naphthyridin-6(7H)-ones. A two-step synthesis of thieno[3,2-c][1,7]naphthyridines involving the isolation of the intermediate imine did not lead to a significant increase in the product yield.
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