Naphthyridines by Cyclization of Pyridine Substrates

This may be done by completion of any one of five bonds, but only two such cyclizations have been successful, as illustrated in the following examples. 

2-A mi no-6-oxo-l-phenyl-4-(iV-phenylcarbamoyl methyl)-1,6-dihydro-3-pyridi-necarbonitrile (15) underwent thermal cyclization to l-amino-6-hydroxy-8-imino-2,7-dipthenyl-2,3,7,8-tetrahydro-2,7-naphthyridin-3(2//)-one (16) (Et3N, Me2NCHO, reflux 60%) also somewhat similar cyclizations.802 

Also other examples, in both the unfused and fused systems.802,960 

4-Styrylpyridine-3-carboxamide (17) underwent cyclization to 3-phenyl-3,4-dihydro-2,7-naphthyridin-1 (2//)-one (18) (P2O5/H3PO4, heat 90%), easily dehydrogenated to 3-phenyl-2,7-naphthyridin-1 (2//)-one (Pd/C, p-cymene, reflux 91%).188,cf 9 

4-Ethoxalylmethylpyridine-3-carbonitrile (19) underwent cyclization to ethyl 1-oxo-1,2-dihydro-2,7-naphthyridine-3-carboxylate (20) (an Amberlite resin, H20, EtOH, reflux 10%) 986 

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