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The synthesis of substituted 1, 8-naphthyridines by the Doebner reaction

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  • The possibility of synthesizing 1, 8-naphthyridines by the Doebner reaction has appeared in many reports. The method involves the introduction of electron releasing groups in the 6-position of 2-amino pyridine to activate the C-3 position. Previously Lewis used 6-methyl-2-amino pyridine with benzaldehyde and pyruvic acid, in an attempt to prepare a 1, 8-naphthyridine. He did not obtain a 1, 8-naphthyridine but an uncyclized product. He attributed his failure to produce a naphthyridine to the weak electron releasing power of the methyl group, which was unable to activate the C-3 position. Nitidandhaprabhas found later that the uncyclized compound lost one molecule of water to give 1(6-methyl-2-pyridyl)-5-phenyl-pyrrolidine-2, 3-dione. Also he found that the use of 2, 6-diaminopyridine in the Doebner reaction did produce a 1, 8-naphthyridine, because of the strong electron releasing power of the amino group. The current work continued the investigation of the effect of electron releasing groups at position 6 in the 2-aminopyridine ring on the formation of 1, 8-naphthyridine. The result showed that 6-hydroxy-2-aminopyridine was also effective in producing ring closure at C-3 to form a 1, 8-naphthyridine. The yield was less than that produced when 2, 6-diamino pyridine was used as the starting material. A second method for the preparation of 1, 8-naphthyridines was developed. Benzalpyruvic acid was prepared and allowed to react with the 6-substituted 2-aminopyridines. By this method the yield of 1, 8-naphthyridines was increased. The effect of different aldehydes in producing 1, 8-naphthyridines by the Doebner reaction was studied. O-methoxy benzaldehyde was used in the reaction in comparision with benzaldehyde. The use of O-methoxy benzaldehyde did produce a better yield. The compounds prepared were substituted 1, 2, 3, 4-tetrahydro-1, 8-naphthyridine-4-carboxylic acid monohydrochlorides. The assigned structures were confirmed on the basis of carbon-hydrogen analysis and the evidence of ultraviolet spectra similar to the spectrum of 6-bromo-2-(methyl amino-methyl)-1, 2, dihydro 1, 8-naphthyridine. This showed two well-defined maxima at 238 and 336 mμ as reported by Zhukova.
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