Reductions of Pyridines and Pyridinium Ions

The earliest investigation of the reduction of pyridinium ions with sodium borohydride was reported by Mathews3 on nicotinamide adenine dinucleotide. This reaction produced a dihydropyridine 

20n The reduction of a pyridine by sodium borohydride was reported by J. Kuthan and E. Janeckova, Collection Czech. Chem. Commun. 29, 1654 (1964). 

Cilento, E. DeCarvalho Filho, and A. C. Giora Albanese, J. Am. Chem. 

On the basis of the foregoing generalizations, the reduction of pyridinium ions containing substituents at the 3- and 5-positions would be expected to give dihydropyridines as products. Such observations have been made by Traber and Karrer32 with 4-pheny 1-1,2,6-trimethyl - 3,5-diethoxy carbonylpyridinium methosul-fate (30) by Wallenfels and Schiily26 with l-(2,6-dichlorobenzyl)- 

5- dimethoxycarbonyl (31a) and 3,5-diaminocarbonyl pyridinium bromides (31b) and by Anderson16 with l-methyI-3,5-diphenyl-pyridinium bromide (32) and l-methyl-3-bromo-5-methoxycarbonyl-pyridinium iodide (33). In each instafice the major or only product was reported to be the 1,2-dihydro-derivative however, the presence of 1,4-dihydropyridine isomers was demonstrated by NMR analysis of the latter two compounds. The last compound, the only example which is unsymmetrically substituted, underwent attack primarily at the position adjacent to the halogen, for only small yields of 1,4- and 

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