Nucleophilic addition to pyridinium salts

The reaction of nucleophiles with pyridinium salts leads to addition, giving dihydropyridines. Attack 

treatment of A-methylpyridinium salts with cyanide produces a mixture of 2- and 4-cyanodi-hydropyridines, with the 2-isomer predominating. 

It is quite difficult to reduce benzene or pyridine, because these are aromatic stmctures. However, partial reduction of the pyridine ring is possible by using complex metal hydrides on pyridinium salts. Hydride transfer from lithium aluminium hydride gives the 1,2-dihydro derivative, as predictable from the above comments. Sodium borohydride under aqueous conditions achieves a double reduction, giving the 1,2,5,6-tetrahydro derivative, because protonation through the unsaturated system is possible. The final reduction step requires catalytic hydrogenation (see Section 9.4.3). The reduction of pyridinium salts is of considerable biological importance (see Box 11.2). 

Note the way we can refer to the unsaturated heterocycle hy considering it as a reduced pyridine, e.g. a 

Nicotinamide adenine dinucleotide reduction of a pyridinium salt 

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Natural product synthesis as a test of newly developed methodologies, for example, partial reduction of furans and pyrroles, nucleophilic addition to pyridinium salts. Os-catalyzed oxidative cyclization for the synthesis of tet-rahydrofurans and pyrrolidines, tethered aminohydroxylation of aUcenes, and ring-closing- and cross-metathesis for the constmction of heteroaromatics 12CC11924. 

Donohoe TJ, Connolly MJ, Walton L. Regioselective nucleophilic addition to pyridinium salts a new route to substituted dihydropyridones. Oro Lett. 2009 1 5562-5565. 

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