Isocyanide addition to pyridinium salts. Efficient entry into substituted nicotinonitrile derivatives

The addition of isocyanides to pyridinium salts is studied. The process takes place efficiently when a carboxamido group is present in the 3 position of the pyridine ring. The outcome of the reaction involves the stabilization of the nitrilium intermediate by the amide, which suffers a mild dehydration, leading regioselectively to beta-cyano-gamma-carbamoyl-1,4-dihydropyridines. In this way, a variety of nicotinamide derivatives were carbamoylated. Extension to quinolinium, isoquinolinium, and N-acylpyridinium salts is also reported. [reaction: see text]