Synthetic applications of N-N linked heterocycles. Part 8. Regiospecific synthesis of 4-(α-acylalkyl)pyridines by attack of lithium enolates of ketones γ to N-(2,6-dimethyl-4-oxopyridin-1-yl)pyridinium salts

The addition of N-(2,6-dimethyl-4-oxopyridin-1-yl)pyridinium salts (2)–(4) to lithium enolates (1) of ketones in tetrahydrofuran at low temperatures gives regiospecifically high yields of 1,4-dihydro-adducts (5)–(7). These are readily isolated and can be decomposed under free-radical conditions, also in high yield, to 4-(α-acylalkyl)pyridines (8)–(10). Unsymmetrical dialkyl ketones give a mixture of two isomeric products, the ratio depending upon reaction conditions and steric factors. αβ-Unsaturated ketones, however, give products resulting from reaction exclusively at the position α′ to the carbonyl group.