Synthetic applications of N–N linked heterocycles. Part 14. The preparation of α-(4-pyridyl)esters and α-(4-pyridyl)nitriles by regiospecific attack of ester and nitrile anions on pyridinium salts

Abstract

Reactions between N-(2,6-dimethyl-4-oxopyridin-1-yl)pyridinium tetrafluoroborate (1) and lithio-derivatives of esters and nitriles give only low yields of 1,4-dihydro-adducts (3) and (7) due apparently to competing proton abstraction from the pyridone methyl groups. This limitation with salt (1) appears only with C–H acids of pK_a > 20. Decomposition of the adducts under free-radical conditions yields α-(4-pyridyl)-esters (4) and -nitriles (8) essentially quantitatively. The same pyridyl-esters and -nitriles may be prepared conveniently in good overall yields by the reaction between the appropriate lithium salts and N-triphenylmethylpyridinium salts. The scope and limitations of the method are discussed. 2-Methyl-N-triphenylmethylpyridinium tetrafluoroborate is reported for the first time.