Synthetic applications of N–N linked heterocycles. Part 16. Reactions between carbanions derived from carbon acids with pKa 7–14 and N-(2,6-dimethyl-4-oxopyridin-1-yl)pyridinium tetrafluoroborate: synthesis of 4-substituted pyridines, and observation of pyridine ringopening reactions
Abstract
Carbanions, derived from carbon acids (5) lying in the pKa range 7–14, add regiospecifically to the pyridinium γ-position in N-(2,6-dimethyl-4-oxopyridin-1-yl)pyridinium tetrafluoroborate (4) to yield 1,4-dihydro-adducts (6). While some intermediates could be fragmented successfully to give 4-substituted pyridines (7), others reverted under similar conditions to the carbon acids (5), due apparently to the presence of traces of water. Anions derived from malononitrile and ethyl cyanoacetate, however, gave ring-opened products (9) resulting from attack at the pyridinium α-position, and while the cyano-ester gave predominantly a 1,4-adduct at –30 °C, this reverted to the ring-opened compound at room temperature. An explanation for this abnormal behaviour is offered.
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