Behavior of Pyridinium Salts Obtained from Derivatives of Pyridinedicarboxylic Acids in Basic Solutions. Addition of Hydroxide or Alkoxide To Form 1,2-Dihydropyridine Intermediates

Article abstract of DOI:10.1021/jo00289a046

The N-alkylated pyridinium salts obtained from the diethyl esters, N-ethyl amides, and N,N-diethyl amides of pyridine-3,5-dicarboxylic acid exhibit ultraviolet absorptions of moderate intensity in the region of 350 nm when dissolved in 95percent ethanol.This absorption increases in intensity on addition of base and disappears on acidification of the solutions.It is not observed in rigorously dried solvents like chloroform or methylene chloride.By 1H NMR spectroscopy it has been shown that a 1,2-dihydropyridine is formed reversibly by addition of hydroxide (or methoxide) to the 2-position of the pyridinium salts.Concurrently with the formation of these intermediates, proton-deuterium exchange (in deuterated solvents) occurs, possibly via betaines formed by base-induced deprotonation at the 2-position of the pyridinium salts.The corresponding derivatives of pyridine-2,5- and -3,5-dicarboxylic acids do not display this behavior.