Pyridine Synthesis via Anodic Oxidation of 1-Acyldihydropyridines

Abstract

The preparation of several substituted pyridines via anodic oxidation of I-acyldihydropyridines is reported.The addition of nucleophiles to I-acylpyridinium salts (1) and subsequent aromatization of the intermediate dihydropyridines (2 and 3) is a very useful method for the synthesis of substituted pyridines (4) and ( s ) .~J Oxidizing agents used for the aromatization of I-acyldihydmpyridines include sulphur, o-chloranil, p-chlaranil.DDQ, silver nitrate, and oxygen.The success of the anidation is quite dependent on the oxidizing agent and Be structure df the I-acyldihydropyridine.In our hands, aromatizadon using sulphur or the chloranils has proven to be quite useful and generaL2 However, reaction with sulphur requires a high boiling solvent (150-2W°C), and oxidation using chloranil gives nonvolatile by-products which must be separated from the desired substituted pyridine.DDQ and silver niwte a= relatively expensive oxidizing agents, and oxygen is effective in only a limited number of cases.As a possible way to circumvent some of these problems, the electrochemical oxidation of 1acyldihydropyridines was considered.The aromatization of 1.4-dihydropyridines using anodic oxidation has been studied and the mechanism of electroaxidation explored.3bHowever, we found only one example of a I-acyl-1.4dihydropyridine electrachemical oxidation.This involved the electrochemical dehydrogenation of I-benzoyl-C(3-.,