Highly selective and facile synthesis of dihydro- and tetrahydropyridine dicarboxylic acid derivatives using electroreduction as a key step

Electroreduction of pyridinedicarboxylic acid derivatives 1a–g in methanol containing ammonium chloride using a divided cell brought about highly selective hydrogenation to give the corresponding dihydropyridines in good yields. From the electrolysis of dimethyl 2,3- and 2,5-pyridinedicarboxylates 1a,c, only the corresponding 1,2-dihydropyridine derivatives 2a,c were obtained in a regioselective manner while that of 2,4-, 2,6-, and 3,4- disubstituted pyridines 1b,d,e afforded the corresponding 1,4-dihydropyridines 2b,d,e selectively in good yields. Further hydrogenation of the resultant dihydropyridines by several methods led to the selective and facile formation of the corresponding tetrahydropyridines 6. Furthermore, Mg-promoted hydrosilylation of the N-acetylated product 5a gave C-silylated tetrahydropyridines in a stereoselective manner.