In aqueous media the one-electron reduction of the title a structure for these conformers and their stability has been studied. Two conformers are unstable, yielding the third compounds on a mercury pool generally gives rise to three dimers, which are distinguishable by HPL chromatography. conformer. A mechanism is proposed for this transformation. From spectroscopic analyses it has been possible to propose Several papers are devoted to the structural analysis of in nearly equal amounts in most cases. The first idea was that regioisomers were obtained, but as carbon atoms 2 and dimeric products obtained from the electrochemical reduction of 1-alkyl-3-carbamoylpyridinium, as NAD ϩ mod-6 are less reactive than carbon atom 4, in addition to the 4,4Ј dimer the 2,4Ј or 2,6Ј possible dimers should have only els (Nicotinamide Adenine Dinucleotide, in the oxidized form), in aqueous media. Generally, the authors claim to been obtained as traces. So we developed a research strategy to look at the structure of the different dimeric species ob-have isolated dimers having the structure of the 1,1Ј,4,4Јtetrahydro-4,4Ј-bipyridine derivative [1] . Underwood [2] [3] , tained from a one-electron reduction of symmetrical 3,5disubstituted pyridinium ions, taken as an NAD ϩ model. and more recently Carelli [4] , have also proposed the formation of 6,6Ј dimers. Although in most cases the electro-
We chose to study the 1-benzyl derivative whose dimers are obtained by a one-electron reduction on a mercury chemical reduction of 1-substituted nicotinamide salts led to the formation of several dimers, the 4,4Ј dimer was ob-pool, well separated on an analytical HPL chromatogram and easily analysed by NMR. Although in this paper we tained as the major product, whilst a minor amount of the 4,6Ј dimer was obtained, the rest being very small quantities give the results obtained for two 3,5-dicarbamoylpyridinium cations (I, II), the properties are similar to those ob-(less than 5%) of 2,2Ј, 2,4Ј, and 6,6Ј dimers. Some of these dimers isomerized into the 4,4Ј dimer, showing that the served for 3,5-disubstituted pyridinium salts described previously [8] [9] . bond between the two dihydropyridine rings is very weak and that the dimer is in equilibrium with its radical [5] . The At the end of this work, some brief results are given on the 1,3-bis[3,5-bis(N-methylcarbamido)pyridinio]propane same holds true for the biological NAD ϩ since three diastereoisomers with the 4,4Ј structure were detected by Car-(III), a bis-pyridinium salt studied in order to corroborate our assumptions on the formation of conformational di-elli [6] and six diastereoisomers by Jaegfeld [7] . These results are explained by the asymmetrical environment of the two mers (see Scheme 1). carbon atoms involved in the bond between the two rings. Moreover, Carelli et al. showed that the ring carbon reactiv-Results and Discussions ity follows the position order 4 > 6 > 2 for 3-monosubsti-