1H-nmr study on the preferred conformations of chiral pyridine-dinucleotide models

Abstract

The synthesis of four chiral NAD + models 1 and their 1,4-dihydro analogs 2 is described. From the temperature dependence of the 1H-nmr spectra it is concluded that for these compounds two preferred conformations I and II, differing slightly in energy, exist. Both conformations are “folded” with the more or less parallel p-anisyl and pyridine groups mutually gauche, but in I the pyridine group is rotated by about 180° as compared with II, thus leading to a conspicuous difference in orientation of the substituent Z (NH 2CO, C 6H 5NHSO 2, (CH 2) 4NSO 2, or (C 4H 8ON)SO 2) in the pyridine ring toward the anisyl group. The most stable conformation (I) has Z closest to the center of the p-anisyl group. In 360-MHz spectra of the dihydropyridines at low temperature (−10°C), slow interconversion of I and II leads to the observation of an XY pattern for the C-4 methylene protons of the 1,4-dihydropyridine system. The anisochronity in this methylene group is caused mainly by the anisotropy of the neighboring p-anisyl group.