Configurational Assignment of Long-Chain Alkylated Pyridinium Aldoxime Bromides by Noesy Experiments

Abstract

A configurational study of long-chain N-alkylated pyridinium aldoxime derivatives and their positional isomers has been carried out by two-dimensional 1H-1H NOESY (nuclear Overhauser effect spectroscopy). Cross peak intensities were observed to be enhanced with increasing mixing time. Mixing times longer than 250 ms result in increasing contribution of spin diffusion that produces unrealistic hydrogen-hydrogen distances. The results of NOE measurements showed significant enhancement in the intensity of iminyl proton resonances on irradiation of hydroxyl proton resonances and vice versa. The chemical shift difference between hydroxyl proton resonances and iminyl proton resonances was found to be ~4 ppm for syn and ~5 ppm for anti configurations. The study reveals that these compounds exist in the E configuration i.e. the syn form in solution. The syn isomer predominates; the anti isomer amounts are 3% (2-oxime), 4% (3-oxime) and 6% (4-oxime).