NMR and DFT analysis of the major diastereomeric degradation product of clopidogrel under oxidative stress conditions

Abstract

In this work, we enriched the major oxidative degradation product of clopidogrel hydrogen sulfate with the addition of 2.0 equiv. of peroxymonosulfate and analyzed the diastereomers in situ with 1D NMR (1H and 13C) and 2D NMR (1H−1H COSY, 1H−13C HSQC and 1H−13C HMBC). We observed that the signal from the CH2 groups at the positions 4, 6 and 7 in the degradation products (S)-5-((S)-1-(2-chlorophenyl)-2‑methoxy-2-oxoethyl)-5‑hydroxy-4,5,6,7-tetrahydrothieno[3,2-c]pyridin-5-ium and their analog diastereomer (5R,10S) are associated with the polarization of the NO bond in the piperidine ring. Depending on the conformation of this cyclic moiety, an intramolecular anisotropic effect between the phenyl ring and the methylene groups H-4 and H-6 affected the multiplicities and the chemical shifts of these oxidative degradation products in the 1H and 13C NMR spectra. We used DFT calculations to understand the pattern of the 1H NMR chemical shifts, the energies of diastereomers conformations and their intramolecular magnetic effects.