Restricted Rotation about the C-N Bond in Thienopyridine Derivatives: NMR and DFT Study

N-acyl tetrahydrothienopyridine derivatives serve as important structural motifs in bioorganic chemistry. Restricted rotation of C-N bond in such molecules gives rise to two distinct chemical environments. Thienopyridine derivatives 4a-4i were synthesized and characterised by ¹H and ¹³C NMR spectroscopy. The NMR spectra reveal a doubling of signals, which suggests the presence of two rotamers in the solution. Variable temperature (VT) ¹H-NMR studies supported this hypothesis. The NOESY analysis confirmed that Zrotamer is present in major form as compared to E-rotamer. The results were further corroborated through density functional theoretic calculations.