Study of the π‐electron distribution in push—pull alkenes by 1H and 13C NMR spectroscopy. Part III—the conformation of 3,4‐dihydro‐4‐oxo‐2H‐thiins

The 1H and 13C NMR spectra of a variety of differently 2,5,6‐trisubstituted 3,4‐dihydro‐4‐oxo‐2H‐thiins (integrated push–pull alkenes) were unequivocally assigned by means of a wide variety of 1D and 2D NMR spectroscopic methods. The NMR parameters thus obtained, together with the results of accompanying force‐field calculations, proved useful in determining the preferred ring conformation of 3,4‐dihydro‐4‐oxo‐2H‐thiin (half‐chair). Additionally, the 13C chemical shift difference, Δδ(C‐5, C‐6), could be employed to indicate the degree of bond polarization of the partial C‐5C‐6 double bond (push–pull character); the barrier to rotation, otherwise used to estimate this parameter characterizing the push–pull alkenes, cannot be applied owing to the ring fusion. Interestingly, NH2 proved less effective in C‐5C‐6 bond polarization than NHPh.