13C NMR spectra and three-dimensional structures of isomeric 3,4-substituted thiophans
The chemical shifts and the direct carbon-proton spin-spin coupling constants for a number of cis and trans isomers of 4-amino-substituted 3-hydroxy(acyloxy)-thiophans, the configurations of the substituents in which and the conformational states were previously established by an independent method, were studied. It was found that in the spectra of the cis isomers the signals of the vicinal 13C atoms, which bear the substituents, are shifted to strong field as compared with the trans isomers (Δδ trans-cis 1.7–4.3 ppm). Conformational γ effects of the substituents on the chemical shifts of the ring 13C atoms were noted. It is shown that a relationship exists between the direct carbon-proton spin-spin coupling constants and the spatial orientations of the acyloxy and acylamino substituents for five-membered saturated rings.