NMR Spectra, MO Calculations of Spin-Spin Coupling Constants and Conformational Analysis of Substituted 1,3-Dioxolanes

Article abstract of DOI:10.1002/mrc.1270130105

The NMR spectra of a number of 2,4-disubstituted 1,3-dioxolanes have been recorded and the proton chemical shifts and coupling constants derived from complete spectral analysis.Vicinal coupling constants were found to be dependent on the substituent at C-4 and this effect is more pronounced for J(trans) of the C-4 - C-5 fragment.These coupling constants also indicate a homogeneous behaviour within a series with either a cis or trans configuration, although the cis behaviour differs from that of trans isomers.This has been interpreted in terms of definite ring conformations in substituted derivatives, while the unsubstituted 1,3-dioxolane undergoes free pesudorotation.Calculations of coupling constants were performed by semi-empirical MO methods, both for unsubstituted 1,3-dioxolane and for C-4 substituted derivatives in a large number of conformations, in order to compare calculated and experimental values; the correct order of J(cis) and J(trans) for 1,3-dioxolane is obtained only by employing energies from ab-inito MO calculations averaged over the pseudorotation circuit.For the C-4 substituted compounds calculated coupling constants were employed in a 'trial and error' process for the identification of the preferred conformations of these compounds; a set of two torsional angles for each compound was derived which allows a tentative description of the geometry of each molecule.A criticism of these geometries is given according to the evidence available on the structure of substituted 1,3-dioxolanes.