13C Nuclear magnetic resonance studies. XIII. 13C Carbonyl carbon shieldings of some β,γ-unsaturated ketones

The 15.1 MHz 13C nuclear magnetic resonance spectra of 13 aliphatic and alicyclic ketones having an olefinic bond in the β,γ position with respect to the carbonyl group have been recorded and the carbonyl shieldings measured. The major aim of this study was to investigate the possibility of detecting homo-conjugative interactions between the π-systems by comparing the carbonyl shieldings with those for the corresponding saturated and α,β-unsaturated systems. Some evidence of homoconjugation is presented on the basis of our previous interpretation of the factors governing carbonyl shieldings. A qualitative assessment of the observed geometrical dependence of the effect is offered.