17O NMR spectra and conformation of cyclic vinyl ethers

The 17O NMR spectra of a number of heterocyclic vinyl ethers were recorded in CDCl3 and DMSO‐d6 solutions. The oxygen chemical shifts reveal substantial differences in the strength of p‐π conjugation between the five‐ and six‐membered ring compounds, especially between those with an exocyclic CC bond. The conformations of the title compounds were estimated by force‐field calculations; the calculated torsional angle of the COCC moiety appears to be a linear function of the 17O NMR shift data, corrected by appropriate structural factors.