The NMR spectra and conformations of dihydropyran derivatives—I: The conformations of 2-alkoxy-5,6-dihydro-α-pyran-6-carboxylic esters

On the basis of NMR spectra of trans- and cis-2-alkoxy-5,6-α-pyran-6-carboxylic esters it was found that, at room temperature, the trans-compounds exist exclusively in the conformation with equatorial carbalkoxy and pseudoaxial alkoxy groups. The cis-isomers appear to be in conformational equilibrium between a form with equatorial carbalkoxy and pseudoequatorial alkoxy groups and that with axial and pseudoaxial substituents. In the latter case, the axial carbalkoxy group is bent out off its normal position by about 15°.