NMR experiments on ketals VI. PMR spectra of alkyl substituted 1, 3‐dioxanes with rigid or anancomeric structures

PMR coupling values and shifts of twenty three 1, 3‐dioxanes with rigid structure have been determined. The coupling data are in accordance with a chair conformation which is less buckled than that of cyclohexane derivatives. It is necessary to take into account the recently recognized influence of free electron pair orbitals on gem and vicinal coupling phenomena (11). An exceptional situation for the equatorial proton on C5is found, not only for the couplings of this proton, but also for its shift. An influence caused by the shielding of the oxygen atoms (O1and O3), their (equatorial)p‐orbitals overlapping with the C‐H orbital, has been shown to be responsible for coupling value variations.