17O NMR spectra of equatorial and axial hydroxycyclohexanes and 5‐hydroxy‐1,3‐dioxanes and their methyl ethers

Abstract

Abstract17O chemical shifts of axial hydroxyl groups in cyclohexanols are upfield of those of corresponding equatorial groups, but in 5‐hydroxy‐1,3‐dioxanes the opposite is observed: the axial OH resonates downfield of the equatorial OH. The situation is the same in the corresponding methyl ethers and is, thus, not a result of intramolecular hydrogen bonding in the axial 5‐hydroxy‐1,3‐dioxane, but appears to parallel the effect on 13C and 19F shifts observed in corresponding equatorial and axial 5‐methyl‐ and 5‐fluoro‐1,3‐dioxanes, which has been attributed to an upfield shifting effect of the antiperiplanar γ‐located heteroatoms. Surprisingly, the reciprocal effect is not seen in the ring 17O shifts of the 5‐hydroxy‐1,3‐dioxanes. A δ compression shift is seen in the 17O spectrum of trans‐3,3,5‐trimethylcyclohexanol (syn‐axial OH and CH3), analogous to the effect earlier reported in 13C spectra. Conversion of four of the alcohols to methyl ethers produces a large upfield effect on the 17O shift, larger in the cyclohexanols than in the 1,3‐dioxane‐5‐ols. Similar upfield shifts have been recorded in the literature; their extent depends on whether the alcohols are primary, secondary or tertiary.