17O NMR spectra of the 1,6‐anhydro‐β‐D‐hexopyranoses and related‐compounds. Determination of configurational effects on the chemical shifts

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Abstract
Well resolved natural‐abundance 17O NMR spectra of the eight 1,6‐anhydro‐β‐D‐hexopyranoses and three deoxy model compounds were recorded in aqueous solution at 90°C. The assignment of the resonances was obtained by chemical shift comparison, O‐acetylation, lanthanide shift reagents (Eu3+) and, more generally applicable, by a computational search for the minimum of the sum of the squared deviations between the observed and calculated shifts. The 17O chemical shifts were evaluated depending on the orientation of the hydroxyl groups, expressed in terms of additive γ‐ and δ‐interactions between the hydroxyl and ring ether groups. The γ‐gauche effects were dominant in a range between −7 and −20 ppm. No appreciable effects have been detected of anti‐periplanar hydroxyl groups on the 17O chemical shifts. The syn‐diaxial δ‐interaction was deshielding between hydroxyl groups, but shielding between a hydroxyl group and the bridge‐ether oxygen. A new type of interaction, a δ‐effect between equatorial hydroxyl oxygens in a planar zig‐zag arrangement, was detected. Arguments are presented suggesting that γ‐gauche effects between oxygens are not related to steric compression.