Conformational analysis reveals a remarkable rigidity of 2,3- and 4,6-O-(S)- and (R)-diphenoyl
(DP) bridged methyl α-D-glucosides, which were used as model compounds to evaluate the atropisomeric features
of the natural ellagitannins, which possess at least one hexahydroxydiphenoyl (HHDP) moiety. The 2,3- and
4,6-O-(S)-DP bridged glucosides with 4C1 pyranose geometries are thermodynamically more stable than their
(R)-DP counterparts, whilst in the 4,6-O-linked galactopyranoses the (R)-DP configuration is preferred. The
chiral scaffold of glucose excerpts a strong atropdiastereoselective effect onto the diphenoyl units, which is
mediated through 10- to 12-membered rings via ester linkages. The calculated results not only explain the
observed (S)-diastereoselectivity of di-esterification reactions of suitably protected racemic hexaoxydiphenic
acids with 4,6-unsubstituted D-glucopyranose derivatives, but also correlate the observed configuration of axially
chiral HHDP-moieties of natural ellagitannins with conformational parameters.
The models and animations shown here visualize the restricted internal motions of diphenoyl moieties resulting
in their conformational stability. The color-coded models map the split-terms of internal strain energy on the atoms
(blue: relaxed molecular parts, yellow-red: strained and bended fragments), showing the build-up of strain energy
as rotation of around the central bond of the diphenoyl residues is attempted.