|TUD Organische Chemie||Immel||Publications||Papers||Abstract 32||View or Print (this frame only)|
S. Immel and K. Khanbabaee
Tetrahedron: Asymmetry 2000, 11, 2495-2507.
Conformational analysis reveals a remarkable rigidity of 2,3-, 4,6-, 3,6-, and 2,4-O-(S)- and (R)-diphenoyl (DP) bridged methyl β-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 3,6- and 2,4-O-linked series with 1C4 glucopyranose geometries the (R)-DP configuration is preferred. The chiral scaffold of glucose exerts 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.