Atropdiastereoisomers of ellagitannin model compounds: configuration, conformation, and relative stability of D-glucose diphenoyl derivatives

Conformational analysis reveals a remarkable rigidity of 2,3-, 4,6-, 3,6-, and 2,4-O-(S)- and (R)-diphenoyl (DP) bridged methyl beta-D-glucosides, whi ch were used as model compounds to evaluate the atropisomeric features of t he natural ellagitannins, which possess at least one hexahydroxydiphenoyl ( HHDP) moiety. The 2,3- and 4,6-O-(S)-DP bridged glucosides with C-4(1) pyra nose geometries are thermodynamically more stable than their (R)-DP counter parts, whilst in the 3,6- and 2,4-O-linked series with C-1(4) glucopyranose geometries the (R)-DP configuration is preferred. The chiral scaffold of g lucose exerts a strong atropdia-stereoselective effect onto the diphenoyl u nits, which is mediated through 10- to 12-membered rings via ester linkages . The calculated results not only explain the observed (S)-diastereoselecti vity of di-esterification reactions of suitably protected racemic hexaoxydi phenic acids with 4,6-unsubstituted D-glucopyranose derivatives, but also c orrelate the observed configuration of axially chiral HHDP-moieties of natu ral ellagitannins with conformational parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.