Biphenyl derivatives substituted in 2- and 2'-position exist in two chiral conformations which differ only by rotation around the central single bond (atropisomers). Rotation is hindered, but not impossible and proceeds rapidly for 2,2'-dimethoxy-1,1'-biphenyl at room temperature. For the corresponding binaphthyl derivative rotation is largely prevented and both enantiomeric conformers may be resolved. The animations show CPK and ball-and-stick type models of both compounds during a full 360° rotation about the 1,1'-bond, in which at least the binaphthyl would have to undergo serious deformations of the naphthyl rings due to steric hindrance. The ball-and-stick models are shown in standard atom colors as well as colored models with the split terms of conformational energy (Van der Waals strain and torsional bending) mapped onto the atoms. Blue colors correspond to relaxed molecular fragments and yellow-red colors indicate strained and bended molecular parts. All structures and energy terms were calculated using the PIMM force-field program.

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.

Details and data were taken from S. Immel, K. Khanbabaee, "Atropdiastereoisomers of Ellagitannin Model Compounds: Configuration, Conformation, and Relative Stability of D-Glucose Diphenoyl Derivatives", Tetrahedron: Asymmetry 2000, 11, 2495-2507.