Exo anomeric interactions

The importance of steric factors in conformational behavior about the axial carbon-heteroatom bond was seen by us (54, 96b) in axial 2-phosphino-l,3-dithianes 44 (Scheme IS) whose rotational isomerism was studied by NMR methods. We had expected that if the exo anomeric interactions in the C—S—C—P( ) system were strong, the preferred rotamers should be T" ... 

In cases of dominant exo-anomeric interactions in R X—A Y segments, there exists a contraction of A-Y bond length and a stretching of A—X bond length that stabilizes the axial or equatorial conformation where it takes place (97CAR1). 

Stereoelectronic effects have long been recognized to influence the configuration and the conformation of acetals, particularly in carbohydrates where these effects were first discovered and discussed in terms of the anomeric and the exo-anomeric effects (1-3). The term anomeric effect introduced by Lemieux in 1958 (A) refers to the tendency of an alkoxy group at C-l of a pyranose ring to assume the axial rather than the equatorial orientation despite unfavorable steric interactions, whereas the term exo-anomeric effect also introduced by the same author (5) concerns the preferred orientation of the 0-R bond of the alkoxy group at the anomeric center. 

The conformation of the backbone of the Klebsiella polysaccharide K23[-(l-3)-a-L-Rha-(l-3)-p-D-Glc-]n has been calculated using a force field which includes terms for non bonded interactions, the exo anomeric effect, and hydrogen bonds [203]. The calculated data were checked using NMR spectroscopy. The P-D-Glc-(l-3)-ot-L-Rha bond shows dihedral angles

[Pg.194]

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