Anomeric center effect

Reduction of aldonolactones and their derivatives with isotopically modified reducing agents leads to sugars labeled at the anomeric center. Glycosides substituted with deuterium or labeled with tritium are widely employed for kinetic isotope-effect measurements, mechanistic studies, isotope-tracing experiments, and so on. 

Figure 13.4 Torsional potential energy surfaces about the two C-O bonds linking the anomeric centers of sucrose at the MM3 level (a), 2-tetrahydrofuranyl-2-tetrahydropyranyl ether at the MM3 level (b), the same ether at the HF/6-31G(d) level (c), and the sum of the difference between the last two with the first (d). Thus, the last surface may be viewed either as the effect of the sucrose hydroxyl groups on the energy surface, evaluated at the MM3 level, added to the framework surface calculated at the ab initio level, or as an MM3 surface that has been partially conected quantum mechanically. Solid triangles represent anomeric torsions in sucrose units found in various X-ray crystal structures. Note that the hybrid surface is the only one that clusters the large majority of these triangles within low-energy contours...

This article deals with the state of knowledge of ring photobromination of carbohydrate derivatives about 12 years after the phenomenon was first observed. The reaction is very unusual in providing means by which hydrogen atoms bonded to carbon atoms of cyclic carbohydrate derivatives may be substituted directly and, in many cases, affording selectivity at ether carbon atoms in preference to the dioxygenated anomeric centers. Some aspects of it have been reviewed from the standpoint of the operation of the captodative effect.24... 

On the basis of the above results and discussion, the glycosides can now be considered. Efforts have been made previously to evaluate the magnitude of the anomeric effect by undertaking equilibration studies between equatorial and axial isomers at the anomeric center in carbohydrates (48), in monosubstituted 2-alkoxytetrahydropyrans (49, 50) and in more rigid systems (51). The anomeric effect has been evaluated to be of the order of 1.2 to 1.8 kcal/mol from these studies. In these evaluations, the conformation of the OR group in the axial and in the equatorial isomer was not considered the influence of the exo-anomeric effect was therefore neglected (3). Nevertheless, these studies demonstrated the importance of the anomeric effect. 

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 MM2 program [52] is by faf the most widely used force field program in the area of hydrocarbon or moderately polar molecules and numerous publications have demonstrated its usefulness. Still the anomeric center had not until recently been properly defined by the force field parameters. Now there are parameters available which include the proper treatment of acetal fragments and hence reproduce the anomeric as well as the exo anomeric effect accurately [53]. Due to the full optimization of the geometry the MM2 program can treat only a limited number of atoms. 

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