Anomeric effect hyperconjugative origin

Fuchs et al. (129). They suggested that when the molecular dipoles of the axial and equatorial conformers are of similar magnitude, the more polar double bond-no bond structure, resulting from hyperconjugative interactions in the axial conformer, 54a (see Scheme 20), will be stabilized in the more polar solvent. This hypothesis would support the importance of the hyperconjugative origin of the anomeric effect. The appearance of a large proportion of the axial conformers of 2-(arylseleno)-l,3-dithianes 37 (Scheme 13), even in... 

According to NBO analysis, the key component of anomeric effect is the negative hyperconjugative interaction between the lone pair of X and the low-lying a C-Y orbital. When both X and Y have at least one lone pair, a more balanced description also involves n <7 c.x donation in the opposite direction. For the 2-alkoxy-substituted tetrahydropyrans, the overall conformational profile originates from the combination of n c-o hyperconjugative interactions, responsible for both the endo- and exo-anomeric effects (Figure 6.56). 

There has been an ongoing controversy regarding the origin of the anomeric effect. Most recently, Mo denied the importance of hyperconjugation based on block-localized... 

We have suggested " that a Class 3 force field be defined as one which contains chemical effects, in addition to the physical effects so far discussed. Chemical effects would include such things as hyperconjugation, the electronegativity effect, the anomeric and Bohlmann effects, and so on. These effects depend upon exactly which atom occupies a position, that is, they depend not only on ordinary mechanical quantities, but also on specific properties of oxygen, for example, compared with nitrogen, or with carbon. These effects can, of course, be properly represented by suitable cross terms in the force constant matrix. The origins of these terms, however, have a definite chemical basis. 

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