Anomeric effect energy associated with

The energy associated with a pure anomeric effect (i.e. one not influenced by other factors) has been estimated at -1.4 kcal mol T17 Putting an OR group axial confers a destabilization of -0.8 kcal mol-1, so the energy difference between 28 and 29 is only about 0.6 kcal mol-1. Therefore, the anomeric effect is small. Bonds will only be significantly weakened when several anomeric effects are superimposed or when they are reinforced by protonation. 

Each of the traditional biomolecular force fields has been p meterized for carbohydrates, and in several instances more than one parameter set may exist, offering alternative levels of sophistication, internal consistency, and compatibility with parameters for.energy expression is very often augmented by a component intended to jreproduce the conformational energies associated with the exo-anomeric effect. 

The paucity of information on the mechanism of reactions, and on the structure of the transition state, and the role of the anomeric effect in its stabilization, constitutes the main reason why qualitative interpretation of reactivity as shown in the aforementioned examples is still very rare. An alternative, more-popular estimation of the relative reaction-rates of con-formers is based on the lone-pair orbital interactions, and their symmetry and energy in the ground state, and could be loosely associated with the perturbation theory of chemical reactivity. ... 

The anomeric effect is widely believed to be caused by hyperconjugation. An axially oriented orbital associated with nonbonding electrons of the ring oxygen can overlap with a tr orbital of the axial exocyclic C—O hemiacetal bond. This effect is similar to that which helps cause the lowest energy conformation of ethane to be the anti conformation (Section 4.8). An anomeric effect will ffequendy cause an electronegative substituent, such as a hydroxyl or alkoxyl group, to prefer the axial orientation. 

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