In organic chemistry, the anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the less hindered equatorial orientation that would be expected from steric considerations.[1] This effect was originally observed in pyranose rings by J. T. Edward in 1955; at that time, N.-J. Chii and R. U. Lemieux began to study the anomerization equilibria of the fully acetylated derivatives of several aldohexopyranoses. The term "anomeric effect" was introduced in 1958.[2] The anomeric effect got its name from the term used to designate the C-1 carbon of a pyranose, the anomeric carbon. Isomers that differ only in the configuration at the anomeric carbon are called anomers.
The anomers of glucopyranose are diastereomers, with the beta anomer on the right having an OH group pointing up equatorially in the lower right-hand corner of the figure, and the alpha anomer on the left having that OH group pointing down axially.
The anomeric effect can be generalized to any system with the general formula R–Y–C–Z, where Y is an atom with one or more electronic lone pairs, and Z is an electronegative atom. The magnitude of the anomeric effect is estimated at about 1–2 kcal/mol in the case of sugars. In this general case, the molecule need not be cyclic. For example, a small molecule that exhibits the anomeric effect and that is often used for theoretical studies is dimethoxymethane. In the case of dimethoxyethane the gauche,gauche conformation is about 3–5 kcal/mol lower in energy (more stable) than the trans,trans conformation—this is about two times as big as the effect in sugars because there are two rotatable bonds that are affected.
Several explanations for the anomeric effect have been proposed.
The simplest explanation is that the equatorial configuration has the dipoles involving both heteroatoms partially aligned, and therefore repelling each other. By contrast the axial configuration has these dipoles roughly opposing, thus representing a more stable and lower energy state.
In 1998, Box's molecular modeling studies of saccharides, and analysis of crystallographic data of monosaccharides from the Cambridge Crystallographic Database, using the molecular mechanics based program STR3DI32, resulted in a refinement of this dipolar hypothesis by showing that the dipolar repulsions originally suggested, above, were reinforced by stabilizing, and significant, C-H...O hydrogen bonds involving the acetal functional group.[3] More recent MO calculations are consistent with this hypothesis.[4] This more comprehensive analysis of the origins of the anomeric effect has also resulted in a better understanding of the related, and equally puzzling, reverse anomeric effect.[5]
An alternative and widely accepted explanation is that there is a stabilizing interaction (hyperconjugation) between the unshared electron pair on the one heteroatom (the endocyclic one in a sugar ring) and the σ* orbital for the axial (exocyclic) C–X bond.
When the exocyclic (in a sugar) atom bears a lone pair of electrons there should also be a similar interaction between that unshared electron pair (of this exocyclic atom) and the σ* orbital of the annular C-O bond. This second interaction, which is a strong feature of the β-anomer (equatorial exocyclic group), should significantly stabilize the β-anomer, and should significantly attenuate the anomeric effect. Thus one would expect that molecules like the glycosyl halides should show small anomeric effects, and have α- and β-anomers of comparable energies. However, it is well known that when the exocyclic atoms bear lone pairs of electrons, the anomeric effect is maximal. Thus, the hyperconjugation hypothesis might contribute to the anomeric effect, but is not the only, or the dominant, stereo-electronic participant.
Some authors also question the validity of this hyperconjugation model based on results from the theory of atoms in molecules.[6]
While most studies on the anomeric effects have been theoretical in nature, the n–σ* (hyperconjugation) hypothesis has also been extensively criticized on the basis that the electron density redistribution in acetals proposed by this hypothesis, is not congruent with the known experimental chemistry of acetals, and, in particular, the chemistry of monosaccharides.[7][8]