Anomeric effect

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. [GoldBookRef|title=Anomeric Effect|file=A00372|year=1996] 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. [cite journal |author=Juaristi, E. |coauthors=Cuevas, G. |title=Recent studies of the anomeric effect |journal=Tetrahedron |year=1992 |volume=48 |pages=5019–5087 |doi=10.1016/S0040-4020(01)90118-8 ] 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".

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In the diagram, both anomers of glucopyranose are identical, except that the "beta" anomer on the right has an OH group pointing up equatorially in the lower right-hand corner of the figure, whereas the "alpha" anomer on the left has 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 exhibit the anomeric effect and that is often used for theoretical studies is dimethoxymethane. In the case of dimethoxyethane the "gauche","gauche" conformation is 3–5 kcal/mol about 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.

Physical origins

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. [cite journal |author=Box, V. G. S. |title=The anomeric effect of monosaccharides and their derivatives. Insights from the new QVBMM molecular mechanics force field |journal=Heterocycles |volume=48 |issue=11 |pages=2389–2417 |year=1998 ] More recent MO calculations are consistent with this hypothesis. [cite journal |author=Takahashi, O. |coauthors=Yamasaki, K.; Hohno, Y.; Ohtaki, R.; Ueda, K.; Suezawa, H.; Umezawa, Y.; Nishio, M. |title=The anomeric effect revisited. A possible role of the CH/n hydrogen bond |journal=Carbohydr. Res. |volume=342 |pages=1202–1209 |year=2007 |doi=10.1016/j.carres.2007.02.032 ] 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. [cite journal |author=Box, V. G. S. |title=Explorations of the Origins of the Reverse Anomeric Effect of the Monosaccharides using the QVBMM (Molecular Mechanics) Force Field |journal=J. Mol. Struct. |volume=522 |pages=145–164 |year=2000 |doi=10.1016/S0022-2860(99)00358-0 ]

An alternative and widely accepted explanation is that there is a stabilizing interaction (hyperconjugation) between the an 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 attenuate the anomeric effect. However, it is well known that when the exocyclic atoms bear lone pairs of electrons, the anomeric effect is maximal.

Some authors also question the validity of this model based on results from the theory of atoms in molecules. [cite journal |author=Vila, A. |coauthors=Mosquera, R. A. |title=Atoms in molecules interpretation of the anomeric effect in the O—C—O unit |journal=J. Comp. Chem. |year=2007 |volume=28 |pages=1516–1530 |doi=0.1002/jcc.20585 ]

While most studies on the anomeric effects have been theoretical in nature, the n–σ* 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. [cite journal |author=Box, V. G. S. |title=The role of lone pair interactions in the chemistry of the monosaccharides. The anomeric effect |journal=Heterocycles |volume=31 |pages=1157–1181 |year=1990 ] [cite journal |author=Box, V. G. S. |title=The role of lone pair interactions in the chemistry of the monosaccharides. Stereo-electronic effects in unsaturated monosaccharides |journal=Heterocycles |volume=32 |pages=795–807 |year=1991 ]

ee also

*Cyclohexane conformation

References