Anomeric and Gauche Effects

Abstract

The anomeric effect and its extensions -the generalised anomeric effect and the gauche effect -are neatly interpreted in terms of competing bonding interactions -n-σ* and σ-σ* -between filled and vacant orbitals. (The reverse anomeric effect is an exception, but the evidence for its existence is not unequivocal.) In a given system this represents just one of several factors controlling conformation, and perhaps reactivity. Its importance is specifically indicated by the changing pattern of bond lengths at the centres concerned. New data -both experimental and calculational -on the structure and conformation of very simple systems -aryl tetrahydropyranyl acetals and 2-substituted ethanol derivatives -are consistent with the dominant role of these stereoelectronic effects.After 15 years we have a fair understanding of the basis of the anomeric effect. Because of its practical importance and fundamental interest it remains a very active area of research. I will discuss recent work on three different but related aspects where basic questions remain to be answered. In all three cases the immediate interest is in conformation. What we are really interested in though is reactivity, and possible applications to reactivity are never far from our thinking. In what terms do we currently understand the basis of the anomeric effect? The first thing to realise -and it applies to almost any problem in Chemistry -is that explanations in terms of a single factor very often have more or less serious limitations. In this case we can manage pretty well with just two, at least as long as we restrict ourselves to structural

Sodium Ion Interactions with Aqueous Glucose: Insights from Quantum Mechanics, Molecular Dynamics, and Experiment

“…28−30 Briefly, the anomeric effect refers to the stability of axial conformations of electronegative substituents at the C1 position in pyranose rings, contrary to what would be expected on the basis of only steric considerations. 31,32 The relative stability of the anomers is a function of the electrostatic environment; in vapor phase and nonpolar solvents, α-glucose is preferred, with the population shifting toward β-glucose as the polarity of the solvent increases. 31,33 As noted above, β-glucose is preferred in aqueous solution.…”

“…Thus, important features such as the oxygen lone pairs are captured, which are considered an important factor in determining energies of stable conformations of glucose. 32,45,46 The QM studies employ implicit solvent and include only the cation, as supported by the aforementioned experimental studies. The MD studies include the anion and cation as well as explicit water.…”

How S–C–N anomeric effects and energetic preference across [S–C–C–O] fragments steer conformational equilibria in 4′-thionucleosides. 1H NMR and ab initio MO study †

“…15 The heterocyclic bases in N-nucleosides drive the two-state N S pseudorotational equilibrium of the constituent β--pentofuranosyl moieties by two counteracting contributions from (i) the inherent steric effect of the nucleobase 16 and (ii) the anomeric effect. 17- 19 We have initiated a conformational study of a series of β--2Ј-deoxy-4Ј-thioribonucleosides 1-4, their ribo counterparts 5-8 and α--4Ј-thioribonucleosides 9 and 10 in order to assess how the electronic changes caused by replacement of oxygen with sulfur change the stereoelectronic effects which govern their conformational degrees of freedom and possibly perturb the pK a values. The conformational preferences of 1-10 in aqueous solutions were examined with the use of temperature-and pH-dependent 1 H NMR spectroscopy.…”