Anomeric effects, in sugars

In a microwave investigation, Lopata and Kuczkowski (Lopata, A. D. Kuczkow-ski, R. L., J. Am. Chem. Soc., 1981, 103, 3304-3309) determined that the equilibrium geometry of FCH2— —CHO was as shown below. Explain the near 90° orientation of the CF bond relative to the plane of the rest of the molecule (the explanation is the same as for the anomeric effect in sugars.)... 

In a probe for the presence of stereoelectronic effects in nucleophilic addition to 12 sterically unbiased ketones, calculations have identified subtle bond length differences in the C-Nu bond of the diastereomeric alcohol products, where Nu- = H-or Me-.304 The calculated differences are weak (<1%) but consistent the bond is longer in the major product, acting as a fossil record of the TS. Using microscopic reversibility, the easier bond to cleave (the longer one) is the easier to form. The effect bears comparison with the kinetic anomeric effect in sugars, where such bond length differences in calculation are borne out in X-ray crystal structures. 

Fig. 7.7 Anomeric effect in sugars (left) preferentially stabilising the axial position of the anomeric C—O bond, and isomer energy difference between 1,3- and 1,4-dioxane at B3LYP/6-31 G(d) as clear evidence against steric (Pauli) repulsion arguments for the anomeric effect.

The MM3 treatment of the anomeric effect was fairly good based on the data available at the time. But better quantum mechanical calculations are now possible, and they indicate that the anomeric energy effect was somewhat larger than was incorporated in MM3. Improvements seen in the MM4 structures discussed here are partly due to improvements in the hydrocarbon section of the force field. Furthermore, torsion-stretch and torsion-bend effects are important in the alcohols and ethers in general, but they were not included in MM3. Since these terms are included in MM4, alcohols and ethers are more accurately treated, and all of this translates into better structures and energies for anomeric compounds as well. Some results of the anomeric effect in sugars will be discussed in Chapter 8. 

Carba-sugars (pseudo-sugars) and their derivatives, chemistry of, 48, 21 - 90 Carbohydrate chemistry anomeric and exo-anomeric effects in, 47, 45-123... 

Luyten I, Thibaudeau C, Sandstrom A, Chattopadhyaya J (1997) The tunable transmission of the aromatic character of the aglycon through the anomeric effect in C-nucleosides drives-its own sugar conformation a thermodynamic study. Tetrahedron 53 6433-6464... 

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