Equilibria in compounds that exhibit the anomeric effect

Equilibrium constant not known in solution crystalline form has all chlorines axial. 

The NMR spectrum in CDQ3 indicates that the all-axial form is strongly favored. The equilibrium constant is not known. 

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in entries 4-6. Single crystal X-ray diffraction studies have unambiguously established that all the chlorine atoms of trans, syn, Irans-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal. Each chlorine in the molecule is bonded to an anomeric carbon, and is subject to the anomeric effect. Equally striking is the observation that all the substituents of the tri-O-acetyl-jS-D-xylopyranosyl chloride shown in entry 5 are in the axial orientation in solution. Here, no special crystal packing forces can be invoked to rationalize the preferred conformation. The anomeric effect of a single chlorine is sufficient to drive the equilibrium in favor of the conformation that puts three acetoxy groups in axial positions. 

A number of explanations have been put forth for the anomeric effect. One suggestion is that an unfavorable dipole-dipole interaction between C-O bonds and the substituent is greater when the substituent is equatorial than when it is axial.  

The dependence of the equilibrium position on the dielectric constant of the medium is consistent with this explanation. The equatorial isomer, by virtue of its higher dipole moment, would be increased in concentration relative to the axial isomer in solvents of higher dielectric constant. 

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Figure 126 Equilibria in compounds that exhibit the anomeric effect.

Scheme 3.1. Equilibria in Compounds That Exhibit the Anomeric Effect... 

In the solid state, tri-O-acetyl-p-D-xylopyranosyl halides adopt the all-equatorial 4Ci conformation, -whereas tri-O-benzoyl-p-D-xylopyranosyl halides crystallize in the all-axial 1C4 conformation, except for tri-O-benzqyl-p-D-xylopyranosyl chloride, which adopts the (skew) conformation. The variations of bond lengths to the anomeric carbon atom are discussed with regard to the anomeric effect. By comparative studies of the conformational equilibria of acetylated and benzoylated 1,5-anhydropentitols on the one hand, and pentopyranoses exhibiting the anomeric effect on the other, data for the non-bonding interactions may be derived. Based on the principle of additivity of these factors, the conformational distribution of structurally related compound may be estimated. In connection with these results, it is noted that two benzoyl groups show a smaller 1,3-diaxial interaction than two acetyl groups the difference between these interactions is about... 

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