Anomeric effect 2-substituted oxanes

The most frequently used measure of the anomeric effect is based on the comparison of the stabihty of 2-substituted oxane (tetrahydropyran THP) and cyclohexane. In general, conformational properties of the oxane ring are similar to those of cyclohexane, with dominance of a chair conformation. It is further presumed that steric interactions in oxane are the same as in cyclohexane, with preference for equatorial positions of bulky substituents. The Gibbs energy of the anomeric effect, AG(AEl), can be expressed as the... 

Fig. 5.—The Standard Equilibrium in (a) Substituted Oxanes and (b) Substituted Cyclohexanes Used in the Definition of the Anomeric Effect by Eq. I.

Axial Preferences and the Gibbs Energy of the Anomeric Effect AG(AE,) (in kJ.moC ) in the 2-Substituted Oxanes ... 

Additional relevant work was presented in 2007 by Takahashi et al. using computational methods in the modeling of substituted oxanes and 1,3-dioxanes. These researchers found that the interatomic distances between axial H4 and axial H6 and the anomeric substituent (OMe, F, Cl, Br) are shorter than the sum of their van der Waals radii, which suggests that the magnitude of the anomeric effect in these heterocycles is due in part to the stabilization provided by C—H—Y hydrogen bonding (Scheme 9) (07CAR1202). 

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