Cyclic systems anomeric effect

In cyclic systems such as 1, the dominant conformation is the one with the maximum anomeric effect. In the case of 1, only conformation lA provides the preferred antiperiplanar geometry for both oxygens. Antiperiplanar relationships are indicated by including lone pairs in the oxygen orbitals. Other effects, such as torsional strain and nonbonded repulsion, contribute to the conformational equilibrium, of course. Normally, a value of about 1.5 kcal/mol is assigned to the stabilization due to an optimum anomeric interaction in an acetal. 

Similar comments apply to cyclic systems (Table 8-2). SYBYL molecular mechanics is completely unsatisfactory for establishing relative conformer stabilities, while MMFF appears to be quite well suited for this purpose. The only unsatisfactory case for the MMFF model is 2-chlorotetrahydropyran, where the noted preference for an axial chlorine (usually attributed to the anomeric effecF) is not reproduced. Caution should be exercised in the application of MMFF to carbohydrates where the anomeric effect may lead to significant conformational preferences. 

Using a binary sensitizing system (phenanthrene P/DCNB /7-dicyanobenzene) in acetonitrile solution, O-aryl glycosides are transacetalized with alcohols after generation of aromatic radical cations [23], According to kinetic anomeric effects, the a-side attack of nucleophiles to cyclic oxocarbenium ions follows scheme 9. 

As indicated at the beginning of this section, heteroatom hyperconjugation was first recognized in carbohydrates. The anomeric effect has been particularly well studied in cyclic systems, such as found in carbohydrates. We return to the anomeric effect in cyclic systems in Topic 2.3. 

The relative magnitude of the anomeric effects in the ap, sc and sc, sc conformers of 11 is then 21.1/14.6 = 1.44, in rather good agreement with the ratio of 1.66 estimated previously for cyclic systems (66) vide supra). 

It is possible that simple repulsion between parallel dipoles in 116 also plays a part in the greater stability of 117. It has been shown that aqueous solvation effects reduce anomeric stabilization in many systems, particularly for tetrahydropyranosyls. In contrast to cyclic acetals, simple acyclic acetlas... 

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