Anomeric Effect in Systems

Based upon the results of the studies of VaseUa et al. [85] and Franck et al. [95], it can be concluded that the reverse anomeric effect does not exist. The electropositive substituent at the anomeric carbon produces the enhanced endo-snometic effect but not a reverse anomeric effect [97]. Non-solvated electropositive axial anomeric substituents such as nitro and sulfone groups show the ent/o-anomeric effect to be much larger than the oxygen anomeric effect, namely, 2.35 kcal/mol for the nitro group and 3.05 kcal/mol for the sulfone group, as opposed to the endo-anomeric effect for the oxygen atom of 0.85-1.54 kcal/mol. 

The NMR studies at low temperature of unsubstituted and the C2 substituted 1,3-oxazines such as 117 (R = H) and 119 (R = CH3) [45] strongly suggest that the conformers 117 and 119 are the major components of the equilibrium mixture 

The NMR study of Af-methyl tetrahydro-1, 3-oxazine such as 121 suggested that conformer 121 with the Af-methyl group in the axial orientation is more stable than conformer 122 wherein the methyl group is equatorially oriented for the same reasons given above [98] (Fig. 2.39). 

If steric repulsion experienced by the methyl and dimethylamino groups increases by the same factor on going from the cyclohexane to dioxane, then the equilibrium constants for the ring inversion of the two compotmds should be the same. At 185°K, cyclohexane conformation in which the NMc2 is axially 

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The concept of the anomeric effect in systems O-C-N is of great importance in chemical behavior of nucleosides, nucleotides, and nucleic acids since it controls the conformation of ribofuranosyl (or deoxyribofuranosyl) ring and with it the reactivity of the W-glycosidic bond. 

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