Anomeric effects in pyranosides and related acetals

The anomeric effect, originally defined as the preference of an electronegative substituent at the anomeric position of pyranosides to stand in an axial rather than an equatorial position, results from the combination of multiple steric, stereoelectronic and medium interactions. A more generalized definition of the enthalpic anomeric effect refers to the gem-dioxy stabilizing effect which makes acetals more stable than their 1,n-dialkoxyalkane isomers (n>1). This effect is far more important than the difference in stability between conformers of acetals as those found in axial and equatorial pyranoses and pyranosides. This review gives an up-dated view of enthalpic, conformational and also kinetic anomeric effects in pyranosides. The latter manifest themselves during heterolytical processes occuring at the anomeric centers of pyranosides and analogous compounds.