Pyranoside anomeric center

The anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a six-membered ring to prefer the axial orientation instead of the less-hindered equatorial orientation that would be expected from steric considerations. For sugars, substituents, such as halide, at the anomeric center of pyranosides prefer to adopt... 

The special electronic structure of the anomeric center in pyranoid derivatives results in experimentally significant differences in molecular geometry (e.g., as reflected by bond lengths and valence angles) about the anomeric carbon atom between a- and 0-pyranoses and a- and /3-pyranosides. In the article by Jeffrey, a discussion of the structural properties of the anomeric center in pyranoses and pyranosides is given, and a comparison is made of crystallographic data of some carbohydrates with the results of theoretical calculations performed on model compounds. Paulsen and co-workers also present x-ray crystallographic data in their discussion of some aspects of the conformational analysis of pentopyranosyl acetates, benzoates, and halides, in comparison with extensive conformational data compiled by Durette and Horton for these compounds in solution. 

The Structural Properties of the Anomeric Center in Pyranoses and Pyranosides... 

In consonance with this heuristic, a phenolic TBS derivative has been shown to migrate to a primary alcohol. A pyranoside anomeric hydroxyl is more acidic than the 2-OH, and thus treatment of the disaccharide with NaH and BnBr results in migration of the silyl group and protection of the anomeric center with a benzyl group. ... 

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