Acetals acyclic, stereoelectronic effects

D-(+)-galactose (15) is an example of the consecutive numbering of the carbon ring atoms in a monosaccharide (disaccharide see p. 253). Carbohydrates can exist in a cyclic and an acyclic structure. For this reason there is a special position in the structure of a monosaccharide, the carbon atom C-l and so called anomeric center. You can see that there is an equilibrium between er-anomer a-15 and / -anomer /3-15 of D-(+)-glucopyranose over the acyclic aldehyde structure 16. Both are cyclic hemi-acetals. The /Tanomer is the preferred conformation, but there are a few effects, like sterical or stereoelectronical effects (anomeric effect, inverse anomeric effect), which have influence on the a /i rate. 

Stereoelectronic factors are also important in the conformational dynamics of acyclic acetals [6] (Cosse-Barbi and Dubois, 1986). Here the usual preference for staggered conformations is supplemented by the anomeric effect (Kirby, 1983), which favours the gauche stereochemistry ( = 60°) about both central C-O bonds, mainly because this allows optimal n-rr overlap between an oxygen lone pair and the antibonding (cr ) orbital of the C-0 bond. Thus the pathway for conformational isomerization suggested by... 

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