An anomeric effect can favour an axial electronegative substituent at a cyclic acetal carbon. Donation from an endocyclic oxygen lone pair into an antiperiplanar exocyclic C–O antibonding orbital is one important contribution. Steric, electrostatic and solvent effects also influence the observed preference.
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The anomeric effect is a stereochemical phenomenon observed in carbohydrate chemistry, specifically in relation to the conformations of pyranose and furanose forms of sugars. It refers to the preference for certain substituents at the anomeric carbon (the carbon that becomes chiral upon the formation of a cyclic structure) to adopt an axial rather than an equatorial position when in a six-membered ring (pyranose) or five-membered ring (furanose).