The Anomeric Effect in Spiroketals
These spiroketal conformations are named according to the position of the oxygen atom on the other six-membered ring. The axial-axial structure displays the anomeric effect twice as each oxgen atom has a lone pair antiperiplanar to the other C-O single bond. The axial-equatorial structure displays the anomeric effect once from its axial oxygen atom and the equatorial-equatorial structure does not display the anomeric effect at all. This gives the order of stability shown above.
Show the oxygen lone pairs and antibonding orbitals on the anomeric carbon for the anomeric effect.