Conformational analysis of glucopyranosylammonium ions: does the reverse anomeric effect exist?

Despite the generality of the anomeric effect (the axial tendency exhibited by many electronegative groups at C1 of a tetrahydropyran), some cationic substituents prefer the equatorial position. This preference is claimed to exceed that due to steric repulsion, and it has been called a «reverse anomeric effect». Such an effect is sometimes invoked to account for relative reactivities and stereoselectivities when there are cationic leaving groups. However, all such substituents have been bulky aromatic rings whose steric factors are difficult to judge, and it is advantageous to compare NHR and NH 2 R + groups of known axial preference