CONFORMATIONAL-ANALYSIS OF GLUCOPYRANOSYLAMMONIUM IONS - DOES THE REVERSE ANOMERIC EFFECT EXIST

Despite the generality of the anomeric effect (the axial tendency exhi bited by many electronegative groups at Cl 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 ca lled a ''reverse anomeric effect''. Such an effect is sometimes invoke d to account for relative reactivities and stereoselectivities when th ere are cationic leaving groups. However, all such substituents have b een bulky aromatic rings whose steric factors are difficult to judge, and it is advantageous to compare NHR and NH2R+ groups of known axial preference. Therefore the proportions of axial anomers of glucopyranos ylamine and some of its N-alkyl, tetra-O-acetyl, and 4,6-benzylidene d erivatives were determined by H-1 NMR in a variety of solvents, includ ing acidic media. This proportion is small, and the assignments were c onfirmed by coupling constants, saturation transfer, reequilibration, and decoupling difference spectroscopy. The data indicate that the shi ft in the position of equilibrium that occurs upon N-protonation is sm all and can be accounted for on the basis of steric effects and a smal l normal anomeric effect. Therefore we conclude that the reverse anome ric effect does not exist.