화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.121, No.29, 6911-6918, 1999
Absence of reverse anomeric effect in glycosylimidazoles
The so-called reverse anomeric effect is the preference of cationic substituents for the equatorial position on a pyranose ring, but it is not consistent with theories of molecular structure. To reinvestigate this, we have measured the N-protonation-induced shifts of the anomeric equilibrium in N-(glycopyranosyl)imidazoles and their tetra-O-acetyl derivatives 1-3 with high precision through an NMR titration method that is applicable to a mixture of alpha and beta anomers. We find a Delta Delta G degrees(beta-->alpha) that is almost always negative, corresponding to a greater preference for the axial position of a protonated imidazolyl group than of an unprotonated group. This preference counters asmall steric effect, arising from hindrance to ionic solvation, that has been measured independently in N-(4-tert-butylcyclohexyl)imidazoles 4. These results are exactly opposite to what is expected from the reverse anomeric effect. We conclude that there is no firm evidence for this effect.