An examination of the purported reverse anomeric effect beyond acetylated N-xylosyl- and N-glucosylimidazoles

Syntheses of six new N-(pentopyranosyl)imidazoles have been achieved, and their conformations were observed with and without protonation. A decisive decrease in J(5',4), consistent with stabilization of the 1C4 conformations, was clearly observed for three N-(pentopyranosyl)imidazoles. As well, no reverse anomeric stabilization was observed for N-(2,3,4-tri-O-acetyl-alpha-D-lyxopyranosyl)imidazole upon protonation. It is suggested that the previous observations of the reverse anomeric effect were due to the slight increase in steric bulk of the imidazole aglycone upon protonation, along with favorable dipolar interactions between ring substituents, and not by a reverse of the anomeric effect.