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 t heir conformations were observed with and without protonation. A decisive d ecrease in J(5',4), consistent with stabilization of the C-1(4) conformatio ns, was clearly observed for three N-(pentopyranosyl)imidazoles. As well, n o reverse anomeric stabilization was observed for N-(2,3,4-tri-O-acetyl-alp ha -D-lyxopyranosyl upon protonation. It is suggested that the previous obs ervations of the reverse anomeric effect were due to the slight increase in steric bulk of the imidazole aglycone upon protonation, along with favorab le dipolar interactions between ring substituents, and not by a reverse of the anomeric effect.