Degradations and Rearrangement Reactions
2.7
411
the anomeric equilibrium on protonation of 283 and 284 reveals that the protonated imidazoyl
group has a small but distinct preference for the axial disposition than does the unprotonated
group; which is the opposite of what the reverse anomeric effect predicts [223,224,225]. Compounds 285 and 286 increase the proportion of their 1 C 4 conformers on N-protonation but not
when the polarity of the solvent is increased as predicted by the reverse anomeric effect [226].
In solution, the α-glycosylpyridinium salt 287 adopts the 1 C 4 conformation and the β-mannosyl compound 288 has the 4 C 1 form; both of them have positively charged groups in an
equatorial position at the anomeric center, indicating the manifestation of the reverse anomeric effect [227] ( > Fig. 8).
Aryl C-glycosides such as 289 and 291 undergo α- to β-anomerization in the presence of an
acid by way of open-chain intermediates [228,229]. Under basic conditions, 293 isomerizes
into 295 through 294 [230] ( > Scheme 59).
⊡ Scheme 59
⊡ Scheme 60