Probing
the Influence of Protecting Groups on the
Anomeric Equilibrium in Sialic Acid Glycosides with the Persistent
Radical Effect
- Publication date
- Publisher
Abstract
A method
for the investigation of the influence of protecting groups
on the anomeric equilibrium in the sialic acid glycosides has been
developed on the basis of the equilibration of <i>O</i>-sialyl
hydroxylamines by reversible homolytic scission of the glycosidic
bond following the dictates of the Fischer–Ingold persistent
radical effect. It is found that a <i>trans</i>-fused 4<i>O</i>,5<i>N</i>-oxazolidinone group stabilizes the
equatorial glycoside, i.e., reduces the anomeric effect, when compared
to the 4<i>O</i>,5<i>N</i>-diacetyl protected
systems. This effect is discussed in terms of the powerful electron-withdrawing
nature of the oxazolidinone system, which in turn is a function of
its strong dipole moment in the mean plane of the pyranose ring system.
The new equilibration method displays a small solvent effect and is
most pronounced in less polar media consistent with the anomeric effect
in general. The unusual (for anomeric radicals) poor kinetic selectivity
of anomeric sialyl radicals is discussed in terms of the planar π-type
structure of these radicals and of competing 1,3-diaxial interactions
in the diastereomeric transition states for trapping on the α-
and β-faces of the radical