O-Glycosides are true acetals in that the -OH
of a hemi-acetal has been substituted by an -OR
group.
In the simplest case, the "OR" maybe
from a simple alcohol such as methanol or ethanol.
In this case, the -OR is referred to
as the aglycone, the "non-sugar" part of the glycoside.
Alternatively, the "OR" can be from
another saccharide so joining two saccharide units together.
The following diagram shows two glycosides of glucose:
methyl β-d-glucopyranoside
sucrose, a disaccharide
So the organic chemistry here, is just that of simple acetals and ketals
(review ?)
The mechanism by which aldehydes or ketones are interconverted to acetals
or ketals is catalysed with aqueous acid BUT not aqueous base.
Hence, glycosides can be hydrolysed with aqueous acid to give the component
saccharides.
Other groups such as -SR or -NR2
can also be used to substitute the -OH
group, such systems are referred to a S-glycosides and N-glycosides
respectively.
