The Chemist’s Way to Synthesize Glycosides - The Sugar Code - page 39

Figure 3.5 1,2 - Trans - and cis - linkages.
3.5.1
Formation of 1,2- Trans - Linkages
To create a 1,2- trans -linkage a participating group in the 2-position, usually an acyl
protection group (for example acetate or benzoate), is utilized. When the donor is
activated and the oxocarbenium ion is starting to form, the free electron pair of
the carbonyl oxygen participate and form a bond to the anomeric center to form
a cyclic acyloxonium ion intermediate (Figure 3.6). When the acceptor now
approaches, the cis -side of the anomeric position is protected by the cyclic ion and
this is why the attack has to come from the trans -side to form a 1,2- trans - linkage.
Another option for the acceptor is to attack the positively charged carbon, which
will lead to formation of an orthoester. Under basic conditions this will be the
main product, but under acidic conditions (read glycosidic conditions) the
orthoester is not stable but will be in equilibrium with the acyloxonium ion
to fi nally give the glycosidic product ( trans -linked !). There is no other prerequisite
for this methodology than having a participating group at the 2- position of
the donor, and this is why this is a simple, general and reliable way to make
1,2 - trans - linkages.
Saccharides with a 2-amino group (most often in the form of an acetamido
group, for example GlcNAc and GalNAc) are abundant moieties in natural carbo-
hydrate structures (please see Chapter 1). Also with this type of donor 1,2- trans -