Chemistry Reference
In-Depth Information
Figure 3.8
Halide - assisted glycosylation.
donors and, if correctly performed, gives excellent
- selectivity. The drawback is
that, due to the lack of promoter, it is a very mild glycosylation methodology,
requiring extended reaction time and sometimes heating, and yields with less
reactive acceptors (for example many secondary hydroxyl groups) are usually not
very high.
α
3.6
Neuraminic Acid and K do - Glycoside Synthesis
Another diffi cult form of glycosides to synthesize in a stereoselective manner is
glycosides with no substituent at all in the position next to the anomeric center -
2-deoxy-glycosides. Such structures found in Nature are Kdo-glycosides, found in
bacterial polysaccharides, and sialic acid glycosides (for structures, please see
Figure 1.6; for relevance of sialic acids in biorecognition, please see Chapters 18,
19, 25 and 27; for relevance of sialic acids in drug design, please see Chapter 28 ).
Nomenclature-wise these are 3-deoxy sugars, since the carboxyl function is C1 and
the anomeric center is C2, but the problem is the same - how to perform a stereo-
selective glycosylation when you have no functional group next to the anomeric
center to use as a handle.
With NeuAc and Kdo the naturally occurring glycosides are both
- linked.
However, due to the different preferred conformations of the pyranose ring of the
two sugars this means that in neuraminic acid glycosides the carboxyl group is in
the axial and the glycosidic linkage in the equatorial position, whereas in Kdo it
is the opposite.
α
