Chemistry Reference
In-Depth Information
Figure 3.16
Synthesis of Le
b
hexasaccharide.
form the new glycosidic linkage. As to the stereoselectivity, 1,2-
trans
- linkages (
β
-
D
-
gluco
and -
galacto
,
-
D
-
manno)
are constructed effi ciently by using 2- participating
groups. 1,2 -
Cis
- linkages (
α
-
D
-
manno
), however, have no
general solution and an optimization for each coupling has to be performed. There
is no universal glycosyl donor, but a variety (for example halide sugars, trichloro-
acetimidates and thioglycosides) to choose from with different properties as stabil-
ity and reactivity. Larger oligosaccharides are preferentially synthesized using a
convergent approach where building blocks are fi rst constructed and then coupled
together to give the target oligosaccharide. This minimizes manipulations on large
structures and allows freedom in the glycosylation sequence. With the techniques
described to master synthesis of glycans, the next step to gain access to potent
ligands for lectins (please see Chapters 15 - 19 for surveys on this protein superfam-
ily) is to optimize their presentation by achieving multivalency. How to do that is
outlined in the following chapter.
α
-
D
-
gluco
and -
galacto,
β
Summary Box
Chemical synthesis can be utilized to get access to required oligosaccharide
structures. Developed methodologies using protecting groups to get regio-
selectivity and also stereoselectivity in the glycosylation reactions in combina-
tion with effi cient glycosyl donor/promoter system allows for the construction
of complex (also labeled and nonnatural) structures.
