Biology Reference
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and the subsequent removal of the protective groups [14]. However,
cationic ring-opening polyaddition of sugar oxazoline monomers
derived from GlcNAc derivatives gave aminopolysaccharides
with relatively controlled structures [15]. For example, a 3,6-
-
dibenzylated sugar oxazoline monomer was polymerized with acid
catalyst via ring-opening addition to give a dibenzyl chitin with DP
is ca. 13 [16]. Then, removal of benzyl groups was carried out to
give chitin [17]. However, complete deprotection did not take place.
Because sugar monomers with protective groups have to be employed
for the chemical synthesis of polysaccharides by polymerization,
subsequent deprotection process is necessarily conducted to
give free polysaccharides. Furthermore, perfect stereocontrol of
glycosidic linkages has not often been achieved.
On the basis of the aforementioned backgrounds and
viewpoints, almost in the past two decades, an enzymatic approach
as a superior method to the traditional chemical one has been
employed for the synthesis of polysaccharides with highly stereo-
and regioselectivities [18-23].
O
1.2
Concept in Synthesis of Polysaccharides
Polysaccharides are produced by the repeated glycosylations
of a glycosyl donor with a glycosyl acceptor to form a glycosidic
linkage [24-26]. Figure 1.3 shows a typical schematic reaction
of glycosylation for possible formation of a disaccharide. For the
design of the substrates, an anomeric carbon (C1) of the glycosyl
donor is activated by introducing a leaving group (X), and a hydroxy
group in the glycosyl acceptor, which participates in the reaction, is
employed as a free form, whereas other hydroxy groups in both the
glycosyl donor and acceptor are protected. There are two important
selectivities that should be controlled to form the glycosidic linkages.
Because two possible geometric isomers, namely,
α
β
-isomers,
are conceived, the control of formation in such two glycosidic
linkages, i.e., stereoselectivity, is one of the two important selectivities
in the glycosylation. The other selectivity is regioselectivity.
Monosaccharide has multiple hydroxy groups that participate in the
formation of the glycosidic linkage. For example, glucose has four
alcoholic hydroxy groups, excluding a hemiacetalic hydroxy group
at C1 position, which can participate in the glycosylation. In the
- and
