Chemistry Reference
In-Depth Information
In another approach avoiding the use of large monomeric functions,
carbohydrate derivatives with an unsaturation included in the sugar ring
or directly linked to a carbon atom of the sugar ring (alkylidene) were
obtained by chemical or by enzymatic dehydration of the protected
substrate under soft conditions.
23
Various protective groups such as
acetate, benzoate or acetonide have been found effective and compatible
with the desired elimination reaction.
Some examples of monomers described in the literature are shown in
Scheme 1 (Type II and III).
These monomers exhibit an endo-cyclic
4,24,25
(named type III) or an
exo-cyclic double bond involving the previously hemiacetalic C atom
26,27
(type II monomer) that can be polymerized or copolymerized by a free-
radical mechanism with comonomers such as N-vinylpyrrolidone, butyl
vinyl ether or vinyl acetate
21
(Scheme 3). This approach was applied to the
synthesis of biocompatible, biodegradable, skin compatible copolymers
designed to biomedical or cosmetic utilization.
26
Poor yields in polymer as well as low polymerization rates were ob-
served due to the steric hindrance of carbohydrate moieties and because
of the electronic effects on the C
¼
C bonds. More ecient polymerization
was achieved by optimizing monomer concentration and/or by selecting
proper comonomer combinations, particularly with respects to mutual
polarity effects.
24,27
An original monomer derived from fructofuranose, with unsaturation
at 5,6-exo-cyclic position was synthesized through a four-step reaction.
28
The reactivity of the corresponding 5,6-exo-fructene in free-radical
homopolymerization as well as in copolymerization was studied in some
detail. Even if this cyclic vinyl ether behaves poorly in homopolymeriza-
tion (low yields and low MW), it was shown to react conveniently in co-
polymerization with alkyl acrylates (Scheme 4), allowing for
the
Type II :
O
OR
3
n
R
3
: Ac
Bz
BOP
O
R
3
O
OR
3
80
°
C,48h
ρ
< 5%
OR
3
R
3
O
R
3
O
O
OR
3
+ AN, NVP,
VA or St
OR
3
with R
4
:
BOP or AIBN
CH
2
N
80
°
C,48h
n
= 10-30%
ρ
O
C
N
OAc
R
4
OR
3
R
3
O
OR
3
O
OAc
AcO
O
=
Type III :
R
1
N
AIBN
m
O
+
n
R
1
O
ρ
= 80%
R
1
OAc
OBu
O
OAc
OAc
Scheme 3 Free-radical polymerization or copolymerization of type II
27
and type III
24
unsaturated monosaccharides leading to polymers with almost 1 C of carbohydrate ring
in polymeric backbone (AN: acrylonitrile, St: styrene, VA: vinyl acetate, NVP: N-vinyl-
2-pyrrolidone).
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