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OH
O
HO
O
OH
HO
O
O
HO
O
HO
O
+
OAc
HO
HO
HO
O
OMe
N
3
O
HO
HO
55
56
5
0.2 equiv Na ascorbate,
0.01 equiv CuSO
4,
water, 70°C, 2 h
OH
O
HO
OH
HO
HO
O
O
OH
HO
N
HO
O
N
OH
O
N
HO
O
HO
HO
5
HO
OH
O
HO
O
O
OH
O
OMe
OH
HO
N
N
O
HO
N
O
HO
OH
O
HO
5
57
SCHEME 5.15
Synthesis of amylopectin analogs on glucose scaffold.
Triazole-linked mimetics of heparosan and chondroitin disaccharide and tetrasac-
charides were synthesized by applying the CuAAC reaction to azido-glucuronic
acid and propargylated N-acetyl glucosamine or N-acetyl galactosamine derivatives
(Scheme 5.18). The transformations were achieved using Cu(I) in the presence of
DIPEA in acetonitrile, leading, for example, to the chondroitin tetrasaccharide analog
67
in 77% yield [37].
Polysaccharides have also been used as starting materials to click sugar residues
using the Huisgen reaction. Thus, regioselective bromination-azidation of curdlan, a
3)-glucan (
71
), afforded 6-azido-6-deoxycurdlan (
72
). Different molecules
bearing a terminal alkyne group, such as
-D-(1
→
-lactoside, were clicked to this scaffold.
Couplings were carried out at room temperature using CuBr
2
in the presence of
ascorbic acid and in dimethyl sulfoxide (DMSO) as solvent. A base (triethylamine,
propylamine, ammonia) was also added and yields were in the range of 63-78%
(Scheme 5.19). The reaction was followed by ATR-FTIR spectroscopy, since azido
groups show a clear and strong IR peak at around 2100 cm
−
1
[38].
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