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OP iv
HO
OPiv
HO
COOMe
CO OMe
O
O
O
O
OMP
O
O
OMP
PivO
MB zO
PivO
MBzO
OMBz
OMBz
NH Ac
N
3
141
143
92%
SO
3
.N M e
3
,DMF,60 °C
96%
OPiv
HO
COOMe
OP iv
Na
3
OSO
O
O
O
COOMe
OMP
PivO
MBzO
O
O
O
OMP
OMBz
PivO
MB zO
N
OMBz
N
3
142
144
NaO
3
SO
Scheme 30
synthesized effectively by coupling the corresponding disaccharide units,
followed by regioselective sulphation.
84
Interestingly, O-sulphation failed when performed on compound 141,
due to the formation of an imine-like structure 142. When sulphation
was performed prior to azide reduction, the 4
0
-sulphate disaccharide 144
was obtained in 96% yield, see Scheme 30.
Maza et al.
85
reported the chemical synthesis of CS and DS oligo-
saccharides using an N-trifluoroacetyl galactosamine building block
containing a 4,6-O-di-tert-butylsilylene group. Using this approach, a CS/
DS-like tetrasaccharide that contains both types of uronic acids,
D
-
glucuronic and
L
-iduronic acid, was synthesized. Eller et al.
86
reported the
combination of an automated carbohydrate synthesizer with a photo-
labile linker cleaved in a continuous-flow photoreactor for the synthesis
of two CS hexasaccharides, corresponding to CS-A and CS-C sequences.
Hydroxyl groups that were not modified during the reaction were per-
manently protected by benzyl ethers, hydroxyl groups that would ultim-
ately be sulphated were masked with levulinyl esters, whereas temporary
protection by Fmoc groups was used for hydroxyl groups of acceptors in
glycosylation reactions. Glycosyl phosphates were employed as donors.
Sulphation was performed on the solid resin support, final deprotection
steps required prior photochemical cleavage of the resin.
2.3.3 Other oligosaccharides. Zlotina et al.
87
reported on the syn-
thesis of fragments of the chain of the fucoidan from Cladosiphon oka-
muranus brown seaweed: disaccharide a-
D
-GlcA-(1
-
2)-a-
L
-Fuc-OPr,
trisaccharide a-
L
-Fuc-(1
2)]-a-
L
-Fuc-OPr, and their se-
lectively O-sulphated derivatives bearing sulpho-groups at O-4 of the
fucose units. Glucuronylation by a series of selectively O-acetylated glu-
curonyl bromides was studied to obtain the target products. It has been
found that a 3-O-acetyl-2,4-di-O-benzyl methyl glucuronate donor was the
most ecient agent for a-glycoside bond formation which could be due
to intramolecular remote participation of 3-O-acetyl group. The synthesis
and biological activities of a library of GAG mimetic oligosaccharides
from dextran was reported.
88
The product was characterized by a global
degree of substitution of carboxymethyl (0.5) and sulphate (1.1). Since
these HS mimetics were resistant to endoglycanases, depolymerization
-
3)-[a-
D
-GlcA-(1
-
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