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Ph
O
O
OLe v
OBn
OBn
BnO
O
O
CF
3
O
O
O
TBS O
BnO
+
HO
O
O
NBnCbz
BnO
O
NPh
OP iv
5
NH T CA
OPiv
NHTCA
101
102
OH
OSO
3
Na
OH
OH
OH
HO
O
O
O
O
O
O
O
NBnCbz
O
NaO
3
SO
BnO
HO
5
OP iv
NHAc
OH
NH Ac
103
Scheme 25
fucoidans.
52
Di-, tetra-, hexa-, octa-, dodeca-, and hexadecafucosides were
mostly obtained by sulphation of oligofucosides derivatives.
53
Sulphation
proceeds via an acid-promoted protocol on polyol with TFA or TfOH
(from 0.5 to 1 equiv per OH group) using Et
3
N.SO
3
complex in DMF.
54
Selective sulphation have also been reported on lactosamine deriva-
tives by Bovin and co-workers
55
and on xylose derivatives by Kosma's
group.
56
Fenner and Kerns published the per or under O-sulphation of
aminoglycoside using Pyr.SO
3
complex or ClSO
3
H.
57
Interestingly chloro-
sulphonic acid showed a better ability to achieve the per-sulphation.
Selective sulphation has been done on glycoconjugates in order to evaluate
them as heparin/HS mimics. Ferro's group described the use of such
compounds with various lipophilic conjugations able to decrease the
anticoagulant activity.
58
The synthesis and sulphation of oligosaccharides
was also described to obtain a glycopeptide with an interesting final
debenzylation process using reagent K (TFA/H
2
O/thioanisole/1,2-ethane-
dithiol/phenol) avoiding the cleavage of sulphate groups.
59
2.3 Oligosaccharides containing both carboxylate and sulphate
groups
The main class of oligosaccharides containing both sulphate and carb-
oxylic groups are GAG. Recent developments include the use of new
protecting groups and/or new leaving groups and promoters for glyco-
sylation reactions, the synthesis of analogues and different chemoenzy-
matic approaches.
2.3.1 Heparan sulphate and heparin. The synthesis of the putative
minimal FGF binding motif HS trisaccharides containing two
L
-iduronic
acid units was reported by F¨gedi's group using an orthogonal protecting
group strategy from suitable protected monosaccharides.
60
Azido groups
were used as glucosamine precursors, whereas acid groups were pro-
tected as t-butyl esters and trichloroacetimidate donors were used for
glycosylation reactions.
A modular synthetic approach reported by Arungundram et al.
61
util-
izes a relatively small number of selectively protected disaccharide
building blocks for the preparation of a library of 12 HS oligosaccharides.
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