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Ph
a) PhCHO
Et
3
SiH
cat. FeCl
3
.
6H
2
O
O
TMSO
TMSO
TMSO
TMSO
Ph
O
OTMS
O
O
O
BnO
TMSO
HO
OTMS
OBn
O
O
O
O
TMSO
HO
60
61
,
61%
b) TBSOTf, 74%
c) HCO
2
NH
4,
Pd/C
65%
PhCHO, Et
3
SiH
cat. FeCl
3
.
6H
2
O
then C
15
H
31
COCl
Ph
O
Ph
O
Ph
O
O
O
BnO
Ph
O
O
O
O
TBSO
OH
O
O
BnO
C
15
H
31
O
OBn
O
O
HO
67
O
HO
d) C
15
H
31
CO
2
H
DCC, DMAP, 85%
Ph
66
, 27%
O
Ph
O
e)
R
CO
2
H, DCC
O
O
BnO
DMAP, 32-64%
TBSO
R
O
O
O
O
O
68
O
C
15
H
31
Scheme 17 Access to key di-O-acylated trehalose intermediate 68.R
=
a small col-
lection of deoxypropionate acyl chains; TBS
=
t-butyldimethylsilyl.
different acyl chains were then incorporated on the trehalose core by
sequential esterification under classical conditions, giving di-O-acyl tre-
halose 68, a key intermediate to access both di- and tetra-O-acylated
sulfoglycolipids.
Installation of the sulfate group at C-2
0
was carried out using the
SO
3
.
pyr complex after desilylation with n-tetrabutylammonium fluoride.
Di-O-acylated sulfoglycolipid analogue 70 was obtained by final depro-
tection of the benzyl and 4,6-O-benzylidene groups with the use of an
excess of anhydrous iron(
III
) chloride,
13
preserving the C-C double bond
(Scheme 18).
45
Some of these Mycobacterium tuberculosis sulfoglycolipid
analogues,
47
simplified in their deoxypropionate acyl chains, showed very
high antigenicity, suggesting their being good candidates as components
of anti-tuberculosis subunit vaccines.
45
For the synthesis of tetra-O-acylated analogues, double regioselective
reductive opening of the 4,6-O-benzylidenes was performed with the
PhBCl
2
/Et
3
SiH combination
48
leading to the two primary alcohols. Double
esterification to 71, desilylation, sulfation and hydrogenolysis of the benzyl
groups, furnished the tetra-O-acylated sulfoglycolipid 72 (Scheme 18).
44
5 Direct synthesis of
b
-
D
-
N
-acetyl glucosamine motifs
using catalytic iron(
III
) triflate
The study of novel chemical methods for stereoselective glycoside syn-
thesis is an important area of glycosciences.
49,50
Among the monomers
found in mammalian carbohydrates, the b-
D
-N-acetyl glucosamine
(GlcNAc) motif was revealed to be the most abundant by statistical an-
alysis of the carbohydrate diversity.
51
For the synthesis of these com-
pounds, the main challenge is the selective formation of the b-glycosidic
linkage.
This
explains
the numerous
b-selective
glycosylation
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