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was kept at room temperature for 3 hours, then concentrated. The residue was eluted
from a column of silica gel with 2:1 AcOEt-cyclohexane, then AcOEt, to give the
triazole-linked tetravalent
C
-glycocluster (77 mg, 56%) as syrup. To a solution of
the latter glycocluster (35 mg, 0.015 mmol) and sugar thiol
1
(67 mg, 0.18 mmol) in
anhydrous CH
2
Cl
2
(0.5 mL) was added DPAP (4.5 mg, 0.018 mmol). The solution
was irradiated (
max
365 nm) at room temperature for 1 hour under magnetic stirring,
then concentrated. The residue was treated with Ac
2
O (1 mL) and pyridine (1 mL) at
room temperature for 2 hours and then concentrated. The residue was eluted from a
column of silica gel with AcOEt, then 2:1 AcOEt-acetone, to give
34
(43 mg, 76%)
as a syrup; [
]
D
=+
20.1 (
c
0.7, CHCl
3
).
3.4 SILSESQUIOXANE-BASED GLYCOCLUSTERS
Cube-octameric silsesquioxanes (COSS, R
8
Si
8
O
12
), more commonly referred to as
polyhedral oligosilsesquioxanes (POSS) [23], are organic-inorganic hybrid materi-
als that are attracting increasing interest due to their presentation of eight equiva-
lent R groups in a globular structure. In this context, in 2004, Lee and coworkers
described the photoinduced reaction of the commercially available octavinyl POSS
35
with
O
-mannosyl and
O
-lactosyl
-thiobutyramides
36a
,
b
to give the corresponding
octavalent glycoconjugates
37a
,
b
(Scheme 3.10) [24].
The rather harsh photochemical conditions (
max
254 nm) as well as the long
reaction time (48 hours) employed by Lee stimulated our group to develop a TEC-
based approach to silsesquioxane-based glycoclusters under mild conditions. It was
also considered to use suitable sugar thiols to give more stable products in respect to
the
O
-glycoside derivatives prepared by Lee. Initial coupling experiments between
35
and excess of a model sugar thiol 1-thio-
-D-glucose (up to 4 eq. per ene of
35
) under
our standard photochemical conditions (
max
365 nm in the presence of DPAP as
radical initiator) were unsatisfactory as only partially glycosylated POSS derivatives
were obtained. This finding together with the observation that Lee employed thiols
in which the sulfhydryl group was attached to the sugar residue through a linker,
RS
SR
O
Si
O
Si
O
Si
O
Si
O
RS
O
SR
H
N
l
254 nm
Si
O
O
Si
Si
O
O
Si
O
S
i
O
O
Si
O
O
O
+
sugar
O
SH
O
O
O
O
S
i
S
i
AIBN, 48 h
O
SR
O
O
RS
Si
O
Si
Si
O
Si
sugar = mannosyl (
36a
), lactosyl (
36b
)
SR
35
(4 eq. x 8 )
RS
70-73%
OH
O
H
O
HO
37a
R =
HO
O
O
N
H
HO
OH
OH
O
O
O
37b
R =
O
O
HO
H
HO
HO
HO
SCHEME 3.10
POSS-based glycoconjugate prepared by Lee via TEC.
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