Synthesis of 2-azidoethyl α-D-mannopyranoside orthogonally protected and selective deprotections

Article abstract of DOI:10.1016/j.tetlet.2006.02.063

We present the synthesis of a fully orthogonally protected mannosyl glycoside 1 and the corresponding methods for selective deprotections. Mannosyl glycoside 1 contains a functionalized linker at the anomeric position to allow for the attachment of carboh

Full text of DOI:10.1016/j.tetlet.2006.02.063

Tetrahedron Letters 47 (2006) 2475–2478
Synthesis of 2-azidoethyl a-D-mannopyranoside
orthogonally protected and selective deprotections
*
´
Jose Juan Reina and Javier Rojo
Grupo de Carbohidratos, Instituto de Investigaciones Quı´micas, CSIC, Av. Ame´rico Vespucio 49, 41092 Seville, Spain
Received 24 January 2006; revised 7 February 2006; accepted 13 February 2006
Available online 28 February 2006
Abstract—We present the synthesis of a fully orthogonally protected mannosyl glycoside 1 and the corresponding methods for selec-
tive deprotections. Mannosyl glycoside 1 contains a functionalized linker at the anomeric position to allow for the attachment of
carbohydrate units to scaffolds in order to prepare carbohydrate multivalent systems.
Ó 2006 Elsevier Ltd. All rights reserved.
Dendritic cell specific ICAM-3 grabbing non-integrin
(DC-SIGN) is a C-type lectin that presents a carbo-
hydrate recognition domain (CRD) at the C-terminus.
This lectin is not only capable of interacting with highly
glycosylated proteins found at the surface of several
pathogens such as viruses, bacteria, fungi and parasites,1
but also recognizes in a calcium dependent way high
mannose structures present in N-glycans of pathogen
glycoproteins. After the discovery of the role that DC-
SIGN plays in the HIV infection process,2 much effort
has been devoted to the synthesis of these mannosylated
structures and related neoglycoconjugates in order to
obtain multivalent carbohydrate systems that mimic
the natural systems.3–7 The epitope present on high
mannose that shows the strongest binding affinity to
DC-SIGN is the glycan Man9GlcNAc2 as represented
in Figure 1.8
drates. Consequently, there is a need to prepare glycans
functionalized via the anomeric position in such a way
as to incorporate the handle necessary to establish the
key arrays.
Here, we present a new and effective synthesis of the
orthogonally protected mannose derivative 1 functional-
ized at the anomeric position with a short and function-
ally versatile spacer moiety. In this synthesis, we have
chosen ‘permanent’ protection for position 4 (O-benzyl
group) and for one end of the linker (the azido group).
These residues will be reduced at the end of the synthetic
sequence by hydrogenolysis (palladium on carbon and
H2 at atmospheric pressure). Positions 2, 3 and 6 (corre-
sponding to the branched positions present in high man-
nose structures, see Fig. 1) have been protected with
levulinic ester (Lev), 4-methoxybenzyl (PMB) and t-
butyldiphenylsilyl (TBDPS) groups, respectively. This
selection of orthogonal groups allows for the preparation
of different branched oligosaccharides with linkage in 2,
3 or 6 position followed by a simple hydrogenolysis to
liberate position 4 and simultaneously provide the amine
function at the terminus of the linker. This terminal
amine provides a means of incorporating theses mannose
derivatives into a multivalent scaffold using classical
strategies such as amide,11–13 urea or thiourea bond for-
mation.14,15 The synthesis of the mannose derivative 1
begins with commercially available peracetylated a-D-
mannose 2. In three steps, and following the procedure
we have published previously based on a modification
of earlier work,11,16 2-azidoethyl a-D-mannopyranoside
(3) was prepared in good yield. Glycosylation of 2 with
2-bromoethanol using a large excess of BF3ÆEt2O as
The synthesis of complex oligosaccharides is usually
based on a well defined selection of protecting groups
in order to achieve efficient protection and deprotection
pathways leading to the target compound. Some exam-
ples of orthogonal protection–deprotection strategies
have been developed for galactose and N-acetyl glucos-
amine both for solution and solid phase synthesis, res-
pectively, with the aim of preparing libraries of
saccharides.9,10 The weak affinity of individual carbohy-
drate–protein interactions has been overcome in nature
by multivalent presentation of the requisite carbohy-
*
Corresponding author. Tel.: +34 954 48 95 68; fax: +34 954 46 05
65; e-mail: javier.rojo@iiq.csic.es
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.063

2476
J. J. Reina, J. Rojo / Tetrahedron Letters 47 (2006) 2475–2478
OH
O
HO
HO
HO
O
HO
O
HO
HO
O
HO
OH
O
O
HO
HO
HO
HO
HO
O
OH
O
O
O
O
O
O
HO
HO
OH
NHAc
HO
O
O
NHAc
HO
HO
HO
HO
HO
O
O
OH
HO
O
HO
HO
HO
O
O
HO
HO
HO
O
OH
Figure 1. Chemical structure of Man9GlcNAc2.
promoter, followed by azide displacement and acetate
deprotection using Zempler conditions with NaOMe
gave glycoside 3 in 63% yield over three steps (Scheme 1).
fied by flash chromatography using as eluent CH2Cl2–
MeOH (9.5:0.5) to give silyl ether 4 in 79% yield. H
1
NMR in CDCl3 of 4 showed typical signals correspond-
ing to the aromatic protons at 7.72–7.68 ppm (m, 4H)
and 7.48–7.38 ppm (m, 6H) and a singlet at 1.01 ppm
for the t-Bu group. The next step in the synthesis was
the protection of position 4 with a benzyl group as a per-
manent protecting group. To achieve this required tran-
sient protection of positions 2 and 3, which was
accomplished by formation of an acetonide with 2,20-
dimethoxy propane and p-TsOH in acetone. Subse-
quently, benzylation of the hydroxyl in position 4 was
achieved by treatment with benzyl chloride and NaH
in dimethylformamide with high yield. Final cleavage
of the acetonide using trifluoroacetic acid in THF at
room temperature and chromatographic purification
on silica gel using as eluent toluene–AcOEt (9:1) gave
The primary hydroxyl group in mannose 3 was pro-
tected with a silyl group by reaction with t-butyl-
diphenylsilyl chloride using the reaction conditions
described in the literature.17 This compound was puri-
OAc
O
OH
O
AcO
AcO
AcO
HO
HO
HO
i
OAc
O
3
2
N3
ii
1
OH
O
mannose 5 in 63% yield over three steps. H NMR in
OH
O
TBDPSO
BnO
TBDPSO
HO
iii
CDCl3 of 5 showed a more complex aromatic region
and a clear AB system for the methylene of the benzyl
group centered at 4.69 ppm. The H3 proton deshielded
at low field 0.15 ppm by the ring effect and H4 moved
to high field. To selectively protect the position 3 we
chose a strategy based on use of a stannylene acetal
between hydroxyl groups in 2 and 3.18 Reaction of 5
with Bu2SnO followed by p-methoxybenzyl chloride
(PMBCl) and Bu4NI in toluene under reflux gave the
mannose derivative 6 protected exclusively in position
3 with the p-methoxybenzyl group. The final intermedi-
ate 6 was purified by chromatography on silica gel using
as eluent toluene–MeOH (94:6) and was obtained in
76% overall yield and as a single regioisomer as con-
HO
HO
O
O
5
4
N3
N3
iv
OH
O
TBDPSO
BnO
PMBO
O
6
N3
v
OLev
O
TBDPSO
BnO
PMBO
1
firmed by NMR spectroscopy. H NMR in CDCl3 of
O
6 showed H2 deshielded at low field and H3 moved to
high field with new singlets for two methylene protons
at 4.63 ppm and for three methoxy protons at 3.83 ppm.
1
N3
Scheme 1. Reagents and conditions: (i) see Ref. 11; (ii) TBDSCl, Im,
DMF, rt, 79%; (iii) (a) 2,20-dimethoxypropane, PPTS, acetone; (b)
BnBr, NaH, DMF; (c) TFA, DCM, 63% in three steps; (iv) (a)
Bu2SnO, toluene, 110 °C; (b) PMBCl, Bu4NI, 76%; (v) levulinic acid,
DCC, DMAP, DCM, 95%.
More significant was the 13C NMR spectrum where C3
moved from 72 to 81 ppm and the position of C2 did
not change.

J. J. Reina, J. Rojo / Tetrahedron Letters 47 (2006) 2475–2478
2477
The protection of the axial hydroxyl group in position 2
represents the final step in the synthesis of the orthogo-
nally protected mannose derivative 1, for which a levu-
linic ester was selected. Although levulinate is not one
of the most common protecting groups used in carbohy-
drate chemistry, the selective deprotection of this unit in
the presence of several functional groups using hydra-
zine acetate makes this group very attractive. Reaction
of 6 with levulinic acid in the presence of 1,3-di-
cyclohexylcarbodiimide (DCC) and 4-dimethylamino-
pyridine (DMAP) using the conditions previously
described in our group provided the fully protected
mannose derivative 1 in 95% isolated yield.19 Com-
pound 1 was completely characterized using mass spec-
trometry, as well as mono- and two-dimensional NMR
experiments.20 The most indicative data were the
deshielding of H2 signal from 4.06 to 5.34 ppm and
the presence of new signals corresponding to the methyl-
ene groups at 2.83–2.61 (m, 4H) and to the methyl group
at 2.51 ppm of the levulinic ester.
ment of glycoside 1 with trifluoroacetic acid in dichloro-
methane at ꢀ20 °C.19 The loss of the p-methoxybenzyl
group was observed in the NMR spectrum by the disap-
pearance of the AB system of the benzyl methylene at
4.53 ppm and the singlet corresponding to the methoxy
group at 3.83 ppm. Also the 1H NMR in CDCl3 showed
that the H3 moved to low field from 3.80 to 4.22 ppm
and 13C NMR showed the shielding of C3 from 55.7
to 70.2 ppm.
Mannose derivative 6 deprotected at position 2 was ob-
tained directly as the last intermediate in the synthetic
pathway leading to the fully protected mannose deriva-
tive 1 (Scheme 1). However, we wanted to demonstrate
the selective removal of this protecting group in the
presence of the other protecting groups selected for this
orthogonally protected mannose. Hydrazine acetate in
methanol at room temperature was used to achieve this
cleavage22 and mannose derivative 6, which was purified
by flash chromatography using toluene–MeOH (100:1)
as eluent, was obtained in 87% yield.
Having synthesized the mannose derivative 1 with the
array of orthogonal protecting groups shown, we have
performed selective deprotections to demonstrate the
orthogonal behavior of the protecting groups used;
these steps are critical for the potential applications of
this mannose in complex and versatile synthesis of oligo-
saccharides. Specific conditions were chosen to achieve a
complete and selective removal of each protecting group
and these are illustrated in Scheme 2.
In conclusion, we have synthesized a functionalized and
fully orthogonally protected mannose glycoside 1. The
selective cleavage of protecting groups from 1 was per-
formed in very good yields and with excellent selectivity.
Final hydrogenation will provide a terminal primary
amine ready to allow the attachment of this carbohy-
drate to a multivalent scaffold. Glycosylation of man-
nose deravative 1 with activated mannosyl donors to
prepare complex mannosyl structures is underway.
The cleavage of the silyl group at position 6 in the man-
nose derivative 1 was easily achieved cleanly and highly
efficiently using tetrabutylammonium fluoride (TBAF)
in THF at room temperature21 or with the HF–Py com-
plex in HOAc–THF as solvent.19 The resulting mannose
derivative 7 was purified by flash chromatography with
hexane–AcOEt (1:1.5) as eluent. In the NMR spectrum
the absence of the t-butyl signal at 1.56 ppm and the
simplification of the aromatic region with the loss of
two phenyl groups were indicative.
Acknowledgements
We would like to thank FIS (PI030093), for financial
support. We also thank Professor Timothy Gallagher,
for critical reading of this manuscript.
References and notes
Mannose derivative 8 with a free hydroxyl group at
position 3 was obtained in quantitative yield by treat-
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OLev
O
OH
O
TBDPSO
BnO
TBDPSO
BnO
iii
PMBO
PMBO
O
O
1
N3
N3
6
ii
i
OLev
O
TBDPSO
BnO
OLev
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HO
HO
BnO
PMBO
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8
N3
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N3
7
Scheme 2. Reagents and conditions: (i) Bu4NF, THF, rt or HF–Py,
HOAc–THF, 95%; (ii) TFA, DCM, ꢀ20 °C, 100%; (iii) N2H4HOAc,
MeOH, rt, 87%.

2478
J. J. Reina, J. Rojo / Tetrahedron Letters 47 (2006) 2475–2478
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(CAr), 128.1 (CAr), 128.0 (CAr), 114.2 (C3 and C5), 98.2
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