Assembly of a Complex Branched Oligosaccharide by Combining Fluorous-Supported Synthesis and Stereoselective Glycosylations using Anomeric Sulfonium Ions

Article abstract of DOI:10.1002/chem.201501844

There is an urgent need to develop reliable strategies for the rapid assembly of complex oligosaccharides. This paper presents a set of strategically selected orthogonal protecting groups, glycosyl donors modified by a (S)-phenylthiomethylbenzyl ether at

Full text of DOI:10.1002/chem.201501844

DOI: 10.1002/chem.201501844
Full Paper
&
Synthetic Methods
Assembly of a Complex Branched Oligosaccharide by Combining
Fluorous-Supported Synthesis and Stereoselective Glycosylations
using Anomeric Sulfonium Ions
Wei Huang,[a, b] Qi Gao,[a] and Geert-Jan Boons*[a, b]
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Abstract: There is an urgent need to develop reliable strat-
egies for the rapid assembly of complex oligosaccharides.
This paper presents a set of strategically selected orthogonal
protecting groups, glycosyl donors modified by a (S)-phenyl-
thiomethylbenzyl ether at C-2, and a glycosyl acceptor con-
taining a fluorous tag, which makes it possible to rapidly
prepare complex branched oligosaccharides of biological im-
portance. The C-2 auxiliary controlled the 1,2-cis anomeric
selectivity of the various galactosylations. The orthogonal
protecting groups, 2-naphthylmethyl ether (Nap) and levu-
linic ester (Lev), made it possible to generate glycosyl ac-
ceptors and allowed the installation of a crowded branching
point. After the glycosylations, the chiral auxiliary could be
removed using acidic conditions, which was compatible
with the presence of the orthogonal protecting groups Lev
and Nap, thereby allowing the efficient installation of 1,2-
linked glycosides. The light fluorous tag made it possible to
purify the compounds by a simple filtration method using
silica gel modified by fluorocarbons. The set of building
blocks was successfully employed for the preparation of the
carbohydrate moiety of the GPI anchor of Trypanosoma
brucei, which is a parasite that causes sleeping sickness in
humans and similar diseases in domestic animals.
also advanced substantially.[7] A host of glycosylating agents,
new linker systems, different solid supports, and a variety of
protecting groups have been carefully evaluated and these ef-
forts have resulted in the first commercially available glycan
synthesizer.
Introduction
It is now well-established that a dense layer of complex carbo-
hydrates covers the surface of all prokaryotic and eukaryotic
cells. These carbohydrates have been implicated in a wide
range of biological processes such as protein folding, fertiliza-
tion, embryogenesis, host-guest interactions, and cell differen-
tiation and mobility.[1] In addition, overwhelming data supports
the relevance of glycosylation in pathogen recognition, inflam-
mation, innate immunity, and the development of autoimmun-
ity and cancer.[2] Although the importance of cell-surface car-
bohydrates in health and disease is widely appreciated, advan-
ces in glycoscience have been slow, due to the staggering
complexity of the glycome.[3] This complexity makes it difficult
to define glycan structures expressed by a given cell type and
complicates the identification of specific glycan recognition de-
terminants of glycan-binding proteins.[4] Libraries of well-de-
fined glycans will make it possible to address these difficulties.
The need for diverse collections of complex glycans has
stimulated the development of fast and convenient methods
for their synthesis.[5] For example, several synthetic strategies
make it possible to assemble complex oligosaccharides from
carefully selected monosaccharide building blocks using a mini-
mal number of chemical steps.[6] Among these strategies, one-
pot multi-step glycosylations, in which several glycosyl donors
are sequentially reacted in the same flask, are particularly at-
tractive and can furnish target oligosaccharides without the
need for protecting group manipulations and isolation and pu-
rification of synthetic intermediates.[6c] Within the past few
years, automated solid-phase oligosaccharide synthesis has
Soluble light fluorous tags offer another attractive means to
simplify the process of oligosaccharide synthesis. In this case,
tagged carbohydrates can easily be separated from nonfluo-
rous-tagged side products by solid-phase extraction using
silica gel modified by fluorocarbons.[8] This generic procedure,
which more closely resembles filtration than chromatography,
depends primarily on the presence or absence of a fluorous
tag and not on the polarity or other molecular features of the
compound. Unlike solid-phase supported synthesis, light fluo-
rous technology does not require large excesses of reagents to
drive the reactions to completion. Fluorous-tagged com-
pounds can easily be analyzed by standard spectroscopic
methods, thereby providing control over the synthesis. Fur-
thermore, efforts are underway to develop a liquid handler to
automate fluorous supported oligosaccharide synthesis.[9] Sev-
eral fluorous versions of protecting groups have been devel-
oped for a variety of functional groups, and thus tags can
easily be installed.[10] Additionally, it is possible to array fluo-
rous-tagged glycans, thereby eliminating the necessity to in-
stall reactive functional groups for glycan immobilization.[11]
Despite the promise of fluorous-supported oligosaccharide
synthesis, it has mainly been employed for the preparation of
relatively simple linear compounds.[10] This limited application
is most likely due to the difficulties of controlling anomeric se-
lectivities in glycosylations and challenges to install branching
points in high yield.[5a,6a,11] In this respect, many complex oligo-
saccharides are branched and, due to steric crowding, the cor-
responding glycosylations are often low-yielding. Furthermore,
1,2-trans-glycosides, such as b-glucosides and b-galactosides,
can reliably be introduced by neighboring-group participation
of an ester-protecting group at C-2 of a glycosyl donor
(Scheme 1a). On the other hand, the installation of 1,2-cis gly-
cosidic linkages, such as a-glucosides and a-galactosides, re-
quires glycosyl donors that have a non-assisting functionality
at C-2, and often these coupling reactions result in mixtures of
anomers.[5a,11] Low-yielding glycosylations and the formation of
[a] W. Huang, Dr. Q. Gao, Prof. Dr. G.-J. Boons
Complex Carbohydrate Research Center
University of Georgia, 315 Riverbend Road
Athens, GA 30602 (USA)
E-mail: gjboons@ccrc.uga.edu
[b] W. Huang, Prof. Dr. G.-J. Boons
Department of Chemistry
University of Georgia
Athens, GA 30602 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501844.
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Scheme 1. Control of anomeric selectivity in glycosylations. a) Neighboring-
group participation by C-2 esters to give a five-membered ring oxocarbeni-
um ion intermediate to form selectively 1,2-trans-glycosides. b) Neighboring
group participation by chiral auxiliary to give a trans-decalin anomeric sulfo-
nium ion intermediate to provide 1,2-cis-glycosides.
anomers defeat the purpose of fluorous support synthesis that
relies on simple filtration protocols for purification.
Recently, we introduced a stereoselective glycosylation ap-
proach based on neighboring-group participation by a (S)-phe-
nylthiomethylbenzyl moiety at C-2 of a glycosyl donor, which
can readily provide 1,2-cis-glycosides (Scheme 1b).[12] Upon ac-
tivation of the donor and formation of an oxacarbenium ion,
the thiophenyl moiety of the C-2 auxiliary participates, result-
ing in the formation of an intermediate sulfonium ion having
a trans-decalin configuration. This stereoisomer is strongly fa-
vored because of the absence of unfavorable gauche interac-
tions. Furthermore, the alternative cis-decalin system places
the phenyl-substituent in an axial position, thereby inducing
unfavorable steric interactions. Displacement of the anomeric
sulfonium ion by a sugar alcohol then results in the formation
of a 1,2-cis-glycoside.
Figure 1. The structure of hexasaccharide 1 of the GPI anchor of T. Brucei
and the monosaccharide building blocks required for its assembly.
Naphthylmethyl ether (Nap)[17] were employed as a convenient
set of orthogonal protecting groups for glycosyl acceptor for-
mation and branching-point installation. The donors 2 and 3,
having participating esters at C-2, were used to install the
mannosyl moieties. Furthermore, it was anticipated that galac-
tosyl donors 4-6, having a chiral auxiliary at C-2, could be em-
ployed for the stereoselective introduction of the challenging
a-galactosides.
We describe here that the use of glycosyl donors modified
by a C-2 (S)-phenylthiomethylbenzyl ether or ester-protecting
group to stereoselectively introduce 1,2-cis- or 1,2-trans-glyco-
sides, respectively, and glycosyl acceptors modified by a fluo-
rous tag can readily provide highly complex branched oligo-
saccharides of biological importance. The strategy was applied
to the preparation of the carbohydrate moiety of the GPI
anchor of Trypanosoma brucei (Figure 1), which is the parasite
that causes sleeping sickness in humans and similar diseases in
domestic animals.[13] The oligosaccharide is composed of
a branched tri-mannoside core, which is a structurally con-
served motif of GPI anchors of many different organisms. It is
further elongated by a-galactosides that are unique to T.
brucei. It is expected that synthetic carbohydrates of different
compositions will aid in the development of therapeutics and
diagnostic for infections caused by this pathogen.[14] Previous
attempts to prepare such oligosaccharides entailed low yield-
ing galactosylations and provided anomeric mixtures.[15]
First, attention was focused on the preparation of galactosyl
donors 4–6 (Scheme 2). It was expected that activation of a tri-
fluoro-N-phenyl imidate of 6 would result in the formation of
an oxacarbenium ion which undergoes neighboring-group par-
ticipation by the (S)-(phenylthiomethyl)benzyl ether leading to
a 1,2-trans anomeric sulfonium ion. Nucleophilic displacement
of the anomeric sulfonium ion by a sugar alcohol will then pro-
vide an a-galactoside.[12a] Alternatively, arylation of the 1,2-oxa-
thiane of compounds such as 4 and 5 will also provide anome-
ric sulfonium ions and such a transformation can easily be ac-
complished by activation the sulfoxide with triflic anhydride
followed by reaction with 1,3,5-trimethoxybenzene.[18] An at-
tractive feature of the 2-oxathianes is that they can be convert-
ed into compounds such as 7 by treatment with benzyne,
which leads to a derivative having a (S)-(phenylthiomethyl)ben-
zyl ether at C-2 and an acetate at the anomeric center.[19] Stan-
dard procedures can then be employed to install an anomeric
imidate for glycosylations.[20] Thus, it was anticipated that 2-ox-
athiane 11 would be an appropriate precursor for the synthesis
of glycosyl donors 4–6.
Results and discussion
The synthesis of building blocks
Thus, thioglycoside 9 was prepared by sequential treatment
of per-O-acetyl-galactose with thiourea and 2-bromoacetophe-
none. The acetyl esters of 9 were cleavage with sodium meth-
oxide in methanol and the resulting tetraol was treated with
We envisaged that building blocks 2–7 and fluorous-tag-modi-
fied benzyl alcohol 8 (Figure 1) would make it possible to as-
semble target compound 1. Levulinic ester (Lev)[16] and 2-
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Assembly of the carbohydrate moiety of the GPI anchor of
T. brucei
First, target compound 1 was prepared by a conventional pu-
rification protocol using silica gel or size exclusion column
chromatography (Scheme 3). In this case, each intermediate
was carefully characterized by two-dimensional NMR spectros-
copy and mass spectrometry. After establishing an appropriate
synthetic protocol, the target compound was resynthesized in
a rapid manner by employing fluorous solid phase extraction
and, in this case, only the fully assembled oligosaccharide was
characterized. The attraction of this approach is that a stream-
lined synthetic protocol for 1 can easily be adapted for the
preparation of many analogs.
Scheme 2. Preparation of building blocks for the GPI anchor carbohydrate
moiety of T. brucei. Reagents and conditions: a) MeONa, MeOH, RT, 1 h, then
p-TSA, MeOH, RT, 18 h; then acetic anhydride, pyridine, RT, 3 h, 73% (for
3 steps), then TiCl4, Et3SiH, DCM, 08C, 8 h, 83%; b) m-CPBA, DCM, À158C,
30 min, 96%; c) NaOMe, MeOH, RT, 1 h, then TBDPSCl, imidazole, DMF, 0 8C,
2 h, 98%; d) BnBr, NaH, DMF, 08C, 1 h, 75%; e) HF-pyridine in pyridine, RT,
18 h, 61%; f) NaH, NapBr, DMF, 08C, 5 h, 95%; g) m-CPBA, DCM, À158C,
30 min, 72%; h) Pb(AcO)4, 1-aminobenzotriazole, DCM, À788C, 1 h, 95%;
i) NH2NH2-AcOH, DMF, 508C, 4 h; then 2,2,2-trifluoro-N-phenyl-acetimidoyl
chloride, DBU, DCM, RT, 1 h, 71%.
Thus, glycosyl donor 2 was coupled with 4-(1H,1H,2H,2H-
perfluorodecyl)benzyl alcohol (8) using N-iodosuccinimide (NIS)
and triflic acid (TfOH) as the activator[23] at À258C to give, after
a reaction time of 30 min, fluorous-tagged mannoside 16 in
high yield. As expected, only the a-anomer was formed due to
neighboring-group participation of the acetyl ester of 2. Next,
the Nap ether of 16 was removed by oxidation with 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the mixture of
DCM and water to give glycosyl acceptor 17, which was cou-
pled with glycosyl donor 5 to provide, after acid mediated re-
moval of the C-2 auxiliary, disaccharide 18. In this glycosyla-
tion, 5 was arylated by treatment with a stoichiometric
amount of triflic anhydride (Tf2O) and 1,3,5-trimethoxybenzene
(TMB) in the presence of 2,6-di-tert-butyl-4-methylpyridine
(DTBMP) in DCM at À408C to form a sulfonium ion intermedi-
ate. Next, glycosyl acceptor 17 was added and the reaction
mixture was allowed to warm to room temperature and, after
a reaction time of 11 h and purification by silica gel column
chromatography, a glycoside product was obtained, having
a (trimethoxyphenylthiomethyl)benzyl ether moiety at C-2. The
latter functionality was cleaved by treatment with 10% tri-
fluoroacetic acid (TFA) in DCM to give glycosyl acceptor 18.
methanol in the presence of camphorsulfonic acid (CSA) to
form a 1,2-oxathiane ketal. Due to the poor solubility of the
latter compound, it was not purified and immediately treated
with trimethylsilyl trifluoromethanesulfonate (TMSOTf) or
BF3OEt2 in the presence Et3SiH to reduce the ketal to a 1,2-oxa-
thiane ether. Although the latter reaction proceeded smoothly
for glucose derivatives,[18a,21] in the case of galactose no reac-
tion occurred. Fortunately, the use of TiCl4 as the Lewis acid in
the presence of Et3SiH gave, after O-acetylation with acetic an-
hydride in pyridine, the target compound 10 in a yield of 83%.
Oxidation of compound 10 using meta-chloroperoxybenzoic
acid (mCPBA) in dichloromethane (DCM) at À158C gave the
galactosyl donor 4. Compound 11 was readily prepared by
treatment of 10 with 1-aminobenzotriazole and Pb(OAc)4 to
generate benzyne for arylation of the 1,2-oxathiane. The latter
compound was treated with hydrazine acetate to remove the
anomeric acetate and the resulting lactol was converted into
an N-phenyl trifluoroacetimidate (6) using 2,2,2,-trifluoro-N-
phenylacetimidoyl chloride in the presence of 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU).[22]
1
Careful analysis by H NMR spectroscopy confirmed that only
the expected a-anomer had formed.
The installation of the a(1,2)-linked galactoside of 1 proved
challenging. Preactivation of 4 followed by the addition of ac-
ceptor 18 did not lead to glycoside formation. A TMSOTf-medi-
ated coupling of 6 with 18 gave only a trace amount of prod-
uct, as shown by MALDI-TOF mass spectrometry. The use of
5 equiv of 6 provided the corresponding trisaccharide in a dis-
appointing yield of 25%. We reasoned that the failures of
these glycosylations was due to the rather low reactivity of C-2
hydroxyl of 18 and the bulky nature of the C-2 auxiliary of gly-
cosyl donors 4 and 6.[24] Therefore, a smaller and more reactive
glycosyl donor was required for this glycosylation. Indeed,
a triflic acid mediated coupling of 7 with 18 led to the forma-
tion of trisaccharide 19 in an isolated yield of 71% and, fortu-
nately, only a trace amount of the unwanted b-anomer was de-
tected. Removal of Nap ether of 19 to give glycosyl acceptor
20 was accomplished by oxidation with DDQ in a mixture of
DCM and water. In this reaction, care had to be taken to avoid
oxidative removal of one of the benzyl ethers and in particular
the use of only a small excess of recrystallized DDQ was critical
The selectively protected galactosyl donor 5 was synthesized
by removal of the acetyl esters of 10 followed by selective sily-
lation of the primary hydroxyl using tert-butyl(chloro)diphenyl-
silane (TBDPSCl) in the presence of imidazole in DMF to give
12. The latter compound was benzylated under standard con-
ditions (!13) followed by removal of the TBDPS ether using
HF-pyridine to give 14, which was converted into Nap ether
15 by alkylation with NapBr in the presence of sodium hydride
in dimethylformamide (DMF). Prior to glycosylation, the 1,2-ox-
athiane 15 was oxidized to the corresponding sulfoxide 5
using mCPBA. The mannosyl donors 2 and 3 were prepared by
standard protecting group manipulations, as detailed in the
Supporting Information.
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charide 24 in an excellent overall
yield of 51% (three steps). In this
case, only the a-anomeric prod-
uct was formed due to neigh-
boring-group participation of
the acetyl ester at C-2 of the gly-
cosyl donor. The overall yield of
the assembly of the hexasac-
charide, starting from the mono-
meric building blocks, was 9%.
Finally, hexasaccharide 24 was
converted into target compound
1 by hydrogenation over Pd/C,
followed by removal of the
acetyl esters using sodium meth-
oxide in methanol.
Fluorous-assisted target glycan
assembly
Having established a robust syn-
thetic approach for the prepara-
tion of 1, the synthesis of this
compound was performed using
a purification-protocol-based flu-
orous solid-phase extraction
(Scheme 4). In this case, each gly-
cosylation was performed twice
to ensure completion of these
critical reactions. Thus, the Nap
ether of 16 was oxidatively re-
moved with DDQ and the result-
ing acceptor 17 was isolated by
fluorous solid phase extraction
(F-SPE) using 20% water in
methanol as the eluent to
remove untagged compounds
and the desired compound was
isolated by elution with acetone.
Scheme 3. The assembly of the GPI anchor moiety of T. brucei. Reagents and conditions: a) NIS, TfOH, DCM,
À258C, 30 min, 89%; b) DDQ, DCM:H2O=10:1, RT, 2 h, 17: 82%; 20: 77%; c) Tf2O, TMB, DTBMP, À408C to RT, then
10% TFA in DCM, RT, 1 h; (18: 87%, a only; 21: 67%, a only); d) TfOH, DCM, À258C to RT, 3 h, 71%, a:b>20:1;
e) TfOH, DTBMP, DCM, À608C to RT, 18 h, then 10% TFA in DCM, RT, 1 h, 76%, a only; f) Ac2O, pyridine, DMAP, RT,
4 h; g) NH2NH2-AcOH, pyridine, RT, 1 h; h) TMSOTf, DCM, À258C to RT, 1 h, 51% over three steps; i) H2, Pd/C,
AcOH, MeOH, RT, 24 h, then MeONa, MeOH, RT, 1 h, 65%.
to avoid overoxidation.[25] a-Galactosylation of 20 was easily
accomplished by preactivation of 4 using Tf2O and TMB in the
presence of DTBMP in DCM at À408C followed by the addition
of glycosyl acceptor 20. The remnant of the auxiliary of the re-
sulting tetrasaccharide was cleaved by treatment with 10% tri-
fluoroacetic acid (TFA) in DCM to give glycosyl acceptor 21 in
an overall yield of 67% as only the a-anomer. Surprisingly,
a glycosylation of 21 with 4 gave a pentasaccharide in a disap-
pointing yield of 20%. Fortunately, a TMSOTf mediated glyco-
sylation of 21 with 6 in DCM gave, after cleavage of the auxili-
ary, pentasaccharide 22 in an overall yield of 76% as only the
a-anomer. The HSQC data of 22 showed that all H1ÀC1 cou-
pling constants were in the range of 171 to 176 Hz confirming
the a-configurations of the glycosidic linkages.
Next, acceptor 17 was coupled with 5 using the standard pre-
activation protocol and, as expected, aqueous workup and
solid-phase extraction resulted in the removal of hydrolyzed
donor and other nonfluorous byproducts. The glycosylation
was repeated and the remnant of the auxiliary was removed
using 10% TFA in DCM to give, after standard fluorous solid
phase extraction, disaccharide 18. The latter compound was
coupled twice with donor 4 using triflic acid as the promoter
to provide trisaccharide 19, which was subjected to DDQ oxida-
tion to remove the NAP ether to provide acceptor 20. Next, the
a(1-6)-galactoside was installed by preactivation of 4 using
Tf2O, TMB, and DTBMP, followed by glycosylation with 20 and,
after repeating the coupling protocol, the remnant of the auxili-
ary was removed by treatment with 10% TFA in DCM to give
tetrasaccharide acceptor 21. This compound was coupled twice
with donor 6 using a standard preactivation protocol to give,
after removal of the C-2 auxiliary and passing the material
through a F-SPE cartridge, pentasaccharide 22. The hydroxyl of
The hydroxyl of 22 was acetylation and the Lev ester of the
resulting compound (23) was removed using hydrazine acetate
to give glycosyl acceptor 24, which was coupled with manno-
syl donor 3 using TMSOTf as the catalyst to provide hexasac-
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potential synthetic problems. For
example, due to the bulky
nature of the auxiliary, a glycosy-
lation of a sterically hindered ac-
ceptor site was challenging and,
in this case,
a conventional
donor had to be used. The at-
traction of the fluorous-support-
ed methodology is that after es-
tablishing a successful synthetic
approach, target compounds
can rapidly be resynthesized by
routine procedures. Also, it
allows for fast preparation of
structural analogs and, for exam-
ple, the approach for fluorous-
supported synthesis of 1 could
easily be adapted to the prepa-
ration of structurally related
compounds. Efforts are under-
Scheme 4. The assembly of the GPI anchor moiety of T. brucei by fluorous solid-phase extraction. Reagents and
conditions: a) DDQ, DCM:H2O=10:1, 2 h; b) Tf2O, TMB, DTBMP, À408C to RT, then 10% TFA in DCM, 1 h; c) TfOH,
DCM, À258C to RT, 3 h; d) TfOH, DTBMP, DCM, À608C to RT, 18 h, then 10% TFA in DCM, 1 h; e) Ac2O, pyridine,
DMAP, 4 h, then NH2NH2-AcOH, pyridine, 1 h; f) TMSOTf, DCM, À258C to RT, 1 h; g) H2, Pd/C, AcOH, MeOH, 24 h,
then NaOMe, MeOH, 1 h.
22 was acetylated and the resulting compound was treated
with hydrazine acetate to remove the Lev ester to give an ac-
ceptor which was subjected to a double coupling with manno-
syl donor 2. After each step, the product was isolated by solid-
phase extraction and immediately used in the next reaction
step. Homogeneous hexasaccharide 25 was obtained after pu-
rification by silica gel and LH-20 size-exclusion column chroma-
tography. This compound was obtained in an overall yield of
16.7%, which corresponds to an 85% yield per reaction step.
The assembly of the hexasaccharide could be completed within
six days. Standard deprotection of 25 gave target compound 1,
the analytical data of which were identical to the compound
prepared by the conventional approach described above.
After establishing a protocol for the efficient fluorous-sup-
ported synthesis of 1, it could easily be adapted to the prepa-
ration of structurally related compounds and, for example,
a pentasaccharide was assembled by appropriate protecting
group manipulations and sequential coupling of 2 with 8 to
give a product that was further extended with 5, 4, 4, and 3,
respectively. The preparation of this compound was completed
within five days.
way to develop a liquid handling system to automate fluorous-
supported synthesis,[9] which will make it possible to further
speedup the process of oligosaccharide assembly.
Experimental Section
General procedure for the preparation of sulfoxide donors 4
and 5 from their corresponding oxathianes 10 and 15
m-CPBA (ꢀ77%, 1.05 equiv) was dissolved in DCM and the result-
ing solution was slowly added to a cooled (À788C) solution of oxa-
thiane in DCM. The reaction mixture was stirred at À788C for
30 min, diluted with DCM (20 mL) and then poured into 10%
Na2S2O3 aqueous solution. The organic layer was washed with aq.
saturated NaHCO3, dried (MgSO4), and filtered; the filtrate was then
concentrated in vacuo. The residue was purified by silica gel
column chromatography.
General glycosylation procedure for oxathiane donors with
various acceptors
Oxathiane donor (1.2 equiv), 1,3,5 trimethoxybenzene (2.5 equiv)
and 2,6-di-tert-butyl-4-methylpyridine (3.0 equiv) were dissolved in
DCM. Molecular sieves (4 Š) were added and the resulting suspen-
sion was cooled to À158C. Trifluoromethanesulfonic anhydride
(1.2 equiv) was added dropwise to the solution and stirring was
continued for 10 min. The reaction mixture was further cooled to
À408C, and a solution of acceptor (1.0 equiv) in DCM, which was
dried over molecular sieves (4 Š), was added dropwise. After a reac-
tion time of 30 min, the reaction was quenched with aq. saturated
NaHCO3 (30 mL). The organic phase was washed with brine
(30 mL), dried (MgSO4), filtered, and the filtrate was concentrated
in vacuo. The residue was purified by silica gel column chromatog-
raphy or Sephadex LH-20 size exclusion chromatography (DCM/
MeOH=1:1, 0.2 mLminÀ1).
Conclusion
We demonstrate here that a set of strategically selected or-
thogonal protecting groups, glycosyl donors modified by
a chiral auxiliary and glycosyl acceptors containing a fluorous
tag, make it possible to rapidly prepare complex branched oli-
gosaccharides of biological importance. After the glycosyla-
tions, the chiral auxiliary could be removed using moderately
strong acidic conditions, which were compatible with the pres-
ence of the orthogonal protecting groups Lev and Nap, there-
by allowing efficient installation of 1,2-cis-linked glycosides.
Previously, the auxiliary-mediated methodology was employed
for the installation of a-glucosides,[12a,c,18] and it is shown here
that it can easily be extended to other monosaccharides such
as galactosides. An exploratory study was required to identify
General procedure for the removal of a C-2 auxiliary
Trifluoroacetic acid was added dropwise to a solution of the glyco-
sylation product in DCM at 08C, adjusting the final concentration
to 10% (v/v). The reaction mixture was stirred for 3 h until TLC in-
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dicated complete consumption of starting material. The reaction
mixture was diluted with DCM and poured into saturated NaHCO3.
The organic layer was dried (MgSO4), filtered, and the filtrate was
concentrated in vacuo. The residue was purified by silica gel
column chromatography or Sephadex LH-20 size exclusion chro-
matography (DCM/MeOH=1:1, 0.2 mLminÀ1).
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General fluorous-supported purification protocol
F-SPE cartridges (FluoroFlash SPE Cartridges, 10 grams, 20 cc tube)
were purchased from Fluorous Technologies. Inc. The fluorous-
tagged compound (200 mg compound per 1 g resin) was loaded
on a F-SPE cartridge using a minimum amount of mixture of water
and DMF (9:1, v:v). The order of elution was 20% water and meth-
anol (3”20 mL), hexane (3”20 mL). The desired fluorous-tagged
compound was obtained by elution with acetone (3”20 mL). The
formation of the desired compound was determined by TCL and
MALDI-TOF. The product containing fractions were concentrated in
vacuo.
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The research was supported by the National Institute of Gener-
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Health (R01GM065248, G.-J.B.).
Keywords: auxiliary
glycosylations · sulfonium ion
·
fluorous tag
·
stereoselective
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Received: May 11, 2015
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Published online on August 6, 2015
Chem. Eur. J. 2015, 21, 12920 – 12926
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