Oligomannan synthesis using ionic liquid supported glycosylation

Article abstract of DOI:10.1021/ol702743x

The synthesis of complex oligosaccharides has been a challenge for researchers. Herein, we describe a strategy for the synthesis of an activated oligomannan 1 that employs ionic liquid (IL) support glycosylation methodology on an IL-tagged mannosyl fluori

Full text of DOI:10.1021/ol702743x

ORGANIC
LETTERS
2008
Vol. 10, No. 1
145-148
Oligomannan Synthesis Using Ionic
Liquid Supported Glycosylation§
Ashish K. Pathak,* Charu K. Yerneni, Zac Young, and Vibha Pathak
Department of Chemistry, Western Illinois UniVersity, Macomb, Illinois 61455
ak-pathak@wiu.edu
Received November 13, 2007
ABSTRACT
The synthesis of complex oligosaccharides has been a challenge for researchers. Herein, we describe a strategy for the synthesis of an
activated oligomannan 1 that employs ionic liquid (IL) support glycosylation methodology on an IL-tagged mannosyl fluoride donor. This
method is capable of rapidly producing linear
r(1f6) oligomannan thioglycosides in a convenient and cost-effective manner without the need
of column purification after each glycosylation step.
Carbohydrates play a pivotal role in many different areas of
chemistry and biology, as they possess a diversity of func-
tionalities and structures. The efficient synthesis of oligosac-
charides has been a challenge of considerable magnitude,
as the traditional synthesis is not only enormously time-
consuming but also requires purification by chromatography
after each glycosylation step.1 The contemporary challenges
in the synthesis of oligosaccharides have led to the develop-
ment of several new strategies for oligosaccharide synthesis.2
Recently developed carbohydrate synthetic techniques are
making it possible to create novel immunogenic carbohy-
drates in sufficient quantities and in pure form. Advances in
polymer-supported solid-phase synthesis have also provided
an effective way of assembling complex oligosaccharides,
and there are some impressive examples of successful
automated syntheses.3 The solid-phase method allows the
removal of excess reagents by simply washing the resins,
thereby minimizing the chromatographic purifications. Al-
though highly successful, polymer-supported solid-phase
synthesis has some serious limitations due to the heteroge-
neous reaction conditions. This has led to an alternative
polymer-supported liquid-phase methodology that restores
homogeneous reaction conditions. Some other limitations of
polymer-supported synthesis include low loading capacity
and limited solubility of the glycosyl donors during the
reaction process (which results in poor yields). Expensive
(2) (a) Seeberger, P. H.; Danishefsky, S. J. Acc. Chem. Res. 1998, 31,
685. (b) Seeberger, P. H.; Haase, W.-C. Chem. ReV. 2000, 100, 4349. (c)
Sears, P.; Wong, C. H. Science 2001, 291, 2344. (d) Wang, C.-C.; Lee,
J.-C.; Luo, S.-Y.; Kulkarni, S. S.; Wen Huang, Y.-W.; Lee, C.-C.; Chang,
K.-L.; Hung, S.-C. Nature 2007, 446, 896. (e) Kamat, M. N.; Rath, N. P.;
Demchenko, A. V. J. Org. Chem. 2007, 72, 6938. (f) Kamat, M. N.; Meo,
C. D.; Demchenko, A. V. J. Org. Chem. 2007, 72, 6947. (g) Carrel, F. R.;
Geyer, K.; Code´e, J. D. C.; Seeberger, P. H. Org. Lett. 2007, 9, 2285. (h)
Geyer, K.; Codee, J. D. C.; Seeberger, P. H. Chem.sEur. J. 2006, 12, 8434.
(i) Ye, X.-S.; Wong, C.-H. J. Org. Chem. 2000, 65, 2410. (j) Zhang, Z.;
Ollmann, I. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.; Wong, C.-H. J. Am.
Chem. Soc. 1999, 121, 734.
§ Part of this work was presented at the 233rd ACS National Meeting at
Chicago, IL, March 25-29, 2007.
(1) Ando, H.; Manbe, S.; Nakahara, Y.; Ito, Y. Angew. Chem., Int. Ed.
2001, 40, 4725.
(3) Solid Support Oligosaccharide Synthesis and Combinatorial Carbo-
hydrate Libraries; Seeberger, P. H., Ed.l Wiley-VCH: New York, 2001;
336pp.
10.1021/ol702743x CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/11/2007

sugar synthons are also often wasted. The inability to monitor
the reaction progress and the lack of good characterization
methods to assess the success of the reaction are also dis-
advantages. At present, high-resolution magic angle spinning
NMR spectroscopy is being used to determine the structures
of polymer-bound oligosaccharides,2a,b but this is a difficult
technique to apply.
disaccharide can be generated upon concentration of the
reaction mixture and washing with a solvent in which the
IL-tagged compounds are not soluble (e.g., diethyl ether) to
clean out the reagents as well as unreacted acceptor glyco-
side. A phase separation clipping of IL using a phase transfer
catalyst will produce pure disaccharide.
Here, we describe a simple method for synthesizing a
homolinear R(1f6) oligomannan thioglycoside on an IL
support in order to eliminate the excessive use of column
chromatographic purification after each glycosylation step.
Ionic liquids (ILs) have aroused considerable interest over
the past decade due to their wide variety of properties. In
this regard, they can be used as solvents and reaction
supports.4 The range of available IL supports makes them
compatible with most common reaction conditions. For
example, ILs have been used as supports for catalyst/reagent
immobilization and for the synthesis of peptides and nucleo-
tides.5 IL-supported glycosylation reactions were recently
reported utilizing trichloroacetamido and sulfoxide glyco-
sides.6 The main features of IL-supported substrates are their
solubility properties, as solubility can be turned on and off
by varying the anions. Thereafter, IL-supported species can
be purified from the reaction mixture by simply washing the
product with a nonpolar solvent. Whenever required, the IL
support can be clipped off and purified by a simple phase
separation technique. In principle, synthesis of almost pure
oligosaccharides can be achieved efficiently with minimal
column chromatographic purification via this approach. An
IL-supported synthesis of disaccharide is illustrated in Figure
1. The IL-tagged glycosyl donor results in an IL-tagged
1
Conventional H NMR at 300 MHz in CDCl3, 13C NMR at
75 MHz in CDCl3, and HR-ESIMS spectral techniques were
also utilized to characterize the products. Previously, the
synthesis of linear R(1f6) oligomannans was reported using
trichloroimidates and allyl glycosides, which required tedious
and careful column chromatographic separations after each
glycosylation step.7
In our approach, the IL support is attached to the donor
or acceptor glycoside depending on the requirements of the
synthetic manipulation. In the present case, a strategy in
which p-thiotolyl glycosides acted as acceptors and an IL-
tagged fluoride glycoside acted as a donor was utilized for
the synthesis of the linear R(1f6) tetramannopyranose
thioglycoside 1 (Figure 2). The donor glycosyl fluoride was
Figure 2. p-Thiotolyl R(1f6) mannopyranose tetrasaccharide.
attached to the IL support via an ester linkage. Whenever
needed, the IL support could be easily detached by stirring
with aqueous saturated NaHCO3 in a H2O/ether (1:1) biphasic
solvent mixture in the presence of phase transfer catalyst.
The synthesis of the mannose building blocks 3 and 4 for
the construction of the target oligomannan is presented in
Scheme 1. The known 1,6-di-O-acetyl-2,3,4-tri-O-benzoyl-
R-D-mannopyranoside (2) was synthesized in one vessel
starting from commercially available D-mannose in an 88%
overall yield.8 The glycoside 2 was then treated with
p-thiocresol in the presence of BF3‚Et2O to produce exclu-
1
Figure 1. Oligosaccharide synthetic strategy using an IL support.
sively the p-thiotolyl glycoside 3. The 300 MHz H NMR
spectrum of 3 in CDCl3 showed the anomeric proton as a
doublet at δ 5.69 ppm (J1,2 ) 1.2 Hz), whereas the 75 MHz
disaccharide upon coupling with a glycosyl acceptor in an
appropriate solvent such as CH2Cl2. Almost pure IL-tagged
(6) (a) Huang, J.-Y.; Leia, M.; Wang, Y.-G. Tetrahedron Lett. 2006,
47, 3047. (b) He, X.; Chan, T. H. Synthesis 2006, 1645.
(7) Zhu, Y.; Kong, F. Carbohydr. Res. 2001, 332, 1.
(8) (a) Heng, L.; Ning, J.; Kong, F. Carbohydr. Res. 2001, 331, 431. (b)
Heng, L.; Ning, J.; Kong, F. J. Carbohydr. Chem. 2001, 20, 285.
(4) Murugesan, S.; Linhardt, R. J. Curr. Org. Synth. 2005, 2, 437.
(5) Miao, W.; Chan, T. K. Acc. Chem. Res. 2006, 39, 897.
146
Org. Lett., Vol. 10, No. 1, 2008

purification by SiO2 column chromatography. The structure
1
of 6 was confirmed by the 300 MHz H NMR spectrum in
Scheme 1. Synthesis of p-Thiotolyl and Fluoride Mannose
CDCl3 (anomeric protons at δ 5.75 and 5.13 ppm as singlets)
and the 75 MHz 13C NMR in CDCl3 (anomeric carbons at δ
98.01 and 86.75 ppm).
Glycosides
The synthesis of p-thiotolyl R(1f6) mannopyranose
tetrasaccharide 1 utilizing the IL-tagged mannopyranosyl
fluoride donor is illustrated in Scheme 3. The reaction
sequence started from glycosyl fluoride 4, which was
selectively deacetylated by the reaction with an AcCl/MeOH/
CH2Cl2 mixture to give the glycosyl fluoride 7. The treatment
of 7 with chloroacetyl chloride in dry CH2Cl2 in the presence
of dry pyridine resulted in a glycoside 8 with 95% yield.
The glycoside 8 was then reacted overnight with N-
methylimidazole in dry CH3CN at reflux to obtain the IL-
tagged mannopyranosyl fluoride. The chloride anion was
exchanged with a hexafluorophosphino anion by the addition
of KPF6 to the reaction mixture and heating the total mixture
overnight at reflux.11 The reaction was cooled to room
temperature and concentrated in vacuo to a solid, which was
then suspended in CHCl3 and filtered. The filtrate was
concentrated to a solid which was purified by washing with
diethyl ether. The resulting solid IL-tagged glycosyl fluoride
9 was dried in vacuo. The 1H NMR spectrum of 9 in CDCl3
showed it to be almost pure; this was further supported by
the HR-ESIMS analysis.
13C NMR spectrum showed the anomeric carbon signal at δ
86.16 ppm supporting the R-glycoside.9 It was further
converted to the 6-O-acetyl-2,3,4-tri-O-benzoyl-R-D-man-
nopyranosyl fluoride (4) using DAST and N-bromosuccin-
imide, in excellent yield. The structure and R-stereochemistry
at the anomeric carbon were confirmed by the 300 MHz 1H
NMR spectrum in CDCl3, which showed the anomeric proton
as a doublet of doublets at δ 5.86 ppm (JH-1,F ) 48.7 Hz,
J1,2 ) 1.8 Hz). This was further supported by a signal at δ
104.73 ppm (JC-1,F ) 224.0 Hz) in the 13C NMR spectrum
of 4 in CDCl3 which was attributable to the anomeric
carbon.10
Following the synthesis of the starting mannose glycosides,
a prototypical glycosylation reaction utilizing selective
anomeric activation of the thioglycosyl acceptor 5, prepared
from thioglycoside 3, and donor glycosyl fluoride 4 was
achieved in high yield (Scheme 2). Selective deacetylation
Next, the IL-supported glycosyl donor 9 was treated with
acceptor glycoside 5 in the presence of the promoters AgClO4
and SnCl2. It resulted in only 55% yield of the desired
disaccharide 10 after workup and purification by washing
with diethyl ether. The glycosylation reaction between 5 and
9 was then performed with the coupling reagents Cp2HfCl2
and AgClO4 in dry CH2Cl2. It produced the IL-tagged
disaccharide 10 in 88% yield after purification by washing
with ether, as described earlier for compound 9. The 300
1
Scheme 2. Prototypical Selective Glycosylation Reaction
MHz H NMR spectrum of IL-tagged disaccharide 10 in
CDCl3 showed the anomeric protons at δ 5.71 and 5.10 ppm
(J1,2 ) 1.4 Hz) as a singlet and a doublet, respectively. In
the 75 MHz 13C NMR spectrum of 10, the anomeric carbons
were observed at δ 98.17 and 86.82 ppm, supporting the
1,2-trans glycosylation.
The IL tag on disaccharide 10 was removed by stirring it
with saturated aqueous NaHCO3 and the phase transfer
catalyst Bu4N+I- in an ether/H2O (1:1) mixture at room
temperature. Concentration of the ether layer gave almost
1
pure disaccharide 11 as shown by the H and 13C NMR
spectra in CDCl3. It was further used for a glycosylation
reaction without purification as an acceptor saccharide.
IL-tagged trisaccharide 12 was synthesized by the coupling
reaction between acceptor disaccharide 11 and glycoside 9,
in a similar manner to the synthesis of disaccharide 10. The
workup and simple washing purification protocol with diethyl
ether produced almost pure IL-tagged trisaccharide 12. The
of the thioglycoside 3 was achieved by the reaction with an
AcCl/MeOH/CH2Cl2 (0.1:20:20 v/v, 12.03 mL/mmol) mix-
ture to produce the acceptor glycoside 5 in 96% yield. The
thioglycoside 5 was then coupled with fluoride glycoside 4
in the presence of AgClO4 and SnCl2, yielding the R(1f6)
disaccharide 6 in 90% yield after the typical workup and
1
300 MHz H NMR spectrum of 12 in CDCl3 showed three
anomeric protons at δ 5.78, 5.20, and 4.80 ppm as singlets.
(9) Rachaman, E. S.; Eby, R.; Schuerch, C. Carbohydr. Res. 1987, 147.
(10) (a) Hall, L. D. AdV. Carbohydr. Chem. 1964, 19, 51. (b) Foster, A.
B.; Hems, R.; Hall, L. D.; Manville, J. F. Chem. Commun. 1968, 158. (c)
Cumpstey, I.; Fairbanks, A. J.; Redgrave, A. J. Monatsh. Chem. 2002, 133,
449.
(11) (a) FT-IR spectra in KBr showed a strong band at 848 cm-1 assigned
to PF6- and C-F absorption at 1165 cm-1. (b) Gordon, C. M.; Holbrey, J.
D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627.
Org. Lett., Vol. 10, No. 1, 2008
147

Scheme 3. IL-Supported Assembly of Homolinear R(1f6) Tetramannopyranose
The presence of three anomeric carbons at δ 98.24, 97.66,
[M + Na]+ corresponding to the molecular formula
115H96O32SNa.
and 86.86 ppm was observed in the 75 MHz 13C NMR
spectrum of 12 in CDCl3. The IL support on 12 was clipped
off by the phase separation technique, as described earlier
for 11, yielding donor trisaccharide 13 in almost pure form
as evidenced from NMR studies. It was used for a further
glycosylation reaction without purification. The trisaccharide
13 was glycosylated with donor IL fluoride 8 to give IL-
tagged tetrasaccharide 14 in 81% yield after purification by
simple washing with ether. The presence of four anomeric
protons in the 300 MHz 1H NMR spectrum of 14 in CDCl3
at δ 5.65, 5.20, 4.94, and 4.80 ppm as singlets supported
the formation of the tetrasaccharide. The IL support from
the tetrasaccharide 14 was removed as described for 11 to
produce the almost pure tetrasaccharide 1 in 86% yield. The
300 MHz 1H NMR spectrum of 1 in CDCl3 showed the four
anomeric protons at δ 5.68, 5.22, 5.01, and 4.81 ppm as
singlets. In the 75 MHz 13C NMR spectrum of 1 in CDCl3,
the four anomeric carbons were observed: δ 98.30, 98.13,
97.78, and 86.86 ppm. Other signals in the 1H NMR spectrum
of 1 were not well-resolved at 300 MHz, but the total number
of protons was found to correspond to the molecular formula.
The final structural confirmation was obtained by FABMS
analysis of tetrasaccharide 1, which showed a peak at 2043.3
C
In conclusion, we have reported a successful, efficient,
and cost-effective synthesis of a tetrasaccharide on an IL
support with minimum column chromatographic purification
as an alternative to solid-phase polymer-supported synthesis.
To our knowledge, this is the first report of a tetrasaccharide
synthesis using an ionic liquid support and may be a very
useful technique to produce oligosaccharides on a large scale.
Currently, other complex oligosaccharides are being syn-
thesized using this technique and will be reported in the near
future.
Acknowledgment. Financial support from University
Research Council, Western Illinois University (WIU), and
NIH Grant R21AI059270 is acknowledged. Z.Y. is thankful
to the College of Arts and Sciences, WIU, for an under-
graduate research award.
Supporting Information Available: Experimental pro-
cedures and characterization data of all new compounds are
available. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Org. Lett., Vol. 10, No. 1, 2008