Efficient synthesis of an α(2,9) trisialic acid by one-pot glycosylation and polymer-assisted deprotection

Article abstract of DOI:10.1021/ol802207e

(Chemical Equation Presented) An efficient synthesis of α(2,9) trisialic acid has been achieved via one-pot glycosylation and polymer-assisted deprotection. The synthesis involves chemo- and regioselective α-sialylation of ethylthiosialoside with the S-be

Full text of DOI:10.1021/ol802207e

ORGANIC
LETTERS
2008
Vol. 10, No. 24
5597-5600
Efficient Synthesis of an r(2,9) Trisialic
Acid by One-Pot Glycosylation and
Polymer-Assisted Deprotection
Hiroshi Tanaka,* Yusuke Tateno, Yuji Nishiura, and Takashi Takahashi*
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan
thiroshi@apc.titech.ac.jp
Received September 22, 2008
ABSTRACT
An efficient synthesis of r(2,9) trisialic acid has been achieved via one-pot glycosylation and polymer-assisted deprotection. The synthesis
involves chemo- and regioselective r-sialylation of ethylthiosialoside with the S-benzoxazolyl (S-Box) sialyl donor. Use of a prelinker to link
an activated ester and a vinyl ether via a carbon chain enables polymer-assisted deprotection of the protected trisialic acids.
Sialic acids, such as Neu5Ac, Neu5Glc, and KDN, are
the most complex monosaccharide units in naturally
occurring oligosaccharides and are frequently located as
monomers at the nonreducing end of oligosaccharides.1
Recent progress in glycobiology suggests that R(2,8) and
R(2,9) di/oligosialic acids may play important roles in the
biological events that occur on the cell surface.2 Chemical
synthesis of these oligosaccharides is highly desirable to
facilitate discovery of their biological roles. However,
R-sialylation is one of the most difficult and challenging
processes in the chemical synthesis of oligosaccharides.3
Recently, modification of the acetamide group at the 5
position of sialyl donors to N,N-diacetyl,4a,k azido,4b
N-TFA,4c,f N-Troc,4d,e,h,j,l N-Fmoc,4e,h N-trichloroacetyl,4e,h,g
N-phthalimide,4i and 5N,4O-carbonyl4m,n,7 groups was shown
to improve the efficiency of R-selective sialylation, providing
several effective methods for the synthesis of various R(2,3)
and R(2,6) sialosides.5 However, synthesis of the R(2,8) or
R(2,9) oligosialic acids remains a difficult task due to the
poor reactivity of sialic acids toward glycosylation. In
addition, oligomers of R-sialosides are less stable under
acidic conditions than conventional sialic acids and, therefore,
require careful treatment during workup and purification of
the final products.6 The synthesis of R(2,9) sialic acids has
(4) (a) Demchenko, A. V.; Boons, G.-J. Tetrahedron Lett. 1998, 39,
3065–3068. (b) Yu, C.-S.; Niikura, K.; Lin, C.-C.; Wong, C.-H. Angew.
Chem., Int. Ed. 2001, 40, 2900–2903. (c) De Meo, C.; Demchenko, A. V.;
Boons, G.-J. J. Org. Chem. 2001, 66, 5490–5497. (d) Ando, H.; Koike, Y.;
Ishida, H.; Kiso, M. Tetrahedron Lett. 2003, 44, 6883–6886. (e) Adachi,
M.; Tanaka, H.; Takahashi, T. Synlett 2004, 609–614. (f) Lin, C.-C.; Huang,
K. T.; Lin, C.-C. Org. Lett. 2005, 7, 4169–4172. (g) Takeda, Y.; Horito, S.
Carbohydr. Res. 2005, 340, 211–220. (h) Tanaka, H.; Adachi, M.;
Takahashi, T. Chem.sEur. J. 2005, 11, 849–862. (i) Tanaka, K.; Goi, T.;
Fukase, K. Synlett 2005, 2958–2962. (j) Tanaka, H.; Nishiura, Y.; Adachi,
M.; Takahashi, T. Heterocycles 2006, 67, 107–112. (k) Crich, D.; Wenju,
L. Org. Lett. 2006, 8, 959–962. (l) Hanashima, S.; Castagner, B.; Esposito,
D.; Nokami, T.; Seeberger, P. H. Org. Lett. 2007, 9, 1777–1779. (m) Farris,
M. D.; De Meo, C. Tetrahedron Lett. 2007, 48, 1225–1227. (n) Crich, D.;
Li, W. J. Org. Chem. 2007, 72, 2387–2391.
(1) Angat, T.; Varki, A. Chem. ReV. 2002, 102, 439–470.
(2) Sato, C.; Kitajima, K. Trends Glycosci. Glycotechnol. 1999, 11, 371–
390.
(5) De Meo, C.; Priyadarshani, U. Carbohydr. Res. 2008, 343, 1540–
1552.
(3) (a) Boons, G.-J.; Demchenko, A. V. Chem. ReV 2000, 100, 4539–
4565. (b) Ando, H.; Imanura, A. Trends Glycosci. Glycotechnol. 2004, 16,
293–303.
(6) Manzi, A. E.; Higa, H. H.; Diaz, S.; Varki, A. J. Biol. Chem. 1994,
269, 23617–23624.
10.1021/ol802207e CCC: $40.75
Published on Web 11/20/2008
2008 American Chemical Society

been reported by using 5-azido4b and N-TFA4 sialyl phos-
phites based on a stepwise strategy. Recently, we reported
the synthesis of R(2,8) tetrasialic acid using the 5N,4O-
carbonyl-protected thiosialoside.7 The 5N,4O-carbonyl pro-
tection of sialic acids enables R sialylation without use of
acetonitrile. However, these effective methods, which are
based on modification of the C5 acetamide group, require
conversion of amino groups into the naturally occurring
N-acetyl derivative (Neu5Ac) or N-glycolyl derivative
(Neu5Glc) via highly polar amino acid intermediates.
Recently, we developed an efficient method for the
synthesis of oligosaccharides that is based one-pot glycosy-
lation and polymer-assisted deprotection.8-10 The one-pot
glycosylation involved sequential chemo- and regioselective
glycosylation to provide oligosaccharides from several simple
building blocks in one pot.11 The polymer-assisted depro-
tection involved deprotection of solid-supported, protected
oligosaccharides followed by their release from the solid
supports. The prelinker 4, composed of an activated ester
and a vinyl ether linked via an ether bond, was used for
quantitative loading of the protected oligosaccharides on to
a solid-support. Deprotection was made easier by supporting
the substrates on solids.12 Herein, we report the synthesis of
the R(2,9) trisialic acid 1 by one-pot glycosylation and
polymer-assisted deprotection using 5N,4O-carbonyl pro-
tected sialyl donors and a new prelinker for polymer-assisted
deprotection.
Scheme 1. Strategy for the Synthesis of the R(2,9) Trisialic
Acid 1 Based on One-Pot Glycosylation and Polymer-Assisted
Deprotection
Our strategy for the synthesis of R(2,9) trisialic acid 1,
based on one-pot glycosylation and polymer-assisted depro-
tection, involves chemo- and regioselective glycosylation of
thiosialoside 7 with the S-Box sialyl donor 6a at the 9
position followed by coupling of the resulting disaccharide
with R-sialoside 87 at the 9 position (Scheme 1). The 5N,4O-
carbonyl protection of building blocks 6-8 is effective for
R-sialylation at the 9 position. The S-Box glycosyl donors
are adaptable to sialylation and can be selectively activated
in the presence of thioglycosides.13,14 The 8,9 diols 7 and 8
were effective as acceptors for R(2,9) sialylation because
the C8 hydroxyl group does not adversely interfere with
glycosylation at the C-9 position.4f The prelinker 3, which
links an activated ester and a vinyl ether via a carbon chain,
was newly designed for polymer-assisted deprotection of the
protected trisialoside 5. The linker 3 undergoes acetalization
with the protected trisialic acid 5 at the 8 and/or 8′ positions;
amidation with an amino group on ArgoPore thereafter
immobilizes the trisaccharide 5 onto a solid support. The
ArgoPore resin enables deprotection of solid-supported,
protected oligosaccharides via debenzylation through Birch
reduction.10 The ester unit in the carbon linker chain of 3
exhibits enhanced stability under basic conditions. In addi-
tion, the reactivity of the vinyl ether of 3 toward acetalization
is improved, and the resulting acetal unit is more easily
hydrolyzed under acidic conditions compared with linker 4
due to the lack of an electron-withdrawing oxygen unit.
The chemical properties of the linkers prepared from the
prelinkers 3 and 4 were compared. The acylated benzy-
lamines 9 and 10 were used as model compounds. Treatment
of 9 at 80 °C for 12 h under basic conditions, which allows
for hydrolysis of both the C1 ester and the 5N,4O-carbonyl
protecting group, had no effect on 9. On the other hand, under
similar conditions, hydrolysis of the R-alkoxyacetamide 10
occurred. Next, we exposed 9 and 10 to 1% trifluo-
romethanesulfonic acid in CH2Cl2 in the presence of H2O
(7) (a) Tanaka, H.; Nishiura, Y.; Takahashi, T. J. Am. Chem. Soc. 2006,
128, 7124–7125.
(8) Tanaka, H.; Yamada, H.; Takahashi, T. Trends Glycosci. Glycote-
chnol. 2007, 19, 183–193
(9) (a) Yamada, H.; Harada, T.; Takahashi, T. J. Am. Chem. Soc. 1994,
116, 7919–7920. (b) Tanaka, H.; Adachi, M.; Takahashi, T. Chem.sEur.
.
J. 2005, 11, 849–862
(10) Tanaka, H.; Ishida, T.; Matoba, N.; Tsukamoto, H.; Yamada, H.;
Takahashi, T. Angew. Chem., Int. Ed. 2006, 45, 6349–6352
.
.
(11) (a) Raghavan, S.; Kahne, D. J. Am. Chem. Soc. 1993, 115, 1580–
1591. (b) Zhang, Z.; Ollmann, I. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.;
Wong, C.-H. J. Am. Chem. Soc. 1999, 121, 734–753. (c) Recent review of
one-pot glycosylation: Wang, Y.; Ye, X.-S.; Zhang, L.-H. Org. Biomol.
Chem. 2007, 5, 2189–2200.
(12) (a) Matsuda, A.; Doi, T.; Tanaka, H.; Takahashi, T. Synlett 2001,
1101–1104. (b) Tanaka, H.; Zenkoh, T.; Setoi, H.; Takahashi, T. Synlett
2002, 1427–1430.
(13) (a) Demchenko, A. V.; Malysheva, N. N.; De Meo, C. Org. Lett.
2003, 5, 455–458. (b) Kamat, M. N.; Demchenko, A. V. Org. Lett. 2005,
7, 3215–3218
.
(14) De Meo, C.; Parker, O. Tetrahedron Asymmetry 2005, 16, 303–
307. (d) Pornsuriyasak, P.; Demchenko, A. V. Tetrahedron Asymmetry 2005,
16, 433–439. (e) Wang, C.-C.; Lee, J.-C.; Luo1, S.-Y.; Kulkarni, S. S.;
Huang, Y.-W.; Lee, C.-C.; Chang, K.-L.; Hung, S.-C. Nature 2007, 446,
896–899
.
5598
Org. Lett., Vol. 10, No. 24, 2008

Table 1. Glycosidation of the 5N,4O-Carbonyl-Protected Sialyl
Donors 6a-d, Which Had Different Leaving Groups
Scheme 3. Synthesis of the R(2,9) Trisialic Acid 1
yield of
R/ꢀ
no. donor
activator
AgOTf
ZrCp2Cl2, AgOTf -30 to rt
AgOTf
AgOTf
T (°C)
12 or 13 ratioa
1
2
3
4
6a
6b
6c
6d
-60 to -40
92
>95:5
-60 to -30
-60 to -40
1
51
87
>95:5
>95:5
a
The ratio was determined on the basis of H spectra.
for 6 h, which are the conditions required for release from
the resin. HPLC analysis of the reaction mixtures showed
that, although 50% of 10 remained intact, 9 had completely
disappeared. These results indicated that prelinker 3 enables
release of the products under less acidic conditions than
prelinker 4.
We next examined glycosylation of the primary alcohol
11 with the 5N,4O-carbonyl protected sialyl donors 6a-d,
which had different leaving groups (Table 1). The S-
benzoxazolyl (S-Box) glycosyl donor 6a underwent R-sia-
lylation with the primary alcohol 11 to provide R-sialoside
12 in excellent yield with R-selectivity. The glycosyl fluoride
Scheme 2
.
Chemical Properties of the Linkers Prepared from
the Prelinkers 3 and 4
Org. Lett., Vol. 10, No. 24, 2008
5599

6b was not activated by in situ generated Zr(OTf)2.15 The
glycosyl bromide 6c underwent smooth glycosidation to
provide disaccharide 13. However, purification of the gly-
cosyl bromide 6c using column chromatography on silica
gel was difficult. The glycosyl chloride 6d exhibited lower
reactivity than 6c. Therefore, we selected the S-Box sialyl
donor 6a as the first glycosyl donor for the one-pot
glycosylation.
esters. After removal of the remaining prelinker 3, the
activated esters of 16 were reacted with 3 equiv of the solid-
supported amine 2 on ArgoPore resin in DMF-CH2Cl2 under
basic conditions for 24 h. Compound 16 disappeared from
the solution to yield the solid-supported protected oligosac-
charides 17. Hydrolysis of the 5N,4O-carbonyl protecting
group, the methyl ethers, and other ester protecting groups
was achieved by treatment with LiOH in dioxane and H2O
at 60 °C for 24 h. Treatment of the resulting amines with
Ac2O, followed by hydrolysis of the partially generated esters
provided the solid-supported acetamides 18. The manipula-
tion for the hydrolysis requires the monitoring of the
N-acylation reaction by cleavage of the product. The solid-
supported protected oligosaccharides 18 were then subjected
to Birch reduction, followed by cleavage from the resin under
mildly acidic conditions to provide the fully deprotected
R(2,9) trisialic acid in 70% yield with 87% purity based on
15. The purity was estimated using HPLC with an evapora-
tive light scattering detector (ELSD). Purification of the
released product provided the R(2,9) trisialic acid 1 in 56%
isolated yield based on 15. It should be noted that the
polymer-assisted deprotection of 15 by using prelinker 4
failed due to release of the sugar from the resin via cleavage
of the amide bond during hydrolysis.
The stepwise synthesis of the protected trisialic acid 5 is
shown in Scheme 2. Treatment of ꢀ-ethylthiosialoside 7 with
1.2 equiv of the S-Box sialoside 6a in the presence of AgOTf
in CH2Cl2 at -60 to -30 °C provided the disaccharide 14
in 88% yield with R/ꢀ ) 95:5. The anomeric ratio was
1
determined based on H spectral data NMR (CDCl3, 400
MHz). The R-anomer r-14 was purified from the mixture
by column chromatography on silica gel. Glycosidation of
the disialoside r-14 with the primary alcohol of 8 was
successfully achieved by activation of 14 with NIS/cat. TfOH
to provide the protected trisialic acid 15 in 86% yield (R/ꢀ
) 69:31). The anomeric configuration of the major product
3
was determined based on the JC1-H3ax coupling constants
(3JC-1,H-3ax ) 4.9, 4.9 and 6.1 Hz, CDCl3, 100 MHz) to be
R17.
One-pot glycosylation using 6a, 7, and 8 was examined.
Chemoselective glycosylation of the thioglycoside 7 with the
donor 6a under the conditions described above provided
disaccharide 14. Subsequent addition of acceptor 8 and NIS
to the reaction vessel afforded the protected trisaccharide
15 as two diastereomers in an overall yield of 79% (R/ꢀ )
64:36) based on 7. The protected R(2,9) trisialic acid R-15
was purified from the mixture of two diastereomers by
column chromatography on silica gel. The spectral data (1H
and 13C NMR) of the major product were identical with those
of the authentic sample prepared by the stepwise synthesis.
Deprotection of the protected trisialic acid was ac-
complished using the polymer-supported strategy. Treatment
of R-sialoside R-15 with 4 equiv of prelinker 3, in the
presence of CSA at 0 °C for 2 h, provided the protected
oligosaccharide 16 as a mixture of mono- and diactivated
In conclusion, we describe the synthesis of the R(2,9)
trisialic acid 1 using one-pot glycosylation and polymer-
assisted deprotection (Scheme 3). The 5N,4O-carbonyl-
protected S-Box sialyl donor underwent R-selective chemose-
lective sialylation of ꢀ-ethythiosialoside 7. Subsequent
activation of the resulting thioglycoside was achieved by
addition of NIS to provide R(2,9) trisialic acid. The prelinker
3, which links an activated ester and a vinyl ether via a
carbon chain, allowed deprotection of solid-supported sialic
acids. These methods can be adapted to deprotection and
derivatization of various sialo-containing oligosaccharides.
Acknowledgment. This work was financially supported
by JSPS Research Fellowships for Young Scientists (DC2).
Supporting Information Available: Experimental pro-
(15) Matsumoto, T.; Maeta, H.; Suzuki, K.; Tsuchihashi, G. Tetrahedron
Lett. 1988, 29, 3567–3570.
1
13C
cedures and H and
NMR spectra. This material is
3
(16) JC-1,H-3ax of R-sialosides is >4 Hz: Haverkamp, J.; Spoormaker,
available free of charge via the Internet at http://pubs.acs.org.
T.; Dorland, L.; Vliegenthart, J. F. G.; Schauer, R. J. Am. Chem. Soc. 1979,
101, 4851–4853.
OL802207E
5600
Org. Lett., Vol. 10, No. 24, 2008