Synthesis of a sialic acid α(2-3) galactose building block and its use in a linear synthesis of sialyl Lewis X

Article abstract of DOI:10.1021/ol0704946

The ubiquity of the sialic acid α(2-3) galactose linkage in oligosaccharides of biological relevance necessitates a building block for the incorporation of this motif into oligosaccharides prepared by modular synthesis. The linear synthesis of the sialyl

Full text of DOI:10.1021/ol0704946

ORGANIC
LETTERS
2007
Vol. 9, No. 9
1777-1779
Synthesis of a Sialic Acid
Galactose Building Block and Its Use in
a Linear Synthesis of Sialyl Lewis X
r(2−3)
Shinya Hanashima, Bastien Castagner, Davide Esposito, Toshiki Nokami,† and
Peter H. Seeberger*
Laboratory for Organic Chemistry, Swiss Federal Institute of Technology (ETH)
Zurich, 8093 Zurich, Switzerland
seeberger@org.chem.ethz.ch
Received February 27, 2007
ABSTRACT
The ubiquity of the sialic acid
incorporation of this motif into oligosaccharides prepared by modular synthesis. The linear synthesis of the sialyl Lewis X tumor-associated
antigen (1) has been accomplished in good yield using a sialic acid (2 3) galactose disaccharide building block. The disaccharide building
block was synthesized efficiently from readily available galactal by a high-yielding and selective sialylation reaction.
r(2−3) galactose linkage in oligosaccharides of biological relevance necessitates a building block for the
r
−
N-Acetylneuraminic acid (Neu5Ac) is the most abundant
member of the sialic acid family in mammals.1,2 In humans,
Neu5Ac is mostly linked to either the C3- or C6-position of
galactose or to the C6-position of N-acetylgalactosamine and
glucosamine. Typically, Neu5Ac is located at the nonreduc-
ing terminus of glycolipids and glycoproteins and thus is
important for interactions with proteins. The tumor-associated
antigen sialyl Lewis X (SLx) 1 contains a sialic acid R(2-
3) galactose moiety and has been implicated in inflammation
and cancer metastasis.3
sialylated oligosaccharides.4 Glycosylations with Neu5Ac
building blocks, especially on the C3-position of galactose,
often result in low yields and anomeric mixtures due to the
hindered anomeric position of sialic acid and the lack of a
participating group on the neighboring C3.5 Given the
ubiquity of terminal R2,3-linked sialyl galactose-capped
oligosaccharides, we sought to find a disaccharide building
block that would give access to these complex carbohydrates
by modular solution or solid-phase synthesis. A suitable
building block should be high yielding and selective in
glycosylation reactions with a complex oligosaccharide
partner and should be amenable to assembly on the solid
phase. Here, we report the efficient synthesis of the sialic
acid R(2-3) galactose building block 11 and its use in a
linear synthesis of sialyl Lewis X.6
Because sialylation by chemical means remains a chal-
lenge, enzymatic glycosylations are often used to construct
† Present address: Department of Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto,
615-8510, Japan.
(1) Angata, T.; Varki, A. Chem. ReV. 2002, 102, 439-469.
(2) Essentials of Glycobiology; Varki, A., Cummings, R., Esko, J., Freeze,
H., Hart, G., Marth, J., Eds.; Cold Spring Harbor Laboratory Press: New
York, 1999.
Sialic acid glycosylating agents carrying amine protecting
groups such as carbamate,7,8 N-TFA,9 azide,10 and imide11,12
(3) (a) Tyrrell, D.; James, P.; Rao, N.; Foxall, C.; Abbas, S.; Dasgupta,
F.; Nashed, M.; Hasegawa, A.; Kiso, M.; Asa, D.; Kidd, J.; Brandley, B.
K. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 10372-10376. (b) Kannagi, R.;
Izawa, M.; Koike, T.; Miyazaki, K.; Kimura, N. Cancer Sci. 2004, 5, 377-
384.
(4) Ernst, B.; Oehrlein, R. Glycoconjugate J. 1999, 16, 161-170.
(5) Recent reviews for chemical sialylation reactions: (a) Boons, G.-J.;
Demchenko, A. V. Chem. ReV. 2000, 100, 4539-4566. (b) Halcomb, R.
L.; Chappell, M. D. J. Carbohydr. Chem. 2002, 21, 723-768. (c) Ando,
H.; Imamura, A. Trends Glycosci. Glycotechnol. 2004, 16, 293-303.
10.1021/ol0704946 CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/06/2007

have shown improved yields and selectivities in glycosyla-
tions. In contrast, few good nucleophiles have been reported.7b
We began our disaccharide building block synthesis by
evaluating different nucleophiles in sialylation reactions
(Table 1). Glycals are excellent precursors for the preparation
blocks. We employed phosphite15 or N-phenyl trifluoro-
acetimidate11 leaving groups on N-Troc-protected building
blocks for the potent reactivity and accessibility of different
sialic acid derivatives such as N-acetyl, N-glycolyl, and free
amine upon N-Troc deprotection.
Sialic acid building blocks 2a and 2b were activated with
TMSOTf (0.15 equiv) in propionitrile16 at -78 °C (Table
1). In the case of diol acceptors 3 and 4, desired disaccharides
6 and 7 were obtained in moderate yield (entries 1 and 2,
Table 1). In contrast, galactal 5 showed a remarkable
improvement in both yield and stereoselectivity (entries 3-5,
Table 1). The use of 1.5 equiv of galactal 5 with donor 2a
provided disaccharide 8 with good selectivity (R/â ) 9.5:1,
entry 3, Table 1). The R-anomer was isolated in 76% yield.
The use of 1.5 equiv of donor 2a provided 8 in 86% yield
with the best selectivity (R/â ) 11:1, entry 5, Table 1). The
use of N-phenyl trifluoroacetimidate 2b gave only low yields
of the desired product due to decomposition of galactal.
Next, the galactal portion of disaccharide 8 was equipped
with a suitable anomeric leaving group (Scheme 1). Disac-
Table 1. Glycosylation of Galactose Nucleophiles 3-5 with
Sialic Acid Building Block 2
building block nucleophile yield of R-anomer
R/â
ratioa
entry
(equiv)
(equiv)
(%)
1
2
3
4
5
2a (1.5)
3 (1.0)
6: 44
7: 56
8: 76
8: 80
8: 86
3:1
8:1
9.5:1
11:1
11:1
2b (1.0)
2a (1.0)
2a (1.2)
2a (1.5)
4 (1.5)
5 (1.5)
5 (1.0)
5 (1.0)
a
1
Determined by H NMR after size-exclusion chromatography.
Scheme 1. Transformation of 8 into Sialyl Galactose Building
Block 11
of glycosylating agents.13 Galactal 5 is an attractive nucleo-
phile in the sialylation reaction and would subsequently be
equipped with an anomeric leaving group. We reasoned that
galactal may be well suited for coupling with sialic acid
because the hydroxyl group on the C3-position is sterically
less hindered, due to the absence of a C2-hydroxyl. In
addition, the C3-hydroxyl group is more nucleophilic than
a typical galactose C3-hydroxyl group, due to its allylic
nature. Few reports on the use of galactals as nucleophiles
in sialylation reactions have been published.14 This is mainly
due to the instability of the olefin under the activation
conditions of the common sialyl thioglycoside building
charide 8 was treated with PhI(OAc)2 and a catalytic amount
of BF3‚Et2O,13a followed by complete acetylation to produce
diacetate 9 with high selectivity in one pot.17 The anomeric
acetate was cleaved with hydrazine acetate to afford hemi-
acetal 10, followed by introduction of the anomeric N-phenyl
trifluoroacetimidate to furnish building block 11 in good
yield.18,19
To demonstrate the viability of disaccharide building block
11, SLx hexasaccharide 16 was synthesized (Scheme 2).
Trisaccharide 12,20 equipped with a protected amine-contain-
ing linker for further conjugation, was glycosylated using
building block 11. The desired pentasaccharide 13 was
obtained as a single anomer in excellent yield. Deprotection
of the levulinoyl group using hydrazine monohydrate in
(6) (a) Kameyama, A.; Ishida, H.; Kiso, M.; Hasegawa, A. J. Carbohydr.
Chem. 1991, 10, 549-560. (b) Gege, C.; Geyer, A.; Schmidt, R. R. Chem.-
Eur. J. 2002, 8, 2454-2463. (c) Zhang, Z.; Niikura, K.; Huang, X.-F.;
Wong, C.-H. Can. J. Chem. 2002, 80, 1051-1054.
(7) (a) Tanaka, H.; Nishiura, Y.; Takahashi, T. J. Am. Chem. Soc. 2006,
128, 7124-7125. (b) Ando, H.; Koike, Y.; Koizumi, S.; Ishida, H.; Kiso,
M. Angew. Chem., Int. Ed. 2005, 44, 6759-6763.
(8) (a) Ando, H.; Koike, Y.; Ishida, H.; Kiso, M. Tetrahedron Lett. 2003,
44, 6883-6886. (b) Tanaka, H.; Adachi, M.; Takahashi, T. Chem.-Eur. J.
2005, 11, 849-862.
(15) Lin, C.-C.; Huang, K. T.; Lin, C.-C. Org. Lett. 2005, 7, 4169-
4172.
(16) (a) Kanie, O.; Kiso, M.; Hasegawa, A. J. Carbohydr. Chem. 1988,
7, 501-506. (b) Schmidt, R. R.; Behrendt, M.; Toepfer, A. Synlett 1990,
694-696.
(17) Treatment with PhI(OAc)2 and BF3‚Et2O produced a mixture of
2-OH and OAc.
(18) (a) Yu, B.; Tao, H. J. Org. Chem. 2002, 67, 9099-9102. (b) Tanaka,
H.; Iwata, Y.; Takahashi, D.; Adachi, M.; Takahashi, T. J. Am. Chem. Soc.
2005, 127, 1630-1631 and references therein.
(19) The phosphate sialyl galactose building block obtained in moderate
yield by the one-pot method described in ref 13b gave poor yield in the
subsequent glycosylation of trisaccharide 12.
(9) De Meo, C.; Demchenko, A. V.; Boons, G.-J. J. Org. Chem. 2001,
66, 5490-5497.
(10) Yu, C.-S.; Niikura, K.; Lin, C.-C.; Wong, C.-H. Angew. Chem.,
Int. Ed. 2001, 40, 2900-2903.
(11) Tanaka, K.; Goi, T.; Fukase, K. Synlett 2005, 2958-2962.
(12) Demchenko, A. V.; Boons, G.-J. Tetrahedron Lett. 1998, 39, 3065-
3068.
(13) (a) Shi, L.; Kim, Y.-J.; Gin, D. Y. J. Am. Chem. Soc. 2001, 123,
6939-6940. (b) Plante, O. J.; Palmacci, E. R.; Andrade, R. B.; Seeberger,
P. H. J. Am. Chem. Soc. 2001, 123, 9545-9554.
(14) (a) Gervay, J.; Peterson, J. M.; Oriyama, T.; Danishefsky, S. J. J.
Org. Chem. 1993, 58, 5465-5468. (b) Martichonok, V.; Whitesides, G.
M. J. Org. Chem. 1996, 61, 1702-1706.
(20) Hanashima, S.; Seeberger, P. H., submitted.
1778
Org. Lett., Vol. 9, No. 9, 2007

Scheme 2. Linear Synthesis of Sialyl Lewis X Hexasaccharide 1
AcOH/pyridine in the presence of allyl alcohol21 set the stage
combining galactal nucleophile 5 and phosphite sialylating
agent 2a. The galactal moiety was readily oxidized and
functionalized to afford N-phenyl trifluoroacetimidyl glyco-
side 11. This new disaccharide building block performed well
in the glycosylation of a trisaccharide, during the synthesis
of SLx hexasaccharide, ready for biological applications. This
disaccharide building block is currently used for the efficient
chemical synthesis of other complex sialylated oligosaccha-
rides in solution and on solid support.
for a final fucosylation. Pentasaccharide 14 was glycosylated
22
with fucose building block 15 in 88% yield using Yb(OTf)3
as a mild activating agent to avoid the formation of
byproducts derived from bisfucosylation.
Global deprotection of hexasaccharide 16 required several
steps. Initially, the Alloc group was exchanged to a Cbz
group followed by treatment with a Zn-Cu couple to remove
the Troc group, accompanied by reduction of the trichloro-
acetamide (TCA), to afford 18 upon acetylation. Finally,
treatment of 18 with sodium methoxide, followed by addition
of water to hydrolyze the methyl ester and hydrogenolysis
with 20% Pd(OH)2/C, gave the desired deprotected SLx 1
ready for conjugation or printing on microarrays via the
amine linker.23
Acknowledgment. This work was supported by the Swiss
National Science Foundation (SNF grant 200020-109555),
ETH Zurich, and JSPS Postdoctoral Fellowships for Research
Abroad (for S.H.). We thank Dr. Yukishige Ito (RIKEN,
Japan) for providing sialic acid.
In summary, we developed an efficient strategy to
synthesize a sialic acid R(2-3) galactose building block by
Supporting Information Available: Experimental pro-
cedures and full spectroscopic data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(21) The allyl alcohol prevents the reduction of the Alloc olefin under
the reaction conditions.
(22) Adinolfi, M.; Iadonisi, A.; Ravida`, A.; Schiattarella, M. Synlett 2004,
275-278.
(23) de Paz, J. L.; Noti, C.; Seeberger, P. H. J. Am. Chem. Soc. 2006,
128, 2766-2767.
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