Stereoselective synthesis of oligo-α-(2,8)-sialic acids

Article abstract of DOI:10.1021/ja0613613

An efficient and elegant synthesis of α(2,8)-oligosialosides is described. The 5-N,4-O-carbonyl-protected sialyl donor undergoes α-sialylation in CH₂Cl₂ to give α(2,8)- and α(2,9)-disialosides in excellent yields. The 5-N,4-O-carbony

Full text of DOI:10.1021/ja0613613

Published on Web 05/13/2006
Stereoselective Synthesis of Oligo-r-(2,8)-Sialic Acids
Hiroshi Tanaka,* Yuji Nishiura, and Takashi Takahashi*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology
2-12-1-S1-35, Oookayama, Meguro, Tokyo 152-8552, Japan
Received February 27, 2006; E-mail: thiroshi@apc.titech.ac.jp; ttak@apc.titech.ac.jp
Polysialic acids composed of the R(2,8) and/or R(2,9) Neu5Ac
units 1 and 2 are frequently located at the nonreducing end of
biologically active glycoconjugates such as glycoproteins and
glycolipids (Figure 1).1 Recent progress in glycobiology suggests
that such oligomers may play important roles in biological events
on the cell surface.2 In terms of structure-activity relationship
studies, the use of chemically pure synthesized oligosialosides as
biochemical probes would be highly desirable because the pos-
sibility that such isolated glycoconjugates could be heterogeneous
and/or contaminated with antigenic compounds cannot be excluded.
Figure 1. Structure of R(2,8)- and R(2,9)-sialosides 1 and 2.
However, oligosialosides 1 and 2 represent difficult and challenging
synthetic targets. The presence of the C1 carboxyl group of sialic
acids reduces the reactivity of the anomeric position toward
glycosidation. A lack of C3 substitutions readily promotes the 2,3
elimination of a sialyl donor during glycosidation, making the
formation of R-isomers difficult.3 Furthermore, the C8 hydroxyl
group on the exocyclic chain exhibits low reactivity toward
glycosylation.
Table 1. Glycosidation of 5-N,4-O-Carbonyl-Protected Sialoside 3
To overcome these problems, the sialyl donors containing a
stereodirecting group at the C3 position, namely OH or SPh, have
been developed for indirect synthetic methods and permit the
preparation of various R(2,8)-disialosides.4 The substituent at the
C3 position on the donors should be stereoselectively installed and
must be removed at the end of the synthesis. On the other hand,
the conversion of the acetamide group at the C5 position of the
sialyl donor to N,N-diacetyl,5 azido,6 N-TFA,7 N-Troc,8,9 N-Fmoc,9
N-trichloroacetyl,9 and N-phthalimide10 groups has been reported
to be effective for direct R-selective sialylation. The coupling of
N,N-diacetyl,7 N-TFA,11 and N-Troc12 sialyl donors and acceptors
directly provided R(2,8)-disialosides in moderate yields and good
selectivity. Furthermore, the 1,5-lactamization of sialic acids was
developed as an alternative route for enhancing the reactivity of
the C8 hydroxyl groups toward glycosylation.13 In these methods,
the use of acetonitrile as a solvent is critical for achieving
R-selective glycosidation.14 However, the utility of these method-
ologies in direct R-sialylation has been mainly demonstrated in the
synthesis of R(2,8)-disialosides. Therefore, an effective method for
the synthesis of oligosialosides continues to be needed.
In this communication, we report on an effective R-selective
sialylation using the 5-N,4-O-carbonyl-protected sialyl donor 3 and
its application to the synthesis of R(2,8)-tetrasialoside 1a. As
illustrated in Table 1, treatment of the 5-N,4-O-carbonyl-protected
R-sialoside 4a15 containing C7 and C8 dihydroxyl groups and 1.5
equiv of 5-N,4-O-carbonyl- and 7,8-O-dichloroacetyl-protected sialy
donor 3 with NIS and a catalytic amount of TfOH in CH2Cl2 at
-78 °C provided R(2,8)-sialoside 5a in 86% yield with complete
R-selectivity. The chloroacetyl protecting groups at the O-7,8
positions proved to be effective for improving the reactivity of the
sialyl donor 3 and can be selective removed to provide 7,8-diols
for the next glycosylation.16 It should be noted that the protec-
tion of the 5-N,4-O-carbonyl group on the thiosialoside enables
equivalent
yield
(%)
entry
of donor 3 (equiv)
acceptor
product
R:â
1
2
3
4
1.5
1.5
1.0
1.0
4a
4b
4c
4d
5a
5b
5c
5d
86
20
quant
96
R onlya
74:26b
R onlya
R onlya
a The R:â ratio was estimated by analysis of 1H NMR spectra. b The
R:â ratio was estimated by HPLC analysis based on refractive index
detection.
R-sialylation to proceed without acetonitrile effects or neighboring-
group participation by any of the auxiliaries.17 The glycosylation
of N-Troc sialoside 4b with donor 3 resulted in a reduced coupling
yield of 5b with moderate R-selectivity. These results indicate that
the cyclic protecting group effectively improved the reactivity of
the C8 hydroxyl group toward glycosylation.18 We next carried out
the glycosylation of primary alcohols in 4c and 4d with 3. The
sialylation of primary alcohols based on the acetonitrile effects
frequently results in poor R-selectivity, compared to the sialylation
of secondary hydroxyl groups such as the C3 hydroxyl group on
galactosides. Treatment of the primary hydroxyl group of 4c with
1.0 equiv of sialyl donor 3 under the same reaction conditions
provided R-sialosides 5c in quantitative yield and excellent selectiv-
ity. Furthermore, glycosylation of the 8,9-dihydroxyl sialyl acceptor
4d with donor 3 afforded R(2,9)-disialoside 5d in 96% yield as a
single product. These results indicated that donor 3 was especially
effective for sialylation of the primary alcohols.
9
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J. AM. CHEM. SOC. 2006, 128, 7124-7125
10.1021/ja0613613 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1 a
a Reagents and conditions: (a) thiourea, 2,6-lutidine, DMF, 70 °C, 85% for 6, 82% for 8. (b) 3 (3.0 equiv), NIS (3.6 equiv), TfOH (0.3 equiv), MS3A,
-78 °C, 89%, R only. (c) 3 (3.0 equiv), NIS (3.6 equiv), TfOH (0.30 equiv), MS3A, -78 °C, 57% R only. (d) Ac2O, Py, CH2Cl2, -50 °C. quant (e)
LiOH‚H2O, H2O, EtOH, 80 °C, 88%. (f) Ac2O, NaHCO3, H2O, 0 °C then NaOMe, MeOH, 64%. (g) Pd(OH)2, H2 (1 atm), MeOH, H2O, 70%.
The structures 5a-d were confirmed as follows. The R config-
uration of sialosides 5a, 5c, and 5d was determined on the basis of
the 3JC1-H3ax coupling constants of the corresponding hydrolysates
of 5a, 5c, and 5d.19 The regioselectivity in the glycosylation of
diols 4a and 4d was estimated by 1H NMR analysis of the acetylated
products from 5a and 5d. The hydrolysate of the N-Troc derivative
5b-R was identical to the hydrolysate of 5a (details are shown in
Supporting Information).
using a simple glycosidation and deprotection protocol. This
coupling method allows the synthesis of the various oligosaccha-
rides containing R(2,8)- and R(2,9)-oligosialoside units, which are
effective biochemical probes for elucidating their biological activi-
ties.
Supporting Information Available: Experimental procedures for
the R-sialylation and full characterization for all compounds. This
material is available free of charge via the Internet at http://pubs.acs.org.
We next conducted the synthesis of R(2,8)-tetrasialosides 1a via
5a (Scheme 1). The removal of the two chloroacetyl groups at the
C7 and C8 positions of 5a provided triol 6 in 85% yield. Treatment
of triol 6 and the 5-N,4-O-carbonyl-protected sialoside 3 (1.5 equiv)
with NIS/TfOH in CH2Cl2 at -78 °C provided R(2,8)-trisialoside
7 in 68%. The use of 3.0 equiv of the glycosyl donor 3 resulted in
the disappearance of acceptor 6 and provided trisaccahride 7 in
89% yield with complete R selectivity. Deprotection of the
chloroacetyl groups on 7 afforded the tetraol acceptor 8 in 82%
yield. Tetrasaccharide formation from 8 using 3.0 equiv of donor
3 under the same reaction conditions provided tetrasaccharide 9 in
57% yield with complete R-selectivity along with the recovered
References
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3
acceptor 8 (31%). The coupling constants JC1-H3ax (5.9, 5.8, 5.4,
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and 5.3 Hz) for 9 indicated that the configuration of all glycosidic
linkages was R. In addition, the regioselectivity of each glycosyl-
1
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(15) The donor 3 was prepared from sialic acid by 10 steps. The details are
described in Supporting Information.
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(17) Recently, Crich and co-wokers have reported on an efficient R-sialylation
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ation step was confirmed by H NMR analysis of the acetylated
product 10.
Deprotection of the R(2,8)-tetrasialoside 9 was examined.
Exposure of the protected tetrasialoside 9 to basic conditions
provided amino acids 11 (88%). Acetylation the resulting amines
provided the N-acetyl derivative 12 in 64% yield. Finally, removal
of the benzyl ethers on 12 by hydrogenolysis using a palladium
catalyst afforded the fully deprotected tetrasialosides 1a.
In conclusion, an efficient and elegant synthesis of R(2,8)-
oligosialosides is described. The 5-N,4-O-carbonyl-protected sialyl
donor undergoes R-sialylation in CH2Cl2 to provide R(2,8)- and
R(2,9)-disialosides in excellent yields. The 5-N,4-O-carbonyl
protecting group was effective for improving the reactivity of the
C8 hydroxyl groups toward glycosylation. Using the sialyl building
block, the synthesis of tetra-R(2,8)-sialic acid could be accomplished
JA0613613
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