An efficient convergent synthesis of GP1c ganglioside epitope

Full text of DOI:10.1021/ja807482t

Published on Web 12/02/2008
An Efficient Convergent Synthesis of GP1c Ganglioside Epitope
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 September 20, 2008; E-mail: thiroshi@apc.titech.ac.jp; ttak@apc.titech.ac.jp2
Scheme 1
Gangliosides are a family of glycolipids, which possess sialic
acids that play important roles in biological events on cell surfaces.1
GP1c glycolipid (1) is one of the most complex c-series ganglio-
sides; it is composed of a tetrasaccharide (Galꢀ(1,3)GalNAcꢀ
(1,4)Galꢀ (1,4)Glc) that bears R(2,8) di- and trisialic acid units.2
Glycolipids that vary in the number of sialic acids are known as
the a-, b-, and c-series of gangliosides.3 Chemical synthesis of these
di/oligo-sialo-containing glycolipids and their derivatives would
facilitate identification of their biological roles. However, synthesis
of both the R (2,8) oligosialic acid unit and the compact and rigid
3,4 dibranched galactoside unit is challenging. Therefore, consider-
able efforts have been directed toward the synthesis of these
complex oligosaccharides.4 Kiso and co-workers reported an
efficient synthesis of the GQ1b ganglioside (2) that has two R(2,8)
disialic acid units. In this method, the disialic acid units were
prepared by hydrolysis of colominic acid.4e Herein, we report an
efficient convergent synthesis of the GP1c ganglioside epitope 3.
We planned the synthesis of the GP1c epitope 3 bearing a C14
alkyl chain instead of ceramide for establishing the methodology
for the synthesis of sugar parts of a-, b-, and c-series of gangliosides.
Scheme 1 shows the convergent strategy for the synthesis of the
GP1c epitope 3. The nonasaccharide 3 was prepared by coupling
the pentasaccharide 6 and the tetrasaccharide donor 4 at the C4
axial hydroxyl group of the latter. The pentasaccharide 6 was
prepared from the R(2,8) trisialylgalactosyl imidate5 8 and the
glucoside 10. The tetrasaccharide donor 4 was prepared by
chemoselective glycosylation of the 2,6-dimethyphenylthiogalac-
toside 9 with the disialyl galactosyl imidate 7. The 2,6-dimethy-
phenylthio leaving group is effective in chemoselective glycosy-
lation without aglycon transfer of thioglycoside 9.6 The tri- and
disialylgalactosides 8 and 7 were prepared from galactoside 11 and
the sialyl donors 13 and 14, which possess a 5N,4O-carbonyl
protecting group, using a simple glycosylation and deprotection
procedure.7 The sialyl donor 13 protected with an isopropylidene
at the C7 and C8 hydroxyl groups was used for R(2,3) sialylation.
The C7 and C8 di-O-chloroacetyl sialyl donor 14 was used for
R(2,8) sialylation to prepare disialylgalactosides 7. Further R(2,8)
sialylation of the resulting disialylgalactosides provided the trisia-
lylgalactosides 8.
The syntheses of the di- and trisialyl building blocks 4 and 6
are shown in Scheme 2. Treatment of both 3,4-dihydroxyl galac-
toside 11 and 1.0 equiv of the sialyl donor 13, which is protected
with an isopropylidene at the C7 and C8 hydroxyl groups, with
NIS/cat. TfOH in CH2Cl2 at -78 °C provided the R(2,3)-sialyl
galactoside 16 in 90% yield with complete R-selectivity. Use of
the di-O-chloroacetyl donor 14 reduced both the yield and
R-selectivity of the coupling product 15. Use of the 4,6-O-
benzylidene galactoside 12 as an acceptor for R(2,3) sialylation
also reduced the yield and selectivity.8 Protection of the resulting
C4 hydroxyl group of 16 with a Troc group, followed by removal
of the acetal, provided the diol 17. Treatment of the diol 17 and
the di-O-chloroacetyl-protected sialyl donor 14 (2.0 equiv) with
NIS/cat. TfOH in CH2Cl2/CH3CN ) 3:2 at -78 °C provided the
R(2,8) disialoside 19 in 91% yield with complete R-selectivity. The
use of pure CH2Cl2 as solvent for R(2,8) sialylation of 17 reduced
the coupling yield of 19 but did not affect the R-selectivity. The
sialyl donor 13 was not effective in the R(2,8) sialylation, and use
of 13 resulted in a reduction in R-selectivity (77%, R:ꢀ ) 54:46).
Deprotection of the chloroacetyl groups on 19 afforded the triol
acceptor 20 in 87% yield. Trisialyl formation from 20 using 3.0
equiv of donor 14 under the same reaction conditions provided the
trisialyl galactoside 22 in 72% yield (R:ꢀ > 90:10). Trisialoside
22 had an R-anomeric configuration, as determined based on the
3JC1-H3ax coupling constants (3JC-1,H-3ax ) 4.9, 4.9, and 6.1 Hz). The
imidates 7 and 8 were prepared in a stepwise procedure, as follows:
(i) removal of the chloroacetyl groups from 19 and 22; (ii)
acetylation of the remaining hydroxyl group; (iii) removal of the
2-trimethylsilylethyl group at the anomeric position; and, (iv)
treatment of the hemiacetal with trichloroacetonitrile to provide the
9
17244 J. AM. CHEM. SOC. 2008, 130, 17244–17245
10.1021/ja807482t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Scheme 3
Reagents and conditions: (a) 4 (2.0 equiv), NIS, cat. TfOH, MS4A, 0
°C, 64%; (b) LiOH ·H2O, Dioxane, H2O, 80 °C; (c) Ac2O, NaHCO3, H2O
then LiOH, H2O; (d) Pd(OH)2, H2, MeOH, H2O, 53% from 26.
step procedure: (i) hydrolysis of acyl protecting groups; (ii)
acetylation of the C5 amino groups; and (iii) hydrogenolysis of
the resulting benzyl ethers to provide the fully deprotected
nonasaccharide 3 in 53% yield based on 26.
Reagents and conditions: (a) 13 (1.0 equiv), NIS, cat. TfOH, CH2Cl2,
In conclusion, we describe an efficient convergent synthesis of
the GP1c epitope 3. The reactivity of the saccharide structure was
tuned by careful selection of a protecting group to match the
requisite glycosidation and glycosylation. Various glycosyl accep-
tors and donors with different oligosialoglycans can be prepared
by use of two 5N,4O-carbonyl-protected thiosialosides. The method
described herein is amenable to the synthesis of a-, b-, and c-series
ganglioside epitopes.
MS3A, -78 °C, 90%, R only; (b) 14 (1.0 equiv), NIS, cat. TfOH, CH2Cl2,
MS3A, -78 °C, 82%, R/ꢀ ) 87:13; (c) (i) TrocCl, Py, CH2Cl2, 96%; (ii)
CSA, MeOH, rt, 92%; (d) 14 (2.0 equiv), NIS, cat. TfOH, CH2Cl2/CH3CN
) 3:2, MS3A, -78 °C, 91%, R/ꢀ ) >95:5; (e) 13 (2.0 equiv), NIS, cat.
TfOH, CH2Cl2/CH3CN ) 3:2, MS3A, -78 °C, 77%, R/ꢀ ) 54:46; (f)
thiourea, 2,6-lutidine, DMF, 60 °C, 87%; (g) Ac2O, Py, CH2Cl2, cat. DMAP,
-50 °C, 92%; (h) (i) TFA, CH2Cl2; (ii) CCl3CN, Cs2CO3, CH2Cl2, 0 °C,
93% from 21, R/ꢀ ) 68:32; (i) 9, TMSOTf, CH2Cl2, MS4A, -78 to -45
°C, 95%; (j) 14 (3.0 equiv), NIS, cat. TfOH, CH2Cl2/CH3CN ) 3:2, MS3A,
-78 °C, 72%, R/ꢀ ) >90:10; (k) thiourea, 2,6-lutidine, DMF, 60 °C, 75%;
(l) A2O, Py, cat. DMAP, CH2Cl2, -30 °C, 87%; (m) (i) TFA, CH2Cl2, (ii)
CCl3CN, Cs2CO3, CH2Cl2, 0 °C, 90% from 24, R/ꢀ ) 84:16; (n) 10,
TMSOTf, CH2Cl2, MS4A, -65 to -40 °C, 93%; (o) Zn dust, AcOH, THF,
0 °C, 94%.
Acknowledgment. This work was financially supported by JSPS
Research Fellowships for Young Scientists DC2.
Supporting Information Available: This material is available free
of charge via the Internet at http://pubs.acs.org.
corresponding imidates 7 and 8. The chloroacetyl esters at the C7
and C8 positions underwent partial hydrolysis during imidate
formation with trichloroacetonitrile. Treatment of the trisialyl
galactosyl donor 8 and glucoside 10 with a catalytic amount of
TMSOTf provided the pentasaccharide 25 in 93% yield. The miner
isomer generated in the trisialylation can be removed in the
purification of acetylated product 24. Selective deprotection of the
Troc group afforded acceptor 6 in 94% yield. In addition,
preparation of the donor 4 was achieved by chemoselective
glycosidation of 7 at the secondary alcohol with 1.2 equiv of
thioglycoside 9 in the presence of a catalytic amount of TMSOTf,
providing the tetrasaccharide 4 in 95% yield.
Synthesis of the GP1c epitope 3 from 4 and 6 is illustrated in
Scheme 3. Treatment of both the pentasaccharide acceptor 6 and
2.0 equiv of the tetrasaccharide donor 4 with NIS and a catalytic
amount of TfOH at 0 °C provided the protected nonasaccharide 26
in 64% yield. In contrast, use of the corresponding 4,6-benzyliden
acetal-protected tetrasaccharide 5 resulted in a complex reaction
mixture. These results indicate that 4,6 di-O-benzyl protection on
the galactoside is important for the synthesis of the compact and
rigid branched structure from the sialyl galactoside. Deprotection
of the protected nonasaccharide 26 was achieved using a three-
References
(1) (a) Rahmann, H.; Jonas, U.; Hildebrandt, H. Trends Glycosci. Glyco-
technol. 1998, 10, 421–437. (b) Kasahara, K.; Sanai, Y. Trends Glycosci.
Glycotechnol. 2001, 13, 587–594.
(2) Miller-Podraza, H.; Månsson, J.-E.; Svennerholm, L. FEBS Lett. 1991, 288,
212–214.
(3) Schwarz, A.; Futerman, A. H. Biochim. Biophys. Acta 1996, 1286, 247–
267.
(4) (a) Ito, Y.; Numata, M.; Sugimoto, M.; Ogawa, T. J. Am. Chem. Soc. 1989,
111, 8508–8510. (b) Ito, Y.; Nunomura, S.; Shibayama, S.; Ogawa, T. J.
Org. Chem. 1992, 57, 1821–1831. (c) Kondo, T.; Tomoo, T.; Abe, H.; Isobe,
M.; Goto, T. Chem. Lett. 1996, 337–338. (d) Ishida, H.; Kiso, M. Trends
Glycosci. Glycotechnol. 2001, 13, 57–64. (e) Ishida, H.-K.; Ishida, H.; Kiso,
M.; Hasegawa, A. Tetrahedron: Asymmetry 1994, 5, 2493–2512. (f) Ando,
H.; Ishida, H.; Kiso, M. J. Carbohydr. Chem. 1999, 18, 603–607. (g) Ando,
H.; Koike, Y.; Koizumi, S.; Ishida, H.; Kiso, M. Angew. Chem., Int. Ed.
2005, 44, 6759–6763. (h) Boons, G.-J.; Demchenko, A. V. Chem. ReV. 2000,
100, 4539–4565. (i) De Meo, C.; Demechenko, A. V.; Boons, G.-J. J. Org.
Chem. 2001, 66, 5490–5497. (j) Tsvetkov, Y. E.; Nifantiev, N. E. Synlett
2005, 1375–1380.
(5) (a) Schmidt, R. R.; Kinzy, W. AdV. Carbohydr. Chem. Biochem. 1994, 50,
21–123. (b) Schmidt, R. R. Angew. Chem., Int. Ed. Engl. 1986, 25, 212–
235.
(6) Li, Z.; Gildersleeve, J. J. Am. Chem. Soc. 2006, 128, 11612–11619.
(7) (a) Tanaka, H.; Nishiura, Y.; Takahashi, T. J. Am. Chem. Soc. 2006, 128,
7124–7125. (b) Farris, M. D.; De Meo, C. Tetrahedron Lett. 2007, 48, 1225–
1227. (c) Crich, D.; Li, W. J. Org. Chem. 2007, 72, 2387–2391.
(8) Details were described in the Supporting Information.
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