A four-component one-pot synthesis of α-Gal pentasaccharide

Article abstract of DOI:10.1021/ol0483246

(Chemical equation presented) A four-component one-pot sequential synthesis of α-Gal pentasaccharide 2 with minimal protecting group manipulations in a very short period of time is described in this paper.

Full text of DOI:10.1021/ol0483246

ORGANIC
LETTERS
2004
Vol. 6, No. 24
4415-4417
A Four-Component One-Pot Synthesis
of -Gal Pentasaccharide
r
Yuhang Wang,† Xuefei Huang,‡ Li-He Zhang,† and Xin-Shan Ye*,†
The State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical
Sciences, Peking UniVersity, Xue Yuan Rd 38, Beijing 100083, China, and Department
of Chemistry, UniVersity of Toledo, 2801 W. Bancroft Street, MS 602,
Toledo, Ohio 43606
xinshan@bjmu.edu.cn
Received August 22, 2004
ABSTRACT
A four-component one-pot sequential synthesis of
period of time is described in this paper.
r-Gal pentasaccharide 2 with minimal protecting group manipulations in a very short
Antibody- and complement-dependent hyperacute rejection
(HAR), as the major barrier to successful pig-to-human
xenotransplantation, is due to the specific interaction of
recipient xenoreactive antibodies with antigens present on
the endothelium of the donor organ, followed by activation
of the complement cascade.1 Carbohydrate structure epitopes
containing a GalR(1 f 3)Gal terminus (R-Gal epitopes) are
established as the main xenoantigens responsible for initiating
the hyperacute rejection of pig organs by humans. The
pentasaccharide R-Gal epitope GalR1-3Galâ1-4GlcNAcâ1-
3Galâ1-4Glcâ-Cer (1) (Figure 1) existing in pig vascular
endothelium binds specifically with human anti-Gal antibod-
ies. It has been identified that the pentasaccharide residue
serves as the binding site for human anti-pig antibodies.2
To overcome the hyperacute rejection, one of the possible
strategies is to use synthetic R-Gal oligosaccharides to
antagonize anti-R-Gal antibodies.3 Such an approach would
require access to a substantial amount of R-Gal oligo-
saccharides and R-Gal analogues.
Several methods were reported for the synthesis of R-Gal
epitope pentasaccharide. Boons et al.4 reported a highly
convergent synthesis of the methyl glycoside of the penta-
saccharide. The Schmidt group synthesized the penta-
saccharide 1 and its methyl glycoside with the trichloro-
acetimidate methodology.5 Wang and co-workers described
a chemoenzymatic approach to the synthesis of R-Gal
epitopes based on the use of recombinant R(1-3)-galacto-
syltransferase.6 However, a highly efficient assembly of the
* To whom correspondence should be addressed. Tel: 86-10-82801570.
Fax: 86-10-62014949.
† Peking University.
‡ University of Toledo.
(1) (a) Fryer, J. P.; Leventhal, J. R.; Matas, A. J. Transplant Immunol.
1995, 3, 21. (b) Sandrin, M. S.; McKenzie, I. FC. Curr. Opin. Immunol.
1999, 11, 527. (c) Mollnes, T. E.; Fiane, A. E. Mol. Immunol. 1999, 36,
269. (d) Cascalho, M. J.; Platt, L. Nat. ReV. Immunol. 2001, 1, 154.
(2) Hallberg, E. C.; Strokan, V.; Cairns, T. D. H.; Breimer, M. E.;
Samuelsson, B. E. Xenotransplantation 1998, 5, 246.
(3) (a) Galili, U. Immunol. Today 1993, 14, 480. (b) Rieben, R.; Von
Allmen, E.; Korchagina, E.; Nydegger, U.; Neethling, F. A.; Kujundzic,
M.; Koren, E.; Bovin, N.; Cooper, D. K. C. Xenotransplantation 1995, 2,
98.
Figure 1. Ceramide pentasaccharide 1 and its derivative 2.
(4) Zhu, T.; Boons, G.-J. J. Chem. Soc., Perkin Trans. 1 1998, 857.
10.1021/ol0483246 CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/27/2004

R-Gal pentasaccharide moiety is still in great demand. Herein
we present a new method for the rapid synthesis of the
pentasaccharide derivative 2 (Figure 1) by a four-component
one-pot strategy. The aminopropyl group7 was incorporated
as a side chain for further derivation. The feature of this
work lies in the integration of three glycosylation steps into
one synthetic operation to furnish the target oligosaccharide
in a few hours without the need for protecting group
manipulation and intermediate isolation.
The one-pot sequential glycosylation strategy8 for the
assembly of oligosaccharides has achieved success as
demonstrated by syntheses of some naturally occurring
oligosaccharides such as Ley,9 fucosyl GM1,10 and Globo H.11
In all of these examples, a three-component coupling strategy
was applied. To enhance the efficiency of the one-pot
approach, we tried to explore the possibility of synthesis of
the R-Gal pentasaccharide by a four-component one-pot
sequential glycosylation method. The retrosynthetic analysis
divided the fully protected target pentasaccharide 3 into four
building blocks: the galactose building blocks 4 and 5, the
glucosamine building block 6, and the lactose building block
7 (Scheme 1). According to the reactivity order of thiogly-
selection of protecting groups was the main concern in the
design of the second and third building blocks, because the
presence of a free hydroxyl group and a thiotoluene
functionality would make them act as both a glycosyl
acceptor and a glycosyl donor. Moreover, their relative
reactivities toward glycosylation should fall between 4 and
7. We finally found that building blocks 5 and 6 were suitable
for the successful one-pot synthesis.
We chose thioglycosides as glycosyl donors because they
are stable enough in most cases and can be activated by a
variety of promoters. To perform an efficient one-pot syn-
thesis, we tried several promoter systems, such as dimethyl-
(thiomethyl)sulfonium triflate (DMTST),12 N-iodosuccin-
imide and triflic acid (NIS/TfOH),13 phenylsulfenyl chloride
and silver triflate (PhSCl/AgOTf),14 and 1-benzenesulfinyl
piperidine and triflic anhydride (BSP/Tf2O).15 Eventually we
found that BSP/Tf2O was the most efficient promoter for
the one-pot glycosylation protocol.
Building blocks 4 and 5 were prepared by literature
procedures.8a The synthesis of building block 6 is shown in
Scheme 2. O-Benzylation of p-methylphenyl 4,6-O-benzyl-
Scheme 2. Synthesis of Building Block 6
Scheme 1. Retrosynthetic Analysis of the Pentasaccharide
Moiety
idene-2-deoxy-2-phthalimido-1-thio-â-D-glucopyranoside8a gave
saccharide 8 in 92% yield. Regioselective reductive ring
opening of the 4,6-O-benzylidene acetal of 8 produced 6 in
90% yield with the 4-hydroxyl group exposed. The synthesis
of building block 7 started from lactose peracetate 9.16
Compound 9 was converted to the â-lactoside 10 in 43%
yield by coupling with benzyl N-(3-hydroxypropyl)-carbam-
ate in the presence of BF3‚Et2O. Saccharide 10 was deacet-
ylated, treated with dibutyltin oxide, and reacted with allyl
bromide to provide regioselectively 3′-O-allyl-protected
lactoside 11. The benzylation of the remaining hydroxyls of
11 was performed under the conditions of sodium hydride
and benzyl bromide in DMF to give compound 12. Removal
of the allyl protecting group of 12 by palladium(II) chloride
yielded building block 7 in 98% yield (Scheme 3).
cosides that the Wong group reported,8a,b building block 4
should have the highest reactivity among the four compo-
nents, and the reducing end component 7 should have no
reactivity because of its O-glycosyl linkage, which cannot
be activated by promoters of thioglycoside donors. The
(5) Gege, C.; Kinzy, W.; Schmidt, R. R. Carbohydr. Res. 2000, 328,
459.
(6) Fang, J.; Li, J.; Chen, X.; Zhang, Y.; Wang, J.; Guo, Z.; Zhang, W.;
Yu, L.; Brew, K.; Wang, P. G. J. Am. Chem. Soc. 1998, 120, 6635.
(7) Liaigre, J.; Dubreuil, D.; Pradere, J.-P.; Bouhours, J.-F. Carbohydr.
Res. 2000, 325, 265.
With all building blocks in hand, the four-component
assembly of fully protected pentasaccharide derivative 3 was
attempted as outlined in Scheme 4. The one-pot synthetic
operation was performed in the presence of the BSP/Tf2O
(8) (a) Zhang, Z.; Ollmann, I. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.;
Wong, C.-H. J. Am. Chem. Soc. 1999, 121, 734. (b) Koeller, K. M.; Wong,
C.-H. Chem. ReV. 2000, 100, 4465. (c) Douglas, N. L.; Ley, S. V.; Lucking,
U.; Warriner, S. L. J. Chem. Soc., Perkin Trans. 1 1998, 51.
(9) Mong, T. K.-K.; Wong, C.-H. Angew. Chem. 2002, 114, 4261.
(10) Mong, T. K.-K.; Lee, H.-K.; Duron, S. G.; Wong, C.-H. Proc. Natl.
Acad. Sci. U.S.A. 2003, 100, 797.
(12) Fugedi, P.; Garegg, P. J. Carbohydr. Res. 1986, 149, 9.
(13) Konradsson, P.; Udodong, U. E.; Fraser-Reid, B. Tetrahedron Lett.
1990, 31, 4313.
(14) (a) Martichonok, V.; Whitesides, G. M. J. Org. Chem. 1996, 61,
1702. (b) Crich, D.; Sun, S. Tetrahedron 1998, 54, 8321.
(15) Crich, D.; Smith, M. J. Am. Chem. Soc. 2001, 123, 9015.
(16) Ravindranathan, K. K. P.; Field, R. A. Carbohydr. Lett. 1998, 3,
179.
(11) Burkhart, F.; Zhang, Z.; Wacowich-Sgarbi, S.; Wong, C.-H. Angew.
Chem. 2001, 113, 1314.
4416
Org. Lett., Vol. 6, No. 24, 2004

Scheme 3. Synthesis of Lactose Building Block 7
Scheme 4. One-Pot Synthesis of Fully Protected
Pentasaccharide 3a and Its Deprotected Product 2
promoter system.15 The first glycosylation built up the
R-linkage17 between the donor 4 and the acceptor 5. In the
following two coupling steps, building blocks 6 and 7 were
sequentially added to the reaction mixture. The reaction
process was monitored by TLC. For the BSP/Tf2O-promoted
reactions, the glycosyl donors must be activated at -70 °C
and the reaction temperature is elevated gradually to room
temperature. The equivalent of BSP we used ranged from
0.5 to 1.0. Finally, the fully protected pentasaccharide
derivative 3 was isolated in 39-41% yield. This finding
corresponds to an average yield of 75% per glycosylation
step. Global deprotection of 3 was performed in four steps
(Scheme 4). The phthalimido functionality was removed in
NH2NH2‚H2O and EtOH under reflux, and the released amino
group was then acetylated with acetic anhydride in pyridine.
The benzoyl functionality was cleaved by treatment with
NaOMe in methanol. The remaining benzyl, benzylidene,
and benzyl carbamate protecting groups were cleaved by
catalytic hydrogenolysis over 10% Pd-C. The target penta-
saccharide 2 was finally obtained in 43% isolated yield in
the form of acetate after purification by C-18 reversed-phase
column chromatography. The characterization of 2 and the
correct anomeric configuration of each glycosidic linkage
a Key: Compound 5 was added as a solid; 6 and 7 were added
in a solution of CH2Cl2, respectively.
In conclusion, using a four-component one-pot sequential
glycosylation method, we have successfully synthesized the
aminopropyl glycoside of the R-Gal pentasaccharide, which
plays an important role in the interaction with human anti-
Gal antibodies and may be highly desired in the research of
xenotransplantation and immunotherapy. A single glycosyl-
ation protocol was used, and only easily available common
saccharide building blocks were needed for the oligosac-
charide assembly. This one-pot strategy should be applicable
to the synthesis of other R-Gal derivatives and other complex
oligosaccharides, as well as the structure-activity relation-
ship study of biologically important carbohydrates.
Acknowledgment. This work was financially supported
by the National Natural Science Foundation of China and
Peking University.
Supporting Information Available: Synthetic procedures
and spectral data of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
were confirmed by its 1D H NMR, 13C NMR and 2D
correlated spectroscopy (HSQC, HMBC, TOCSY), and
HRMS analysis.18
OL0483246
(18) Selected NMR data for the five anomeric protons and carbons: 1H
NMR (500 MHz, D2O) δ 5.14 (d, J ) 4.0 Hz, 1H, H-1′′′′), 4.70 (d, J ) 8.5
Hz, 1H, H-1′′′), 4.54 (d, J ) 8.0 Hz, 1H, H-1′), 4.50 (d, J ) 8.0 Hz, 1H,
H-1), 4.42 (d, J ) 8.0 Hz, 1H, H-1′′); 13C NMR (125 MHz, D2O) δ 103.7
(C-1′′), 103.5 (C-1), 103.4 (C-1′′′), 102.8 (C-1′), 96.2 (C-1′′′′).
(17) To determine the configuration of the newly formed glycosyl bond,
we once isolated the disaccharide and the R-linkage was demonstrated by
its proton NMR analysis. Selected 1H NMR data for one of the two anomeric
protons: δ 4.98 (d, J ) 3.0 Hz, 1H).
Org. Lett., Vol. 6, No. 24, 2004
4417