Olefin self-metathesis as a new entry into xenotransplantation antagonists bearing the Galili antigen

Article abstract of DOI:10.1039/b111352f

A hexameric disaccharide cluster bearing the terminal Galα related xenotransplantation antigen was constructed using a sequence of ruthenium carbenoid catalyzed olefin self-metathesis of monoallylated tribenzyl pentaerythritol followed, after interconvers

Full text of DOI:10.1039/b111352f

Olefin self-metathesis as a new entry into xenotransplantation
antagonists bearing the Galili antigen
Bingcan Liu and René Roy*
Department of Chemistry, Centre for Research in Biopharmaceuticals, University of Ottawa, Ottawa, ON,
Canada K1N 6N5. E-mail: rroy@science.uottawa.ca; Fax: +1 613 562 5170
Received (in Cambridge, UK) 12th December 2001, Accepted 28th January 2002
First published as an Advance Article on the web 19th February 2002
A hexameric disaccharide cluster bearing the terminal Gala
related xenotransplantation antigen was constructed using a
sequence of ruthenium carbenoid catalyzed olefin self-
metathesis of monoallylated tribenzyl pentaerythritol fol-
lowed, after interconversion of benzyl ethers into para-
functionality was accomplished by complete hydrogenolysis
(H2, Pd/C, 90%) followed by p-iodobenzylation in 45% yield
(p-IPhCH2Br, TBAI, NaH, DMF, rt, 6 h) according to Scheme
2.
The final preparation of the hexameric perglycosylated
cluster 8 is illustrated in Scheme 3. Sonogashira11 cross-
coupling of peraryliodide 5 with known prop-2-ynyl disaccha-
ride 65 under palladium-catalyzed conditions in the absence of
copper cocatalyst (1.3 eq 6/iodide, (Ph3P)2PdCl2, TEA, DMF,
60 °C, 5 h) provided fully protected hexamer 7 in 75% yield. As
previously observed,5,12 it was remarkable that the efficiency of
the cross-coupling could provide multiple carbohydrate attach-
ments in a single step reaction. Finally, complete deprotection
of both acetyl and benzoyl ester protecting groups in 7 to afford
8 was accomplished under catalytic transesterification condi-
tions using NaOMe in MeOH (95%). Interestingly, compound 8
was readily soluble in water, a condition judged essential for
biological activity. Therefore, the choice of a conformationally
restricted glycluster having several aryl–alkyne functionalities
does not seem to impair water-solubility, as long as disaccha-
rides or higher oligosaccharides are attached to it. This is in
striking contrast to a previously made pergalactosylated cluster
into which the hydrophilicity of the glycan portion was
insufficient to counterbalance the lipophilic linkers.12
Preliminary modelling of the above cluster showed pairs of
pentaerythritol substituents to be perpendicular to one another
and facing opposite directions, thus allowing easy access to
antibody binding sites. The relative topology of this new family
of glycoclusters is anticipated to be critical to fully explore
receptor specificities.13
iodobenzyl ethers, by
a
single step Sonogashira
cross-coupling of six prop-2-ynyl glycosides onto a hex-
americ aryl iodide scaffold.
Successful pig liver-to-human xenotransplantations are criti-
cally hampered by hyperacute organ rejection due to the
presence of natural human antibodies (1–2% IgG, 3–8% IgM)
recognizing cell surface Gala1-3Galb epitopes (Galili antigen)
covering the epithelial lining of xenografts.1,2 To overcome this
situation and to allow organ ‘accommodation’, strategies such
as immune suppression and serum perfusion through affinity
columns that remove circulating anti-Gala antibodies have been
accomplished.3 Analogously, intravenous Gala-OR antagonist
therapies have witnessed some successes, at least in baboon
allografts.4 Due to the low affinity of circulating antibodies
against monomeric epitopes, few clustered Galili antigens have
been prepared5–7 in order to rely on the well-established
increased avidity of multivalent carbohydrate scaffolds.8,9
Using a pentaerythritol platform (1) functionalised with a
unique allyl ether group, olefin self-metathesis provided, as
expected,10 a hexameric cluster (4)† according to Scheme 1.
Thus, substoichiometric benzylation of tetraol 1 (BnBr, NaH,
DMF) provided tribenzyl ether 2 as a thick syrup in 65% yield
after silica gel column chromatography. Allylation of the
residual hydroxy group (allyl bromide, NaH, DMF, rt) gave
fully protected ether 3 in 90% yield. Treatment of olefin 3 with
In conclusion, an effective combination of olefin self-
metathesis coupled with a single step multiple Sonogashira
cross-coupling strategy allowed an excellent entry toward novel
multivalent carbohydrate clusters.
We thank the National Science and Engineering Research
Council of Canada (NSERC) for financial support. B Liu is a
recipient of an OGSST award.
a
catalytic amount of ruthenium carbenoid (Cl2Ru(P-
Cy3)2 = CHPh (GrubbsA catalyst), CH2Cl2, rt, 12 h) afforded
homodimer 4 as an inseparable mixture of trans/cis stereoi-
somers (10+1) in 85% yield.
Interconversion of the perbenzylated ether 4 into p-iodo-
benzylated ether 5 with concomitant reduction of the alkene
Scheme 1 (a) Benzyl bromide, NaH, DMF, rt, 6 h, 65%; (b) allyl bromide,
NaH, DMF, rt, 6 h, 90%; (c) GrubbsA catalyst, CH2Cl2, rt, 12 h, 85%, E/Z
= 10+1.
Scheme 2 (a) H2, Pd/C, solvent, 24 h, 90%; (b) pIPhCH2Br, TBAI, NaH,
DMF, rt, 6 h, 45%.
594
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Scheme 3 (a) (Ph3P)2PdCl2, TEA, DMF, 60 °C, 6 h, 75%; (b) NaOMe, MeOH, rt, 2 d, 95%.
126.7 and 120.6 (ar), 101.1 (C-1), 95.0 (C-1A), 85.1 and 84.4 (acetylenic),
Notes and references
†
77.2, 74.1, 71.9, 70.3, 68.7, 67.8 and 64.1 (C-4, C-5, C-2A, C-3A, C-4A, C-5A,
PhCH2, -CH2O-, C(CH2OR)4), 60.5 (C-6), 60.1 (C-6A), 56.6 (C-1B), 45.5
(C(CH2O)4), 25.1 (-CH2-). MALDI-TOF-MS calcd for C146H198O74 (M +
Na+) 3158.2, found 3158.3.
Physical data for representative compounds, NMR assignments are
based on COSY and HMQC experiments (500 MHz).
1
3: H NMR (CDCl3) d 7.25 to 7.32 (m, 15H, ar), 5.83 to 5.90 (m, 1H,
vinyl), 5.11 to 5.25 (m, 2H, vinyl), 4.48 (s, 6H, benzylic), 3.93 to 3.95 (m,
2H. allylic), 3.56 (s, 6H, C(CH2OR)4), 3.53 (s, 2H, C(CH2OR)4); 13C NMR:
d 138.9 (ar), 135.2 (vinyl), 128.2, 127.3, and 127.2 (ar), 116.1 (vinyl), 73.3
(benzylic), 72.3 (allylic), 69.3 (C(CH2OR)4), 45.6 (C(CH2OR)4). FAB-MS
calcd for C29H34O4 (M + H+) 447.25, found 447.32.
4: 1H NMR (CDCl3) d 7.23 to 7.32 (m, 30H, aryl), 5.68 (m, 2H, vinyl),
4.48 (s, 12H, benzylic), 3.91 (m, 4H. allylic), 3.56 (s, 12H, C(CH2OR)4),
3.53 (s, 4H, C(CH2OR)4); 13C NMR: d 138.9 (ar), 129.0 (vinyl), 128.2,
127.3 and 127.2 (ar), 73.3 (benzylic), 71.4 (allylic), 69.4 and 65.4
(C(CH2OR)4), 45.6 (C(CH2OR)4). ESI-MS calcd for C56H64O8(M + H+)
865.4, found 865.2.
5: 1H NMR (CDCl3) d 7.58 (d, 12H, J = 8 Hz, ar), 6.96 (d, 12H, J = 8
Hz, ar), 4.39 (s, 12H, benzylic), 3.44 (s, 12H, C(CH2OR)4), 3.40 (s, 4H,
C(CH2OR)4), 3.33 (br s, 4H, 2CH2O-), 1.50 (br s, 4H, 2CH2-); 13C NMR:
d 138.9 (ar), 137.7, 129.6 and 127.3 (ar), 72.9 (benzylic), 71.6 (-CH2O-),
69.7 (C(CH2OR)4), 45.8 (C(CH2OR)4), 26.7 (-CH2-). ESI-MS calcd for
C56H60O8I6 (M + NH4+) 1639.4, found 1639.2.
7: 1H NMR (CDCl3) d 7.98 to 8.02 (m, 24 H, ar), 7.49 to 7.55 (m, 12 H,
ar), 7.39 to 7.47 (m, 12 H, ar), 7.13 to 7.28 (m, 36 H, ar), 5.56 (dd, J = 8.0,
10.0, 6 H, H-2), 5.45 (br d, J = 2.1, 4 H, H-4), 5.20 (d, J < 1, 6 H, H-1A),
5.19 (dd, J = 3.5, 10.4, 6 H, H-2A), 5.03 (dd, J = 3.3, 10.4, 6 H, H-3A), 4.95
(d, J = 8.0, 6 H, H-1), 4.89 (br d, J = 2.1, 6 H, H-4A), 4.58 (s, 12H, H-1B),
4.56 (dd, J = 6.2, 11.0, 6H, H-6a), 4.42 (s, 12H, benzylic), 4.39 (dd, J =
7.0, 11.0, 6 H, H-6b), 4.10 (dd, J = 3.1, 10.0, 6H, H-3), 4.05 (t, J = 6.9, 6H,
H-5), 3.97 (t, J = 6.6, 6H, H-5A), 3.73 (dd, J = 6.5, 11.2, 6H, H-6aA), 3.66
(s, 12H, 3.66 (dd, J = 6.5, 11.2, 6H, H-6bA), 3.52 (s, 12H, C(CH2OR)4), 3.46
(s, 4H, C(CH2OR)4), 3.36 (s, 4H, 2CH2O-), 2.20, 2.03, 1.99, 1.86 and 1.73
(5s, 90H, CH3CO), 1.54 (br s, 4H, 2CH2-); 13C NMR: 170.2, 169.8, 169,7
and 169.3 (CH3CO), 166.0 and 165.0 (PhCO), 139.6, 133.4, 133.3, 132.1,
132.0, 131.7, 129.7, 129.4, 129.3, 128.5, 128.4, 127.1, 126.9 and 121.0 (ar),
98.9 (C-1), 93.7 (C-1A), 86.7 and 83.5 (acetylenic), 73.7 (C-3), 72.8
(benzylic), 71.3 (-CH2O-), 70.9 (C-5), 70.0 (C-2), 69.7 and 69.6
(C(CH2OR)4), 67.6 (C-4A), 66.9 (C-3A), 66.5 (C-2A), 66.4 (C-5A), 65.1 (C-4),
61.7 (C-6), 61.2 (C-6A), 56.8 (C-1B), 45.6 (C(CH2O)4), 26.4 (-CH2-), 20.7,
20.5, 20.4 and 20.3 (CH3CO). MALDI-TOF-MS calcd for C290H308O116 (M
+ Na+) 5668.8, found 5668.5.
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8: 1H NMR (D2O) d 7.34 (br s, 12H, ar), 7.06 (br s, 12H, ar), 5.21 (br s,
6H, H-1A), 4.82 (br s, 12H, PhCH2 or H-1B), 4.63 (br s, 12H, PhCH2 or H-
1B), 4.58 (br s, 6H, H-1), 4.23 (br s, 12H, H-5 and H-5A), 4.06 (br s, 6H, H-
4A), 4.02 (br d, J = 10.8, 6H, H-3A), 3.94 (dd, J = 2.9, 10.1, 4H, H-2A),
3.63–3.91 (m, 42H, H-2, H-3, H-4, H-6 and H-6A); 3.42 (m, 20H,
C(CH2OR)4, –CH2O-), 1.52 (br s, 4H, –CH2-); 13C NMR: d 138.6, 131.3,
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