Chemoselective glycosylations of sterically hindered glycosyl acceptors

Article abstract of DOI:10.1016/S0040-4039(02)02334-1

Unexpected intermolecular aglycon transfer in chemoselective glycosylations between activated thioglycosyl donors and deactivated thioglycosyl acceptors could be avoided by employing a glycosyl acceptor that has a bulky anomeric dicyclohexylmethanethio group. The methodology was applied to the synthesis of a protected fragment of an oligosaccharide released from the jelly coat glycoprotein of X. leavis.

Full text of DOI:10.1016/S0040-4039(02)02334-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9429–9431
Chemoselective glycosylations of sterically hindered glycosyl
acceptors
Richard Geurtsen and Geert-Jan Boons*
Complex Carbohydrate Research Center, University of Georgia, 220 Riverbend Road, Athens, GA 30602-4712, USA
Received 19 July 2002; accepted 11 October 2002
Abstract—Unexpected intermolecular aglycon transfer in chemoselective glycosylations between activated thioglycosyl donors and
deactivated thioglycosyl acceptors could be avoided by employing a glycosyl acceptor that has a bulky anomeric dicyclohexyl-
methanethio group. The methodology was applied to the synthesis of a protected fragment of an oligosaccharide released from
the jelly coat glycoprotein of X. leavis. © 2002 Elsevier Science Ltd. All rights reserved.
Carbohydrates are key to many of the events leading to
fertilization1–3 and examples include recognition of egg
by sperm,4 induction of the acrosome reaction, fusion
of sperm and egg and the formation of the fertilization
layer.
Gal glycosyl donor (1B) followed by fucosylation of the
two internal galactosides.
Herein we report the synthesis of properly protected
gallibiose donors 12 and 13 using a chemoselective
glycosylation strategy, whereby the anomeric reactivity
of thioglycosyl donors and acceptors is controlled by a
combination of properly selected protecting groups and
the use of either an anomeric thioethyl or bulky dicy-
clohexylmethanethio group.
The South African clawed toad, X. laevis, is a useful
model animal in which to study phenomena associated
with fertilization and early development. In this organ-
ism, the interaction of the novel carbohydrate-binding
protein XL35 with the O-glycans of a jelly coat protein
(JCP) enrobing the egg, is considered key in the preven-
tion of polyspermy.5–7 O-Glycans released upon mild
base treatment of JCP samples derived from the
oocytes of six individual X. laevis toads have been
found by NMR spectroscopy to comprise twenty three
different structural motifs, eleven of which are unique.8
Heptasaccharide 1 is one of the most complex oligosac-
charides isolated from the jelly coat glycoprotein and is
composed of an unusual dimeric H-antigen substituted
by an a-galactoside (Fig. 1). The oligosaccharide is
attached to the protein through an O-linked N-acetyl
galactosamine moiety, which is further extended by a
N-acetyl glucosamine residue.
An attractive feature of thioglycosyl donors and accep-
tors is that they can be assembled into oligosaccharides
by a chemoselective glycosylation strategy.10,11 This
approach is based on the observation that protecting
group patterns can control anomeric reactivities of
thioglycosides. Thus, it is possible that a thioglycosyl
donor can be coupled with a thioglycosyl acceptor of
lower anomeric reactivity to give
a well-defined
product. In a subsequent glycosylation, the resulting
Previously, we reported a polymer-supported synthesis
of protected tetrasaccharide 1A using a novel two direc-
tional glycosylation strategy.9 It was expected that the
synthesis of 1 could be completed by extension of this
tetrasaccharide by a properly protected Gal-a-(1“4)-
Figure 1. A heptasaccharide (1) isolated from the jelly coat
glycoprotein of X. laevis.
* Corresponding author. Tel.: +1-706-542-9161; fax: +1-706-542-4412;
e-mail: gjboons@ccrc.uga.edu
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02334-1

9430
R. Geurtsen, G.-J. Boons / Tetrahedron Letters 43 (2002) 9429–9431
In order to reduce the formation of side products,
glycosyl acceptor 7 was used in the coupling with 2.
The rationale of using this acceptor was that it has a
benzyl ether at C-3, which should increase glycosyl
accepting properties. Unfortunately, coupling of 2
with 7, using Br2/AgOTf as the activator gave disac-
charide 8 in low yield and the product was contami-
nated with trisaccharide and trehalose.
product can act as a glycosyl donor and react with a
thioglycosyl acceptor that has even a lower reactivity.
This process can be repeated to give rather complex
structures without the need for intermediate protect-
ing group chemistry. Recently, the usefulness of this
approach was expanded by the creation of a database
that makes it possible to predict the relative reactivi-
ties of a large number of p-methylphenyl thioglyco-
sides.12 In general, an electron-donating ether sub-
stituent at C-2 activates and an electron-withdrawing
ester functionality deactivates the reactivity of an
anomeric-leaving group.
Next, an orthogonal glycosylation between fluoride 9
and acceptor 7 was investigated. It is well known that
an anomeric fluoride can be activated in the presence
of a thioglycosyl acceptor to give a product that can
immediately be used in a subsequent glycosylation.20
It was expected that coupling of reactive ethyl thio-
glycoside 2, which has an activating benzyl ether at
C-2, with ethyl thioglycoside 3, which has a deactivat-
ing levulinoyl ester (Lev) at C-2,13–15 would give di-
saccharide 4 in high yield (Scheme 1). This disaccha-
ride could then immediately be used as a glycosyl
donor and furthermore the Lev ester at C-2 would
allow incorporation of a fucoside. Surprisingly, cou-
pling of 2 with 3 in the presence of N-iodosuccini-
mate/trimethylsilyl triflate (NIS/TMSOTf) as the
promoter16 gave apart from the expected disaccharide
4 (yield: 35%) substantial quantities of trehalose and
a trisaccharide. The use of the mild activator iodo-
nium dicollidine perchlorate (IDCP)17 gave a some-
what higher yield but the product was still
contaminated with side products and furthermore 4
was formed as an inseparable mixture of anomers.
The best results were obtained by in situ conversion
of thioglycoside 2 into the corresponding bromide fol-
lowed by a silver triflate promoted glycosylation (Br2,
AgOTf, TMU, toluene)18,19 with glycosyl acceptor 5,
which has a more deactivating benzoyl group at C-2,
to give disaccharide 6 as only the a-anomer in a yield
of 48%. However, this glycosylation gave also sub-
stantial quantities of trehalose and trisaccharide. Fur-
thermore, mixtures of anomers were obtained when
others solvents than toluene were used. In this
respect, it is important to note that use of toluene is
not compatible with IDCP due to insolubility of the
activator.
Unfortunately,
a
Zr(Cp)2Cl2/AgOTf
mediated
coupling21 of 9 with 7 gave a mixture of products.
Interestingly, ethyl 2,3,4,6-tetra-O-benzylthiogalac-
toside (2) was isolated as a main product. Thus, it
appears that after activation of 9, the resulting
oxacarbenium ion reacts with the anomeric ethyl thio
group of 7 rather than with its alcohol.22 In this par-
ticular case, this mode of reaction is prevalent
because of the low reactivity of the C-4 hydroxyl of
7. The activation of the anomeric center of 7 may
subsequently lead to self-condensation. Probably, the
trisaccharides observed in the glycosylations described
above result from further glycosylation of the
hydroxyl of the self-condensed disaccharide. Alterna-
tively, it can not be excluded that trisaccharide for-
mation arises from activation of disaccharides 4, 6
and 8 by a sugar oxacarbenium ion followed by con-
densation with an acceptor. It is, however, unlikely
that acceptors 3, 5 and 7 or disaccharides 4, 6 and 8
are directly activated by the promoter because of the
much higher reactivity of glycosyl donor 2.
It was anticipated that a glycosyl acceptor that has
an anomeric dicyclohexylmethanethio group would be
less prone to be activated by a sugar oxacarbenium
ion.23,24 In this case, the bulky aglycon of the glycosyl
acceptor would block attack by an oxacarbenium ion.
Indeed, AgOTf/Br2 mediated coupling of ethyl
thiogalactoside 2 with dicyclohexylmethyl thiogalac-
toside 10 gave disaccharide 1225 in a good yield of
73% as only the a-anomer. As expected, coupling of
2 with acceptor 11, which has a Lev ester at C-2,
gave 13 in a good yield of 71% (Scheme 2). Finally,
to demonstrate that 12 is an appropriate glycosyl
donor, it was coupled with 14 using the promoter
NIS/TMSOTf and trisaccharide 1526 was isolated in
an excellent yield of 91%.
In conclusion, chemoselective glycosylations of ethyl
thioglycosyl acceptors that have hydroxyls of low
reactivity are prone to self-condensation. Employing a
glycosyl acceptor that has
a bulky dicyclohexyl-
methanethio group at its anomeric center can prevent
this unwanted reaction. The new methodology was
applied to the synthesis of properly protected galli-
biose fragment 13, which will be a key building block
in the synthesis of heptasaccharide 1.
Scheme 1.

R. Geurtsen, G.-J. Boons / Tetrahedron Letters 43 (2002) 9429–9431
9431
14. Zhu, T.; Boons, G. J. Chem. Eur. J. 2001, 7, 2382–2389.
15. Zhu, T.; Boons, G. J. Tetrahedron: Asymmetry 2000, 11,
199–205.
16. Veeneman, G. H.; van Leeuwen, S. H.; van Boom, J. H.
Tetrahedron Lett. 1990, 31, 1331–1334.
17. Veeneman, G. H.; van Boom, J. H. Tetrahedron Lett.
1990, 31, 275–278.
18. Sato, S.; Mori, M.; Ito, Y.; Ogawa, T. Carbohydr. Res.
1986, 155, C6–C10.
19. Kihlberg, J. O.; Leigh, D. A.; Bundle, D. R. J. Org.
Chem. 1990, 55, 2860–2863.
20. Kanie, O.; Ito, Y.; Ogawa, T. J. Am. Chem. Soc. 1994,
116, 12073–12074.
21. Suzuki, K.; Maeta, H.; Suzuki, T.; Matsumoto, T. Tetra-
hedron Lett. 1989, 30, 6879–6882.
22. Yu, H.; Yu, B.; Wu, X. Y.; Hui, Y. Z.; Han, X. W. J.
Chem. Soc., Perkin Trans. 1 2000, 9, 1445–1453.
23. Geurtsen, R.; Coˆte´, F.; Hahn, M. G.; Boons, G. J. J.
Org. Chem. 1999, 64, 7828–7835.
Scheme 2. Reagents and conditions: (i) Br2, AgOTf, TMU, 4
,
,
A MS, toluene; (ii) NIS, TMSOTf toluene, 4 A MS.
24. Geurtsen, R.; Holmes, C. S.; Boons, G. J. J. Org. Chem.
1997, 62, 8145–8154.
25. All new compounds gave satisfactory NMR spectro-
scopic, mass spectroscopic and elemental analytical data.
Selected data for 12: [h]2D4 +115.30 (c 1); MALDI-TOF
MS: m/z=1231.2 [M+Na], 1247.5 [M+K]; 13C NMR
(CDCl3, 75 MHz): l 166.31, 165.84, 165.79 (3 C6H5CO),
138.82, 138.73, 138.50, 138.12, 133.13, 133.02, 129.89,
129.81, 129.75, 128.38–127.39 (4 C6H5CH2, 3 C6H5CO),
129.53, 129.48 (2 C6H5CO), 100.80 (C-1%), 85.66 (C-1),
79.06 (C-3%), 76.50 (C-4), 75.89 (C-2%/C-4%), 74.85 (C-2%/C-
4%), 71.53 (C-3), 69.82, 69.41, 68.92 (C-2, C-5, C-5%),
74.94, 74.22, 72.86, 72.56 (4 C6H5CH2), 67.62 (C-6%),
62.32 (C-6), 60.07 (SCH(C6H11)2), 41.26, 39.52, 32.05,
31.57, 30.04, 29.38, 26.46, 26.46, 26.32, 26.21
Acknowledgements
This work was supported by the NIH Resource Center
for Biomedical Complex Carbohydrates (P41-
RR05351).
References
1. Mengerink, K. J.; Vacquier, V. D. Glycobiology 2001, 11,
37R–43R.
2. Takasaki, S.; Mori, E.; Mori, T. Biochim. Biophys. Acta
1
(SCH(C6H11)2); H NMR (CDCl3, 300 MHz): l 7.99 (d,
Gen. Subj. 1999, 1473, 206–215.
2H, 1 C6H5CO, J=7.2 Hz), 7.99 (d, 4H, 2 C6H5CO,
J=7.8 Hz), 7.62–7.10 (m, 29H, 4 C6H5CH2, 3 C6H5CO),
5.77 (d, 1H, H-1), 5.52 (dd, 1H, H-3, J3,4=2.4 Hz), 4.96
3. Dell, A.; Morris, H. R.; Easton, R. L.; Patankar, M.;
Clark, G. F. Biochim Biophys. Acta Gen. Subj. 1999,
1473, 196–205.
(d, 1H, H-1%, J1%,2%=3.3 Hz), 4.90–4.62 (m, 8H,
3
4. Vo, L. H.; Hedrick, J. L. Biol. Reproduct. 2000, 62,
766–774.
5. Roberson, M. M.; Barondes, S. H. J. Biol. Chem. 1982,
257, 7520–7524.
6. Lee, J. K.; Buckhaults, P.; Wilkes, C.; Teilhet, M.; King,
M. L.; Moremen, K. W.; Pierce, M. Glycobiology 1997, 7,
367–372.
7. Nishihara, T.; Wyrick, R. E.; Working, P. K.; Chen, Y.
H.; Hedrick, J. L. Biochemistry 1986, 25, 6013–6020.
8. Strecker, G.; Wieruszeski, J. M.; Plancke, Y.; Boilly, B.
Glycobiology 1995, 5, 137–146.
C6H5CH2, H-6a, H-5, H-6b), 4.50 (d, 1H, H-4), 4.46 (d,
1H, C6H5CH2, Jgem=−11.1 Hz), (dd, 1H, H-5%, J=8.6,
J=5.3 Hz), 4.17 (dd, 1H, H-3%, J2%,3%=10.2, J3%,4%=2.4 Hz),
4.10 (s br, 1H, H-4%), 4.07 (dd, 1H, H-2%), 3.97 (s, 2H,
C6H5CH2), 3.37 (t, 1H, H-6%a, J=8.7 Hz), 2.88 (dd, 1H,
H-6%b, J5%,6%b=5.1 Hz), 2.41 (t, 1H, SCH(C6H11)2, J=5.4
Hz), 2.0–0.7 (m, 22H, SCH(C6H11)2).
26. Selected data for 15: MALDI-TOF MS m/z=1483 [M+
1
Na]; H NMR (CDCl3, 600 MHz): l 7.01–8.00 (m, 50H,
7 C6H5CH2, 3 C6H5CO, aromatic), 5.80 (t, 1H, J2%,3%=7.7
Hz, H-2%), 5.20 (dd, 1H, J3%,4%=2.8 Hz, H-3%), 4.91 (d, 1H,
9. Geurtsen, R.; Boons, G. J. Eur. J. Org. Chem. 2002,
1473–1477.
J1¦,2¦=3.4 Hz, H-1%), 4.87–4.69 (m, 11H, 5 C6H5CH2),
4.77(q, 2H, H-6a%, H-6b%), 4.63 (s, 2H, C6H5CH2), 4.50 (s,
2H, C6H5CH2), 4.47 (s, 1H, H-1), 4.39 (d, 1H, J4%,5%=2.56
Hz, H-4%), 4.34 (m, 1H, H-5¦), 4.24 (d, 2H, J=10.2 Hz,
H-6a,b), 4.19 (dd, 1H, J3¦,4¦=2.8 Hz, H-3¦), 4.10 (s, 1H,
H-4¦), 4.04 (dd, 1H, J2¦,3¦=10.6 Hz, H-2¦), 4.02 (m, 1H,
J=7.9 Hz, H-5%), 3.76 (dd, 1H, J=9.1 Hz, J=2.8 Hz,
H-3), 3.71(m, 1H, H-5), 3.66 (t, 1H, J2,3=9.1 Hz, H-2),
3.33 (m, 1H, J=8.8 Hz, H-6a¦), 2.82 (q, 1H, H-6b¦).
10. Boons, G. J. Tetrahedron 1996, 52, 1095–1121.
11. Garegg, P. J. Adv. Carbohydr. Chem. Biochem. 1997, 52,
179–205.
12. Zhang, Z.; Ollmann, I. R.; Ye, X. S.; Wischnat, R.;
Baasov, T.; Wong, C. H. J. Am. Chem. Soc. 1999, 121,
734–753.
13. Hassner, A.; Strand, G.; Rubinstein, M. J. Am. Chem.
Soc. 1975, 97, 1614–1615.