Homodimerization of hyaluronan and heparan sulfate derivatives by olefin metathesis reaction

Article abstract of DOI:10.1016/S0040-4039(02)02482-6

Hyaluronan and heparan sulfate disaccharides of the type β-D-glucuronic acid-(1→3)-N-acetyl-β-D-glucosamine and α-L-iduronic acid-(1→4)-N-acetyl-β-D-glucosamine, respectively, with an n-pentenyl group at the reducing end have been synthesized. Homodimeriz

Full text of DOI:10.1016/S0040-4039(02)02482-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 89–91
Homodimerization of hyaluronan and heparan sulfate derivatives
by olefin metathesis reaction
Shyam M. Rele, Suri S. Iyer and Elliot L. Chaikof*
Laboratory of Biomolecular Materials Research, Emory University School of Medicine, Atlanta, GA 30322, USA
Received 25 September 2002; revised 2 November 2002; accepted 5 November 2002
Abstract—Hyaluronan and heparan sulfate disaccharides of the type b-
D
-glucuronic acid-(1“3)-N-acetyl-b-D-glucosamine and
a-L
-iduronic acid-(1“4)-N-acetyl-b-
D
-glucosamine, respectively, with an n-pentenyl group at the reducing end have been
synthesized. Homodimerization of these derivatives using Grubbs catalyst furnished dimerized disaccharides separated by a C8
spacer arm. © 2002 Elsevier Science Ltd. All rights reserved.
Glycosaminoglycans (GAG), such as heparan sulfate
(HS) and hyaluronan (HA), sequester and modulate the
activity of a variety of soluble, membrane-bound, and
matrix proteins and thereby regulate a number of bio-
logical processes, such as cell growth, migration and
proliferation, bacterial and viral recognition events, as
well as anticoagulation and cancer metastasis.1,2 While
the GAG biosynthetic pathway is quite complex, the
end result is a family of polysaccharides consisting of
repeating disaccharide units. Significantly, the recent
identification that smaller oligosaccharide sequences3,4
may be responsible for many of the unique biological
activities of these parent polysaccharides holds the
promise of generating relatively small molecule GAG
equivalents through a total synthesis strategy.5 For
example,1,3,4 discrete HS oligosaccharides induce basic
fibroblast growth factor (bFGF) dimerization and acti-
vation and, likewise, potentiate the activity of
antithrombin III (AT-III), a protease inhibitor of blood
coagulation. Similarly, HA derived saccharide
sequences have been implicated in embryonic develop-
ment, cancer, as well as angiogenesis and wound heal-
ing.2 With this in mind, we have postulated that the
tethering of structurally defined HA and HS based
fragments by a neutral molecular spacer might provide
a strategy for the synthesis of related molecular ago-
nists and/or antagonists of GAG dependent events.
Olefin metathesis6,7 has become an attractive method
for the construction of complex molecules due to the
high reactivity, operational simplicity and remarkable
functional group tolerance of Grubbs ruthenium cata-
lyst.7 For example, as applied to carbohydrate chem-
istry, olefin cross metathesis has been recently used by
Roy et al. to synthesize sialoside monosaccharide
derivatives.6a In this report, we utilize olefin metathesis
Scheme 1. Reagents and conditions: (a) TMSOTf, CH2Cl2, 0–25°C, 3.5 h, 80%; (b) CH3COSH, 25°C, 24 h, 70%.
Keywords: heparan sulfate; hyaluronan; disaccharides; alkene cross metathesis.
* Corresponding author. Fax: 1-404-727-3660; e-mail: echaiko@emory.edu
0040-4039/03/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02482-6

90
S. M. Rele et al. / Tetrahedron Letters 44 (2003) 89–91
as a convenient approach for the homodimerization
of more complex HA and HS derived disaccharides
synthesized with a pendant pentenyl group. In this
manner, a series of fully protected higher ordered
dimerized (Gemini ) disaccharides separated by a C8
spacer arm were generated.
via the n-pentenyl spacer arm. Subsequent reduction
of the azido functionality to acetamido using
CH3COSH afforded the disaccharide 89 in 39% yield
over three steps.
With the requisite disaccharides in hand, we pro-
ceeded with the cross metathesis reaction. The pro-
The synthesis of the desired hyaluronan disaccharide
containing a pentenyl group at the anomeric position
was accomplished in 65% yield over two steps start-
tected disaccharides 4 and 8 were refluxed in
dichloromethane with 5–10 mol% of Grubbs catalyst
for 24 h under an argon atmosphere. After quenching
with ethyl vinyl ether, the products were flash chro-
matographed which provided the resulting divalent
disaccharides 9 and 10 in 90% yield, respectively, as
shown in Scheme 3.
ing from the b-
Briefly, condensation of the b-
1 with the b- -glucuronic acid donor 2 in the pres-
D
-glucuronic acceptor 18 (Scheme 1).
D
-glucosamine acceptor
D
ence of a catalytic amount of TMSOTf afforded the
fully protected disaccharide 3 in 80% yield. The azido
group was reduced to the N-acetyl group using thiol-
acetic acid and the product 49 was obtained in 70%
yield.
The E:Z ratio could not be determined in the 1H
NMR spectrum due to overlapping resonance signals
around 5.0 ppm. However, HMQC experiments of 9
revealed two peaks at 128.0 and 126.5 ppm that cor-
responded to the E- and Z-isomers, respectively.11
The area under the peaks was calculated and corre-
sponded to an E:Z ratio of 4:1. A similar result was
obtained for the divalent disaccharide 10.
The synthesis of the fully protected heparan sulfate
disaccharide is shown in Scheme 2. The iduronic acid
donor 5 was synthesized in 14 steps starting from 1,3
D
-glucuronolactone.3b,10 Condensation of 5 with the
b- -glucosamine acceptor 6 in the presence of 5
D
mol% of TMSOTf resulted in the formation of the
fully protected disaccharide 7 in 55% yield. Further
transformation included replacement of the levulinyl
group by a p-methoxybenzyl moiety.3a This substitu-
tion was critical since delevulinylation prevents fur-
ther elongation of the disaccharide through the
levulinyl end and, as a consequence, provides selective
control over the process of olefenic homodimerization
In conclusion, the synthesis of disaccharides of
hyaluronan and heparan sulfate possessing a n-pen-
tenyl group at the reducing glucosamine end has been
accomplished. Olefin metathesis of these protected
GAG (HA and HS) derivatives has been performed
for the first time using Grubbs catalyst under mild
conditions to yield novel and complex divalent disac-
charides linked by a C8 spacer.
Scheme 2. Reagents and conditions: (a) TMSOTf, CH2Cl2, −20 to 25°C, 3.5 h, 55%; (b) i. NH2NH2·HOAc, pyridine, 15 min, 90%,
ii. CH3OC6H4CH2OC(ꢀNH)CCl3, TfOH, CH2Cl2, 2 h, 60%, iii. CH3COSH, 25°C, 24 h, 70%.
Scheme 3.

S. M. Rele et al. / Tetrahedron Letters 44 (2003) 89–91
91
Acknowledgements
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This work was funded by grants from the NIH. The
authors wish to thank Dr. Shaoxiong Wu of the Emory
NMR center for help with the HMQC experiments.
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5.19–5.06 (m, 4H), 4.97–4.91 (vt, 2H, J=8.1 Hz), 4.80 (d,
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J=10.3 Hz, J=4.5 Hz), 3.85–3.63 (m, 10H), 3.58 (s, 6H),
3.56–3.47(m, 4H), 3.07 (q, 2H, J=8.7 Hz), 2.04 (m, 2H),
1.98 (s, 18 H), 1.95–19.3 (s, br, 6H), 1.80 (m, 2H) 1.75 (m,
2H), 1.60 (m, 2H), 1.23 (m, 2H); HRMS (FAB) calcd for
C64H82O30N2 (M++Na) 1381.00, found 1381.50.
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