A short route to L-iduronic acid building blocks for the syntheses of heparin-like disaccharides

Article abstract of DOI:10.1016/j.tetlet.2003.08.092

The effective preparation of differentially protected L-iduronic acid derivatives, as building blocks for the synthesis of heparin-like oligosaccharides, is described in less than nine steps starting from readily available 1,2-O-isopropylidene-6,3-D-glucu

Full text of DOI:10.1016/j.tetlet.2003.08.092

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7767–7770
A short route to
L
-iduronic acid building blocks for the syntheses
of heparin-like disaccharides
Weijun Ke, Dennis M. Whitfield,* Manjinder Gill, Suzon Larocque and Siu-Hong Yu
Institute for Biological Sciences, National Research Council, Ottawa, ON, Canada K1A 0R6
Received 17 July 2003; revised 20 August 2003; accepted 21 August 2003
Abstract—The effective preparation of differentially protected
heparin-like oligosaccharides, is described in less than nine steps starting from readily available 1,2-O-isopropylidene-6,3-
curonolactone. The pivaloyl group was used as a permanent protecting group of hydroxyl groups. Two heparin-like disaccharides
with different sulfation pattern have been prepared by using these -iduronic acid building blocks.
Crown Copyright © 2003 Published by Elsevier Ltd. All rights reserved.
L
-iduronic acid derivatives, as building blocks for the synthesis of
D
-glu-
L
Glycosaminoglycans (GAGs) and their binding proteins
are integral to many physiological processes including
inflammation, cell proliferation, and blood coagula-
tion.1,2 At present there is lack of specific inhibitors
suitable for the intervention in GAG-mediated diseases.
Cardiotoxins (CTXs) from cobra venom are highly
basic, slightly curved, all b-sheet polypeptides capable
of inducing general cytotoxic effect on many cell
types.3,4 They cause severe tissue necrosis and local
gangrene in humans by an unknown mechanism.5 In
vitro GAG-binding studies of cardiotoxins (CTXs)
established CTXs as specific GAG-binding toxins
toward different types of GAG chains.6 The molecular
basis of CTX-GAG interactions remain elusive.
than that of glucuronic acid derivatives since neither
-iduronic acid nor -idose are available as starting
materials. The preparation of -ido synthons is a key
point in glycoaminoglycan oligosaccharide synthesis
and there is a constant need for their efficient prepara-
tion.8 In this paper, we report a short synthetic route
for preparing L-iduronic acid building blocks by which
two disaccharides with different sulfation patterns were
prepared.
L
L
L
After examining the literature and some experimenta-
tion, it was decided that the best route to
acid building blocks was to start from the commercially
available crystalline 1,2-O-isopropylidene-6,3- -glu-
L-iduronic
D
curonolactone 1.9–11 The stereochemistry at C-5 was
inverted to the O-5 pivaloate 2 via triflate intermedi-
ate.9 Then taking advantage of an O-5 to O-3 migra-
tion side reaction noted in Ref. 9, we were able to
prepare multigram quantities of methyl ester 3 under
the catalysis of the organic base triethylamine at 0°C.
We have also shown that acetate and benzoate groups
undergo this migration, but for our preparative pur-
poses the pivaloate is the most useful. Hydrolysis of the
isopropylidene acetal 3 was then performed in TFA–
We have recently initiated a collaborative project
directed to understand at the molecular level the bind-
ing specificity of GAGs to the CTXs in order to be able
to identify specific inhibitors. To perform structure–
activity relationship studies, we decided to synthesize a
variety of monosaccharide building blocks, which could
be coupled to form oligosaccharides that would serve to
determine the key structural features necessary for
binding to the cobra toxin proteins (CTXs).
H2O 9:1 to afford the
L
-iduronic acid derivative 4
according to Sinay¨ et al.12 The key intermediate 513 was
then obtained by regio- and stereoselective silylation
with dimethylthexylsilyl chloride (TDSCl) and imida-
zole, (CH2Cl2, −20°C) to afford almost exclusively the
b-silyl glycoside in 35% yield from 3 (Scheme 1).14
L
-Iduronic acid is a typical component of glycosamino-
glycans (GAGs), where it plays an important role in
various biological processes.7 The preparation of
L
-
iduronic acid derivatives is much more complicated
1
Analysis of the H NMR (chloroform-d) spectrum of 5
indicate a preferred 1C4 conformation for this com-
pound (J4,5=1.6 Hz (4C1: J4,5 >4.2 Hz)10a and J1,2=1.0
Hz). This route is significantly shorter and higher yield-
Keywords: glycosylations; carbohydrates; heparin; disaccharides.
* Corresponding author. Tel.: +1-613-993-5265; fax: +1-613-952-9092;
e-mail: dennis.whitfield@nrc-cnrc.gc.ca
0040-4039/$ - see front matter Crown Copyright © 2003 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.08.092

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W. Ke et al. / Tetrahedron Letters 44 (2003) 7767–7770
was very slow and did not allow for successful monoac-
ylation. However, we developed the use of catalytic
scandium trifluoromethanesulfonate, Sc(OTf)3, that
allowed for acylation to give a mixture of O-4, O-2
monoacylates and diacylate.15 For example, pivaloyla-
tion of the key intermediate 5 with 1.1 equiv. of pival-
oic anhydride at 10°C for 1.5 h gave a mixture of O-4
pivaloate 6, O-2 pivaloate 8, O-4 and O-2 dipivaloate 9
and traces of unreacted diol 5. Pure pivaloates 6, 8 and
9 could easily be separated from the mixture on an
MPLC chromatography column eluting with
Hex:EtOAc 10:1 (6:8:9=9:6:5). The ratio of 6, 8 and 9
could be varied by the reaction temperature, time and
even solvent condition. Acetylation of 6 under the
catalysis of Sc(OTf)3 at room temperature overnight
gave a mixture of a anomer and b anomer of the
corresponding O-2 acetylated silyl glycoside 7.16 Silica
gel chromatography afforded pure a-anomer and b-
anomer (a:b=5:8). Glycoside 7 was then deprotected
with CH3COOH and Bu4NF (THF, 10–15°C) to give
the intermediate 10. Standard protecting group manip-
ulation allowed for the formation of the trichloroace-
timidate 1217 in 55% yield by treatment of intermediate
10 with CCl3CN under the catalysis of DBU at 0°C.
The O-2 acetylated donor 12 allows for the preparation
of O-2-sulfated L-iduronic acid containing oligosaccha-
1
rides. H NMR analysis of building block 12 revealed a
1C4 conformation as judged by the coupling constant
(J4,5=2.3 Hz and J1,2 <1.0 Hz). Interestingly, a NOESY
experiment showed that donor 12 has an a configura-
tion (a strong NOE between H-1 and H-2, but not to
H-5, was observed). Similarly, donor 1318 was also
obtained from the tripivaloate 9 via the intermediate
11. 1H NMR analysis of building block 13 also revealed
1
a predominant C4 conformation (J1,2=3.1 Hz, J2,3
=
4.4 Hz, J3,4=4.6 Hz, J4,5=3.5 Hz). This L-iduronic acid
building block can be used to make only N-sulfated or
N-acetylated oligosaccharides as well as for testing the
glycosylation chemistry. Other variants can easily be
constructed from 5.
To test the protecting group strategy and the glycosyla-
tion chemistry, donor 12 was coupled with acceptor 14
at −20°C under promotion by triethylsilyltrifluoro-
methanesulfonate (TESOTf) to yield disaccharide 16 in
85% isolated yield (Scheme 2). Similarly, the disaccha-
ride 19 was also obtained by coupling donor 12 with
acceptor 1519 at −15 to −12°C in a yield of 64%. The
key to the deprotection is the selective removal of the
O-acetyl groups in the presence of O-pivaloyl groups.
When treating 16 with 7.5% HCl in methanol,20 the
reaction was slow and the yield was not satisfactory due
to migration of the pivaloyl group from O-4 of
iduronic acid to O-2. To ensure that the O-acetate of
disaccharide 16 could be cleaved in the presence of the
O-pivaloate in an effective way, the organic base DBU
was tried and a methanol solution of 16 was treated at
−20°C with 0.1 M DBU to give deacetylated disaccha-
ride 17 with the yield of 55%.21 Interestingly, treatment
of disaccharide 19 with DBU under the same condition
did not lead to the deacetylated disaccharide 20 effec-
tively. Fortunately, the deacetylated disaccharide 20
Scheme 1. Reagents and conditions: (a) i. Tf2O, Py; ii.
NaOPiv, DMF; (b) Et3N, MeOH, 0°C, O/N; (c) 90% TFA, rt,
3 h; (d) imidazole, CH2Cl2, −20°C, O/N; (e) Piv2O, Sc(OTf)3,
CH2Cl2; (f) Ac2O, Sc(OTf)3, CH2Cl2, rt, O/N; (g) CH3COOH,
Bu4NF, THF, 10–15°C, 5 h; (h) CCl3CN, DBU, CH2Cl2, 0°C,
1 h.
ing than any previously reported route to similar
iduronic acid derivatives.
L-
The OH-2 and OH-4 of sugar 5 have similar reactivi-
ties, but for our purposes they must be differentially
protected. It should be noted that conventional O-
acylation with acyl chlorides or anhydrides in pyridine

W. Ke et al. / Tetrahedron Letters 44 (2003) 7767–7770
7769
L
-iduronic acid building block developed by this syn-
thetic route, and their binding properties with CTX A3
are being studied. The detailed syntheses and binding
studies will be reported in another paper. Thus, an
effective synthetic strategy was developed for preparing
heparin-like oligosaccharide chains with defined size,
sequence and charge distribution.
Acknowledgements
This research was supported by NSC Grant 89-2311-B-
007-045 and an NSC-NRC Joint Research Project
Grant (N-001). Ken Chan is thanked for obtaining the
mass spectra. Suzon Larocque is thanked for her assis-
tance in performing the NMR studies. This is NRC
paper c 42477.
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Scheme 2. Reagents and conditions: (a) TESOTf, 4 A MS,
CH2Cl2, 3 h; (b) 7.5% HCl, MeOH, 0°C, O/N; (c) SO3:NMe3,
DMF, 60°C, O/N; (d) 0.1 DBU/MeOH, −20°C, 3 h; (e) i.
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L
-iduronic acid building blocks was devel-
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oped. Two heparin-like disaccharides with different sul-
fation patterns have been prepared by using the

7770
W. Ke et al. / Tetrahedron Letters 44 (2003) 7767–7770
13. 5: Colorless syrup, [h]D=+27.2 (c 1.0 CHCl3); H NMR
(399.89 MHz; CDCl3, 25°C): l 5.22 (br t, 1H), 4.93 (d,
J=1.0 Hz, 1H), 4.32 (d, J=1.6 Hz, 1H), 3.87 (m, 1H),
3.77 (s, 3H), 3.59 (m, 1H), 3.14 (br, 1H), 1.63 (m, 1H),
1.20, 1.20 (2×s, 2×9H), 0.85 (m, 12H), 0.25, 0.19 (2×s,
2×3H); 13C NMR (50.32 MHz; CDCl3, 22.1°C): l 175.88,
168.35, 93.49, 74.64, 69.00, 68.63, 66.99, 52.24, 38.75,
33.93, 27.02, 24.98, 20.16, 19.89, 18.54, 18.34, −2.02,
−3.69; MS: m/z=435.2 [M+H]+. 1H and 13C NMR for all
other compounds were obtained under the same condi-
tions unless otherwise noted.
27.08, 26.93, 20.72; MS: m/z=401.3 [M−OTCI]+.
18. 13: White solid, [h]D=−57.3 (c 1.0 CHCl3); H NMR: l
1
1
8.81 (s, 1H), 6.51 (d, J=3.1 Hz, 1H), 5.26 (br t, 1H), 5.21
(br t, 1H), 5.07 (br t, 1H), 4.89 (d, J=3.5 Hz, 1H), 3.75
(s, 3H), 1.22, 1.21, 1.19 (3×s, 3×9H); 13C NMR: l 176.57,
176.46, 176.06, 167.19, 159.77, 94.56, 90.54, 69.13, 67.80,
67.49, 67.09, 52.62, 38.99, 38.92, 27.22; MS: m/z=443.3
[M−OTCI]+.
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16. 7: a-anomer: colorless syrup, [h]D=+38.5 (c 1.01 CHCl3);
1H NMR: l 5.12 (br t, 1H), 5.03 (d, J=1.8 Hz, 1H), 4.98
(m, 1H), 4.90 (m, 1H), 4.49 (d, J=2.5 Hz, 1H), 3.73 (s,
3H), 2.06 (s, 3H), 1.61 (m, 1H), 1.23, 1.19 (2×s, 2×9H),
0.85 (m, 12H), 0.22, 0.15 (2×s, 2×3H); 13C NMR: l
176.67, 175.28, 169.40, 167.17, 93.08, 72.86, 67.90, 66.72,
66.08, 52.21, 38.81, 33.92, 27.10, 26.95, 25.00, 20.87,
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[M−OSithexyl]+; b-anomer: colorless syrup, [h]D=−52.9
(c 1.01 CHCl3); 1H NMR: l 5. 39 (d, J=2.6 Hz 1H), 5.10
(br t, 1H), 5.07 (br t, 1H), 4.87 (d, J=3.3 Hz, 1H), 4.70
(br t, 1H), 3.75 (s, 3H), 2.03 (s, 3H), 1.63 (m, 1H), 1.21,
1.18 (2×s, 2×9H), 0.86 (m, 12H), 0.19, 0.17 (2×s, 2×3H);
13C NMR: l 176.69, 176.45, 169.33, 168.50, 92.93, 69.43,
67.46, 67.16, 67.10, 52.40, 38.81, 38.78, 33.82, 27.06,
26.97, 25.04, 20.85, 20.04, 20.01, 18.48, −2.46, −3.21; MS:
m/z=561.4 [M+H]+.
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1
23. 22: White solid. H NMR (D2O): l 5.13 (br s, 1H), 4.97
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1H), 4.04 (br t, 1H), 4.01 (br t, 1H), 3.91 (m, 2H), 3.84
(br t, 1H), 3.82 (br t, 1H), 3.74 (br t, 1H), 3.45 (s, 3H),
3.25 (dd, J=3.3, 10.1 Hz, 1H).
1
24. 23: White solid. H NMR (D2O): l 5.16 (br s, 1H), 5. 04
(d, J=3.5 Hz, 1H), 4.74 (d, J=2.7 Hz, 1H), 4.34 (br t,
2H), 4.26 (br t, 1H), 4.05 (br t, 1H), 4.00 (m, 2H), 3.75
(br t, 1H), 3.69 (br t, 1H), 3.44 (s, 3H), 3.30 (dd, J=3.5,
9.9 Hz, 1H).
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1
17. 12: White solid, [h]D=−49.7 (c 1.88 CHCl3); H NMR: l
8.86 (s, 1H), 6.49 (s, 1H), 5.13 (m, 2H), 4.97 (m, 1H), 4.89
(d, J=2.3 Hz, 1H), 3.74 (s, 3H), 2.07 (s, 3H), 1.23, 1.18
(2×s, 2×9H); 13C NMR: l 176.48, 176.26, 168.92, 167.33,
159.80, 94.00, 90.62, 68.12, 66.12, 52.62, 38.89, 38.85,