Novel efficient routes to heparin monosaccharides and disaccharides achieved via regio- and stereoselective glycosidation

Article abstract of DOI:10.1021/ol036390m

A new methodology for the synthesis of heparin building blocks has been developed. We describe novel efficient routes to both L-iduronic acid and D-glucuronic acid acceptors. Glycosylation with thioglycosides donors gave corresponding disaccharides in a regio- and stereoselective fashion. An improved approach to synthesizing azido-glucose thioglycoside donor to render azido-sugar from mannose via nucleophilic substitution is described.

Full text of DOI:10.1021/ol036390m

ORGANIC
LETTERS
2004
Vol. 6, No. 5
723-726
Novel Efficient Routes to Heparin
Monosaccharides and Disaccharides
Achieved via Regio- and Stereoselective
Glycosidation
Henry N. Yu, Jun-ichi Furukawa, Tsuyoshi Ikeda, and Chi-Huey Wong*
Department of Chemistry and Skaggs Institute for Chemical Biology, The Scripps
Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
wong@scripps.edu
Received December 8, 2003
ABSTRACT
A new methodology for the synthesis of heparin building blocks has been developed. We describe novel efficient routes to both L-iduronic
acid and D-glucuronic acid acceptors. Glycosylation with thioglycosides donors gave corresponding disaccharides in a regio- and stereoselective
fashion. An improved approach to synthesizing azido-glucose thioglycoside donor to render azido-sugar from mannose via nucleophilic
substitution is described.
Heparin belongs to the family of glycosaminoglycans
(GAGs). It is a linear polymer consisting of repeating units
of 1 f 4-linked pyranosyluronic acid and 2-amino-2-
deoxyglucopyranose (glucosamine) residues (Figure 1).1 The
uronic acid residues consist of either L-idopyranosyluronic
acid (L-iduronic acid) or D-glucopyanosyluronic acid (D-
glucuronic acid). As it is the most acidic biopolymer in
nature, heparin interacts with a large number of proteins,
which regulate many important biological activities such as
antithrombotic, antiatherosclerotic, artherogenic, antiinflam-
matory, angiogenic, antiangiogenic, and antiviral activities,
cancer, and Alzheimer’s disease.2 With the exception of the
antithrombin III-heparin interaction,3 in which the minimal
sequence of heparin required for binding is a pentasaccharide,
most structure-activity relationships (SARs) between pro-
teins and heparin are poorly understood. This is mainly due
to the lack of pure defined sequences of heparin available.
Since the first total synthesis of this heparin pentasaccharide
was reported,4 numerous synthetic methodologies have been
disclosed for the preparation of heparin-type oligosaccha-
(1) (a) Casu, B. Carbohydr. Chem. Biochem. 1985, 43, 51-134. (b)
Comper, W. D. Heparin and Related Polysaccharides; Gordan and Breach,
1981; Vol. 7.
(2) (a) Capila, I.; Linhardt, R. J. Angew. Chem., Int. Ed. 2002, 41, 390-
421. (b) Conrad, H. E. Heparin Binding Proteins, Academic Press: New
York, 1998.
Figure 1. Schematic view of heparin.
10.1021/ol036390m CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/11/2004

resulting 1C4 conformation, locked by a 1,2-acetal, generally
gave R-selectivity in glycosylation reactions. Unfortunately,
iduronic acid 2 isomerizes to pyranose and furanose forms
during installation of the 1,2-cyclic acetal systems.10 Thus,
our first goal was to develop a new effective strategy for
L-iduronic acid synthesis that would yield the pyranose form
exclusively.
Inexpensive D-glucuronolactone 4 was converted to methyl
tetra-O-acetyl-R-D-glucopyranuronate 5a according to a
reported “two-step in one-pot” procedure11 (Scheme 1). Free
Scheme 1. Preparation of Iduronic Acid 10
Figure 2. Common method for the synthesis of iduronic acid and
an improved scheme developed by Seeberger and co-workers.
rides.5 However, an easy and truly practical route to heparin
oligosaccharides is still elusive. Good synthetic methods for
the construction of heparin-based oligosaccharides, along
with knowledge of the specific structural requirements for
the various actions of heparin, could allow “tailor-made”
sequences of the heparin template to be prepared for specific
therapeutic applications.6 In an effort to employ our effective
programmable one-pot synthetic strategies7 for the prepara-
tion of heparin and its analogues, we now describe short and
straightforward routes to various synthetically relevant mono-
and disaccharide building blocks.
Of the three saccharide residues, L-iduronic acid is the most
challenging to synthesize.8 Initially, we evaluated the most
common protocol4a for its preparation, which started from
diacetone glucose 1 (Figure 2). After nine synthetic steps, it
yielded the hemiacetal mixture 2. Subsequent acetylation of
2 gave four isomers consisting of an R/â mixture of pyranose
and furanose forms, which required difficult chromatography
to separate the desired pyranose product. A recent report9
provided a shorter route to the acceptor 3a, where the
radical bromination12 at C-5 using NBS and UV light in
refluxing CCl4 gave bromide 6a in good yield. Subsequent
isomerization at C-5 by free radical reduction13 of 6a with
tributyltin hydride gave a 1:3 ratio of D-gluco and L-ido (7a)
isomers. Attempts to invert the C-5 carbon center with
NaBH4 or catalytic hydrogenation resulted in decomposition
of the starting material. To investigate the isomerization
outcome for the â anomer, compound 6b was prepared.
Treatment of 6b with tributyltin hydride in refluxing benzene
gave a slightly higher ratio of D-gluco and L-ido isomers
(1:2, 80%). Iduronic acid derivatives (7a and 7b) were then
converted to glycosyl bromide 8. Without purification,
bromide 8 was subjected to treatment with sodium borohy-
(3) (a) Petitou, M.; Duchaussoy, P.; Driguez, P.-A.; G. Jaurand, J.;
Herault, J.-P.; Lormeau, J.-C.; van Boeckel, C. A. A.; Herbert, J.-M. Angew.
Chem., Int. Ed. 1998, 37, 3009-3014. (b) Petitou, M.; Herault, J.-P.; Bernat,
A.; Driguez, P.-A.; Duchaussoy, P.; Lormeau, J.-C.; Herbert, J.-M. Nature
1999, 398, 417-422.
(4) (a) Jacquient, J.-C.; Petitou, M.; Duchaussoy, P.; Lederman, I.; Choay,
J.; Torri, G.; Sinay¨, P. Carbohydr. Res. 1984, 130, 221-241. (b) Sinay¨, P.;
Jacquient, J.-C.; Petitou, M.; Duchaussoy, P.; Lederman, I.; Choay, J.; Torri,
G. Carbohydr. Res. 1984, 132, C5-C9.
(5) (a) Dreef-Tromp, C. M.; Basten, J. E. M.; Broekhoven, M. A.; van
Dinther, T. G.; Petitou, M. C.; van Boeckel, A. A. Bioorg. Med. Chem.
Lett. 1998, 8, 2081-2086. (b) Petitou, M.; Duchaussoy, P.; Driguez, P.-
A.; Jaurand, G.; He´rault, J.-P.; Lormeau, J.-C.; van Boeckel, C. A. A.;
Herbert, J.-M. Angew. Chem., Int. Ed. 1998, 37, 3009-3014. (c) Kovensky,
J.; Duchaussoy, P.; Bono, F.; Salmivirta, M.; Sizun, P.; Herbert, J.-M.;
Petitou, M.; Sinay¨, P. Bioorg. Med. Chem. 1999, 7, 1567-1580. (d) van
Boeckel, C. A. A.; Petitou, M. Angew. Chem., Int. Ed. Engl. 1993, 32,
1671-1818. (e) Haller, M. F.; Boons, G.-J. Eur. J. Org. Chem. 2002, 2033-
2038. (f) de Paz, J.-L.; Ojeda, R.; Erichardt, N.; Ma´rtin-lomas, M. Eur. J.
Org. Chem. 2003, 68, 3308-3324. (g) Kuberan, B.; Lech, M. Z.; Beeler,
D. L.; Wu, Z. L. L.; Rosenberg, R. D. Nat. Biotechnol. 2003, 21, 1343-
1346.
(6) Lever, R.; Page, C. R. Nat. ReV. Drug DiscoV. 2002, 1, 140-148.
(7) (a) Zhang, Z.; Ollmann, R. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.;
Wong, C.-H. J. Am. Chem. Soc. 1999, 121, 734-753. (b) Mong, T. K.-K.;
Lee, H.-K.; Duro´n, S. G.; Wong, C.-H. PNAS 2003, 100, 797-802.
(8) Pellisser, H. Org. Prep. Proced. Int. 2002, 34, 441-465 and
references therein.
(10) As we finished this manuscript, two methods were reported as notes
to circumvent the isomerization problem: (a) Lohman, G. J. S.; Hunt, D.
K.; Ho¨germeier, J. A.; Seeberger, P. H. J. Org. Chem. 2003, 68, 7559-
7561. (b) Lohman, G. J. S.; Seeberger, P. H. J. Org. Chem. 2003, 68, 7541-
7543.
(11) Bollenback, G. N.; Long, J. W.; Benjamin, D. G.; Lindquist, J. A.
J. Am. Chem. Soc. 1955, 77, 3310-3315.
(12) Ferrier, R. J.; Furneaux, R. H. J. Chem. Soc., Perkin Trans. 1 1980,
1535-1539.
(9) Orgueira, H. A.; Bartolozzi, A.; Schell, P.; Litjens, R. E.J. N.;
Palmacci, E. R.; Seeberger, P. H. Chem. Eur. J. 2003, 9, 140-169.
(13) (a) Chiba, T.; Sinay, P. Carbohydr. Res. 1986, 151, 379-389. (b)
Medakovic´, D. Carbohydr. Res. 1994, 253, 299-300.
724
Org. Lett., Vol. 6, No. 5, 2004

dride in acetonitrile to give the 1,2-ethylidene acetal 9
smoothly in 70% yield. Similar reaction conditions were first
developed by Betaneli et al. for the preparation of 1,2-O-
ethylidene derivatives of other common pyranoses,14 but such
conditions have not been widely adopted.15 It is of interest
to note that while (R)- and (S)-mixtures are reported in most
monosaccharides, only the (S)-isomer was isolated here,
where the (S)-configuration was unambiguously assigned by
NOE experiments. Compound 9 was then smoothly depro-
tected by transesterification to afford diol 10 (95%).
blocks, a similar strategy was pursued for the syntheses of
the glucuronic acid-based disaccharides. To this end, bromide
19 was treated with NaBH4 and t-Bu4NI in acetonitrile to
give acetal 20 in low yield (5-10%) (Scheme 4). Changing
Scheme 4
With the diol 10 in hand, a regio- and stereoselective
glycosidation was attempted (Scheme 2). We were gratified
Scheme 2. Regio- and Stereoselective Glycosidation with Diol
10
solvents or reaction temperatures did not improve the yield,
and 1,2-O-ethylidene formation with â-glycosyl chloride 2216
was also unsuccessful. We therefore turned our attention to
a selective oxidation procedure starting from readily available
starting material 24a and 24b (Scheme 5).14
Scheme 5. Glycosidation with Diol 25a/25b
to find that the coupling reaction went swiftly to render
desired 1 f 4 R-linked disaccharide 12 in 57% yield. To
investigate whether a 4,6-O-benzylidene acetal had any effect
on the glycosidation outcome, a different donor 13 was
examined. Again, glycosidation was regio- and stereoselec-
tive; a 7:1 ratio of disaccharide 14 (63%) and 15 was
observed. The high regioselectivity of this reaction demon-
strated that 4-OH is more nucleophilic than 3-OH. These
hydroxyls can also be easily differentiated by installing
orthogonal protecting groups (Scheme 3). The 3-OAc of
Scheme 3
Selective oxidation of the primary hydroxyl groups was
accomplished using 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) and NaOCl under phase-transfer conditions with
tetrabutylammnonium chloride in NaHCO3 (aq) and EtOAc.17
Subsequent formation of the methyl ester using H+ resin in
methanol gave diol acceptors 25a or 25b in 62 and 60%
yields, respectively. Regio- and stereoselective glycosylation
compound 9 could be selectively removed by treatment with
Mg(OMe)2 in MeOH at 0 °C (f 16, 88%), followed by
esterification, to give 17 (86%) or 18 (80%).
Following our initial success in the preparation of the
L-iduronic acid acceptor and the various disaccharide building
Org. Lett., Vol. 6, No. 5, 2004
725

was then attempted with donor 11 and acceptor 25a. This
gave the desired 1 f 4 R-linked disaccharide 26a, along
with the regioisomer 27a, in a 3:2 ratio. No â-anomer was
found.9 Coupling of donor 11 and acceptor 25b was also
investigated. Similar results were obtained (data not shown).
Collectively these data demonstrated that the chirality of the
ethylidene carbon (R or S) plays no role in determining the
coupling outcome. The striking difference in regioselectivity
between iduronic acid 10 and its epimer 25a/b might arise
from the different intramolecular hydrogen bonding pattern.18
In compound 10, the hydrogen bond 4-OH f O-2 enhances
the nucleophilic reactivity of the 4-OH, which results in
excellent regioselective glycosidation. On the other hand,
possible intramolecular hydrogen bonding 3-OH f Od
C(OMe) in 25a/b counterbalances the nucleophilic reactivity
in 4-OH. Thus, the glycosidation outcome is not as regiose-
lective as the ido isomer.
Scheme 6. Preparation of Donor 11
A short route for the preparation of azidoglucosyl donor
11 has also been developed (Scheme 6). An SN2-like reaction
was carried out with mannosyl bromide 28 according to a
published procedure.19 Only â-thiomannoside 29 was ob-
tained, which was purified by crystallization (95%). The
following deacetylation product 30 also crystallized spon-
taneously (98%). The 3-OH was selectively protected as the
p-methoxybenzyl ether after di-n-butyltin oxide activation20
(f 31, 65%), which was followed by direct benzylidenation
(f 32, 89%). Azidoglucosyl donor 11 was formed through
a two-step sequence; triflation of the free hydroxyl was
followed by nucleophilic substitution with NaN3 in DMF,
rendering 11 in excellent yield (95%). It is noteworthy that
when the SN2 reaction was carried out with the R-glycosides,
a mixture of azidosugar and an elimination side product was
obtained.
In summary, we have developed efficient routes to
L-iduronic and glucuronic acid derivatives suitable for
glycosylation. Both heparin disaccharide building blocks
were synthesized, with 12 and 14 being achieved in a regio-
and stereoselective fashion, while 26a was formed in
moderate yield. A new and short route to azidoglucosyl donor
11 is also presented.
(14) Betaneli, B.; Ovchinnikov, M. V.; Backinowsky, L. V.; Kochetkov,
N. K. Carbohydr. Res. 1982, 107, 285-291.
(15) (a) Kong, F.; Wang, W. Carbohydr. Res. 1989, 315, 117-127. (b)
Kong, F.; Du, J.; Shang, H. Carbohydr. Res. 1987, 162, 217-225.
(16) (a) Ernst, B.; Winkler, T. Tetrahedron Lett. 1989, 30, 3081-3084.
(b) Ernst, B.; De Mesmaeker, A.; Wagner, B.; Winkler, T. Tetrahedron
Lett 1990, 31, 6167-6170.
Acknowledgment. We thank the NIH for support of this
work.
(17) Davis, N. J.; Flitsch, S. L. Tetrahedron Lett. 1993, 34, 1181-1184.
(18) (a) Ojeha, R.; de Paz, J. L.; Mart´ın-Lomas, M.; Lassaletta, J. M.
Synlett 1999, 8, 1316-1318.
Supporting Information Available: Synthetic details and
characterization for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(19) Yu, H. N.; Ling, C.-C.; Bundle, D. R. J. Chem. Soc., Perkin Trans.
1 2001, 832-837.
(20) Yu, H. N.; Ling, C.-C.; Bundle, D. R. Can. J. Chem. 2002, 80,
1131-1140.
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