Synthesis of Heparin Oligosaccharides

Article abstract of DOI:10.1021/ja038244h

An efficient preparation of rare 2-O-benzoyl-3-O-benzyl-1,6-anhydro-β-l-idopyranose from commercially available diacetone α-d-glucose in five straightforward steps is described here. With this key building block in hand, the total syntheses of heparin oligosaccharides with three, five, seven, and nine sugar units are successfully carried out. Copyright

Full text of DOI:10.1021/ja038244h

Published on Web 12/19/2003
Synthesis of Heparin Oligosaccharides
Jinq-Chyi Lee,‡ Xin-An Lu,† Suvarn S. Kulkarni,† Yuh-Sheng Wen,† and Shang-Cheng Hung*,†
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, and Department of Chemistry,
National Tsing Hua UniVersity, Hsinchu 300, Taiwan
Received August 31, 2003; E-mail: schung@chem.sinica.edu.tw
Scheme 1
Heparin and its structurally related heparan sulfate, the linear
sulfated polysaccharides belonging to the family of glycosami-
noglycans, play significant roles in a diverse set of biological
processes, including blood coagulation, virus infection, cell growth,
inflammation, wound healing, tumor metastasis, lipid metabolism,
and diseases of the nervous system.1 Heparin is widely used as an
anticoagulant drug in clinic2 and contains a trisulfated disaccharide
repeating unit (Scheme 1) as the major component consisting of
alternating D-glucosamine and L-iduronic acid with R1 f 4 linkages.
Enzymatic degradation of heparin with heparinase gives a mixture
of oligosaccharides 1 with even sugar units invariably having an
unsaturated D-glucuronic acid residue at the nonreducing end.3 Effort
has been invested to determine the minimal structural requirements,
mostly involving tetra- (n ) 1) to decasaccharide (n ) 4), for
biological activities.4 With the discovery of increasing numbers of
heparin-binding proteins, there is a need to characterize the
molecular properties, within the proteins and heparin, responsible
for specific recognition. Toward fulfilling this goal, we have
explored herein the synthesis of heparin oligosaccharides 2-5.
The frequently encountered problems in the synthesis of heparin
Scheme 2
molecules5 include the generation of rare L-idose,6 the differentiation
of all hydroxy groups on each sugar residue, the stereocontrol in
the construction of all R-glycosidic bonds, the cleavage of multi-
protecting groups, and the transformation of multifunctional groups.
Our retrosynthetic plan of target compounds 2-5 entails two
building blocks, an elongation and termination disaccharide unit 6
and a starting D-glucosamine unit 7. The 2-naphthylmethyl group
(2-NAP),7 which is used to block the C4′-hydroxyl of 6, allows
chemoselective deprotection with DDQ during chain-enhancement
and simultaneous removal along with the permanent benzyl groups
(P) in the final termination process. The ester protecting groups
(P1) not only offer anchimeric assistance to generate exclusive 1,2-
intermediate. Regioselective benzoylation of 10 provided the
trans-glycosidic linkages, but also can be selectively removed to
corresponding 2-ester 1111 in 85% yield, exclusively.
free those hydroxyls which would ultimately carry sulfonate groups.
With the key synthon 11 in hand, we proceeded to synthesize
In addition, a temporary protection (P2) is needed to mask the
heparin oligosaccharides 2-5, as outlined in Scheme 3. The cheaply
primary hydroxyl on L-idose that could be oxidized to carboxylic
available D-glucosamine hydrochloride was first transformed into
acid.
the 1,3-diol 12 (75%) employing a combination of amino-azido
An efficient synthesis of the L-idopyranosyl sugar 11 is illustrated
conversion at C2 and 4,6-O-naphthylidenation. Regioselective O1-
in Scheme 2. The 5,6-diol 8,8 generated from commercially
benzoylation of compound 12 with 1-N-(benzyloxy)benzotriazole
available diacetone R-D-glucose in two known steps, underwent
(BzOBT)12 followed by O3-benzylation afforded the product 13,
one-pot benzoylation-mesylation to yield the corresponding furanose
which was subjected to sequential O6-ring opening,13 O6-benzoy-
9 (81%) as a single isomer. Treatment of compound 9 with t-BuOK
lation, and anomeric debenzoylation to provide the 1-alcohol 14.
in t-BuOH followed by addition of a 1:2 mixture of 0.6 N H2-
Transformation of 14 into the corresponding trichloroacetimidate
SO4(aq) and diglyme and subsequent heating at elevated temperature
and further coupling with the acceptor 11 led to the R-linked
(160 °C) for 16 h led to the 2,4-diol 10 (52%) in a one-pot manner.
disaccharide 15r and its â-isomer 15â in 61% and 11% yields,
The formation of 10 presumably takes place through an intramo-
respectively. The absolute configuration of 15â was unambiguously
lecular SN2 substitution to produce the C5-epimerized L-ido
determined through its X-ray single-crystal analysis (see Supporting
epoxide,9 which concomitantly gets hydrolyzed in acidic media10
Information). Cu(OTf)2-catalyzed acetolysis of 15r delivered the
and eliminates a water molecule via 3-O-benzyl-L-idopyranose
1,6-diacetate 16 (88%), which upon selective O1-deacetylation using
ammonia was similarly converted to the imidate 17 (77%). The
4-alcohol 18, prepared by selective O6-benzoylation (Bz2O, Et3N,
† Academia Sinica.
‡ National Tsing Hua University.
9
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J. AM. CHEM. SOC. 2004, 126, 476-477
10.1021/ja038244h CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
Supporting Information Available: Experimental procedures, 1H
NMR spectra, and X-ray structural information for compound 15â (PDF
and CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
97%) of methyl 2-azido-3-O-benzyl-2-deoxy-R-D-glucopyrano-
side,14 was coupled with 17 to provide the R-linked trisaccharide
19 (89%) as a single isomer. Further chain-elongation sequence,
involving removal of the O4-NAP using DDQ and subsequent
glycosylation with the disaccharide donor 17, posed no problems
and furnished the pentasaccharide 20 (58%), exclusively. While
reaction of compound 19 with HBF4‚Et2O15 gave the corresponding
6′-alcohol in excellent yield, simultaneous removal of two acetyl
groups in 20 proved testing and compelled us to rethink our present
strategy.
References
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Levulinoyl (Lev) esters were thought to be better options for
our purpose. Direct levulinolysis of 15r did not work. Alternatively,
deacetylation16 of 16 afforded the 1,6-diol (81%), which was reacted
with Lev2O in pyridine to get the ester 21 (97%). A similar reaction
sequence of anomeric deprotection and imidate formation led to
the glycosyl donor 22 (53% in two steps), which was coupled with
18 in a likewise manner to construct the R-linked trisaccharide 23
(84%). The elongation cycle was then repeated thrice to assemble
the penta-, hepta-, and nonasaccharides 24, 25, and 26, respectively.
Cleavage of the Lev groups in 23-26 followed by oxidation using
TEMPO,17 individually, furnished the acids 27-30 in good overall
yields. The corresponding O-sulfates, obtained by consecutive
deacetylation and O-sulfonation of 27-30, underwent hydrogenoly-
sis to reduce the OBn, O-2-NAP, and N3 groups and subsequent
N-sulfonation to provide the desired target molecules 2-5, respec-
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In summary, we successfully developed a straightforward
synthesis of the L-idopyranosyl sugar 11 as a valuable building block
and carried out the total synthesis of heparin oligosaccharides 2-5.
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Acknowledgment. We thank Professor Chun-Chen Liao for his
helpful discussions. This work was supported by the National
Science Council of Taiwan R.O.C. (NSC 91-2113-M-001-004).
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