A new route to L-iduronate building-blocks for the synthesis of heparin- like oligosaccharides

Article abstract of DOI:10.1055/s-1999-2810

The effective preparation of a conveniently protected L-iduronic acid derivative, as a building block for the synthesis of heparin-like oligosaccharides, is described in five steps starting from readily available D-glucuronolactone.

Full text of DOI:10.1055/s-1999-2810

1316
LETTER
A New Route to L-Iduronate Building-blocks for the Synthesis of Heparin-like
Oligosaccharides
Rafael Ojeda, José L. de Paz, Manuel Martín-Lomas, José M. Lassaletta*
Grupo de Carbohidratos, Instituto de Investigaciones Químicas, CSIC-USe, c/ Americo Vespuccio s/n, Isla de la Cartuja, E-41092 Seville,
Spain
Fax +34-(9)5-4460565; E-mail: jmlassa@cica.es
Received 17 May 1999
Abstract: The effective preparation of a conveniently protected L-
TfO
O
HO
O
O
iduronic acid derivative, as a building block for the synthesis of he-
parin-like oligosaccharides, is described in five steps starting from
readily available D-glucuronolactone.
O
O
a (i)
O
O
O
O
O
Keywords: carbohydrates, benzylation, regioselectivity, silylation
3
2
a (ii)
Heparin and heparan sulfate are highly sulfated gly-
cosaminoglycans (GAGs) involved in many important bi-
ological functions such as blood coagulation, cell growth
and differentiation, angiogenesis, etc.1 As illustrated by
Petitou and van Boeckel during their pioneering studies
on the anticoagulant activity of heparin,2 a detailed
knowledge of the mechanisms of action and establishment
of the minimal structural requirements necessary for the
activity of these molecules requires fragments having well
defined structures. Due to the heterogeneity of these
GAGs, however, the purification of such fragments from
natural sources is a difficult task, and the materials avail-
able are still heterogeneous. This fact has stimulated inter-
est in the chemical synthesis of highly homogeneous
oligosaccharides containing the repeating sequences pre-
sumably responsible for the biological activity.
PivO
MeO2C
PivO
O
O
O
b (i)
O
O
O
O
O
HO
4
5
b (ii)
HO
MeO2C
PivO
MeO2C
O
O
c
O
O
O
O
BnO
BnO
6
7
d
We have recently initiated a project directed to under-
standing the mechanisms of activation of the fibroblast
growth factor (FGF) family of proteins by heparin/hepa-
ran sulfate.3 In order to perform structure-activity rela-
tionship studies in this field, we decided to prepare a
variety of synthetic oligosaccharidic fragments belonging
to the ‘regular region’ of heparin and analogues differing
in the number and position of sulfate groups. Consequent-
ly, we needed an appropriate building block for the intro-
duction of the iduronate residues. However, a survey of
the literature reveals that the few existent reports on the
synthesis of these compounds describe long, tedious
routes to afford the final products in low overall yields.4,5
Therefore, we decided to investigate a new strategy for the
synthesis of L-iduronic acid derivatives suitable for hep-
arin-like oligosaccharide synthesis.
MeO2C
HO
MeO2C
HO
O
O
e
OTDS
OH
BnO
OH
BnO
OH
1
8
Scheme 1 Reagents and conditions: a) i) Tf2O, Py; ii) NaOPiv, DMF
(94%). b) i) 1% Et3N, MeOH, -60 Æ -40 °C; ii) PhCHN2, HBF4, -40
°C (58%) or BnOC(=NH)CCl3, TfOH (cat.), 0 °C (50%). c) i) KOH,
MeOH/H2O; ii) IMe, K2CO3, DMF (60%). d) TFA/H2O (9:1) (75%).
e) TDSCl, imidazol, -30 Æ -20 °C (80%)
2 and O-3 should be orthogonally removable); d) O-1
should also be orthogonally protected, since activation of
the anomeric position should allow further chain elonga-
tions by reaction with glycosyl acceptors. Compound 1
was chosen as the target, since it fulfilled all of these re-
quirements (see the conclusions below). Our approach
starts with inexpensive, commercially available D-glucu-
This target should ideally possess the following character-
istics: a) the carboxylate function should be in fact, or in-
troduced at an early stage of the synthesis in order to avoid
the oxidation of advanced L-ido derivatives; b) selective
glycosylations at position 4 should be feasible; c) the pro-
tecting groups should allow for selective 2-O-sulfation af-
ter the glycosylation steps (i.e., the protecting groups at O-
Synlett 1999, No. 8, 1316–1318 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
A New Route to L-Iduronate Building-blocks for the Synthesis of Heparin-like Oligosaccharides
1317
ronolactone. This material was easily transformed into the a strong OH-4 Æ O-2 hydrogen bond. This interesting sit-
corresponding isopropylidene derivative 2 in high yield uation has convenient practical consequences because as
following a known procedure.6 The remaining free OH a result of the steric hindrance around OH-2 and of the dif-
group at position 5 was then activated by esterification ference in nucleophilic reactivity due to the presence of
with triflic anhydride, and the resulting crude triflate 37 this hydrogen bond, it can be predicted that attack of elec-
was then used without purification for the reaction with trophilic species (such as glycosyl oxocarbenium ions)
sodium pivaloate8 (Scheme 1). In this way, the corre- will occur regioselectively at OH-4.14
sponding 5-O-pivaloyl L-iduronolactone 4 could be iso-
lated in an excellent 94% overall yield from 2. Taking
advantage of the relatively high resistance to basic condi-
NOE
tions exhibited by the pivaloyl protecting group, the open-
H
H
OBn
O
4JH-2, H-4 = 1.1 Hz
3JH-4, OH = 11.5 Hz
3JH-2,O H 0 Hz
ing of the lactone ring was selectively performed using
methanolic 1% Et3N (-60 Æ -40 °C) as the reagent.9 The
resulting crude material consists of the desired product 5
plus a small amount (2-5%) of unreacted lactone. In spite
of the substantially different chromatographic mobilities
of both compounds, the former could not be fully purified,
as variable amounts of 4 were systematically detected as
byproduct after flash chromatography.10 Therefore, the
crude material was used in the next step without further
purification. The introduction of the desired benzyl pro-
tecting group was the key transformation in the synthesis,
as the usual basic conditions had to be avoided in order to
prevent the easy lactone ring-formation. First experiments
were carried out using phenyldiazomethane11 as the re-
agent and HBF4 as the catalyst.12 In this way, the desired
product 6 could be isolated in acceptable 58% overall
yield from 5. However, the high excess of reagent needed
to reach this result, on one hand, and the difficult, tedious
purification resulting from the large amounts of phenyl
carbene-derived by-products, on the other, constitute seri-
ous drawbacks in this method. Therefore, the benzylation
using benzyl trichloroacetimidate13 as the reagent in the
presence of catalytic amounts of TfOH was also investi-
gated. This second approach afforded the same product 6
in slightly lower yield (50%, overall), but it should be con-
sidered as the method of choice due to the lower costs and
easier purification of the product. Removal of the pivaloyl
protecting group at O-5 was effected by means of KOH in
MeOH/H2O followed by re-methylation (IMe/K2CO3/
DMF) of the carboxylate moiety to afford known5 com-
pound 7 in 60% yield. Hydrolysis of the isopropylidene
acetal was then performed in TFA-H2O 9:1 to afford the
L-Iduronic acid derivative 8 according to Sinaÿ et al.5 The
required building block 1 was finally obtained by regio-
MeO2C
H
OTDS
H
O
O
H
H
E+
1
Experimental:
Methyl 3-O-benzyl-1,2-isopropylidene-5-O-pivaloyl-b-L-id-
ofuranuronate (6). To a solution of 4 (53 g, 0.176 mol) in dry
MeOH (500 mL) at -60 (C was added Et3N (0.25 mL, 1.76 mmol).
The reaction was allowed to warm to 40 (C and stirred overnight.
CH2Cl2 (400 mL) was added and the mixture was washed with satd.
NH4Cl (2 ( 300 mL) and H2O (2 ( 300 mL), dried (Na2SO4) and con-
centrated. To a cooled (0 °C) solution of this residue in dry CH2Cl2
(1.6 L) were added benzyltrichloroacetimidate (freshly distilled,
18.6 mL, 1.1 mol) and triflic acid (0.053 mol, 0.17 M in dry Et2O).
The reaction mixture was allowed to warm to rt and stirred over-
night. The mixture was neutralised (Et3N) and concentrated. The re-
sulting residue was purified by flash chromatography (7:1 hexane-
ethyl acetate) to afford 6 (37.2 g, 50%) as an oil. [α]23D +12.3 (c 1,
1
CH2Cl2); H NMR (500 MHz, CDCl3): d 7.33-7.24 (m, 5H, Ph),
5.96 (d, J 4.1, 1H, H-1), 5.43 (d, J 5.8, 1H, H-5), 4.69 (dd, J = 5.1,
5.8 Hz, 1H, H-4), 4.64 (dd, J = 1.2, 4.1 Hz, 1H, H-2), 4.63 (d, J =
11.6 Hz, 1H, CHPh), 4.51 (d, J = 11.6 Hz, 1H, CHPh), 4.18 (dd, J
= 1.2, 5.1 Hz, 1H, H3), 3.67 (s, 3H, COOMe), 1.47 (s, 3H, isopro-
pylidene), 1.33 (s, 3H, isopropylidene), 1.22 (s, 9H, CMe3); 13C
NMR (125 MHz, CDCl3): d 177.1, 168.6, 137.1, 128.4, 127.9,
127.7, 112.9, 105.3, 83.8, 82.8, 78.9, 72.4, 70.7, 52.4, 38.6, 27.3,
27.0, 26.9. Anal. Calc. for C22H30O8: C 62.55%; H 7.16%. Found: C
62.23%; H 7.16%.
Methyl 3-O-benzyl-1-O-dimethylthexylsilyl-b-L-idopyranur-
onate (1). To a solution of 8 (2.8 g, 9.39 mmol) and imidazole (1.4
g, 20.6 mmol) in CH2Cl2 (10 mL) at -30ºC was added dimethylhex-
ylsilyl chloride (2.8 mL, 12.2 mmol). After stirring for 72 h, MeOH
(2 mL) was added and the mixture was diluted with CH2Cl2 (50
and stereoselective silylation with dimethylthexylsilyl mL), washed with water (2 x 20 mL), dried (Na2SO4) and concen-
trated. The residue was purified by flash chromatography (toluene-
chloride (TDSCl) and imidazole, (CH2Cl2, -30 Æ -20 °C)
ethyl acetate 6:1) to yield 1 (3.3 g, 80 %) as an oil. [α]D23 +28.2 (c
to afford almost exclusively the b-silyl glycoside in an ex-
1.0, CHCl3); 1H NMR (500 MHz, CDCl3): d 7.27-7.29 (m, 5H, Ph),
cellent 80 % yield. Analysis of the 1H NMR (chloroform-
5.02 (d, J = 1.1 Hz, 1H, H-1), 4.66 (d, J = 11.8 Hz, 1H, CHPh), 4.58
d) spectrum of 1 indicate a preferred 1C4 conformation for
(d, J = 11.8 Hz, 1H, CHPh), 4.47 (d, J = 1.45 Hz, 1H, H-5), 3.99 (m,
this compound (J1,2 = 1.1 Hz; J2,3 ª J3,4 ª 3.1 Hz; J4,5 = 1.4
1H, H-4), 3.95 (dd, J = 3.1, 3.0 Hz, 1H, H-3), 3.78 (s, 3H, COOMe),
3.77 (d, J = 11.45 Hz, 1H, OH-4), 3.67 (m, 1H, H-2), 2.79 (s, 1H,
OH2), 1.63 (m, J = 6.8 Hz, 1H, CHMe2), 0.88 (d, J = 6.8 Hz, 3H,
CHMe), 0.86 (d, J = 6.8 Hz, 3H, CHMe), 0.85 (s, 3H, SiCMe), 0.84
(s, 3H, SiCMe), 0.26 (s, 3H, SiMe), 0.18 (s, 3H, SiMe); 13C NMR
(125 MHz, CDCl3): d 169.3, 137.4, 128.6, 128.2, 127.7, 93.5, 75.4,
74.3, 72.6, 69.4, 67.4, 52.1, 34.1, 25.0, 20.2, 20.0, 18.6, 18.5. Anal.
Calc. for C22H36O7Si: C 59.97 %; H 8.24 %. Found: C 60.05 %; H
8.30 %.
4
Hz, J2,4 = 1.1 Hz) (Figure 1). Additionally, 2D ROESY
NMR spectroscopy showed a strong NOE between H-1
and H-5, indicating their close proximity, which is in
agreement with the assigned conformation, and confirms
the expected (configuration at the anomeric center. More-
over, the coupling constants, J4,OH-4 = 11.5 Hz and J2,OH-2
ª 0.0 Hz suggest that such a conformation is stabilised by
Synlett 1999, No. 8, 1316–1318 ISSN 0936-5214 © Thieme Stuttgart · New York

1318
R. Ojeda et al.
LETTER
(6) Kitihara, T.; Ogawa, T.; Naganuma, T.; Matsui, M. Agr. Biol.
Chem. 1974, 38, 2189; Bashyal, B. P.; Chow, H.-F.; Fellows,
L. E.; Fleet G.W.J. Tetrahedron 1987, 43, 415.
(7) Csuk, R.; Hönig, H.; Nimpf, H.; Weidmann, H. Tetrahedron
Lett. 1980, 21, 2135.
(8) Sodium pivaloate was prepared from NaOH and pivalic acid
(slight excess), and dried in vacuo (0.3 mbar, 20 h).
(9) Increasing the amount of Et3N or raising temperature led to the
unexpected, partial pivaloyl migration from O-5 to O-2.
(10) Presumably, some catalytic activity by the silica-gel promoted
a partial lactonization during chromatography.
(11) X. Creay, Organic Syntheses 1985, 64, 207.
(12) Liotta, L. J.; Ganem, B. Tetrahedron Lett. 1989, 30, 4759.
(13) Iversen, T.; Bundle, D. R.; J. Chem. Soc., Chem. Commun.
1981, 1240.
Acknowledgement
We thank the Spanish ‘Ministerio de Educacion y Cultura’ for fi-
nancial support (project PB96-0820) and for a fellowship to J.L.P.
We also thank the Spanish Research Council (CSIC) for a fel-
lowship to R.O.
References and Notes
(1) Linhardt, R. J.; Toida, Y. In Carbohydrates as Drugs;
Witczak, Z.; Nieforth, K. Eds.; New York 1998; pp 277-341.
(2) Review: van Boeckel, C. A. A.; Petitou, M. Angew. Chem. Int.
Ed. Engl. 1993, 32, 1671.
(3) Lassaletta, J. M.; Martín-Lomas, M.; Nieto, P.; de Paz, J. L.;
Ojeda, R.; Angulo, J. XIXth International Carbohydrate
Symposium, San Diego 1998, Commun. BP-041.
(4) Tabeur, C.; Machetto, F.; Mallet, J.-M.; Duchaussoy, P.;
Petitou, M.; Sinaÿ, P. Carbohydrate Res. 1996, 281, 25, and
references cited therein. Chiba, T.; Sinaÿ, P. Carbohydrate
Res. 1986, 151, 379. Bazin, H. G.; Kerns, R. J.; Linhardt, R. J.
Tetrahedron Lett. 1997, 38, 923. Bazin, H. G.; Kerns, R. J.;
Linhardt, R. J. J. Org. Chem. 1999, 64, 144.
(14) This essential prediction has been further confirmed during
the synthesis of several oligosaccharides. See ref. 3.
Article Identifier:
1437-2096,E;1999,0,08,1316,1318,ftx,en;L06999ST.pdf
(5) Jacquinet, J.-C.; Petitou, M.; Duchaussoy, P.; Lederman, I.;
Choay, J.; Torri, G.; Sinaÿ, P. Carbohydrate Res. 1984, 130,
221.
Synlett 1999, No. 8, 1316–1318 ISSN 0936-5214 © Thieme Stuttgart · New York