Sulfo-Protected Hexosamine Monosaccharides: Potentially Versatile Building Blocks for Glycosaminoglycan Synthesis

Article abstract of DOI:10.1021/ol035882w

(Equation Presented) 2,2,2-Trifluorodiazoethane was investigated as a reagent for sulfo group protection on hexosamine monosaccharides. The synthesis of glucosamine and galactosamine building blocks fully differentiated for glycosaminoglycan synthesis and

Full text of DOI:10.1021/ol035882w

ORGANIC
LETTERS
2003
Vol. 5, No. 25
4839-4842
Sulfo-Protected Hexosamine
Monosaccharides: Potentially Versatile
Building Blocks for Glycosaminoglycan
Synthesis
Nathalie A. Karst,‡ Tasneem F. Islam,‡ and Robert J. Linhardt*
Departments of Chemistry, Biology and Chemical and Biological Engineering,
Rensselaer Polytechnic Institute, Troy, New York 12180
linhar@rpi.edu
Received September 26, 2003
ABSTRACT
2,2,2-Trifluorodiazoethane was investigated as a reagent for sulfo group protection on hexosamine monosaccharides. The synthesis of
glucosamine and galactosamine building blocks fully differentiated for glycosaminoglycan synthesis and the synthesis of glycosyl donors are
described. The compatibility of trifluoroethylsulfonate under a variety of reaction conditions has also been investigated.
2,2,2-Trifluorodiazoethane, introduced for sulfonic acid
protection,1 was first used as a reagent for sulfate ester
protection in carbohydrates by Flitsch and co-workers in
1997.2 Simple hexoses, protected as 2,2,2-trifluoroethylsul-
fonates, were shown to be stable under a variety of reaction
conditions and could be removed by using base to recover
the corresponding sulfonated hexoses. In our efforts toward
the chemoenzymatic synthesis of glycosaminoglycans (GAGs),
we decided to investigate the preparation of monomer
building blocks carrying sulfo groups protected as trifluo-
roethylsulfonates. In this approach a sulfo group is either
already present in the carbohydrate or introduced at the
beginning of the synthesis. Tedious manipulations of sulfo
monoester, particularly difficult at the higher oligosaccharide
level, are eliminated, thus facilitating protecting group
manipulation and purification, giving more straightforward
access to oligosaccharides of interest.3 We planned to begin
by investigating the use of trifluoroethylsulfonate in the
preparation of hexosamine monomer building blocks, which
can be used in the synthesis of chondroitin sulfate, dermatan
sulfate, and heparin oligosaccharides. The typical sulfation
patterns in hexosamine residues naturally occurring in GAGs
directed us toward the selection of our primilary targets
(Table 1). Monosaccharide buiding blocks were built with
Table 1. Most Common Sulfation Patterns in GAGsa
D-glucosamine in HP/HSb
D-galactosamine in CS/DSb
GlcNp6S
Ga lNp4S
Ga lNp6S
Ga lNp4,6S
GlcNp3,6S
GlcNpS6S
GlcNpS3,6S
a Chosen targets are shown in bold. a HP: heparin. HS: heparan sulfate.
CS: chondroitin sulfate. DS: dermatan sulfate. In HP/HS N can be
substituted with acetyl or sulfo and in CS/DS N is always substituted with
acetyl.
‡ Present address: Department of Chemistry, University of Iowa, Iowa
City, Iowa 52242.
(1) Messe, C. O. Synthesis 1984, 1041-1042.
(2) Proud, A. D.; Prodger, J. C.; Flitsch, S. L. Tetrahedron Lett. 1997,
38, 7243-7246.
orthogonal protection compatible with elongation at both the
reducing and nonreducing ends (the C-4 of glucosamine and
(3) Karst, N.; Linhardt, R. J. Curr. Med. Chem. 2003, 10, 1993-2031.
10.1021/ol035882w CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/12/2003

the C-3 of galactosamine). Hydroxyl groups were protected
by using various combinations of ester or ether protecting
groups and both azido and N,N-dimethylmaleimido series
were studied for their compatibility with the trifluoroethyl-
sulfonate group.
Scheme 2
Starting from D-glucosamine hydrochloride, compound 1
was prepared as described in the literature4 and subsequently
treated with benzoyl chloride to afford the 3-benzoylated
derivative 2 in good yield (Scheme 1). Cleavage of the
Scheme 1
ethylsulfonate moiety and the 4,6-diol 7 was obtained as the
only product in good yield. When diol 7 was then submitted
to a 2-step sulfonation/sulfo-protection sequence compound
8 was obtained in 67% yield.
Monosaccharide building block 12, with ether protecting
groups, was next synthesized to broaden the scope of our
protection chemistry (Scheme 2). Introduction of the p-
methoxybenzylidene at the 4,6-position of 9, followed by
benzylation at the 3-position gave 10. Treatment of 10 with
dibutylborane triflate and 1 M borane solution in THF6
regioselectively opened the benzylidene ring and provided
compound 11 in good yield and stereoselectivity. The
remaining free 6-position was sulfonated and sulfo-protected
to afford building block 12 in 71% overall yield. The
p-methoxybenzyl could later be selectively removed, under
acidic conditions, to give access to glycosylation acceptor
13.
benzylidene acetal gave the 4,6-diol 3, which was selectively
6-O-sulfonated with sulfur trioxide-trimethylamine complex
in N,N-dimethylformamide.
Treatment of the intermediate sulfo monoester with an
excess of a fresh solution of trifluorodiazoethane5 in aceto-
nitrile in the presence of citric acid afforded the desired
6-trifluoroethylsulfonate (sulfo diester) derivative 4 in 68%
overall yield. The reaction proceeded smoothly after 1-2
days at room temperature requiring excess citric acid for the
reaction to go to completion. No side products were detected,
demonstrating the azido group to be compatible with the
conditions of the sulfo-protection reaction. The selectivity
of the trifluorodiazoethane for the sulfo ester was also
verified as the 4-hydroxyl group remained untouched in this
reaction. Chloroacetylation of the free 4-hydroxyl group
provided the first GAG building block, 5. Acceptor 4 could
be later recovered from building block 5 by selective
cleavage of the 4-chloroacetyl group with hydrazine acetate.
Preparation of disulfo derivative 8 was next undertaken
from the common intermediate 1 (Scheme 2). 3-O-Sulfona-
tion followed by treatment with trifluorodiazoethane afforded
derivative 6 in 61% yield. Conditions for the cleavage of
the benzylidene acetal were compatible with the trifluoro-
Scheme 3
(4) La Ferla, B.; Prosperi, D.; Lay, L.; Russo, G.; Panza, L. Carbohydr.
Res. 2002, 337, 1333-1342.
D-Glucosamine hydrochloride was used to prepare 1,3,4,6-
O-acetyl-2-deoxy-2-dimethylmaleimido â-D-glucopyranoside
14,7 which was treated with p-methoxyphenol in the presence
of catalytic trifluoromethanesulfonic acid and transesterified
to afford the â-MP derivative 15 (Scheme 4). Benzylidena-
(5) Typical Procedure. Monosaccharide sulfo ester (500 mg) in solution
in acetonitrile (5 mL) was treated with a fresh solution of 2,2,2-
trifluorodiaozethane (40 mL) prepared as described in ref 2. (CAUTION:
This reagent should be considered as potentially explosive and highly toxic.)
Citric acid (2 g) was added and the reaction mixture was stirred at room
temperature until TLC analysis showed complete consumption of the starting
material (1-2 days). The solution was filtered over Celite and concentrated.
The residue, dissolved in dichloromethane, was washed successively with
water, a saturated solution of sodium bicarbonate, and water, dried (MgSO4),
filtered, and concentrated. The product was purified by silica gel chroma-
tography.
(6) Jiang, L.; Chan, T.-H. Tetrahedron Lett. 1998, 39, 355-358.
(7) Aly, M. R. E.; Castro-Palomino, J. C.; Ibrahim, E.-S. I.; El-Ashry,
E.-S. H.; Schmidt, R. R. Eur. J. Org. Chem. 1998, 11, 2305-2316.
4840
Org. Lett., Vol. 5, No. 25, 2003

Scheme 4
Scheme 6a
tion, leaving the 3-position differentiated, was followed by
sulfonation and sulfo-protection to afford derivative 17 in
70% overall yield. No side products were detected during
these reactions and both the MP and DMM protecting groups
were stable under the reaction conditions.
a Reagents and conditions: (i) Me3N‚SO3 (3-4.5 equiv), DMF;
50 °C; (ii) CF3CHN2, citric acid, MeCN.
Preparation of target compound 22 was next undertaken
(Scheme 5). Triol 18, obtained from 14 by literature
Activation of the anomeric position and preparation of the
glycosyl donors was next studied. Selective removal of the
TDS10 group was found to be more troublesome than
expected. Indeed, the trifluoroethylsulfonate group, when
present at the 6-position, acted as a good leaving group under
basic conditions in both GlcN and GalN series and the
corresponding 1,6-anhydro sugars were recovered as side
products. When excess acetic acid was added to tetrabutyl-
ammonium fluoride, in the case of compound 5, or a milder
reagent such as trihydrofluoride triethylamine was used, the
corresponding hemiacetals could be obtained in good yields
(Scheme 7).
Scheme 5
Scheme 7
methods,8 was benzylidenated and benzoylated affording 19
in 75% overall yield. Cleavage of the benzylidene acetal,
selective 6-O-sulfonation, and sulfo-protection afforded the
expected compound 21. Chloroacetylation at the 4-position
gave access to the desired building block 22 in good yield.
The azido GalN derivative 23 was prepared from D-
galactosamine hydrochloride according to known procedures9
and benzoylated at the 3-position to afford common inter-
mediate 24 (Scheme 6). Regioselective opening of ben-
zylidene acetal under different conditions gave access to
either 6- or 4-benzylated compounds in good yield and with
good selectivity. The corresponding 4,6-diol could be
obtained in 77% yield from cleavage of the benzylidene ring.
The intermediate free hydroxyl derivatives were sulfonated
and sulfo-protected to afford the corresponding 4-, 6-, and
4,6-trifluoroethylsulfonates 25, 26, and 27 in good to
moderate yields.
(8) Chiesa, M. V.; Schmidt, R. R. Eur. J. Org. Chem. 2000, 21, 3541-
3554.
(9) Herzner, H.; Eberling, J.; Schultz, M.; Zimmer, J.; Kunz, H. J.
Carbohydr. Chem. 1998, 17, 759-776.
Activation of the 6-trifluoroethylsulfonate hemiacetals
under basic conditions to prepare trichloroacetimidates also
Org. Lett., Vol. 5, No. 25, 2003
4841

mediate was treated with trichloroacetonitrile and catalytic
DBU to afford a separable mixture of R- and â-trichloro-
acetimidate 31 in 59% overall yield.
Scheme 8
Preliminary glycosylation attempts, achieved by the cou-
pling of R-fluoride 30 and R-imidate 31 with the 6-hydroxyl
acceptor 32, showed promising results (Scheme 8). Despite
the electron-withdrawing character of the trifluoroethylsul-
fonate, which disarmed the glycoside donor, encouraging
yields were obtained in both series.
In conclusion, mono- and disulfo-protected hexosamine
building blocks were prepared in good yields. Trifluoro-
ethylsulfonate moieties were shown to be compatible with
both azido and N,N-dimethylmaleoyl groups and to a wide
array of protection and deprotection chemistry. These 6-O-
trifluoroethylsulfonate derivatives can rearrange under basic
conditions so that care must be taken to avoid basic
conditions in the preparation of glycosyl donors. Glycoside
fluorides are easily prepared in the presence of the sulfo
protecting group, and fluoride donors have shown good
behavior in both glycopeptide and oligosaccharide synthesis
and can be activated by using a large range of reaction
conditions.11 Additional glycosylation studies are currently
underway on the applications of these donors in GAG
synthesis and the results of these studies will be reported in
due course.
led to partial loss of the sulfo-protecting group. Milder bases
such as cesium carbonate did not afford the trichloroace-
timidates. In contrast, halogens, including fluoride, were
easily introduced at the anomeric position of 6-sulfo-
protected derivatives. Thus, treatment of the hemiacetals with
dimethylammonium sulfur trifluoride afforded the corre-
sponding fluorides 28, 29, and 30 in good yield.
Removal of the TDS group in the 4-sulfo-protected series
was achieved with tetrabutylammonium fluoride in the
presence of an excess or equimolar amount of acetic acid.
Since the configuration of the GalN sugar did not allow easy
intramolecular substitution and ring closure, no side product
was observed under those conditions. The hemiacetal inter-
Supporting Information Available: Spectral data for
compounds 4, 8, 12, 17, 21 and 25-27. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL035882W
(10) The abbreviation used are the following: DMM, dimethylmaleimido;
TDS, thexyldimethylsilyl.
(11) For a review on glycosyl fluoride see: Toshima, K. Carbohydr.
Res. 2000, 327, 15-26.
4842
Org. Lett., Vol. 5, No. 25, 2003