A galabiose-based two-dimensional scaffold for the synthesis of inhibitors targeting Pk- and P-antigen binding proteins

Article abstract of DOI:10.1016/S0040-4039(03)00475-1

A disaccharide scaffold based on galabiose (Galα1-4Gal) was synthesized. Four different acceptors were evaluated in the α-galactosylation and a relationship between the nucleophilicity, yield, and α/β-selectivity was found. The scaffold contains two ortho

Full text of DOI:10.1016/S0040-4039(03)00475-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2785–2787
A galabiose-based two-dimensional scaffold for the synthesis of
inhibitors targeting Pk- and P-antigen binding proteins
Jo¨rgen Ohlsson and Ulf J. Nilsson*
Bioorganic Chemistry, Lund University, PO Box 124, SE-221 00 Lund, Sweden
Received 17 December 2002; revised 10 February 2003; accepted 20 February 2003
Abstract—A disaccharide scaffold based on galabiose (Gala1–4Gal) was synthesized. Four different acceptors were evaluated in
the a-galactosylation and a relationship between the nucleophilicity, yield, and a/b-selectivity was found. The scaffold contains
two orthogonal derivatisation sites, i.e. at O-2% and the anomeric position, and as proof of concept, one derivatised galabioside
was synthesized. Compounds based on this galabiose-scaffold are potential inhibitors of P- and Pk-antigen binding proteins.
© 2003 Elsevier Science Ltd. All rights reserved.
The surface of an animal cell is covered with glyco-
proteins and glycolipids1,2 and it is well known that
pathogenic bacteria, viruses and toxins attach to these
glycoconjugates.3,4 This attachment is often a prerequi-
site for infection and/or toxicity and thus interesting as
a target for the development of novel ways to treat
bacterial and viral infections. Well-known examples,
where carbohydrate-structures are exploited as attach-
ment mediators by pathogens, are the Pk and P-antigen
(Gala1–4Galb1–4Glc and GalNAcb1–3Gala1–4Galb1–
4Glc, respectively). These carbohydrate structures are
in various stages of infection bound by uropathogenic
Escherichia coli,5,6 Streptococcus suis,7,8 parvovirus
B19,9 Shiga toxin from Shigella dysenteria10 and Vero-
toxins from E. coli.11 A majority of these binding events
have been demonstrated to require the galabiose
(Gala1–4Gal) disaccharide present as a core structure
in both the P- and Pk-antigen.
adhesins of uropathogenic E. coli.15 Herein we describe
the synthesis of a novel galabiose-based scaffold 1 (Fig.
1) that allows orthogonal introduction of substituents
at O-2% and C-1.
A galactose donor with a non-participating protecting
group at O-2 was required in the synthesis of the
scaffold 1. To meet this requirement, a p-methoxybenz-
yl ether was chosen as protection group at O-2,
because it possesses the additional feature of being
removable under selective mildly acidic conditions.16
Furthermore, a galactose acceptor with HO-4 unpro-
tected and an azide at the anomeric position was
required. a-Galactosylations of galactopyranose HO-4
are normally favoured by having a per-benzylated
donor and a 2,3,6-tri-O-benzoyl-protected acceptor.17
However, changing the 2-O-benzyl of the donor for a
2-O-p-methoxybenzyl (i.e. 1018) and having an azide at
the anomeric position of the acceptor (i.e. acceptor 6)
resulted in a low yield of the desired disaccharide,
although with retained a-selectivity. In order to investi-
gate this a-galactosylation further, a series of acceptors
(6–9) with an increasing number of benzyl-groups was
synthesized (Scheme 1) and evaluated. The synthesis of
acceptors 6–9 (Scheme 1) started from galactosyl azide
219 by introduction of a 4,6-O-benzylidene acetal
In the search for inhibitors of carbohydrate–lectin
interactions, the introduction of substituents on core
carbohydrate-structures as scaffolds has been success-
fully employed.12–15 Within this context, a properly
designed galabiose derivative can be used as a scaffold
in the assembly of a combinatorial collection of com-
pounds, aimed at finding inhibitors of P and Pk-binding
proteins. We recently reported the use of a galabiose
scaffold functionalised with a primary alkyl bromide at
C-1 and an allyl ether at O-3% in the combinatorial
assembly of 20 galabiose-based inhibitors of PapG
* Corresponding author. Tel.: +46-46-2228211; fax: +46-46-2228209;
e-mail: ulf.nilsson@bioorganic.lth.se
Figure 1. Scaffold 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00475-1

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J. Ohlsson, U. J. Nilsson / Tetrahedron Letters 44 (2003) 2785–2787
Scheme 1. Reagents and conditions: (a) a,a-dimethoxytoluene, p-TSA, CH3CN. (b) BzCl, pyridine. (c) BnBr, NaH, DMF. (d) i.
80% aqueous AcOH, 80°C, ii. BzCl, pyridine, −10°C, 0.5 h. (e) NaCNBH3, THF, MS AW-300, HCl–Et2O (pH 2), 0°C, 4 h. (f)
i. 80% aqueous AcOH, 80°C, ii. BzCl, pyridine, −10°C, 0.5 h. (g) NaCNBH3, THF, MS AW-300, HCl–Et2O (pH 2), 0°C, 30 min.
Table 1. NIS/TMSOTf-Promoted glycosylations of 6–9
through treatment of 2 with a,a-dimethoxytoluene and
with 10a
a catalytic amount of p-toluenesulfonic acid in freshly
distilled acetonitrile, which gave 3 in 95% yield. Com-
pound 3 was either benzoylated by treatment with
benzoyl chloride in pyridine to give galactoside 4 in
86% yield, or benzylated by treatment with benzyl
bromide and sodium hydride in DMF to give galac-
toside 5 in 86% yield. The azide 4 was converted to
acceptor 6 by hydrolysis of the benzylidene acetal in
aqueous acetic acid at 80°C, followed by selective 6-O-
benzoylation with benzoyl chloride in pyridine at
−10°C. The galactosyl acceptor 7 was prepared from 4
by reductive opening of the 4,6-O-benzylidene ring with
sodium cyanoborohydride in ethereal hydrogen chlo-
ride solution20 in 93% yield. Finally, compounds 8 and
Entry
Acceptor
Product
11
Yield (%)
a/b
9 were prepared from 5 as described for 6 and 7, in 78%
and 82% yields, respectively.
1
2
3
4
6 R=R%=Bz
7 R=Bz, R%=Bn 12
8 R=Bn, R%=Bz 13
49
67
76
87
\25/1
\25/1
12/1
Two trends could be observed when the four galactosyl
acceptors 6–9 were a-galactosylated with the thiogly-
coside 10 under N-iodosuccinimide/trimethylsilyl
trifluoromethanesulfonate21,22 promotion, to give the
galabiosides 11–14 (Table 1). First, the more benzyl
groups on the acceptor, thereby increasing the acceptor
nucleophilicity, led to increased yields. The major side
product was identified as the succimido glycoside corre-
sponding to 10 and with the more nucleophilic accep-
tors, this side reaction was suppressed. However, a
second trend was that increasing the number benzyl
groups on the galactosyl acceptor was accompanied by
lowered a/b-selectivity. The introduction of two or
three benzyls (i.e. entry 3 and 4, Table 1) led to the
formation of the b-anomer, which was unseparable
from the a-anomer. This is a typical example of the
need for fine-tuning reaction conditions, i.e. matching
acceptor nucleophilicity with donor reactivity, in glyco-
sylations that proceed via the in situ anomerisation
mechanism.23
9 R=R%=Bn
14
8/1
a Reaction conditions: N-Iodosuccinimide, TMSOTf, MS-AW300,
CH2Cl2:Et2O, 1:2, −50°C.
Compound 1 is ideal for the preparation of structurally
diverse galabiose derivatives by alkylation of the unpro-
tected HO-2%, followed by simultaneous removal of the
benzyl groups and reduction of the azide and final
N-acylation. In order to demonstrate the strategy
(Scheme 2), the scaffold 1 was treated with methyl
iodide and sodium hydride in DMF, yielding the inter-
mediate galabioside 16 in 90% yield. The galabioside 16
was hydrogenolyzed over Pd/C in MeOH/HCl. N-
Acylation was accomplished by the addition of Na2CO3
followed by dropwise addition of a solution of benzoyl
chloride in THF. Purification on a Sep-Pak C18 car-
tridge afforded galabioside 17 along with about 15%
methyl benzoate, which was easily removed with con-
ventional flash chromatography yielding 17 in 56%
yield.
The galactosyl acceptor 7 was chosen for further reac-
tions since it provided the highest yield without detri-
mental formation of the b-anomer. Debenzoylation of
galabioside 12 in methanolic sodium methoxide, fol-
lowed by benzylation gave the benzyl protected gal-
abioside 15 in 87% yield (Scheme 2). The
p-methoxybenzyl group of 15 was smoothly cleaved
with 2% trifluoroacetic acid in dichloromethane,16
affording the target scaffold 1 in 95% yield.
In summary, we have reported an efficient synthesis of
the galabiose scaffold 1. Compound 1 can serve as a
source for collections of diverse galabiose derivatives by
orthogonal derivatization at HO-2% and C-1. Such gal-
abiosides are potential inhibitors of Pk- and P-antigen

J. Ohlsson, U. J. Nilsson / Tetrahedron Letters 44 (2003) 2785–2787
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Scheme 2. Reagents and conditions: (a) i. NaOMe, MeOH,
CH2Cl2, ii. NaH, BnBr, DMF. (b) 2% TFA, CH2Cl2, 0°C. (c)
NaH, MeI, DMF. (d) i. H2, HCl (aq), Pd/C, MeOH, ii. BzCl,
Na2CO3, THF.
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Acknowledgements
This work was supported by grants from the Swedish
Research Council and from the program ‘Glycoconju-
gates in Biological Systems’ sponsored by the Swedish
Foundation for Strategic Research.
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