Novel Galf-disaccharide mimics: synthesis by way of 1,3-dipolar cycloaddition reactions in water

Article abstract of DOI:10.1016/j.tetasy.2008.08.006

Galactofuranose-disaccharide analogues incorporating a 1,4-dideoxy-1,4-imino-d-galactitol moiety were obtained by 1,3-dipolar cycloadditions of a galacto-configured cyclic nitrone with arabino- or galacto-furanosides carrying a C-vinyl or O-allyl substituent. Remarkably, the cycloadditions could be performed highly efficiently and stereoselectively in water using unprotected nitrone and sugar-derived dipolarophile as reaction partners. The resulting pseudo-disaccharides are analogues of the Galf-Galf motifs found in mycobacterial cell-wall glycans and therefore constitute mimics of the acceptor substrates of mycobacterial Galf-transferases.

Full text of DOI:10.1016/j.tetasy.2008.08.006

Tetrahedron: Asymmetry 19 (2008) 1999–2002
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Novel Galf-disaccharide mimics: synthesis by way of 1,3-dipolar
cycloaddition reactions in water
*
Virginie Liautard, Valérie Desvergnes , Olivier R. Martin
Institut de Chimie Organique et Analytique, UMR 6005, University of Orléans and CNRS, Rue de Chartres, 45067 Orléans Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2008
Accepted 6 August 2008
Galactofuranose-disaccharide analogues incorporating a 1,4-dideoxy-1,4-imino-D-galactitol moiety were
obtained by 1,3-dipolar cycloadditions of a galacto-configured cyclic nitrone with arabino- or galacto-fur-
anosides carrying a C-vinyl or O-allyl substituent. Remarkably, the cycloadditions could be performed
highly efficiently and stereoselectively in water using unprotected nitrone and sugar-derived dipolaro-
phile as reaction partners. The resulting pseudo-disaccharides are analogues of the Galf-Galf motifs found
in mycobacterial cell-wall glycans and therefore constitute mimics of the acceptor substrates of myco-
bacterial Galf-transferases.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Within a research program dedicated to the search of new com-
pounds as potential inhibitors of mycobacterial galactan biosyn-
The cell wall of Mycobacteria is an impenetrable barrier, which
provides the organism with great protection towards its environ-
ment. It consists of the so-called mAGP complex, a three-layer
assembly of biomolecules (mycolic acids, arabinogalactans and
peptidoglycans) and of the lipopolysaccharide LAM (lipoarabino-
mannan).1 The central glycan section of the mAGP complex
comprises a unique galactofuran core, which contains about 30
thesis, we have already reported the preparation and evaluation
of several novel UDP-Galf-like compounds, incorporating a 1,4-imi-
nogalactitol moiety as UGM inhibitors.9 We have now extended
our investigations to the analogues of galactan fragments, mimick-
ing the acceptor substrate of the mycobacterial Galf-transferases.
Very few examples of iminosugar-containing oligo-arabino- or
galacto-furanosides have been reported so far.10 Herein, we report
the first results of our synthetic studies towards novel Galf-disac-
charide analogues incorporating a 1,4-dideoxy-1,4-iminogalactitol
moiety linked to O-5, C-5 or O-6 of the Galf unit, having the general
formula shown in Figure 1.
D
-galactofuranosyl (Galf) residues linked by alternating b-(1?5)
and b-(1?6) bonds, and connected to the peptidoglycan base by
an -Rha-(1?3)- -GlcNAc-1-P tether. Of particular interest is
a-
L
a-D
the fact that galactose in its furanose form is found only in prokary-
otes and some lower eukaryote organisms.2 Galf-containing oligo-
saccharides are essential for the survival and infectivity of
mycobacteria:3 the biosynthesis of galactofuranose-containing gly-
cans has thus become a promising target for the development of
new antimycobacterial agents.4
The repeating Galf unit is formed by an unprecedented ring-
contraction of UDP-Galp into UDP-Galf catalysed by UDP-Gal
mutase (UGM).5 The sugar nucleotide is then substrate of Galf-
transferases6 for the formation of the glycofuranoside linkages in
the growing glycan chain. The recent work by Lowary7 and Besra8
has shed light on these elusive transferases and on the mechanism
of galactan formation: remarkably, two Galf-transferases appear to
be sufficient enough to account for the assembly of the entire cell-
wall Galf-containing glycan of mycobacteria, both of which are
behaving as bifunctional transferases.
HO
OH
H
(CH2)n
HO
N
O
HOH2C
OCH3
OH
OH
HO
OH
HO
n = 0 or 1
Figure 1. Target disaccharide analogues.
2. Results and discussion
Our synthetic approach is based on the coupling of the sugar
entities by a 1,3-dipolar cycloaddition process11 between the imi-
nogalactitol-derived nitrone 1 and
L
-arabino- or
D
-galacto-furano-
* Corresponding author. Tel.: +33 238 49 45 81; fax: +33 238 41 72 81.
E-mail address: olivier.martin@univ-orleans.fr (O. R. Martin).
Present address: Institut des Sciences Moléculaires, UMR 5255, Université
Bordeaux 1 and CNRS, 351 Cours de la Libération, 33405 Talence Cedex, France.
sides, carrying vinyl group as
a
a
dipolarophile. A
major
advantage of this methodology is the very high b-stereoselectivity
of the cycloadditions onto nitrone 1, as shown in our previous
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.08.006

2000
V. Liautard et al. / Tetrahedron: Asymmetry 19 (2008) 1999–2002
studies,9b which ensures the proper b-configuration of the pseudo-
galactofuranose component of the disaccharide analogues. In the
first experiment, we reacted 5-O-allyl-L-arabinofuranoside deriva-
We therefore decided to investigate the cycloaddition process
using deprotected reaction partners; cycloaddition reactions of this
type are indeed compatible with free OH groups,19 and can be per-
formed in water.20 We first prepared the unprotected 6-O-allyl-b-
tive 212 with nitrone 1 (Scheme 1). The reaction gave the desired
cycloaddition product 3,13 resulting from an anti-exo cycloaddition
stereochemistry, as the major product, in 49% (isolated) yield
(dr P 92%, 1H NMR of crude product). The isoxazolidine ring was
reductively opened with Zn in the presence of catalytic copper ace-
tate14 to afford protected pseudo-disaccharide 4 in good yield.
In order to reach disaccharide mimics, which more closely
resemble the structure of the parent Galf disaccharides, we also
D
-galactofuranoside 9 from the free galactoside21 and examined
its reaction with nitrone 1 (Scheme 3). The reaction could still be
run in toluene at reflux temperature, and afforded the cycloaddi-
tion product 10 in 35% yield. We then proceeded to deprotect nit-
rone 1. In spite of the sensitivity of this compound, we were able to
cleave all the benzyl groups from 1 to give 8 in good yield (66%)
using a large excess of BCl3,22 a reagent that already proved to be
useful in the deprotection of other benzylated iminosugar deriva-
tives in our work including our UDP-Galf mimics.9 The reaction
of 8 with 9 was then investigated in water at 60 °C;23 although
the reaction was slow, it afforded the desired, unprotected cycload-
dition product 11 in a yield of 51%, with excellent stereoselectivity.
The N-O bond in 11 could be cleaved using Raney Ni24 to give pseu-
do-1,6-linked galactofuranobioside 12, thus avoiding the separa-
tion of the final product from metal salts. The cycloaddition of 8
with 9 is noteworthy, in that it provides a means of coupling two
free sugar units by a carbon-carbon linkage in water, and thus
opens a concise approach to a diversity of disaccharide mimics.
While 1,3-dipolar cycloadditions of sugar-derived nitrones11 have
been used to prepare disaccharide mimics including an imino-C-
disaccharide,25 this is the first example of coupling of two free su-
gar derivatives by such a reaction in water.
investigated the cycloaddition of 1 with the 5-C-vinyl-L-arabino-
furanoside derivative 515; the cycloaddition proceeded here highly
stereoselectively and in a quite satisfactory yield to give the bicy-
clic product 6 (Scheme 2). The N–O bond in 6 was cleaved under
the same conditions to give the protected b-C-linked Galf-disac-
charide mimics 7. While a diversity of imino-C-disaccharides has
already been reported, in particular by Vogel et al.,16 this com-
pound is one of the rare examples of C-linked imino-disaccharides
incorporating two five-membered ring units;17 in its deprotected
form, compound
7 has a structure that closely mimics a
galactofuranobioside.
At this point, debenzylation of compounds 4 and 7 was at-
tempted in order to obtain the corresponding pseudo-disaccha-
rides as free glycosides for biological assays. This step met,
however, with unexpected difficulties: catalytic hydrogenation un-
der a wide variety of conditions (Pd/C, Pd(OH)2, etc.), cleavage un-
der dissolving metal conditions (Na/NH3) and other methods (BCl3/
CH2Cl2)18 either did not provide the final product or promoted deg-
radation reactions; the BCl3-technique was clearly unsatisfactory
here because of the sensitivity of the glycosidic linkage under these
conditions.
We also explored the synthesis of structures that mimic 1,5-
linked galactofuranose disaccharides. For this purpose, the 5-O-al-
lyl-b-D-galactofuranoside 17 was prepared from methyl b-D-galac-
tofuranoside 13,26 by way of derivative 14, as shown in Scheme 4.
Acetylation of 14, then cleavage of the isopropylidene group fol-
lowed by tritylation27 gave compound 15, which carries a free
OCH3
O
O
N
O
OBn
O
N
OBn
O
OCH3 toluene
BnO
OBn
O
110 °C
OBn
12-14 h
OBn
BnO
OBn
BnO OBn
OBn
BnO
49%
OBn
2
1
3
Zn, Cu(OAc)2
AcOH/H2O
H
N
OCH3
O
O
80 °C
76%
OH
OBn
OBn
BnO OBn
BnO
4
Scheme 1.
HO
O
OCH3
OBn
O
N
OBn
toluene
110 °C
O
N
OBn
O
OCH3
BnO
OBn
HO
OBn
BnO
14 h
64%
OBn
BnO
OBn
OBn
BnO
1
5
6
OBn
OH
OH
Zn, Cu(OAc)2
AcOH/H2O
OH
OH
HO
H
N
O
O
O
OCH3
OBn
O
OCH3
OH
OH
HO
OBn
80 °C, 1 h 30
86%
OH
BnO
BnO
OBn
β-Galf-(1-6)-Galf-OMe
7
Scheme 2.

V. Liautard et al. / Tetrahedron: Asymmetry 19 (2008) 1999–2002
2001
OCH3
OH
O
1: toluene
O
N
OR
OH
110 °C, 14 h
35%
O
O
OCH3
OH
O
N
OR
RO
OH
O
OR
8: H2O
HO
OH
RO
OR
60 oC, 10 d
51%
OR
OR
1 R = Bn
8 R = H
10 R = Bn
11 R = H
9
OCH3
OH
O
OCH3
OH
O
H2
Ni Raney
OH
O
OH
OH
O
OH
H
N
N
O
H2O,16 h
86%
OH
OH
OH
HO
OH
OH
HO
OH
OH
11
12
Scheme 3.
1. Py, Ac2O
2. AcOH,
H2O/THF
50 °C 73%
OH
OH
O
O
O
O
OCH3
O
OCH3
O
OCH3
O
OH
3. Pyridine
TrCl
CSA, DMF
48 h, 0 °C
OTr
AcO
HO
OH
OAc
HO
OH
16 h, 60 °C
61%
13
14
15
89%
1. AcOH 80%
reflux
O
O
Pd(PPh3)4,
O
82%
OCH3
O
OCH3
OH
Methylallylcarbonate
THF, reflux, 3 h
59-68%
OTr
AcO
2. MeONa,
MeOH-CH2Cl2
99%
OH
OAc
HO
16
17
Scheme 4.
HO
O
HO
O
OCH3
O
OCH3
O
H2O, 5 d
O
N
OH
OH
HO
HO
OH
H
N
60°C
H2
8 + 17
HO
OH
72%
OH
HO
Raney Ni
16h,
91%
OH
OH
OH
18
OH
19
Scheme 5.
OH group at C-5. Allylation of this OH group proved to be a chal-
lenge: the alkylation was successful by Pd-catalysed allyl transfer
onto this position, to give 16, a process28 that is particularly conve-
nient for the allylation of OH groups in sensitive environments
such as sugar derivatives carrying esters or aminosugar derivatives
carrying carbamates. Removal of the trityl and acetyl groups in 16
afforded the selectively 5-O-allylated galactofuranose derivative
17, in 22% overall yield from 13. The cycloaddition of 17 with free
nitrone 8 afforded bicyclic product 18 in 72% yield after 5 d at
60 °C, with excellent stereoselectivity (Scheme 5).
disaccharide 19 in two steps only from free monosaccharidic part-
ners and in 66% overall yield. Compound 19 is a mimic of the
b-(1?5)-linked galactobioside motif found in mycobacterial galac-
tans. Pseudo-disaccharides 12 and 19 are also analogues of homo-
DMDP-7-O-apioside, a natural product isolated from the leaves of
bluebells (Hyacinthoides non-scripta).29
3. Conclusion
In conclusion, we have demonstrated in this study that complex
structures of significant biological interest could be readily reached
The N–O bond in 18 was cleaved as in compound 11 using Ra-
ney Ni. This remarkable sequence of reactions afforded pseudo-

2002
V. Liautard et al. / Tetrahedron: Asymmetry 19 (2008) 1999–2002
13. All new compounds gave correct elemental analyses and/or HRMS data.
14. Karanjule, N. S.; Markad, S. D.; Sharma, T.; Sabharwal, S. G.; Puranik, V. G.;
Dhavale, D. D. J. Org. Chem. 2005, 70, 1356–1363.
by the 1,3-dipolar cycloaddition of two free sugar derivatives, car-
rying appropriate functional groups in water; in addition, by using
this method, we have achieved the synthesis of new types of pseu-
do-disaccharides incorporating both an imino-galactofuranose and
a normal galactofuranose unit, and thus mimicking fragments of
mycobacterial galactans. Such compounds are potential inhibitors
of the enzymes involved in the biosynthesis of mycobacterial
cell-wall glycans. Biological investigations on these compounds
are currently in progress, and the results will be reported in due
course.
15. Experimental procedure for 5: To a solution of methyl 2,3-di-O-benzyl-b-L-
arabino-pentodialdofuranoside: Tadahiro, I.; Hiromichi, S. Chem. Pharm. Bull.
1967, 15, 132–135; Hiromichi, S.; Tadahiro, I. Chem. Pharm. Bull. 1968, 16,
1129–1132 (170 mg, 0.50 mmol) in THF (3.4 mL) was added, at 0 °C, a 1 M
solution of vinylmagnesium bromide in THF (2 mL). After stirring for 3 h at 0 °C
and 12–14 h at rt, the reaction was quenched by adding saturated aqueous
ammonium chloride. The mixture was extracted with ethyl acetate. The
organic phase was separated, washed with brine and dried over sodium
sulfate. Solvents were evaporated, and purification on silica gel afforded
compound 5 (130 mg, 71%) as a mixture of separable diastereoisomers (dr
28:72).
16. For a review on imino-C-disaccharides synthesis: (a) Robina, I.; Vogel, P.
Synthesis 2005, 675–702; (b) Vogel, P.; Gerber-Lemaire, S.; Juilleret-Jeannerat,
L. Imino-C-Disaccharides and Analogues: Synthesis and Biological Activity. In
Iminosugars: From Synthesis to Therapeutic Application; Compain, P., Martin, O.
R., Eds.; Wiley: Chichester, 2007; pp 87–123; For related imino-C-
disaccharides precursors see: (c) Cardona, F.; Goti, A.; Brandi, A. Org. Lett.
2003, 5, 1475–1478; (d) Cardona, F.; Lalli, C.; Faggi, C.; Goti, A.; Brandi, A. J. Org.
Chem. 2008, 73, 1999–2002.
17. (a) Marquis, C.; Cardona, F.; Robina, I.; Wurth, G.; Vogel, P. Heterocycles 2002,
56, 181–208; (b) Dondoni, A.; Giovannini, P. P.; Perrone, D. J. Org. Chem. 2002,
67, 7203–7214.
18. Williams, D. R.; Brown, D. L.; Benbow, J. W. J. Am. Chem. Soc. 1989, 111, 1923–
1925.
Acknowledgement
Dr. J. C. Jacquinet is warmly acknowledged for discussions and
advice.
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8
and
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