Synthesis of ethyl 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)-beta-D- galactofuranoside present in motif E of the Mycobacterium tuberculosis cell wall.

Article abstract of DOI:10.1021/ol0002976

[figure: see text] The stereocontrolled synthesis of the trisaccharide ethyl 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)- beta-D-galactofuranoside present in motif E of the Mycobacterium tuberculosis cell wall is described.

Full text of DOI:10.1021/ol0002976

ORGANIC
LETTERS
2001
Vol. 3, No. 3
321-323
Synthesis of Ethyl
5-O-(r-D-arabinofuranosyl)-6-O-
(â-D-galactofuranosyl)-â-D-galactofuranoside
Present in Motif E of the Mycobacterium
tuberculosis Cell Wall
Mukund K. Gurjar,* L. Krishnakanth Reddy, and Srinivas Hotha
National Chemical Laboratory, Pune 411 008, India
gurjar@dalton.ncl.res.in
Received October 3, 2000
ABSTRACT
The stereocontrolled synthesis of the trisaccharide ethyl 5-O-(r-D-arabinofuranosyl)-6-O-(â-D-galactofuranosyl)-â-D-galactofuranoside present
in motif E of the Mycobacterium tuberculosis cell wall is described.
Three million deaths occur every year due to tuberculosis,
and perhaps it constitutes the single largest lethal disease of
the world.1 The mycobacterium species, namely Mycobac-
terium tuberculosis and M. aVium and to lesser extent M.
kansasii, are associated with AIDS, and therefore, in recent
years, they have attracted unprecedented attention. The
impact of tuberculosis, as an opportunistic infection, on
developing countries has been phenomenal because of
increasing incidences of AIDS in these countries.2-4
The unique cell wall structure of M. tuberculosis contains
two major polysaccharides, lipoarabinomannan (LAM) and
arabinogalactan (AG), in which both arabinose and galactose
residues are found in the furanose form.5
The five major motifs of AG, namely motifs A-E (1 -
5), have been isolated and characterized6 as shown in Figure
1. Synthesis of these motifs provides opportunities to
investigate, in detail, their role in immunological response
arising from mycobacterial infection. The structural analysis
revealed that motifs A,7a,8a B, and C8c contain only arabino-
furanose residues, whereas motif D has exclusively galacto-
furanoses. Motif E is novel and unique because of the
presence of the both arabinofuranose and galactofuranose
residues in its structural framework. In addition, the oli-
gosaccharide structure of motif E is sterically complicated
because both the 5- and 6- positions of the reducing
(5) McNeil, M.; Wallner, S. J.; Hunter, S. W.; Brennan, P. J. Carbohydr.
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Verlag: Berlin, 1996; Chapter 1, p 215.
(2) Spinney, L. New Sci. 1996, 8.
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Immune Defic. Syndr. 1991, 4, 516.
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R.; Brennan, P. J. Biochemistry 1995, 34, 4257. (b) Wolucka, B. A.; McNeil,
M. R.; de Hofffman, E.; Chojnacki, T.; Brennan, P. J. J. Biol. Chem. 1994,
269, 23328. (c) McNeil, M. R.; Daffe, M.; Brennan, P. J. J. Biol. Chem.
1990, 265, 18200. (d) Daffe, M.; Brennan, P. J.; McNeil, M. J. Biol. Chem.
1990, 265, 6734. (e) Lee, R. E.; Mikusˇova´, K.; Brennan, P. J.; Besra, G. S.
J. Am. Chem. Soc. 1995, 117, 11829.
10.1021/ol0002976 CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/13/2001

acetyl-R-D-arabinofuranose (9)10 with tri-n-butyltin ethoxide
in refluxing ClCH2CH2Cl and subsequent exposure of 10
with CCl3CN-DBU-CH2Cl2 at 0 °C (Scheme 2).
Scheme 2
A coupling reaction11 between 8 and 11 was carried out,
with BF3:OEt2 as an activator, to give the disaccharide 12
which was isolated in 70% yield and characterized after
Zemple´n deacetylation12 (Scheme 3). The 1H NMR and 13
C
Scheme 3
Figure 1.
galactofuranose component are linked with arabinofuranosyl
and galactofuranosyl residues, respectively. The synthesis
of the pentaarabinofuranoside of motif A (1) was first
described from our laboratories. This communication reports
the first stereocontrolled synthesis of 5-O-(R-D-arabinofura-
nosyl)-6-O-(â-D-galactofuranosyl)-â-D-galactofuranose as eth-
yl glycoside 6 (Figure 1) belonging to motif E.
Our first concern was the preparation of the aglycone (8)
in which the 2- and 3-positions are blocked with permanent
protecting benzyl ethers while the 6-position is blocked with
a temporary protecting TBDPS ether. The known ethyl 2,3-
di-O-benzyl-â-D-galactofuranose 7,9 obtained from D-galac-
tose in 5 steps, was treated with TBDPS-chloride in the
presence of imidazole to give the 6-O-TBDPS derivative 8
(Scheme 1).
NMR spectra of 13 were in complete agreement with the
assigned structure.13 The location of the signal due to C-1′
at δ 104.5 in the 13C NMR spectrum of 13 confirmed the
R-linkage at the newly formed glycosidic bond.13 Compound
13 was transformed into the 2,2′,3,3′,5′-penta-O-benzyl
Scheme 1
(7) (a) Gurjar, M. K.; Hotha, S.; Mereyala, H. B. J. Chem. Soc., Chem.
Commn. 1998, 685. Other interesting reports on mycobacteria: (b) Gurjar,
M. K.; Adhikari, S. Tetrahedron 1997, 53, 8629. (c) Gurjar, M. K.; Reddy,
K. R. J. Chem. Soc., Perkin Trans 1 1993, 1265. (d) Gurjar, M. K.; Saha,
U. K. Bioorg. Med. Chem. Lett. 1993, 3, 697. (e) Gurjar, M. K.; Mainkar,
P. S. Carbohydr. Res. 1993, 239, 297. (f) Gurjar M. K.; Saha, U. K.
Tetrahedron Lett. 1992, 33, 4979. (g) Gurjar, M. K.; Mainkar, A. S.
Tetrahedron 1992, 48, 6729. (h) Gurjar, M. K.; Saha, U. K. Tetrahedron
1992, 48, 4039. (i) Gurjar, M. K.; Reddy, K. R. Carbohydr. Res. 1992,
226, 232. (j) Gurjar, M. K.; Viswanadham, G. Tetrahedron Lett. 1991, 32,
6191. (k) Gurjar, M. K.; Viswanadham, G. J. Carbohydr. Chem. 1991, 10,
481.
(8) (a) D’Souza, F. W.; Lowary, T. L. Org. Lett. 2000, 2, 1493. (b)
D’Souza, F. W.; Ayers, J. D.; McCarren, P. R.; Lowary, T. L. J. Am. Chem.
Soc. 2000, 122, 1251. (c) Ayers, A. D.; Lowary, T. L.; Morehouse, C. B.;
Besra, G. S. Bioorg. Med. Chem. Lett. 1998, 8, 437.
2,3,5-Tri-O-acetyl-D-arabinofuranosyl trichloroimidate (11),
as the glycosylating agent, was obtained by selective
deprotection of the 1-O-acetyl group of 1,2,3,5-tetra-O-
322
Org. Lett., Vol. 3, No. 3, 2001

derivative (14) by using NaH and benzyl bromide in DMF.
Removal of the temporary TBDPS protecting group with 1
M Bu4NF solution in THF gave the aglycone (15) containing
the 6-hydroxyl (Scheme 3).
107.8. Finally, compound 19 was deprotected by hydro-
genolysis over 10% Pd(OH)2/C in methanol to give the target
molecule 6 (Scheme 5).
The glycosidation reaction of 15, with substituted galacto-
furanosyl derivatives, turned out to be a difficult proposition
because trichloroacetaimidate methodology,11 SEt promoted
glycosidation,14 and Helfrich reaction14 failed in our hands.
Finally, Fraser-Ried’s n-pentenyl glycosidation technique15
was successful. Thus, 1,2,3,5,6-penta-O-acetyl-â-D-galacto-
furanose 1616 and 4- pentene-1-ol in the presence of a
catalytic amount of BF3:OEt2 in CH2Cl2 at 0 °C gave the
â-pentenyl glycoside derivative 17 (Scheme 4).
Scheme 5
Scheme 4
1
The high resolution H NMR spectrum of 6 showed
resonances due to anomeric protons at δ 4.89, 4.93, and 5.10
as singlets. The chemical shifts of anomeric carbons were
revealed at δ 106.5, 107.5, and 108.2 in the 13C NMR
spectrum. In addition, the FAB-MS and elemental analysis
of 6 were in complete agreement with the assigned structure.
In conclusion, this communication reports the first syn-
thesis of a novel trisaccharide present in motif E of
Mycobacterium tuberculosis. The arabinogalactan caused
profound interest for two fundamental reasons, (1) it appears
to be essential for viability and (2) three out of the four
sugars, namely araf, galf, and rhamp, are not found in
humans. Only recently it has been established that etham-
butol, the drug of choice for TB since 35 years, is involved
in the inhibition of biosynthetic pathway of arabinan, thus
establishing it to be a valuable target for the discovery of
new drugs.6e,17
The coupling reaction between 15 and 17 in the presence
of N-iodosuccinamide and triflic acid (cat.) in CH2Cl2 at
ambient temperature for 48 h gave the trisaccharide 18. The
Zemplen deacetylation of 18 provided the penta-O-benzy-
lated derivative 19 which was analyzed by the 1H NMR and
13C NMR spectral data. For example, in the 1H NMR
spectrum of 19, signals due to three anomeric protons were
observed at δ 4.85, 4.95, and 5.17 as singlets. The 13C NMR
spectrum showed anomeric carbons at δ 105.3, 107.0, and
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Carbohydr. Res. 1989, 185, 27.
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(15) (a) Mootoo, D. R.; Konradsson, P.; Udodong, U.; Fraser-Ried, B.
J. Am. Chem. Soc. 1988, 110, 5583. (b) Konradsson, P.; Mootoo, D. R.;
McDevitt, R. E.; Fraser-Reid, B. J. Chem. Soc., Chem. Commun. 1990,
270. (c) Fraser-Reid, B.; Udodong, U. E.; Wu, Z.; Ottosson, H.; Merritt, J.
R.; Rao, C. S.; Roberts, C.; Madsen, R. Synlett. 1992, 927.
(16) Chittenden, G. J. F.; Carbohydr. Res. 1972, 25, 35.
Acknowledgment. L.K.R. and S.H. thank CSIR (New
Delhi) for financial assistance in the form of a Senior
Research Fellowship.
Supporting Information Available: 1H and 13C NMR
spectra of 6, 13, 14, and 19. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL0002976
(17) (a) Deng, L.; Mikusˇova´, K.; Robuck, K. G.; Scherman, M.; Brennan,
P. J.; McNeil, M. R. Antimicrob. Agents Chemother. 1995, 39, 694.
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