Synthesis of pentaarabinofuranosyl structure motif A of Mycobacterium tuberculosis

Article abstract of DOI:10.1039/a707796c

The first synthesis of motif A, the branched chain arabinofuranosyl pentasaccharide [t-β-Araf-(1 → 2)-α-D-Araf]₂-3,5-α-D-Araf-(1 → 5) which constitutes the major humoral immunological epitope in the arabinogalactan cell wall of Mycobacterium tu

Full text of DOI:10.1039/a707796c

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Synthesis of pentaarabinofuranosyl structure motif A of Mycobacterium
tuberculosis
Hari Babu Mereyala, Srinivas Hotha and Mukund K. Gurjar
Indian Institute of Chemical Technology, Hyderabad 500 007, India
The first synthesis of motif A, the branched chain arabino-
furanosyl pentasaccharide [t-b-Araf-(1 ? 2)-a-D-Araf]2-
3,5-a-D-Araf-(1 ? 5) which constitutes the major humoral
immunological epitope in the arabinogalactan cell wall of
Mycobacterium tuberculosis is described.
motif A is the major humoral immunological epitope of
arabinogalactan vis a vis whole mycobacteria.2d Our interest in
the chemistry of compounds derived from M. tuberculosis has
previously resulted in the synthesis3 of oligosaccharide frag-
ments of glycolipids and glycopeptide cell wall segments. In
this report, we communicate the first synthesis of the branched
chain arabinofuranosyl pentasaccharide [t-b-Araf-(1 ? 2)-a-d-
Araf]2-3,5-a-d-Araf-(1 ? 5) which forms the crucial part of
structural motif A of the M. tuberculosis cell wall.
Tuberculosis (TB) continues to affect developing countries as
8000000 new cases and 3000000 deaths occur every year.1 As
a consequence of the HIV epidemic, the occurrence of TB
particularly in developed countries has risen sharply. The
etiological agent, Mycobacterium (M.) tuberculosis has been
extensively investigated and Fig. 1 represents a schematic
diagram of macro structural motifs (A–E) of cell wall
arabinogalactan.2a The fine structure of the cell wall of
mycobacteria allows us to understand drug and solute impene-
trability, antigen processing and presentation by accessory cells,
and aspects of immunopathogenesis. The structurally unusual
and biologically significant arabinofuranosyl residue of motif A
(1, Fig. 1) is responsible for the antigenicity of arabinoga-
lactan.2a It is speculated that, in part or complete, structural
Synthesis of 1a was initiated from d-arabinose which was
transformed into 5-O-tert-butyldiphenylsilyl-1,2-O-(propane-
2,2-diyl)-b-d-arabinofuranose (2) in two steps.4 Subsequent
desilylation using Bun NF in THF at ambient temperature gave
4
1,2-O-(propane-2,2-diyl)-b-d-arabinofuranose (3).5 Reaction
of 3 with NaH–BnBr in DMF protected both the hydroxy groups
to afford the 3,5-di-O-benzyl derivative 4. Conversion of 4 into
the n-pentenyl glycoside was effected with pent-4-en-1-ol in the
presence of TsOH to obtain a 1:1 anomeric mixture of a,b-
n-pentenyl glycosides (5 and 6) which were separated by silica
gel column chromatography (Scheme 1). In another sequence,
OH
O
O
O
O
OH
O
~
■
●
■
●
■
●
O
O
HO
O
O
HO
O
HO
OH
O
OH
O
O
O
HO
H3CO
O
O
O
OH
O
HO
O
HO
O
■
●
O
HO
7
My
¡
OH
1
■
●
◆
●
◆
●
◆
8
8
My
My
¡
1
1
1
1
1
1
.
.
My
My
My
¡
1
1
1
1
1
1
1
1
.
8
My
¡
My
8
8
.
.
1
1
1
1
My
8
My
¡
My
My
.
1
1
1
1
1
1
¡
My
¡
My
8
8
1
1
My
¡
My
1
●
■
●
1
1
1
1
1
1
.
8
.
¡
8
1
¡
¡
1
1
8
.
Rha
8
¡
GlcNAc
P
MurNGl
GlcNAc MurNGl
GlcNAc MurNGl GlcNAc MurNGl GlcNAc
GlcNAc MurNGl GlcNAc MurNGl GlcNAc MurNGl
Fig. 1 Schematic diagram of the proposed illustration of the macro structural motifs of the cell wall arabinogalactan. My, Mycolic acid; (!) t-b-d-Araf; (8)
2-a-d-Araf; (.) 3, 5-a-d-Araf; ( ~ ) t-b-d-Galf; (-) 6-b-d-Galf; (5) 5-b-d-Galf; (/) 5,6-b-d-Galf; GlcNAc, N-acetylglucosamine; Rha, rhamnose;
MurNGl, N-glycolylmuramic acid.
Chem. Commun., 1998
685

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tions. In addition, the 13C NMR spectrum of 10 showed
resonances due to anomeric carbons at dC-1 100.1, dC-1A 105.3
and dC-1B 105.5.
HO
TBDPSO
HO
O
O
O
O
O
HO
O
O
a
Ref. 4
OH
The OH group at C-5 of compound 10 was glycosylated again
with donor 9 under the conditions reported above. However, in
this reaction, a 3:2 mixture of a- and b-pentasaccharides (11a
and 11b) was formed. The major a-anomeric product (11a) was
isolated by silica gel column chromatography, hydrogenolysis
of which over Pd(OH)2/C at normal temperature and pressure
for 12 h gave the required pentasaccharide 1a. The structure of
1a was fully characterised by 1H, 13C NMR and FABMS
analysis.9
In conclusion it is pertinent to mention that resistance to the
current regime of anti-TB drugs is developing rapidly and
therefore there is a constant need to discover new drugs. It is
reported that (S,S)-ethambutol inhibits arabinan biosynthesis
and therefore the arabinan segment of the cell wall provides an
attractive target for development of new drugs because of the
xenobiotic status of the human host. The present synthesis of the
pentaarabinofuranoside of structure motif A of M. tuberculosis
cell wall opens a new vista in this direction.
HO
OH
OH
D-Arabinose
2
3
b
Ref. 6
BnO
BnO
BnO
O
O
O
BnO
O
OH
OBn
OBn
4
7
c
d
BnO
+
BnO
SPy
O
HO
O
O
HO
O
BnO
O
OBn
OBn
OBn
8
5
6
Scheme 1 Reagents and conditons: (a) Bun4NF, THF, room temp., 2 h,
96%; (b) NaH,BnBr, DMF, 0 °C–room temp., 83%; (c) Pent-4-en-1-ol,
TsOH, CH2Cl2, 60 °C, 2 h, 85%; (d) PySSPy, Bun3P, CH2Cl2, room temp.,
30 min, 98%
S. H. thanks CSIR, New Delhi, for a Junior Research
Fellowship.
Notes and References
2,3,5-tri-O-benzyl-a,b-d-arabinofuranose (7)6 was transformed
into the corresponding S-(2-pyridyl)-1-thiofuranoside 8 by
* E-mail: gurjar@csiict.ren.nic.in
reacting with 2,2A-dithiodipyridyl and Bun P in CH2Cl2.7
3
1 Current Topics in Microbiology and Immunology: Tuberculosis, ed.
T. M. Shinnick, Springer-Verlag, Berlin, 1996, vol. 215.
The coupling reaction of 5 with 8 was promoted8 by the
protocol developed in our laboratory, according to which 5%
MeI in dry CH2Cl2 was used as an activator to give the
2 (a) G. S. Besra, K.-H. Khoo, M. R. McNeil, A. Dell, H. R. Morris and
P. J. Brennan, Biochemistry, 1995, 34, 4257; (b) B. A. Wolucka, M. R.
McNeil, E. de Hoffmann, T. Chojnacki and P. J. Brennan, J. Biol. Chem.,
1994, 269, 23 328; (c) M. R. McNeil, M. Daffe and P. J.Brennan, J. Biol.
Chem., 1990, 265, 18 200; (d) M. Daffe, P. J. Brennan, M. McNeil,
J. Biol. Chem., 1990, 265, 6734; (e) M. McNeil, S. J. Wallner, S. W.
Hunter, P. J. Brennan, Carbohydr. Res., 1987, 166, 299; (f) R. E. Lee, K.
Mikusova, P. J. Brennan and G. S. Besra, J. Am. Chem. Soc., 1995, 117,
11 829.
3 M. K. Gurjar and S. Adhikari, Tetrahedron, 1997, 53, 8629; H. B.
Mereyala and B. R. Gaddam, Proc. Indian Acad. Sci. (Chem. Sci.), 1994,
106, 1225; M. K. Gurjar and K. R. Reddy, J. Chem. Soc., Perkin Trans.
1, 1993, 1269; M. K. Gurjar and U. K. Saha, Bioorg. Med. Chem. Lett.,
1993, 3, 697; M. K. Gurjarand P. S. Mainkar, Carbohydr. Res., 1993, 239,
297; M. K. Gurjar and U. K. Saha, Tetrahedron Lett., 1992, 33, 4979;
M. K. Gurjar and A. S. Mainkar, Tetrahedron, 1992, 48, 6729; M. K.
Gurjar and U. K. Saha, Tetrahedron, 1992, 48, 4039; M. K. Gurjar and
K. R. Reddy, Carbohydr. Res., 1992, 226, 232; M. K. Gurjar and
G. Viswanadham, Tetrahedron Lett., 1991, 32, 6191; M. K. Gurjar and
G. Viswanadham, J. Carbohydr. Chem., 1991, 10, 481.
1
b-disaccharide 9. Its structure was confirmed by H and 13C
NMR spectroscopy (Scheme 2).
The O-glycosylation of 24 with the above formed n-pentenyl
disaccharide 9 was induced in the presence of iodonium
dicollidine perchlorate (IDCP)9 in CH2Cl2, followed by desilyl-
ation of the coupled product with Bun NF in THF, resulted in
4
the isolation of the trisaccharide 10 whose newly formed
glycosidic linkage was confirmed as having an a-configuration
by the 1H NMR spectrum. For example, the characteristic
resonances due to H-1A was located at d 5.05 as a singlet,
whereas H-1 and H-1B protons appeared as doublets at d 4.90
and 5.75, respectively, as expected for b-anomeric configura-
HO
BnO
BnO
O
O
O
O
O
O
O
O
b,c
a
O
+
4 O. Dahlman, P. J. Garegg, H. Mayer and S. Schramek, Acta Chem.
Scand., Ser. B, 1986, 40, 15.
5 C. Genu-Dellac, G. Gosselin and J.-L. Imbach, Carbohydr. Res., 1991,
216, 249.
6 P. Finch, G. M. Iskander and A. H. Siriwardena, Carbohydr. Res., 1991,
210, 319.
5
8
BnO
O
BnO
O
BnO
BnO
BnO
BnO
BnO
BnO
9
10
d
7 H. B. Mereyala and G. V. Reddy, Tetrahedron, 1991, 47, 6435.
8 R. U. Lemieux and A. R. Morgan, Can. J. Chem., 1965, 43, 2190.
9 Selected data for 9: dH(200 MHz, CDCl3) for anomeric protons: 5.05 (d,
J 4.6), 5.15 (d, J 4.1); dC(50 MHz, CDCl3) for anomeric carbons: 98.5,
100.2; FABMS: 823 (M + Na)+. For 10: dH(200 MHz, CDCl3) for
anomeric protons: 4.90 (d, J 4.6), 5.05 (s), 5.76 (d, J 4.6); dC(50 MHz,
CDCl3) for anomeric carbons: 100.1, 105.3, 105.5; FABMS: 928 (M +
Na)+. For 11a: dH(200 MHz, CDCl3) for anomeric protons: 4.92 (d, J
4.6), 5.04 (s), 5.18 (d, J 4.7), 5.29 (s), 5.70 (d, J 4.8); dC(50 MHz, CDCl3)
for anomeric carbons: 100.1, 100.4, 105.2, 105.5, 106.0; FABMS: 1643
(M + Na)+. For 11b: dH(200 MHz, CDCl3) for anomeric protons: 4.90 (d,
J 4.70), 5.02 (s), 5.09 (d, J 4.65), 5.29 (d, J 4.8), 5.70 (d, J 4.7); dC (50
MHz, CDCl3) for anomeric carbons: 98.3, 100.2, 100.6, 105.1, 105.5;
FABMS: 1643 (M + Na)+. For 1a: dH(400 MHz, D2O) for anomeric
protons: 4.90 (d, J 4.20), 5.10 (d, J 4.65), 5.15 (s), 5.28 (s), 6.00 (d, J 4.8);
dC(50 MHz, D2O) for anomeric carbons: 102.0, 102.1, 106.3, 106.5,
106.9; FABMS: 741 (M + Na)+.
RO
O
O
RO
O
O
O
O
RO
O
RO
RO
OR
O
O
O
O
O
+
OR
O
O
O
O
RO
O
RO
RO
O
OR
RO
O
O
RO
O
RO
RO
RO
RO
O
RO
OR
OR
11a R = Bn
1a R = H
11b R = Bn
e
Scheme 2 Reagents and conditions: (a) 5% MeI in CH2Cl2, 57 °C, 4 Å MS
powder, 15 h, 69%; (b) IDCP CH2Cl2, 4 Å MS powder, 24 h, 62%; (c)
Bun4NF, THF, room temp., 3 h, 95%; (d) I(s-Collidine)2ClO4, CH2Cl2, 4 Å
MS powder, 12 h, 70%; (e) Pd(OH)2/C, MeOH,H2, room temp., 12 h,
97%
Received in Cambridge, UK, 29th October 1997; 7/07796C
686
Chem. Commun., 1998