Chemical synthesis of cyclic galactooligofuranosides isolated from enzymatic degradation products of cell wall arabinogalactan of Mycobacterium tuberculosis

Article abstract of DOI:10.1021/ol800530u

(Chemical Equation Presented) Synthesis of cyclic tetra-, hexa- and octasaccharides containing alternating (1→5)-β- and (1→6)-β-galactofuranosyl linkages has been achieved by intramolecular cycloglycosylation of corresponding linear sugars and by cyclooligomerization of 1,6-linked and 1,5-linked disaccharides. In particular, cyclooligomerization of the (1→6)-β-galactofuranosyl disaccharide provides an efficient way to secure all three cyclic sugars in one operation.

Full text of DOI:10.1021/ol800530u

ORGANIC
LETTERS
2008
Vol. 10, No. 12
2373-2376
Chemical Synthesis of Cyclic
Galactooligofuranosides Isolated from
Enzymatic Degradation Products of Cell
Wall Arabinogalactan of Mycobacterium
tuberculosis
Kwan Soo Kim,* Bo-Young Lee, Sung Ho Yoon, Hyo Jin Jeon, Ju Yuel Baek, and
Kyu-Sung Jeong
Center for BioactiVe Molecular Hybrids and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, Korea
kwan@yonsei.ac.kr
Received March 8, 2008
ABSTRACT
Synthesis of cyclic tetra-, hexa- and octasaccharides containing alternating (1f5)-ꢀ- and (1f6)-ꢀ-galactofuranosyl linkages has been achieved
by intramolecular cycloglycosylation of corresponding linear sugars and by cyclooligomerization of 1,6-linked and 1,5-linked disaccharides.
In particular, cyclooligomerization of the (1f6)-ꢀ-galactofuranosyl disaccharide provides an efficient way to secure all three cyclic sugars in
one operation.
In recent years, a great deal of effort has been devoted to
the elucidation of the cell wall structure1 of Mycobacterium
tuberculosis, the causatiVe agent of human tuberculosis, due
to the emergence of multidrug resistant strains.2 One of the
major structural components of the cell wall of M. tubercu-
losis is the complex of mycolic acid, arabinogalactan, and
peptidoglycan (mAGP complex), which plays a crucial role
in the survival and pathogenicity of M. tuberculosis. Active
structural investigations have revealed the structural relation-
ship of arabinogalactan (AG) with other parts of the mAGP
complex and the detailed structure of AG, which consists of
D-arabinan and D-galactan.3 Ethambutol, an effective anti-
tuberculosis drug, inhibits the polymerization step of D-
arabinan biosynthesis4 while the D-galactan moiety was
suggested to be essential for the growth and viability of
(1) For selected reviews on the structure of mycobacterial cell wall, see:
(a) Brennan, P. J. Tuberculosis 2003, 83, 91–97. (b) Lowary, T. L. In
Glycoscience: Chemistry and Chemical Biology; Springer-Verlag: Berlin,
2001; 2005-2080. (c) Chatterjee, D. Curr. Opinion Chem. Biol. 1997, 1,
579–588. (d) Brennan, P. J.; Nikaido, H. Annu. ReV. Biochem. 1995, 64,
29–63.
(3) (a) Daffe, M.; Brennan, P. J.; McNeil, M. J. Biol. Chem. 1990, 265,
6734–6743. (b) McNeil, M.; Daffe, M.; Brennan, P. J. J. Biol. Chem. 1990,
265, 18200–18206. (c) Besra, G. S.; Khoo, K.-H.; McNeil, M. R.; Dell,
A.; Morris, H. R.; Brennan, P. J. Biochemistry 1995, 34, 4257–4266. (d)
Lee, R. E. B.; Li, W.; Chatterjee, D.; Lee, R. E. Glycobiology 2005, 15,
139–151.
(2) For multi-drug resistance of M. tuberculosis strains, see: (a)
Nettleman, M. D. JAMA 2005, 293, 2788–2790. (b) Long, R. CMAJ 2000,
163, 425–428. (c) Pablos-Mendez, A; Raviglione, M. C.; Laszlo, A.; Binkin,
N.; Rieder, H. L.; Bustreo, F.; Cohn, D. L.; Weezenbeek, C. S. B, L.; Kim
S., J.; Chaulet, P.; Nunn, P. N. Eng. J. Med. 1998, 338., 1641–1649.
(4) (a) Takayama, K.; Kilburn, J. O. Antimicrob. Agents Chemother.
1989, 33, 1493–1499. (b) Mikusov, K.; Slayden, R. A.; Besra, G. S.;
Brennan, P. J. Antimicrob. Agents Chemother. 1995, 39, 2484–2489.
10.1021/ol800530u CCC: $40.75
Published on Web 05/22/2008
2008 American Chemical Society

mycobacteria, and thus, the Galf metabolism is a potential
target for development of new antituberculosis drugs.5 During
the structural study of AG, Brennan and co-workers unex-
pectedly identified novel cyclic galactooligosaccharides 1,
2, and 3 among the degradation products of AG by
extracellular enzymes isolated from the bacterium Cellu-
lomonas sp.6 Compounds 1-3 are attractive synthetic targets
due to their structural uniqueness, their potential ability to
form inclusion complexes, and their potential inhibitory
activities on the Galf metabolism of M. tuberculosis.
Although there have been reports on synthetic cyclic ga-
lactofuranosides by Kochetkov and co-workers,7 they all
contain a single type of the Galf linkage such as (1f6)-ꢀ-,
(1f3)-ꢀ-, or (1f5)-ꢀ-linkage, while compounds 1-3 have
alternating (1f5)-ꢀ- and (1f6)-ꢀ-Galf linkages. Neither
their molecular geometries nor their capabilities to form
inclusion complexes have been investigated probably due
to low yields in the Kochetkov’s synthesis of the cyclic
sugars.
glycosides and 2′-carboxybenzyl (CB) glycosides8 would be
used here extensively for the construction of linear oligosac-
charides and their cyclization. Retrosynthesis of 4-6 leads
to disaccharide 9, and further analysis of disaccharide 7-9
provides monosaccharides 10-13, which would be synthe-
sized from a common starting materail 14.
The synthesis commenced with preparation of four
monosaccharide building blocks 10-13 from known com-
pound 149 (see the Supporting Information). We decided to
synthesize at first each of the target molecules 1-3 by
cyclization of linear sugars 4-6, respectively (method A).
Coupling of 10 with 11 was carried out by sequential addition
of Tf2O and 10 to a solution of 11 in the presence of 2,6-
di-tert-butyl-4-methylpyridine (DTBMP) in CH2Cl2 at -78
°C to give desired ꢀ-disaccharide 9 as shown in Scheme 2.
Scheme 2
Herein, we describe synthesis of cyclic sugars 1-3 by
employing two different methodologies, one by intramo-
lecular cycloglycosylation of linear oligosaccharides 4-6
(method A in Scheme 1) and another by cyclooligomerization
of (1f5)-ꢀ-disaccharide 7(method B-1) and (1f6)-ꢀ-
Scheme 1
Compound 9 was converted into disaccharide donor 15 by
selective hydrogenolysis and into disaccharide acceptor 16
by desilylation. Coupling of 15 and 16, however, afforded
desired tetrasaccharide 17 in only 27% yield along with the
self-condensed ester8b of 15 in 52% yield. The poor results
with the glycosyl donor 15 in the glycosylation of 16 led us
to examine other glycosyl donors such as glycosyl fluorides.
We have previously shown that CB glycosides could be
readily converted into glycosyl fluorides by treatment with
Tf2O and HF/pyridine9 or Tf2O and (diethylamino)sulfur
trifluoride (DAST).10 Both HF/pyridine and DAST with
Tf2O, were not quite satisfactory for the conversion of 15
into galactosyl fluoride 18, while a combination of Tf2O,
bis(2-methoxyethyl)aminosulfur trifluoride (Deoxofluor),11
and HF/pyridine cleanly converted 15 into 18.12
Glycosylation of 16 with the fluoride donor 18 using SnCl2
and AgClO4 in ether13 at -10 °C afforded exclusively the
desired ꢀ-tetrasaccharide 17 in 62% yield (Scheme 3).
(7) (a) Backinowsky, L. V.; Nepogodiev, S. A.; Kochetkov, N. K.
Carbohydr. Res. 1989, 185, C1-C3. (b) Kochetkov, N. K.; Nepogodiev,
S. A.; Backinowsky, L. V. Tetrahedron 1990, 46, 139–150. (c) Nepogodiev,
S. A.; Backinowsky, L. V.; Kochetkov, N. K. Russ. Chem. Bull. 1993, 42,
1418–1422.
disaccharide 8 (method B-2). The latent-active glycosylation
strategy employing 2′-(benzyloxycarbonyl)benzyl (BCB)
(8) (a) Kim, K. S.; Kim, J. H.; Lee, Y. J.; Lee, Y. J.; Park, J. J. Am.
Chem. Soc. 2001, 123, 8477–8481. (b) Kim, K. S.; Kang, S. S.; Seo, Y. S.;
Kim, H. J.; Lee, Y. J.; Jeong, K.-S. Synlett 2003, 1311–1314.
(9) Lee, Y. J.; Lee, B.-Y.; Jeon, H. B.; Kim, K. S. Org. Lett. 2006, 8,
3971–3974.
(5) (a) Kremer, L.; Dover, L. G.; Morehouse, C.; Hitchin, P.; Everett,
M.; Morris, H. R.; Dell, A.; Brennan, P. J.; McNeil, M. R.; Flaherty, C.;
Ducan, K.; Besra, G. S. J. Biol. Chem. 2001, 276, 26430–26440. (b) Pan,
F.; Jackson, M.; Ma, Y.; McNeil, M. J. Bacteriol. 2001, 183, 3991–3998.
(6) McNeil, M. R.; Robuck, G.; Harter, M.; Brennan, P. J. Glycobiology
1994, 4, 165–173.
(10) Lee, Y. J.; Baek, J. Y.; Lee, B.-Y.; Kang, S. S.; Park, H.-S.; Jeon,
H. B.; Kim, K. S. Carbohydr. Res. 2006, 341, 1708–1716.
2374
Org. Lett., Vol. 10, No. 12, 2008

Scheme 3
Scheme 4
Compound 17 was then transformed to alcohol 19 by
desilylation and subsequent selective hydrogenolysis of 19
afforded CB tetrasaccharide 4 bearing both glycosyl donor
and acceptor functions. Cycloglycosylation of 4 in CH2Cl2
(1.0 mM) employing Tf2O in the presence of DTBMP at
-78 °C afforded cyclic tetrasaccharide 20 in 91% yield.
Removal of benzoyl groups of 20 by NaOMe and remaining
benzyl groups by hydrogenolysis provided fully deprotected
cyclic sugar 1. Anomeric carbon chemical shifts at δ 105.1,
105.5, 105.9, and 106.5 of the linear tetrasaccharide 4 clearly
indicated that its four glycosyl linkages were in the ꢀ-con-
figuration. On the other hand, the 13C NMR spectrum of the
cyclic tetrasaccharide 20 showed only two anomeric carbon
peaks at δ 106.7 and 106.8 due to its symmetric nature. Two
anomeric proton signals at δ 5.06 (d, J ) 1.2 Hz) and 5.16
and two anomeric carbon peaks at δ 107.6 and 106.8 for 1
also indicated that all glycosyl bonds are ꢀ-linkages.9,14
The BCB tetrasaccharide 17 was transformed to CB
tetrasaccharide 21 by selective hydrogenolysis (Scheme 4).
Sequential addition of Tf2O and 21 to a solution of
disaccharide acceptor 16 in the presence of DTBMP in
CH2Cl2 at -20 °C afforded exclusively the desired ꢀ-hexaga-
lactoside 22 in 77% yield. Desilylation of 22 and subsequent
hydrogenolysis of the resulting BCB glycoside afforded CB
hexasaccharide 5 bearing both glycosyl donor and acceptor
functions. Cycloglycosylation of 5 was carried out in dilute
solution (1.0 mM in CH2Cl2) -78 °C to afford cyclic
hexasaccharide 23 in 79% yield. Removal of the benzoyl
groups of 23 and subsequent hydrogenolysis of benzyl groups
afforded cyclic hexagalatofuranoside 2. The 400 MHz NMR
spectra of 2 in CDCl3 showed two anomeric proton signals
at δ 5.07 and 5.25 as singlets and two anomeric carbon peaks
at δ 109.6 and 110.7.
(Scheme 5). Removal of TBS group in 24 followed by
selective hydrogenolysis gave CB octasaccharide 6. Cy-
Scheme 5
cloglycosylation of 6 by treatment with Tf2O in dilute CH2Cl2
afforded desired cyclic octasaccharide 25 in 66% yield.
Deprotection of the 20 benzoyl groups of 25 by NaOMe
followed by hydrogenolysis of benzyl groups provided
deprotected cyclic octasaccharide 3.
We then performed synthesis of cyclic oligosaccharides
1-3 directly from disaccharides 7 and 8 by cyclooligomer-
Linear octasaccharide 24 was prepared in 61% yield by
glycosylation of tetrasaccharide 19 with CB tetrasaccharide
21 using Tf2O in the presence of DTBMP at -20 °C
Org. Lett., Vol. 10, No. 12, 2008
2375

ization (method B). The 1,5-linked disaccharide 7 was
obtained by desilylation of 18 (see the Supporting Informa-
tion) and the 1,6-linked disaccharide 8 was prepared starting
from 12 and 13 as shown in Scheme 6. Thus, glycosylation
Table 1. Cyclooligomerization of Disaccharides 7 and 8
Scheme 6
product (yield, %)
20 23 25
entry
disaccharide
conc (mM)
1
2
3
4
5
6
7
7
7
8
8
8
1
4
20
4
20
40
70
69
69
23
28
20
3
8
15
23
32
31
0
0
0
16
16
18
of 13 with 12 gave BCB disaccharide 26, which was then
converted into CB disaccharide 27 by hydrogenolysis. After
conversion of 27 into digalactosyl fluoride 28 using Tf2O,
Deoxofluor, and HF/pyridine, the levulinyl group of 28 was
removed to give the disaccharide 8.
Cyclooligomerizations of 7 and 8 were accomplished
employing SnCl2 and AgClO4 as promoters at a few different
concentrations in ether as shown in Table 1. The reaction
with the 1,5-linked disaccharide 7 afforded cyclic tetrasac-
charide 20 as the major product and cyclic hexasaccharide
23 as the minor product without generation of cyclic
octasaccharide 25 (entries 1-3 in Table 1). On the other
hand, cyclooligomerization of 1,6-linked disaccharide 8
provided not only 20 and 23 but also a substantial amount
of 25 (entries 4-6). Especially, the reaction at 40 mM
concentration afforded 25 in up to 18% yield. Thus, we have
established a way to secure substantial amounts of all three
cyclic sugars 1, 2, and 3 for further studies.
In summary, synthesis of natural cyclic oligosaccharides
1-3 has been achieved. In particular, cyclooligomerization
of (1f6)-ꢀ-disaccharide 8 provides efficiently all three cyclic
sugars 1-3 in one operation. Studies on inclusion complexes
and the selective functionalization of these cyclic sugars are
currently underway.
Acknowledgment. This work was supported by a grant
from KOSEF through the Center for Bioactive Molecular
Hybrids (CBMH). B.-Y.L., S.H.Y., H.J.J., and J.Y.B.
acknowledge fellowships from the BK 21 program from the
Ministry of Education and Human Resources Development.
(11) Lal, G. S.; Pez, G. P.; Pesaresi, R. J.; Prozonic, F. M.; Cheng, H.
J. Org. Chem. 1999, 64, 7048–7054.
(12) A proposed mechanism for the conversion of 15 into 18 is as
follows. Addition of Tf2O to 15 would give a glycosyl triflate mixed
anhydride, and then upon addition of Deoxofluor, the triflate would be
displaced by the fluoride anion of Deoxofluor to provide a glycosyl carboxy
fluoride intermediate. Finally, HF/pyridine would facilitate the lactonization
of the carboxy fluoride to generate a glycosyl oxocarbenium ion, which
would readily react with fluoride ion to afford glycosyl fluoride 18. In fact,
we were able to isolate the glycosyl carboxy fluoride intermediate.
(13) For SnCl2/AgClO4 as the promoter for glycosyl fluoride, see:
Mukaiyama, T.; Murai,Y.; Shoda, S. Chem. Lett. 1981, 431.
(14) Gelin, M.; Ferrieres, V.; Plusquellec, D. Eur. J. Org. Chem. 2000,
1423–1431.
Supporting Information Available: Experimental pro-
cedures, characterization data, and copies of H and 13C
1
NMR spectra for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL800530U
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Org. Lett., Vol. 10, No. 12, 2008