Arabinofuranosides from mycobacteria: Synthesis of a highly branched hexasaccharide and related fragments containing β-arabinofuranosyl residues

Article abstract of DOI:10.1021/jo010910e

The synthesis of 11 oligosaccharides (4-14) containing β-arabinofuranosyl residues is reported. The glycans are all fragments of two polysaccharides, arabinogalactan and lipoarabinomannan, which are found in the cell wall complex of mycobacteria. In the preparation of the targets, the key step was a low-]temperature glycosylation reaction that installed the β-arabinofuranosyl residues with good to excellent stereocontrol.

Full text of DOI:10.1021/jo010910e

892
J . Org. Chem. 2002, 67, 892-903
Ar a bin ofu r a n osid es fr om Mycoba cter ia : Syn th esis of a High ly
Br a n ch ed Hexa sa cch a r id e a n d Rela ted F r a gm en ts Con ta in in g
â-Ar a bin ofu r a n osyl Resid u es
Haifeng Yin, Francis W. D’Souza, and Todd L. Lowary*
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
lowary.2@osu.edu
Received September 10, 2001
The synthesis of 11 oligosaccharides (4-14) containing â-arabinofuranosyl residues is reported.
The glycans are all fragments of two polysaccharides, arabinogalactan and lipoarabinomannan,
which are found in the cell wall complex of mycobacteria. In the preparation of the targets, the key
step was a low-temperature glycosylation reaction that installed the â-arabinofuranosyl residues
with good to excellent stereocontrol.
In tr od u ction
Over the past decade, diseases arising from mycobac-
terial infections have reemerged as global health threats.1
Mycobacterium avium infections are common in HIV-
positive individuals,2 and tuberculosis, which results from
infection by Mycobacterium tuberculosis, kills 3 million
people annually.1,3 In the future, this number is almost
certain to rise given the worldwide increase in drug-
resistant strains of this organism. A recent World Health
Organization survey of 35 countries (including the United
States) revealed that between 2 and 42% of all isolates
of M. tuberculosis are resistant to one or more antitu-
berculosis agents.4 Statistics such as these underscore
the importance of identifying new therapies for the
treatment of these diseases.5
The cell wall complex of mycobacteria, like that of their
close relatives in the Actinomycetes family (the Coryne-
bacteria, Nocardia, and Rhodococcus genera), is a unique
assembly of carbohydrates and lipids.6 The major carbo-
hydrate components are two polysaccharides, an ara-
binogalactan (AG) and a lipoarabinomannan (LAM),
which are unusual in that all of the galactose and
arabinose is present in the furanose ring form. Both
polysaccharides have at their nonreducing termini the
hexasaccharide motif shown in Figure 1. This moiety can
either be unsubstituted (1) or covalently attached to other
functionalities. In the AG (2), a portion of these hexa-
saccharide motifs is esterified at the four primary hy-
droxyl groups with long chain branched lipids (mycolic
acids). Together these lipids and the polysaccharide
comprise the mycolyl-AG complex, which plays an im-
F igu r e 1. Hexasaccharide motif found at the nonreducing
termini of mycobacterial arabinogalactan (AG) and lipoarabi-
nomannan (LAM).
portant role in preventing the passage of antibiotics into
the organism and in protecting it from its environment.7
In the LAM (3), this hexasaccharide is often capped with
short mannopyranosyl oligosaccharides, which are be-
lieved to be involved in the infection process through
interaction with human mannose binding receptors.8 In
addition to these roles, these polysaccharides (in par-
ticular LAM)9 have been implicated in a number of
immunological events that occur upon infection by my-
cobacteria. It has been postulated that hexasaccharide
1 is an important player in these processes.10 In support
of this hypothesis is the report that removal of the
arabinofuranose residues by treatment of the polysac-
(1) Farmer, P.; Bayona, J .; Becerra, M.; Furin, J .; Henry, C.; Hiatt,
H.; Kim, J . Y.; Mitnick, C.; Nardell, E.; Shin, S. Int. J . Tuberc. Lung
Dis. 1998, 2, 869.
(2) (a) Horsburgh, C. R. J r, J . Infect. Dis. 1999, 179, S461. (b)
Klepser, M. E.; Klepser, T. B. Drugs 1997, 53, 40. (c) Wright, J .
Pharmacotherapy 1998, 18, 738.
(3) Bloom, B. R.; Murray, C. J . Science 1992, 257, 1055.
(4) Pablos-Mendez, A.; Raviglione, M. C.; Laszlo, A.; Binkin, N.;
Reider, H. L.; Bustreo, F.; Cohn, D. L.; Lambregts-van Weezenbeek,
C. S. B.; Kim, S. J .; Chaulet, P.; Nunn, P. N. Engl. J . Med. 1998, 338,
1641. (Erratum N. Engl. J . Med. 1998, 339, 139).
(5) (a) Barry, C. E., III; Slayden, R. A.; Sampson, A. E.; Lee, R. E.
Biochem. Pharmacol. 2000, 59, 221. (b) Barry, C. E., III; Biochem.
Pharmacol. 1997, 54, 1165.
(7) (a) Connell, N. D.; Nikaido, H. Membrane permeability and
transport in Mycobacterium tuberculosis. In Tuberculosis: Pathogen-
esis, Protection and Control; Bloom, B. R., Ed.; American Society for
Microbiology: Washington, DC, 1994; p 333. (b) Besra, G. S.; Khoo,
K.-H.; McNeil, M. R.; Dell, A.; Morris, H. R.; Brennan, P. J . Biochem-
istry 1995, 34, 4257.
(8) Schlesinger, L. S. Curr. Top. Microbiol. 1996, 215, 71.
(9) Chatterjee, D.; Khoo, K. H. Glycobiology 1998, 8, 113. (b)
Strohmeier, G. R.; Fenton, M. J . Microbes Infect. 1999, 1, 709.
(6) Brennan, P. J .; Nikaido, H. Annu. Rev. Biochem. 1995, 64, 29.
10.1021/jo010910e CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/10/2002

Arabinofuranosides from Mycobacteria
J . Org. Chem., Vol. 67, No. 3, 2002 893
charides with arabinofuranosidases abolishes many of
their immunomodulatory properties.10c
In contrast to the large number of investigations
dedicated to solving the â-mannoside problem, relatively
few investigations have addressed methods for the ste-
reoselective formation of â-arabinofuranosides, or the
stereochemically related â-fructofuranosides. Early work
in this area was done by Fletcher and Glaudemans who,
in 1965, reported that methanolysis of 2,3,5-tri-O-benzyl-
R-D-arabinofuranosyl chloride afforded a 92:8 â: R ratio
of methyl glycosides.18 On the basis of kinetic data, it was
proposed that the reaction proceeded through an SN1-
ion-pair mechanism. Over the subsequent 35 years, only
sporadic syntheses of â-arabinofuranosides were re-
ported.19 In most cases only glycosides of simple alcohols
were prepared and often as R: â mixtures. The publica-
tion10a,20 of the structure of 1 has prompted renewed
interest in this area, and new methods for the prepara-
tion of â-arabinofuranosides have begun to appear.
Among these are the application of the Fletcher and
Glaudemans methodology to the preparation of simple
â-arabinofuranosides,21 the use of intramolecular agly-
cone delivery (IAD),22 the glycosylation of 2,3-anhydro-
D-lyxofuranosyl thioglycoside and sulfoxides,23 and the
glycosylation of alcohols by thioglycosides at low tempera-
ture.13c,d These studies have led to the successful syn-
thesis of hexasaccharide 413c and two related penta-
saccharides.22b,24 New methods for the formation of
â-fructofuranosides have also been described recently.
These include the application of the IAD method,25 the
use of glycosyl phosphites,26 and the development of a
new siloxane protected glycosyl donor that provides
â-fructofuranosides with a high degree of stereoselectiv-
ity.27
The ability of mycobacteria to synthesize both AG and
LAM is critical to their viability. Inhibitors of the
enzymes that assemble these polysaccharides should
therefore be lethal to these organisms. Indeed, one of the
drugs used to treat tuberculosis, ethambutol, has been
shown to block the formation of the arabinan portions of
both glycans.11 There has been increasing interest in
identifying compounds that act like ethambutol by in-
hibiting the arabinosyltransferases involved in cell wall
assembly.12 Over the past few years, as part of a program
directed at the identification of inhibitors of mycobacte-
rial arabinosyltransferases, we have reported the syn-
thesis of a number of oligosaccharide fragments of AG
and LAM.13 These studies were undertaken in order to
provide substrates that could be used not only to eluci-
date arabinan biosynthesis13a,14 but also to determine the
immunological importance of these glycans.
Among our targets was the hexasaccharide motif
shown in Figure 1. We anticipated that the assembly of
this oligosaccharide would be complicated by the presence
of the two â-arabinofuranosyl residues (residues E and
F). These glycosidic bonds are stereochemically analogous
to the â-D-mannopyranosyl linkages found in mammalian
glycoconjugates. The stereoselective formation of these
glycosides has been a long-standing problem in oligosac-
charide synthesis.15 The 1,2-cis relationship between the
substituents at C1 and C2 prohibits the use of glycosyl
donors with C2 acyloxy groups because 1,2-trans glyco-
sides are produced due to neighboring group participa-
tion. The use of donors with nonparticipating groups at
C2 also gives predominantly 1,2-trans glycosides because,
in the absence of neighboring group participation, both
stereoelectronic and steric effects favor the R-glycoside.
Solutions to the â-mannoside problem have been explored
for a number of years, and many elegant synthetic
methods have resulted from these investigations.15 Among
the most straightforward and general is a method
developed by Crich and co-workers,16 which has recently
been applied to the synthesis of an octasaccharide
comprised of eight â-(1f2)-linked mannopyranose resi-
dues.17
In a previous communication,13c we reported the syn-
thesis of hexasaccharide 4 via a very efficient and
convergent route. We describe here a full account of that
synthesis and the application of this methodology to the
preparation of all fragments of this glycan that contain
a â-arabinofuranose residue (5-14, Figure 2). The work
reported here complements our earlier synthetic inves-
tigations, in which we synthesized all fragments of 4 that
contained only R-arabinofuranosyl residues.28 These oli-
gosaccharides will be used in investigations focused on
(16) (a) Crich, D. Chemistry of Glycosyl Triflates: Synthesis of
â-Mannopyranosides. In Glycochemistry: Principles, Synthesis and
Applications; Wang, P. G., Bertozzi, C. R., Eds.; Marcel-Dekker: New
York, 2001; p 53. (b) Crich, D.; Sun, S. Tetrahedron 1998, 54, 8321. (c)
Crich, D.; Sun, S. J . Am. Chem. Soc. 1998, 120, 435. (d) Crich, D.;
Sun, S. J . Am. Chem. Soc. 1997, 119, 11217. (d) Crich, D.; Sun S. J .
Org. Chem. 1997, 62, 1198.
(10) . (a) Chatterjee, D.; Bozic, C. M.; McNeil, M.; Brennan, P. J . J .
Biol. Chem. 1991, 266, 9652. (b) Chatterjee, D.; Roberts, A. D.; Lowell,
K.; Brennan, P. J .; Orme, I. M. Infect. Immun. 1992, 60, 1249. (c)
Misaki, A.; Azuma, I.; Yamamura, Y. J . Biochem. (Tokyo) 1977, 82,
1759.
(11) (a) Mikusova´, K.; Slayden, R. A.; Besra, G. S.; Brennan, P. J .
Antimicrob. Agents Chemother. 1995, 39, 2484. (b) Deng, L.; Mikusova´,
K.; Robuck, K. G.; Scherman, M.; Brennan, P. J .; McNeil, M. R.
Antimicrob. Agents Chemother. 1995, 39, 694. (c) Khoo, K.-H.; Douglas,
E.; Azadi, P.; Inamine, J . M.; Besra, G. S.; Mikusova´, K.; Brennan, P.
J .; Chatterjee, D. J . Biol. Chem. 1996, 271, 28682.
(12) (a) Reynolds, R. C.; Bansal, N.; Rose, J .; Friedrich, J .; Suling,
W. J .; Maddry, J . A. Carbohydr. Res. 1999, 317, 164. (b) Maddry, J .
A.; Suling, W. J .; Reynolds, R. C. Res. Microbiol. 1996, 147, 106. (c)
Bouix, C.; Bisseret, P.; Eustache, J . Tetrahedron Lett. 1998, 39, 825.
(d) Maddry, J . A.; Bansal, N.; Bermudez, L. E.; Comber, R. N.; Orme,
I. M.; Suling, W. J .; Wilson, L. N.; Reynolds, R. C. Bioorg. Med. Chem.
Lett. 1998, 8, 237. (e) Ha¨usler, H.; Kawakami, R. P.; Mlaker, E.; Severn,
W. B.; Stu¨tz, A. E. Bioorg. Med. Chem. Lett. 2001, 11, 1679.
(13) . (a) Ayers, J . D.; Lowary, T. L.; Morehouse, C. B.; Besra, G. S.
Bioorg. Med. Chem. Lett. 1998, 8, 437. (b) D’Souza, F. W.; Cheshev, P.
E.; Ayers, J . D.; Lowary, T. L. J . Org. Chem. 1998, 63, 9037. (c)
D′Souza, F. W.; Lowary, T. L. Org. Lett. 2000, 2, 1493. (d) Yin, H.
Lowary, T. L. Tetrahedron Lett. 2001, 42, 5829.
(17) Crich, D.; Li, H.; Yao, Q.; Wink, D. J .; Sommer, R. D.; and
Rheingold, A. L. J . Am. Chem. Soc. 2001, 123, 5826.
(18) (a) Glaudemans, C. P. J .; Fletcher, H. G. J . Am. Chem. Soc.
1965, 87, 2456. (b) Glaudemans, C. P. J .; Fletcher, H. G. J . Am. Chem.
Soc. 1965, 87, 4636.
(19) (a) Finch, P.; Siriwardena, A. H. Glycoconj. J . 1989, 6, 477. (b)
Dourtoglou, A.; Gross, B. J . Carbohydr. Chem. 1983, 2, 57. (c) Sadeh,
S.; Zehavi, U. Carbohydr. Res. 1982, 101, 152. (d) Granata, A.; Perlin,
A. S. Carbohydr. Res. 1980, 86, 305. (e) Zissis, E.; Gladuemans, C. P.
J . Carbohydr. Res. 1976, 50, 292.
(20) Daffe, M.; Brennan, P. J .; McNeil, M. J . Biol. Chem. 1990, 265,
6734.
(21) Subramaniam, V.; Lowary, T. L. Tetrahedron 1999, 55, 5965.
(22) (a) De´sire´, J .; Prandi, J . Carbohydr. Res. 1999, 317, 110. (b)
Sanchez, S.; Bamhaoud, T.; Prandi, J . Tetrahedron Lett. 2000, 41, 7447.
(23) Gadikota, R. R.; Callam, C. S.; Lowary, T. L. Org. Lett. 2001,
3, 607.
(24) Mereyala, H. B.; Hotha, S.; Gurjar, M. K. J . Chem. Soc., Chem.
Commun. 1998, 685.
(14) Lee, R. E.; Brennan, P. J .; Besra, G. S. Glycobiology 1997, 7,
1121.
(15) (a) Gridley, J . J .; Osborn, H. M. I. J . Chem. Soc., Perkin Trans.
1 2000, 1471. (b) Barresi, F.; Hindsgaul, O. Front. Nat. Prod. Res. 1996,
1, 251.
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35, 4763.
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8869.

894 J . Org. Chem., Vol. 67, No. 3, 2002
Yin et al.
F igu r e 2. Synthetic targets; rings have been lettered to facilitate comparison with 1.
Ch a r t 1. Bu ild in g Block s Used for th e
P r ep a r a tion of 4
elucidating the biosynthetic pathway by which mycobac-
terial AG and LAM are assembled and in the character-
ization of antibodies generated against these polysac-
charides.
Resu lts a n d Discu ssion
Syn th esis of Hexa sa cch a r id e 4. A. Str a tegy. In
designing a synthetic route to 4 we endeavored to develop
an approach that could be readily applied to the synthesis
of all of the oligosaccharides in Figure 2. Such a require-
ment ruled out the possibility of a block synthesis, e.g.,
the simultaneous attachment of both Araf-â-(1f2)-Araf
disaccharide fragments (residues DE and CF) to the AB
disaccharide. We also wanted to avoid the somewhat
cumbersome IAD method. Instead, we proposed to rapidly
assemble this molecule from the AB disaccharide frag-
ment (15, Chart 1) via the stepwise addition of the
arabinofuranose residues in pairs. Thioglycoside 16 was
to serve as the precursor to the C and D residues; rings
E and F were to be added via thioglycoside 17. Although
very efficient, this approach requires that both â-ara-
binofuranosyl residues be introduced simultaneously. At
the time we started this work, there were no examples
of two 1,2-cis-â-glycosides being installed in a single
step.29 We were, however, hopeful that conditions could
be found under which this transformation could be
successfully carried out.
B. Syn th esis of Bu ild in g Block s. The synthesis of
4 began with the preparation of building blocks 15-17.
Disaccharide 15 is known.28 The preparation of 16 and
17 is illustrated in Scheme 1.
To access 16 (Scheme 1A), we started with the diac-
etate 18.30 Conversion to the thioglycoside was carried
(29) A later synthesis of pentasaccharide 26 (Figure 3, ref 22b) did
install both â-linked residues in a single step through the use of the
IAD protocol.
(28) D’Souza, F. W.; Ayers, J . D.; McCarren, P. R.; Lowary, T. L. J .
Am. Chem. Soc. 2000, 122, 1251.
(30) Ning, J .; Kong, F. Carbohydr. Res. 1997, 300, 355.

Arabinofuranosides from Mycobacteria
J . Org. Chem., Vol. 67, No. 3, 2002 895
Sch em e 1a
imide and silver triflate activation, a series of reaction
conditions were investigated for the coupling of 24 with
an excess of thioglycoside 17. Through these studies it
was determined that the temperature was critical to the
stereoselectivity. When the glycosylation was carried out
at either -40 °C, 0 °C, or room temperature, a number
of glycoside products were formed. However, when the
reaction was carried out at -78 °C, the conversion
proceeded cleanly, affording the desired hexasaccharide
25 in 81% yield. In an effort to determine if any
hexasaccharide possessing solely R-arabinofuranosyl link-
ages had been formed, all of the fractions obtained from
the chromatography column during purification were
1
screened by H NMR spectroscopy. None of this product
was detected. We have since found that the same degree
of stereocontrol can be achieved by initiating the reaction
at -78 °C and then allowing the reaction to warm to 0
°C. Under these conditions the reaction proceeds more
quickly, with the characteristic orange color of these
reactions developing as the solution reaches -60 °C.
The stereochemistry of the newly formed glycosidic
linkages could be easily determined by NMR spectros-
copy.34 The 13C chemical shift of â-arabinofuranosyl
residues is upfield relative to the R-isomers (100-104
ppm vs 105-109 ppm). In the 13C NMR spectrum of 25,
six anomeric carbons were apparent (106.75, 106.63,
105.95, 105.84, 100.39, and 100.01 ppm). Further support
a
(a) EtSH, BF3-OEt2, CH2Cl2, 0 °C, 80%; (b) NaOCH3, CH3OH,
rt, 97%; (c) (ClAc)2O, NaHCO3, DMF, rt, 83%; (d) Ac2O, pyridine,
DMAP, 0 °C, quantitative; (e) p-TolSH, BF3-OEt2, CH2Cl2, 0 °C,
92%.
1
for the structure came from the H NMR spectrum. Four
out by reaction with ethanethiol and boron trifluoride
etherate, which provided 19 as a 4:1 R: â mixture in 80%
yield. The products were separated by chromatography
and the R-isomer then deprotected (in 97% yield) upon
treatment with sodium methoxide. The resulting alcohol,
20, was converted to 16 in 83% yield upon reaction with
chloroacetic anhydride and sodium bicarbonate.
Thioglycoside 17 was synthesized (Scheme 1B) in two
steps from the commercially available hemiacetal 21.
First, treatment of 21 with acetic anhydride and pyridine
provided 22 in quantitative yield. Without further puri-
fication, 2230 was then reacted with p-thiocresol and
boron trifluoride etherate, which afforded 17 as a 4:1 R: â
mixture in 92% yield.
C. Glycosyla tion s a n d Dep r otection . After all of the
building blocks were in hand, conditions for the assembly
of the hexasaccharide were explored (Scheme 2). Coupling
of disaccharide 15 with 2.4 equiv of thioglycoside 16
proceeded without incident, upon activation with N-
iodosuccinimide and silver triflate. The desired tetrasac-
charide, 23, was obtained in 74% yield. A number of
conditions were explored for the selective removal of the
chloroacetate group in the presence of the benzoate
esters. These included the use of thiourea,31 hydra-
zinedithiocarbonate,32 tetrabutylammonium fluoride, and
hydrazine acetate.33 After considerable optimization, it
was found that the best results were obtained with
hydrazine acetate in a mixture of dichloromethane and
methanol at 40 °C. Under these conditions, deprotection
of tetrasaccharide 23 afforded a 91% yield of diol 24.
With an efficient route to tetrasaccharide 24 in place,
the stage was set for the key reaction, the addition of
both â-arabinofuranosyl residues. Using N-iodosuccin-
of the anomeric hydrogens appeared as doublets or
3
singlets with J H1,H2 ) 0-1.7 Hz; the remaining two were
3
split into doublets with J H1,H2 ) 4.6 Hz. For R-arabino-
3
furanosides, J H1,H2 values of 0-2 Hz are typical, while
this coupling constant is significantly larger for the
â-anomer (3-5 Hz).35 Furthermore, an HMQC spectrum36
1
of 25, correlated the H resonances with J H1,H2 ) 4.6 Hz
with the 13C resonances at 100.39 and 100.01 ppm.
The stereoselectivity of this glycosylation reaction is
impressive, especially when considering the relative ease
with which the reaction can be carried out. For example,
the key step in the previous synthesis of pentasaccharide
26 (Figure 3)22b was the simultaneous introduction of both
â-arabinofuranosyl residues via the highly stereoselective
IAD method. The product of this reaction was obtained
in 23% yield. The method described here is procedurally
more simple than the IAD protocol and the yield of the
desired product is significantly higher. Moreover, the
glycosyl donor used in the glycosylation is more straight-
forwardly accessed.
Through the use of this method it was possible to
synthesize 25 in multi-milligram quantities. Conversion
to the deprotected oligosaccharide was achieved in two
steps by treatment of 25 with sodium methoxide in
methanol followed by hydrogenolysis of the benzyl ethers.
Hexasaccharide 4 was obtained 86% yield from 25.
Syn th esis of Oligosa cch a r id es 5-14. A. Str a tegy
a n d P r ep a r a tion of Bu ild in g Block s. Once the syn-
thesis of 4 was complete, we turned our attention to the
preparation of smaller fragments of this glycan. However,
before proceeding with the synthesis of these oligosac-
(34) Mizutani, K.; Kasai, R.; Nakamura, M.; Tanaka, O.; Matsuura,
(31) Naruto, M.; Ohno, K.; Naruse, N.; Takeuchi, H. Tetrahedron
Lett. 1979, 251.
H. Carbohydr. Res. 1989, 185, 27.
1
(35) Although J C,H values are unambiguous determinants of 1,2-
(32) Van Boeckel, C. A. A.; Beetz, T. Tetrahedron Lett. 1983, 24,
3775.
(33) Udodong, U. E.; Rao, C. S.; Fraser-Reid B. Tetrahedron 1992,
48, 4713.
cis-â-glycosides in pyranosides, in conformationally unrestricted fura-
nose rings this parameter is not a reliable indicator of anomeric
stereochemistry (see ref 34).
(36) Mu¨ller, L. J . Am. Chem. Soc. 1979, 101, 4481.

896 J . Org. Chem., Vol. 67, No. 3, 2002
Yin et al.
Sch em e 2a
a
(a) 16, N-iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 74%; (b) NH2NH2-H2O, HOAc, CH3OH, CH2Cl2, 40 °C, 91%; (c) 17,
N-iodosuccinimide, silver triflate, CH2Cl2, -78 °C, 81%; (d) NaOCH3, CH3OH, rt, 97%; then H2, Pd/C, HOAc, H2O, 86%.
Ch a r t 2. Bu ild in g Block s Used for th e
P r ep a r a tion of 5-14
F igu r e 3. Pentasaccharide 26 synthesized by Prandi and co-
workers (ref 22b).
charides, we decided to modify the strategy. Identical to
the method developed for the preparation of 4, the key
reaction in our routes to 5-14 remained the introduction
of the â-arabinofuranosyl residues via low-temperature
glycosylation of the appropriate acceptor species with
thioglycoside 17. The most significant difference in
strategy was the replacement of the benzoate ester
protecting groups on rings A and B with benzyl ethers.
This substitution not only eliminated the need for the
selective cleavage of the chloroacetate groups in the
presence of the benzoate esters but it also simplified the
final deprotection for most of the targets (see below). With
With the necessary building blocks in hand, we could
these goals in mind, we chose the building blocks shown
in Scheme 3B, both 31 and 32 were synthesized from
alcohol 34,28 in two and three steps, respectively. Ben-
zylation of alcohol 34 followed by cleavage of the siloxane
protecting group afforded 31 in 89% overall yield. The
primary alcohol in 31 was subsequently silylated using
tert-butylchlorodiphenylsilane and pyridine, providing a
76% yield of 32.
then assemble oligosaccharides 5-14. As discussed be-
low, although all of the key glycosylation reactions
involving thioglycoside 17 proceeded to give the â-ara-
binofuranoside as the major product, the stereoselectivi-
ties were generally not as high as those observed in the
synthesis of 25. In many cases, the corresponding R-gly-
coside was also produced in 10-25% yield. These undes-
ired stereoisomers could be separated by chromatography
and were not characterized.
B. Syn th esis of Disa cch a r id e 5. The synthesis of
disaccharide 5 (Scheme 4) was achieved in two steps from
alcohol 30 and thioglycoside 17. Coupling of these two
species under the promotion of N-iodosuccinimide and
silver triflate afforded 35 in 73% yield. In this and
following glycosylation reactions, the stereochemistry of
in Chart 2 as precursors to all of the oligosaccharides
shown in Figure 2. The synthesis of 17 was described
above (Scheme 1B) and 27,37 29,38 and 3039 were prepared
as previously reported. The preparation of 28, 31, and
32 is discussed below.
Access to 28 was achieved from methyl glycoside 3039
in two steps (Scheme 3A). First, the hydroxyl group in
30 was benzoylated to provide 33 (in 90% yield). This
product was subsequently converted, in 72% yield, to
thioglycoside 28 under standard conditions. As illustrated
(37) Callam, C. S.; Gadikota, R. R.; Lowary, T. L. J . Org. Chem.
2001, 66, 4549.
(38) Montgomery, J . A.; Shortnacy, A. T.; Thomas, H. J . J . Med.
Chem. 1974, 17, 1197.
(39) Ning, J .; Kong, F. Carbohydr. Res. 2001, 330, 165.

Arabinofuranosides from Mycobacteria
J . Org. Chem., Vol. 67, No. 3, 2002 897
Sch em e 3a
Introduction of the â-arabinofuranosyl residue was car-
ried out through the use of 17 as was done for the
synthesis of 35. A 54% yield of trisaccharide 38 was
obtained after chromatography. The product was hydro-
genated, affording 6 in 76% yield.
The preparation of trisaccharide 7, which is illustrated
in Scheme 6, proceeded along lines similar to the syn-
thesis of 6. Coupling of thioglycoside 28 and alcohol 32
provided disaccharide 39 in 60% yield. The benzoate ester
was removed, giving 40 in 68% yield, and the resulting
alcohol glycosylated with 17. This reaction produced
trisaccharide 41 in 63% yield. Reaction of the protected
glycan with tetrabutylammonium fluoride and then
hydrogen and palladium on carbon provided 7 in 62%
yield over the two steps.
D. Syn th esis of Tetr a sa cch a r id es 8 a n d 9. The
preparation of tetrasaccharide 8 is shown in Scheme 7.
Glycosylation of 29 with 27 provided 42 in 84% yield. This
disaccharide was the converted to triol 43 upon reaction
with sodium methoxide in methanol. The product was
formed in 95% yield and the primary hydroxyl group was
then protected as a trityl ether. Benzylation of the
remaining secondary hydroxyl groups afforded 45 and the
trityl ether was then removed, affording 46 in 48% yield
over the three steps from 43. The remaining R-arabino-
furanosyl linkage was installed by reaction of 46 with
thioglycoside 28. Produced in 88% yield was trisaccharide
47, which was then converted to alcohol 48 in 79% yield
under standard conditions. The final glycosyl residue was
then added via the reaction of 17 and 48 to provide
tetrasaccharide 49 in 52% yield. Conversion of 49 into 8
was achieved in one step and 79% yield by hydrogenolysis
of the benzyl ethers.
a
(a) BzCl, pyridine, rt, 90%; (b) p-TolSH, BF3-OEt2, CH2Cl2, 0
°C, 72%; (c) NaH, BnBr, DMF, 0 °C; then n-Bu4NF, THF, rt, 89%;
(d) t-BuPh2SiCl, pyridine, rt, 76%.
Sch em e 4a
The synthesis of tetrasaccharide 9 was carried out as
illustrated in Scheme 8. Disaccharide 43 was treated with
1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane and pyridine
to provide 50 in 68% yield. This product was then
converted to benzyl ether 51 and the siloxane protecting
group cleaved upon treatment with tetrabutylammonium
fluoride. The product was isolated in 53% overall yield
from 50. The primary alcohol was then protected as a
tert-butyldiphenylsilyl ether and the resulting alcohol
was glycosylated with thioglycoside 28. The resulting
product, trisaccharide 54, was obtained in 75% yield.
Cleavage of the benzoyl group afforded, in 80% yield,
alcohol 55, which was in turn reacted with 17 under the
standard conditions for the formation of â-arabinofura-
nosides. Tetrasaccharide 56 was isolated in 60% yield.
Deprotection of 56 to give 9 was achieved in two steps
and 65% overall yield.
a
(a) 17, N-iodosuccinimide, silver triflate, CH2Cl2, -78 f 0 °C,
73%; (b) H2, Pd/C, CH3OH, 79%.
the newly formed glycosidic linkage was established via
1H and 13C NMR spectroscopy as described previously.
Disaccharide 35 was subsequently deprotected by reac-
tion with hydrogen and palladium on carbon to afford 5
in 79% yield.
C. Syn th esis of Tr isa cch a r id es 6 a n d 7. As outlined
in Scheme 5, the preparation of trisaccharide 6 began
with the glycosylation of methyl glycoside 29 with
thioglycoside 28. The reaction afforded, in 91% yield,
disaccharide 36, which was next subjected to Zemple´n
deacylation. Alcohol 37 was obtained in 91% yield.
Sch em e 5a
a
(a) N-Iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 91%; (b) NaOCH3, CH3OH, rt, 91%; (c) 17, N-iodosuccinimide, silver triflate,
CH2Cl2, -78 f 0 °C, 54%; (d) H2, Pd/C, CH3OH, 76%.

898 J . Org. Chem., Vol. 67, No. 3, 2002
Yin et al.
Sch em e 6a
a
(a) N-Iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 60%; (b) NaOCH3, CH3OH, rt, 68%; (c) 17, N-iodosuccinimide, silver triflate,
CH2Cl2, -78 f 0 °C, 63%; (d) n-Bu4NF, THF, rt, then H2, Pd/C, CH3OH, 62%.
Sch em e 7a
a
(a) N-Iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 84%; (b) NaOCH3, CH3OH, rt, 95%; (c) TrCl, pyridine, rt; (d) BnBr, NaH, DMF,
0 °C; (e) n-Bu4NF, THF, rt, 48%, three steps from 43; (f) 28, N-iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 88%; (g) NaOCH3, CH3OH,
rt, 79%; (h) 17, N-iodosuccinimide, silver triflate, CH2Cl2, -78 f 0 °C, 52%; (i) H2, Pd/C, CH3OH, 79%.
E. Syn th esis of Tetr a sa cch a r id es 10 a n d 11 a n d
P en ta sa cch a r id e 12. The most efficient route to 10 and
11 involved a common intermediate, 58 (Scheme 9).
Alcohol 31 was glycosylated with an excess of thioglyco-
side 28, which afforded trisaccharide 57 in 82% yield. The
benzoyl groups were removed under basic conditions,
resulting in a 78% yield of 58. This diol was then
glycosylated with 1 equiv of thioglycoside 17. This
reaction produced a mixture of tetrasaccharides, 59 and
60, which were separated by chromatography. The
isolated yields of the products were 41% (59) and 26%
(60). Both products were then deprotected by hydro-
genolysis of the benzyl ethers under standard conditions,
in 83% and 86% yields, respectively.
The structures of the products produced from the
glycosylation of diol 58 with a limiting amount of 17 were
determined through the use of 13C NMR spectroscopy,
by inspection of the chemical shifts of the anomeric
carbons on residues C and D (Figure 4). Access to a panel
of protected oligosaccharide derivatives (e.g., 38-41, 47-
49, 56) allowed us to determine markers for the attach-
ment of a â-arabinofuranosyl moiety to residue C vs
residue D. When residue C possesses a free hydroxyl
group at C2, the chemical shift of the anomeric carbon
(C1C) is between 108.0 and 108.5 ppm. Benzoylation or
glycosylation of this functionality results in a 2.5 ppm
upfield shift of the C1C resonance. A similar trend is seen
for residue D. In oligosaccharides in which this ring has
a 2-hydroxyl group, C1D resonates between 109.0 and
109.5 ppm. If this position is glycosylated or benzoylated
the chemical shift of C1D is shifted upfield (to 106.6-107.0
ppm). The 1 ppm difference between the chemical shift
C1D and C1C when these rings possess a C2-hydoxyl group
is consistent for all compounds investigated and allowed
us to determine the structures of 59 and 60 (Figure 4).
Additional data supporting these chemical shifts as
markers for determining the regioselectivity of this
glycosylation can be found in the Supporting Information
(Figure S1).
The synthesis of pentasaccharide 12 also involved
intermediate 58 (Scheme 10). Glycosylation of this trisac-
charide diol with 2 equiv of 17 provided 61 in 63% yield.
The benzyl groups were then removed by hydrogenolysis,
affording 12 in 86% yield.

Arabinofuranosides from Mycobacteria
J . Org. Chem., Vol. 67, No. 3, 2002 899
Sch em e 8a
a
(a) 1,3-Dichloro-1,1,3,3-tetraisopropyldisiloxane, pyridine, rt, 0 °C, 68%; (b) NaH, BnBr, DMF, 0 °C; (c) n-Bu4NF, THF, rt, 53% from
50; (d) t-BuPh2SiCl, pyridine, rt, 68%; (e) 28, N-iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 75%; (f) NaOCH3, CH3OH, rt, 80%; (g) 17,
N-iodosuccinimide, silver triflate, CH2Cl2, -78 f 0 °C, 60%; (h) n-Bu4NF, THF, rt, then H2, Pd/C, CH3OH, 65%.
Sch em e 9a
a
(a) 28, N-iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 82%; (b) NaOCH3, CH3OH, rt, 78%; (c) 17, N-iodosuccinimide, silver triflate,
CH2Cl2, -78 f 0 °C, 41% 59 and 26% 60; (d) H2, Pd/C, CH3OH, 83%; (e) H2, Pd/C, CH3OH, 86%.
F . Syn th esis of P en ta sa cch a r id es 13 a n d 14 a n d
Hexa sa cch a r id e 4. The route used for the preparation
of 13 and 14 (Scheme 11) was analogous to that used for
the preparation of 10 and 11. First, disaccharide 52,
prepared as outlined in Scheme 8, was glycosylated with
an excess of thioglycoside 28. The resulting product,
tetrasaccharide 62, was produced in 89% yield. The
benzoyl groups in 62 were then cleaved (84%) and the
resulting diol was glycosylated with a limiting amount
of 17. This reaction afforded three chromatographically
separable products, hexasaccharide 64 and pentasaccha-
rides 65 and 66 in 21%, 28%, and 10% yield, respectively.
Differentiation of the structures of 65 and 66 was carried
out as described above for 59 and 60. All three products
were deprotected upon reaction with hydrogen and
palladium on carbon, to afford oligosaccharides 4, 13, and
14.
In conclusion, we report here the synthesis of a panel
of oligosaccharides containing â-arabinofuranosyl resi-
dues. The key reaction in the formation of the targets is
a low-temperature glycosylation of alcohols with a readily
available thioglycoside (17) promoted by N-iodosuccin-
imide and silver triflate. Under these conditions, the
â-arabinofuranosides are produced with good to excellent
stereoselectivity. The method has been used to synthesize
a hexasaccharide motif found at the nonreducing end of
two mycobacterial cell wall polysaccharides. In addition,
all fragments of this oligosaccharide containing a â-ara-
binofuranosyl residues have been synthesized.
Although in the preparation of 5-14 the stereocontrol
of the key glycosylation reactions is not as high as in the
synthesis of 4, the desired products are nevertheless
produced as the major products. The sensitivity of the
stereocontrol to the coupling partners employed in the

900 J . Org. Chem., Vol. 67, No. 3, 2002
Yin et al.
F igu r e 4. 13C NMR spectroscopic markers used to determine structures of oligosaccharides 59, 60, 65, and 66.
Sch em e 10a
under UV light or by charring with 10% H2SO4 in EtOH.
Solvents were evaporated under reduced pressure and below
40 °C (bath). Column chromatography was performed on silica
gel 60 (40-60 µM). Iatrobeads refers to a beaded silica gel 6RS-
8060, which is manufactured by Iatron Laboratories (Tokyo).
The ratio between silica gel and crude product ranged from
100 to 50:1 (w/w). Optical rotations were measured at 22 ( 2
1
°C. H NMR spectra were recorded at 400, 500, or 800 MHz,
and chemical shifts are referenced to either to TMS (0.0,
CDCl3) or HOD (4.78, D2O). 13C NMR spectra were recorded
at 100 or 125 MHz, and 13C chemical shifts are referenced to
internal CDCl3 (77.00, CDCl3) or external dioxane (67.40, D2O).
Elemental analyses were performed by Atlantic Microlab Inc.,
Norcross, GA. Electrospray mass spectra were recorded on
samples suspended in mixtures of THF with CH3OH and
added trifluoroacetic acid or NaCl. Analytical data for all new
compounds (1H NMR, 13C NMR, elemental analysis, HRMS,
[R]D) is provided in the Supporting Information.
Eth yl 2-O-Acetyl-3,5-d i-O-ben zyl-1-th io-D-a r a bin ofu r a -
n osid e (19). To a solution of 18 (3.80 g, 9.17 mmol) in dry
CH2Cl2 (10 mL) at 0 °C was added dropwise ethanethiol (0.75
mL, 10.13 mmol). After stirring for 10 min, boron trifluoride
etherate (1.39 mL, 10.97 mmol) was added dropwise. After 2
h, Et3N (3 mL) was added and the reaction mixture was
concentrated. The residue was then purified by chromatogra-
phy (hexanes/EtOAc, 5:1) to provide the product 19 (3.07 g,
80%) as two separable anomers in a 4:1 R:â ratio as oils.
Eth yl 3,5-Di-O-ben zyl-1-th io-r-D-ar abin ofu r an oside (20).
To a solution of 19 (R-isomer) (1.85 g, 4.44 mmol) in dry CH3-
OH (10 mL) was added dropwise 0.1 M methanolic sodium
methoxide (4 mL). After 2 h, the reaction mixture was
neutralized with prewashed Amberlite IR-120 (H+) resin,
filtered, and concentrated. The residue was purified by chro-
matography (hexanes/EtOAc, 4:1) to give 20 (1.61 g, 97%) as
an oil.
Eth yl 3,5-Di-O-ben zyl-2-O-ch lor oa cetyl-1-th io-r-D-a r a -
bin ofu r a n osid e (16). Alcohol 20 (150 mg, 0.40 mmol) was
dissolved in dry DMF (5 mL), and then NaHCO3 (50 mg, 0.60
mmol) and chloroacetic anhydride (103 mg, 0.60 mmol) were
added. The reaction mixture was stirred for 2 h and was then
diluted with CH2Cl2. The organic layer was washed with water,
dried (Na2SO4), ad concentrated and the residue chromato-
graphed (hexanes/EtOAc, 6:1) to give 16 (150 mg, 83%) as an
oil.
a
(a) 17, N-iodosuccinimide, silver triflate, CH2Cl2, -78 f 0 °C,
63%; (b) H2, Pd/C, CH3OH, 86%.
glycosylations is reminiscent of many syntheses of â-m-
annosides, prior to the development of the Crich method
described in the Introduction.16 Often, seemingly small
changes in the structure of the donor and acceptor result
in significant alterations in â: R ratios.40 Clearly ad-
ditional work is needed to provide a general and highly
stereoselective â-arabinofuranoside synthesis.
Glycans 4-14 will be of much use in biosynthetic
studies of hexasaccharide 1 as well as in investigations
of the specificity of various antibodies that have been
raised against mycobacterial AG and LAM. It is also
likely that many of these oligosaccharides will serve as
substrates for the enzymes that further elaborate hex-
asaccharide 1. These include the mycolyltransferases,
which add the mycolic acids to this hexasaccharide in AG
and the mannosyltransferases that glycosylate this motif
in LAM. The use of these glycans in a range of biochemi-
cal studies is underway.
1-O-Acetyl-2,3,5-tr i-O-ben zyl-1-D-a r a bin ofu r a n ose (22).
To a solution of 2,3,5-tri-O-benzyl-D-arabinofuranose41 (21, 5.0
g, 11.9 mmol) in dry pyridine (10 mL) at 0 °C was added acetic
anhydride (1.35 mL, 14.31 mmol) dropwise. A catalytic amount
of DMAP was added, and the solution was stirred for 4 h at 0
°C. The reaction mixture was then diluted with CH2Cl2 and
then washed with 5% HCl, a saturated solution of NaHCO3,
and then water. The organic layer was dried (Na2SO4), filtered,
Exp er im en ta l Section
Gen er a l Meth od s. Solvents were distilled from the ap-
propriate drying agents before use. Unless stated otherwise
all reactions were carried out at room temperature and under
a positive pressure of argon and were monitored by TLC on
silica gel 60 F254 (0.25 mm, E. Merck). Spots were detected
(40) Paulsen, H. Angew. Chem., Intl Ed. Engl. 1982, 21, 155.
(41) Commercially available from Pfanstiel Laboratories.

Arabinofuranosides from Mycobacteria
J . Org. Chem., Vol. 67, No. 3, 2002 901
Sch em e 11a
a
(a) 28, N-iodosuccinimide, silver triflate, CH2Cl2, 0 °C, 89%; (b) NaOCH3, CH3OH, rt, 84%; (c) 17, N-iodosuccinimide, silver triflate,
CH2Cl2, -78 f 0 °C, 21% 64, 28% 65, and 10% 66; (d) H2, Pd/C, HOAc, H2O, 67%; (e) H2, Pd/C, CH3OH, 75%; (f) H2, Pd/C, CH3OH, 71%.
and concentrated to give the product 2230 in quantitative yield
as an oil. The product was used in the next step without
further purification.
Hexa sa cch a r id e 25. Tetrasaccharide diol 24 (255 mg, 0.21
mmol) and thioglycoside 17 (436 mg, 0.83 mmol) were dis-
solved in dry CH2Cl2 (6 mL). The solution was stirred under
N2 for 30 min with activated, powdered molecular sieves (4 Å,
2.0 g) at -78 °C, and then N-iodosuccinimide (186 mg, 0.83
mmol) and silver triflate (64 mg, 0.25 mmol) were added. After
being stirred for 90 min at this temperature, Et3N was added.
The reaction mixture was diluted with CH2Cl2 and filtered
through Celite. The filtrate was washed successively with a
saturated aqueous solution of Na2S2O3, water, brine, and dried
(Na2SO4). After evaporation of the solvent, chromatography
of the residue (hexanes/EtOAc, 3:1 f 2:1) gave 25 (343 mg,
81%) as an oil.
Hexa sa cch a r id e 4. A solution of 25 (184 mg, 0.09 mmol)
in dry CH2Cl2 (2 mL) and dry CH3OH (6 mL) was treated with
a catalytic amount of 0.1 M methanolic sodium methoxide.
After stirring for 4 h, the reaction mixture was neutralized
with prewashed Amberlite IR-120 (H+) resin, filtered, and
concentrated. The resulting residue was purified by chroma-
tography (hexanes/EtOAc, 2:1 f 1:2). The residue was dis-
solved in AcOH/H2O (4:1, 5 mL) and hydrogenolyzed over 10%
Pd/C (60 mg) for 3 h. The reaction mixture was filtered through
Celite and concentrated, and the residue was purified by
chromatography on Iatrobeads (CH2Cl2:CH3OH:H2O, 60:35:5)
to give 4 (64 mg, 86%) as a white solid.
Meth yl 2-O-Ben zoyl-3,5-d i-O-ben zyl-r-D-a r a bin ofu r a -
n osid e (33). To a solution of 30 (400 mg, 1.16 mmol) in dry
pyridine (5 mL) at room temperature was added benzoyl
chloride (150 µL, 1.25 mmol). The reaction mixture was stirred
for 2 h and then concentrated. The residue was dissolved in
CH2Cl2, and the resulting organic solution was washed with
brine and concentrated. Purification by chromatography (hex-
anes/EtOAc, 4:1) afforded 33 (490 mg, 90%) as an oil.
p -Cr esyl 2-O-Ben zoyl-3,5-d i-O-b en zyl-1-t h io-r-D-a r a -
bin ofu r a n osid e (28). To a solution of 34 (2.2 g, 4.91 mmol)
in dry CH2Cl2 (80 mL) was added thiocresol (620 mg, 5 mmol).
The solution was stirred for 10 min at 0 °C, and boron
trifluoride etherate (1.2 mL, 10 mmol) was added dropwise.
p-Cr esyl 2,3,5-Tr i-O-ben zyl-1-th io-D-a r a bin ofu r a n osid e
(17). Acetate 22 (1.64 g, 3.55 mmol) was dissolved in dry CH2-
Cl2 (10 mL), and the solution was cooled to 0 °C. p-Thiocresol
(485 mg, 3.90 mmol) was added, and after stirring for 10 min,
boron trifluoride etherate (540 µL, 4.26 mmol) was added
dropwise. The solution was stirred for 1 h at 0 °C before Et3N
(590 µL, 4.23 mmol) was added, and then the reaction mixture
was diluted with CH2Cl2. After washing with a saturated
solution of NaHCO3, the organic layer was dried (Na2SO4) and
concentrated to give a residue that was chromatographed
(hexanes/EtOAc, 8:1) to give 17 (1.73 g, 4:1 R: â mixture, 92%)
as an oil.
Tetr a sa cch a r id e 23. To a solution of disaccharide alcohol
15 (533 mg, 0.88 mmol) and donor 16 (948 mg, 2.10 mmol)
in dry CH2Cl2 (10 mL) was added powdered molecular
sieves (4 Å, 1.0 g). The reaction mixture was stirred for 20
min at 0 °C, and then N-iodosuccinimide (568 mg, 2.52 mmol)
and silver triflate (162 mg, 0.63 mmol) were added. After
stirring for 2 h at 0 °C, Et3N was added. The reaction mixture
was then diluted with CH2Cl2 and filtered through Celite. The
filtrate was washed successively with a saturated solution of
Na2S2O3, water, and brine. The organic phase was dried (Na2-
SO4), filtered, concentrated, and purified by chromatography
(hexanes/EtOAc, 5:1 f 2:1) to give 23 (894 mg, 74%) as an
oil.
Tetr a sa cch a r id e 24. To a stirred solution of tetrasaccha-
ride 23 (414 mg, 0.30 mmol) in dry CH2Cl2 (5 mL) and dry
CH3OH (2 mL) were added acetic acid (171 µL, 2.99 mmol)
and hydrazine monohydrate (145 µL, 2.99 mmol). The solution
was stirred at 40 °C for 3 h and then cooled to room
temperature before being concentrated. The residue was taken
up in CH2Cl2 and washed with water. The organic phase was
then dried (Na2SO4), filtered, concentrated, and purified by
chromatography (hexanes/EtOAc, 3:1 f 1:1) to give 24 (335
mg, 91%) as an oil.

902 J . Org. Chem., Vol. 67, No. 3, 2002
Yin et al.
After 2 h at 0 °C, Et3N (2 mL) was added and the solution
was concentrated. The residue was purified by column chro-
matography (hexanes/EtOAc, 6:1) to provide product 28 (1.96
g, 72%) as an oil.
(380 mg, 1.70 mmol) and AgOTf (100 mg, 0.39 mmol) in CH2-
Cl2 (30 mL) as described for the preparation of 36. Purification
by chromatography (hexanes/EtOAc, 4:1) yielded 39 (840 mg,
60%) as an oil.
Meth yl 2-O-Ben zyl-r-D-a r a bin ofu r a n osid e (31). To a
solution of 35 (1.50 g, 3.0 mmol) in dry DMF (5 mL) at 0 °C
were added sodium hydride (60% dispersion in oil, 240 mg, 6
mmol) and benzyl bromide (370 µL, 3.1 mmol). After stirring
at 0 °C for 2 h, CH3OH (1 mL) was added and the solvent was
evaporated. The resulting residue was dissolved in EtOAc, and
this organic solution was washed with water and brine and
then dried (Na2SO4). The solvent was evaporated, the residue
was dissolved in THF (10 mL), and n-Bu4NF (200 mg) was
added. After stirring overnight, the solvent was evaporated
and the product purified by chromatography (hexanes/EtOAc,
1:1) to give 31 (580 mg, 89%) as an oil.
Met h yl 2-O-Ben zyl-5-O-t-b u t yld ip h en ylsilyl-r-D-a r a -
bin ofu r a n osid e (32). To compound 31 (600 mg, 2.36 mmol)
in pyridine (10 mL) was added, dropwise, tert-butylchlo-
rodiphenylsilane (730 µL, 2.8 mmol). The solution was stirred
24 h and the solvent evaporated. Purification by chromatog-
raphy (hexanes/EtOAc, 4:1) yielded 32 (890 mg, 76%) as an
oil.
Disa cch a r id e 35. Alcohol 30 (700 mg, 2.0 mmol) and
thioglycoside 17 (1.1 g, 2.1 mmol) were dissolved in CH2Cl2
(50 mL) and stirred with 4 Å molecular sieves at room
temperature for 15 min. The reaction mixture was then cooled
to -78 °C, and NIS (520 mg, 2.3 mmol) and AgOTf (130 mg,
0.51 mmol) were added in succession. The temperature of the
reaction was allowed to rise to 0 °C over 3 h, and then Et3N (2
mL) and Celite were added. The suspension was stirred for
15 min and then filtered. The filtrate was concentrated and
the resulting residue purified by chromatography (hexanes/
EtOAc, 4:1) to afford 35 (1.12 g, 73%) as an oil.
Disa cch a r id e 5. Oligosaccharide 35 (400 mg, 0.52 mmol)
was dissolved in CH3OH (20 mL), and 10% Pd/C (150 mg) was
added. Hydrogen was bubbled through the suspension at room
temperature, and the reaction was monitored by TLC. After
the hydrogenation was complete, the solution was filtered
through Celite and the filtrate was concentrated. The residue
was purified by chromatography on Iatrobeads (CH2Cl2/CH3-
OH, 8:1) to give 5 (122 mg, 79%) as an oil.
Disa cch a r id e 36. Alcohol 29 (550 mg, 1.60 mmol) and
thioglycoside 28 (970 mg, 1.8 mmol) were dissolved in CH2Cl2
(30 mL), and 4 Å molecular sieves (1 g) were added. The
solution was stirred for 15 min and cooled to 0 °C. After
stirring for an additional 15 min, NIS (450 mg, 2 mmol) and
AgOTf (130 mg, 0.5 mmol) were successively added to the
reaction mixture. The progress of the reaction was followed
by TLC, and after the disappearance of the alcohol, Et3N (2
mL) was added followed by Celite. This suspension was stirred
for another 15 min, before being filtered. The filtrate was
concentrated, and the resulting residue was purified by
chromatography (hexanes/EtOAc, 6:1), yielding 36 (1.1 g, 91%)
as an oil.
Disa cch a r id e 37. Disaccharide 36 (820 mg, 0.68 mmol) was
dissolved in a 0.1 M solution of NaOCH3 in CH3OH (10 mL)
and stirred for 2 h at room temperature. The reaction mixture
was then neutralized with prewashed Amberlite IR-120 (H+)
resin and filtered. The filtrate was concentrated, and the
residue was purified by chromatography (hexanes/EtOAc, 6:1)
to provide 37 (600 mg, 91%) as an oil.
Tr isa cch a r id e 38. Alcohol 37 (600 mg, 0.91 mmol) and
thioglycoside 17 (500 mg, 0.95 mmol) were coupled using NIS
(230 mg, 1 mmol) and AgOTf (50 mg, 0.2 mmol) in CH2Cl2 (20
mL) as described for the preparation of 35. Purification by
chromatography (hexanes/EtOAc, 6:1) yielded 38 (520 mg,
54%) as an oil.
Tr isa cch a r id e 6. Oligosaccharide 38 (470 mg, 0.44 mmol)
was debenzylated via hydrogenation over 10% Pd/C (150 mg)
in CH3OH (20 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 6 (145 mg, 76%) as an oil.
Disa cch a r id e 40. Compound 39 (560 mg, 0.62 mmol) was
debenzoylated in a 0.1 M solution of NaOCH3 in CH3OH (20
mL) as described for the preparation of 37. Purification by
chromatography (hexanes/EtOAc, 4:1) yielded 40 (340 mg,
68%) as an oil.
Tr isa cch a r id e 41. Alcohol 40 (340 mg, 0.42 mmol) and
thioglycoside 17 (260 mg, 0.50 mmol) were coupled using NIS
(120 mg, 0.55 mmol) and AgOTf (50 mg, 0.19 mmol) in CH2-
Cl2 (30 mL) as described for the preparation of 35. Purification
by chromatography (hexanes/EtOAc, 4:1) yielded 41 (320 mg,
63%) as an oil.
Tr isa cch a r id e 7. Oligosaccharide 41 (320 mg, 0.26 mmol)
was dissolved in THF (20 mL), and n-Bu4NF (100 mg, 0.40
mmol) was added. The solution was stirred at room temper-
ature for 6 h and then concentrated. The crude product was
purified by chromatography (hexanes/EtOAc, 4:1) to give the
desilylated compound, which was immediately debenzylated
via hydrogenation over 10% Pd/C (100 mg) in CH3OH (10 mL)
as described for the preparation of 5. Purification by chroma-
tography (CH2Cl2/CH3OH, 8:1) on Iatrobeads yielded 7 (68 mg,
62%) as an oil.
Disa cch a r id e 42. Alcohol 29 (2.60 g, 7.56 mmol) and
thioglycoside 27 (4.50 g, 7.92 mmol) were coupled using NIS
(1.85 g, 8.2 mmol) and AgOTf (200 mg, 0.8 mmol) in CH2Cl2
(30 mL) as described for the preparation of 36. Purification
by chromatography (hexanes/EtOAc, 2:1) yielded 42 (5.01 g,
84%) as an oil.
Disa cch a r id e 43. Disaccharide 42 (3.20 g, 4.06 mmol) was
debenzoylated in a 0.1 M solution of NaOCH3 in CH3OH (20
mL) as described for the preparation of 37. Purification by
chromatography (hexanes/EtOAc, 2:1) yielded 43 (1.84 g, 95%)
as an oil.
Disa cch a r id e 46. Compound 43 (1.25 g, 2.93 mmol) and
trityl chloride (1.18 g, 4 mmol) were dissolved in pyridine (60
mL) and stirred overnight. The reaction mixture was concen-
trated, and the residue was extracted with EtOAc (2 × 30 mL).
The organic extracts were then washed with brine (2 × 20 mL)
and concentrated to provide crude 44. The residue was
dissolved in DMF/THF (3 mL/20 mL), and the solution was
cooled to 0 °C before sodium hydride (60% dispersion in oil,
0.8 g, 20 mmol) and benzyl bromide (1 mL, 8.1 mmol) were
added. The suspension was stirred at 0 °C for 2 h, and then
CH3OH was added slowly. The reaction mixture was concen-
trated, and the resulting residue was extracted with EtOAc
(100 mL). The organic extract was washed with water and
brine and then evaporated to dryness to provide crude 45. The
residue was dissolved in CH3OH/CH2Cl2 (10 mL/40 mL) and
p-TsOH (0.4 g, 2.3 mmol) was added. After stirring for 3 h at
room temperature, the solution was concentrated and ex-
tracted with CH2Cl2. The organic extract was washed with
brine and water and then dried over Na2SO4 before being
concentrated. The resulting crude product was purified by
chromatography (hexanes/EtOAc, 4:1) to yield 46 (910 mg,
48%) as an oil.
Tr isa cch a r id e 47. Alcohol 46 (200 mg, 0.32 mmol) and
thioglycoside 28 (180 mg, 0.33 mmol) were coupled using NIS
(77 mg, 0.34 mmol) and AgOTf (8 mg, 0.07 mmol) in CH2Cl2
(20 mL) as described for the preparation of 36. Purification
by chromatography (hexanes/EtOAc, 6:1) yielded 47 (210 mg,
88%) as an oil.
Tr isa cch a r id e 48. Compound 47 (210 mg, 0.21 mmol) was
debenzoylated in a 0.1 M solution of NaOCH3 in CH3OH (10
mL) as described for the preparation of 37. Purification by
chromatography (hexanes/EtOAc, 4:1) yielded 48 (150 mg,
79%) as an oil.
Tetr a sa cch a r id e 49. Alcohol 48 (120 mg, 0.12 mmol) and
thioglycoside 17 (75 mg, 0.14 mmol) were coupled using NIS
(35 mg, 0.15 mmol) and AgOTf (10 mg, 0.04 mmol) in CH2Cl2
(20 mL) as described for the preparation of 35. Purification
by chromatography (hexanes/EtOAc, 4:1) yielded 49 (88 mg,
52%) as an oil.
Disa cch a r id e 39. Alcohol 32 (0.74 g, 1.5 mmol) and
thioglycoside 28 (700 mg, 1.3 mmol) were coupled using NIS

Arabinofuranosides from Mycobacteria
J . Org. Chem., Vol. 67, No. 3, 2002 903
Tetr a sa cch a r id e 8. Oligosaccharide 49 (88 mg, 0.06 mmol)
was debenzylated via hydrogenation over 10% Pd/C (32 mg)
in CH3OH (20 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 8, (27 mg, 79%) as an oil.
preparation of 35. Purification by chromatography (hexanes/
EtOAc, 4:1) yielded 59 (330 mg, 41%) and 60 (210 mg, 26%)
as oils.
Tetr a sa cch a r id e 10. Oligosaccharide 59 (280 mg, 0.22
mmol) was debenzylated via hydrogenation over 10% Pd/C (90
mg) in CH3OH (10 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 10 (98 mg, 83%) as an oil.
Tetr a sa cch a r id e 11. Oligosaccharide 60 (190 mg, 0.14
mmol) was debenzylated via hydrogenation over 10% Pd/C (60
mg) in CH3OH (20 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 11 (67 mg, 86%) as an oil.
P en ta sa cch a r id e 61. Diol 58 (510 mg, 0.59 mmol) and
thioglycoside 17 (62 mg, 1.18 mmol) were coupled using NIS
(340 mg, 1.5 mmol) and AgOTf (90 mg, 0.35 mmol) in CH2Cl2
(40 mL) as described for the preparation of 35. The product
was purified by chromatography (hexanes/EtOAc, 4:1) to afford
61 (610 mg, 63%) as an oil.
Disa cch a r id e 50. Compound 43 (1.80 g, 3.78 mmol) was
dissolved in pyridine (20 mL) and 1,3-dichloro-1,1,3,3-tetra-
isopropyldisiloxane (1.40 g, 4.44 mmol) was added dropwise.
The reaction mixture was stirred for 8 h at room temperature
and then concentrated. Purification by chromatography (hex-
anes/EtOAc, 3:1) yielded 50 (1.84 g, 68%) as an oil.
Disa cch a r id e 52. Compound 50 (1.84 g, 2.56 mmol) was
dissolved in dry DMF (10 mL) and cooled to 0 °C. To this
solution were added sodium hydride (60% dispersion in oil,
200 mg, 5 mmol) and benzyl bromide (355 µL, 3 mmol). After
stirring at 0 °C for 2 h, CH3OH (1 mL) was added. The solvent
was evaporated and the resulting residue was diluted with
EtOAc. This organic solution was washed with water and brine
and then dried (Na2SO4) and concentrated. The resulting oil
was dissolved in THF (10 mL), n-Bu4NF (200 mg) was added,
and the solution was stirred overnight at room temperature.
The solvent was then evaporated, and the product was purified
by chromatography (hexanes/EtOAc, 1:3) to afford 52 (770 mg,
53%) as an oil.
P en ta sa cch a r id e 12. Oligosaccharide 61 (590 mg, 0.35
mmol) was debenzylated via hydrogenation over 10% Pd/C (200
mg) in CH3OH (20 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 12 (208 mg, 86%) as an oil.
Disa cch a r id e 53. To a solution of 52 (500 mg, 0.88 mmol)
in pyridine (20 mL) was added, dropwise, tert-butylchlo-
rodiphenylsilane (300 mg, 1.09 mmol). The solution was stirred
for 36 h and the solvent evaporated. The product was purified
by chromatography (hexanes/EtOAc, 4:1) to yield 53 (480 mg,
68%) as an oil.
Tetr a sa cch a r id e 62. Compound 52 (1.10 g, 1.94 mmol) and
thioglycoside 28 (2.16 g, 4 mmol) were coupled using NIS (900
mg, 4.0 mmol) and AgOTf (200 mg, 0.8 mmol) in CH2Cl2 (30
mL) as described for the preparation of 36. Purification by
chromatography (hexanes/EtOAc, 5:1) yielded 62 (2.40 g, 89%)
as an oil.
Tr isa cch a r id e 54. Compound 53 (430 mg, 0.53 mmol) and
thioglycoside 28 (350 mg, 0.65 mmol) were coupled using NIS
(150 mg, 0.65 mmol) and AgOTf (40 mg, 0.16 mmol) in CH2-
Cl2 (10 mL) as described for the preparation of 36. Purification
by chromatography (hexanes/EtOAc, 6:1) yielded 54 (490 mg,
75%) as an oil.
Tr isa cch a r id e 55. Compound 54 (470 mg, 0.39 mmol) was
debenzoylated in a 0.1 M solution of NaOCH3 in CH3OH (20
mL) as described for the preparation of 37. Purification by
chromatography (hexanes/EtOAc, 2:1) yielded 55 (350 mg,
80%) as an oil.
Tetr a sa cch a r id e 56. Alcohol 55 (350 mg, 0.31 mmol) and
thioglycoside 17 (190 mg, 0.35 mmol) were coupled using NIS
(84 mg, 0.37 mmol) and AgOTf (20 mg, 0.08 mmol) in CH2Cl2
(20 mL) as described for the preparation of 35. Purification
by chromatography (hexanes/EtOAc, 4:1) afforded 56 (280 mg,
60%) as an oil.
Tetr a sa cch a r id e 9. Compound 56 (280 mg, 0.18 mmol) was
dissolved in THF (10 mL), and n-Bu4NF (58 mg, 0.22 mmol)
was added. The solution was stirred at room temperature
for 6 h and then concentrated. The crude product was purified
by chromatography (hexanes/EtOAc, 6:1) to give the desily-
lated compound, which was debenzylated via hydrogenation
over 10% Pd/C (100 mg) in CH3OH (10 mL) as described
for the preparation of 5. Purification by chromatography
(CH2Cl2/CH3OH, 8:1) on Iatrobeads yielded 9 (66 mg, 65%) as
an oil.
Tr isa cch a r id e 57. Diol 31 (280 mg, 1.1 mmol) and thiogly-
coside 28 (1.30 g, 2.4 mmol) were coupled using NIS (590 mg,
2.6 mmol) and AgOTf (100 mg, 0.4 mmol) in CH2Cl2 (40 mL)
as described for the preparation of 36. Purification by chro-
matography (hexanes/EtOAc, 4:1) yielded 57 (970 mg, 82%)
as an oil.
Tr isa cch a r id e 58. Compound 57 (820 mg, 0.76 mmol) was
debenzoylated in a 0.1 M solution of NaOCH3 in CH3OH (20
mL) as described for the preparation of 37. Purification by
chromatography (hexanes/EtOAc, 4:1) yielded 58 (520 mg,
78%) as an oil.
Tetr a sa cch a r id e 63. Compound 62 (1.4 g, 1 mmol) was
debenzoylated in a 0.1 M solution of NaOCH3 in CH3OH (20
mL) as described for the preparation of 37. Purification by
chromatography (hexanes/EtOAc, 5:1) yielded 63 (990 mg,
84%) as an oil.
Hexa sa cch a r id e 64, P en ta sa cch a r id e 65, a n d P en -
ta sa cch a r id e 66. Diol 63 (750 mg, 0.63 mmol) and thiogly-
coside 17 (470 mg, 0.89 mmol) were coupled using NIS (250
mg, 1.1 mmol) and AgOTf (70 mg, 0.27 mmol) in CH2Cl2 (20
mL) as described for the preparation of 35. The products were
purified by chromatography (hexanes/EtOAc, 6:1) to afford 64
(260 mg, 21%), 65 (280 mg, 28%), and 66 (100 mg, 10%) as
oils.
Hexa sa cch a r id e 4. Oligosaccharide 64 (220 mg, 0.11
mmol) was debenzylated via hydrogenation over 10% Pd/C (70
mg) in a 4:1 mixture of acetic acid and water (20 mL) as
described for the preparation of 5. Purification by chromatog-
raphy (CH2Cl2/CH3OH, 8:1) on Iatrobeads yielded 4 (61 mg,
67%) as a white solid. The NMR data for this compound were
identical to those of the compound obtained by deprotection
of 25.
P en ta sa cch a r id e 13. Oligosaccharide 65 (250 mg, 0.16
mmol) was debenzylated via hydrogenation over 10% Pd/C (80
mg) in CH3OH (20 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 13 (83 mg, 75%) as an oil.
P en ta sa cch a r id e 14. Oligosaccharide 66 (100 mg, 0.06
mmol) was debenzylated via hydrogenation over 10% Pd/C (30
mg) in CH3OH (20 mL) as described for the preparation of 5.
Purification by chromatography (CH2Cl2/CH3OH, 8:1) on Ia-
trobeads yielded 14 (29 mg, 71%) as an oil.
Ack n ow led gm en t. This work was supported by the
National Institutes of Health (AI44045-01) and the
National Science Foundation (CHE-9875163).
Su p p or tin g In for m a tion Ava ila ble: Analytical data for
all new compounds, NMR spectra of selected compounds, figure
comparing anomeric chemical shifts in protected oligosaccha-
rides. This material is available free of charge via the Internet
at http://pubs.acs.org.
Tetr a sa cch a r id e 59 a n d Tetr a sa cch a r id e 60. Diol 58
(550 mg, 0.63 mmol) and thioglycoside 17 (330 mg, 0.63 mmol)
were coupled using NIS (160 mg, 0.71 mmol) and AgOTf (32
mg, 0.13 mmol) in CH2Cl2 (20 mL) as described for the
J O010910E