Synthesis of oligosaccharides of motifs D and E of arabinogalactan present in mycobacterium tuberculosis

Article abstract of DOI:10.1021/jo010180a

Syntheses of the ethyl glycosides of 5-O-(β-D-galactofuranosyl)-β-D-galactofuranose and 5-O-(α-D-arabinofuranosyl)-6-O-(β-D-galactofuranosyl) -β-D-galactofuranose present in motifs D and E of Mycobacterium tuberculosis arabinogalactan, respectively, have been presented. The pentenylmediated O-glycosylation reaction was utilized to obtain the disaccharide of motif D. The first coupling reaction to prepare the inner disaccharide portion of motif E was accomplished by trichloroacetamidate method while the installation of the terminal sugar by pentenyl glycosylation approach was successful.

Full text of DOI:10.1021/jo010180a

J . Org. Chem. 2001, 66, 4657-4660
4657
Syn th esis of Oligosa cch a r id es of Motifs D a n d E of
Ar a bin oga la cta n P r esen t in Mycoba cter iu m tu ber cu losis
Mukund K. Gurjar,* L. Krishnakanth Reddy, and Srinivas Hotha
National Chemical Laboratory, Pune 411 008, India
gurjar@dalton.ncl.res.in
Received February 16, 2001
Syntheses of the ethyl glycosides of 5-O-(â-D-galactofuranosyl)-â-D-galactofuranose and 5-O-(R-D-
arabinofuranosyl)-6-O-(â-D-galactofuranosyl)-â-D-galactofuranose present in motifs D and E of
Mycobacterium tuberculosis arabinogalactan, respectively, have been presented. The pentenyl-
mediated O-glycosylation reaction was utilized to obtain the disaccharide of motif D. The first
coupling reaction to prepare the inner disaccharide portion of motif E was accomplished by
trichloroacetamidate method while the installation of the terminal sugar by pentenyl glycosylation
approach was successful.
In tr od u ction
mendous opportunities to understand their role in the
survival and pathogenicity of these organisms. Motifs A,
B, and C are composed of arabinofuranose units having
subtle differences between them with respective to O-
glycosidic linkages.8 Motif D has solely galactofuranose
residues but motif E is unique in that it contains both
arabinofuranose and galactofuranose residues. Although
motifs A and B oligosaccharides are branched-chain, the
branched-chain oligosaccharide of motif E is sterically
more demanding because both 5 and 6 positions of the
reducing end galactofuranose unit are linked with ara-
binofuranosyl and galactofuranosyl residues, respec-
tively.
The synthesis of the sugar components of AG of M.
tuberculosis has been a topic of immense activity.9-11 The
first major contribution appeared from this laboratory
and reported the synthesis of the pentaarabinofuranoside
of motif A.9a Subsequently Lowary and co-workers re-
ported10 a series of publications on motifs A and B and
studied their ring conformation by NMR techniques. Very
recently, Prandi et al. also published11 the synthesis of
the pentaarabinofuranosyl structure of motif A. In this
report we wish to reveal our recent findings12 on the
stereocontrolled synthesis of ethyl 5-O-(R-D-arabinofura-
nosyl)-6-O-(â-D-galactofuranosyl)-â-D-galactofuranoside (6)
The social and economical burden due to mycobacterial
diseases such as leprosy and tuberculosis are of unprec-
edented nature particularly as it relates to developing
countries.1 In spite of enormous efforts to develop anti-
infective drug molecules, tuberculosis still constitutes the
leading killer disease.2 Seven million new cases and three
million deaths occur every year due to tuberculosis, and
with AIDS becoming epidemic in many developing coun-
tries, this mortality figure is bound to increase at an
alarming rate.3
Mycobacterium (M.) tuberculosis, a causative agent of
tuberculosis, contains on its cell wall surface many
different groups of oligosaccharides which form the topic
of interest for chemists and biochemists alike.4 However,
recent studies on M. tuberculosis are targeted toward two
major polysaccharides, lipoarabinomannan (LAM) and
arabinogalactan (AG). The unique feature of LAM and
AG is attributed to arabinose and galactose residues
present in the furanose forms.5 LAM and AG are neces-
sary for the survival of M. tuberculosis, and interfering
with the biosynthesis of those polysaccharides is an
idealistic approach for identifying anti-TB drugs.6 Etham-
butol is a drug of choice for the last 35 years for
tuberculosis, but only recently it has been demonstrated
that ethambutol exhibits inhibitory power to disrupt the
biosynthesis of arabinan portions of LAM and AG.7
Five major motifs of AG, namely motifs A-E, have
been identified. Synthesis of these motifs provides tre-
(7) (a) Lee, R. E.; Mikusˇova´, K.; Brennan, P. J .; Besra, G. S. J . Am.
Chem. Soc. 1995, 117, 11829. (b) Deng, L.; Miku?ova´, K.; Robuck, K.
G.; Scherman, M.; Brennan, P. J .; McNeil, M. R. Antimicrob. Agents
Chemother. 1995, 39, 694.
(8) (a) Besra, G. S.; Khoo, K.-H.; McNeil, M. R.; Dell, A.; Morris, H.
R.; Brennan, P. J . Biochemistry 1995, 34, 4257. (b) Wolucka, B. A.;
McNeil, M. R.; de Hoffman, 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) McNeil, M. R.; Robuck, K. G.;
Harter M.; Brennan, P. J . Glycobiology 1994, 4, 165.
(1) (a) Opravil, M. Infection 1997, 25, 56. (b) Millard, F. J . J . Royal
Soc. Med. 1996, 89, 497. (c) Efferen, L. S.; Hyman, C. L. Curr. Opin.
Pulm. Med. 1996, 2, 236. (d) Sudre, P.; ten Dam G.; Kochi, A. Bull.
World Health Organization 1992, 70, 149. (e) Huebner, R. E.; Castro,
K. G. Annu. Rev. Med. 1995, 46, 47.
(2) Lee, R. E.; Brennan, P. J .; Besra, G. S. Tuberculosis. In Current
Topics in Microbiology and Immunology; Shinnick, T. M., Ed.; Springer-
Verlag: Berlin, 1996; Chapter 1, p 215.
(3) (a) Valainis, G. T.; Cardona, I. M.; Greer, D. L. J AIDS, J .
Acquired Immune Defic. Syndr. 1991, 4, 516. (b) Spinney, L. New Sci.
1996, 8. (c) Gilbert, G. L. Med. J . Aust. 1996, 154, 121.
(4) (4) Brennan, P. J .; Nikaido, H. Annu. Rev. Biochem. 1995, 64,
29.
(5) McNeil, M.; Wallner, S. J .; Hunter, S. W.; Brennan, P. J .
Carbohydr. Res. 1987, 166, 299.
(6) Lee, R. E.; Brennan, P. J .; Besra, G. S. Glycobiology 1997, 7,
1121.
(9) (a) Gurjar, M. K.; Hotha, S.; Mereyala, H. B. J . Chem. Soc.,
Chem. Commun. 1998, 685. Other interesting reports on mycobacte-
ria: (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.
(10) (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.
(11) Sanchez, S.; Bamhoud, T.; Prandi, J . Tetrahedron Lett. 2000,
41, 7447.
(12) Gurjar, M. K.; Reddy, L. K.; Hotha, S. Org. Lett. 2001, 3, 321.
10.1021/jo010180a CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/02/2001

4658 J . Org. Chem., Vol. 66, No. 13, 2001
K Gurjar et al.
Sch em e 2
Sch em e 3
F igu r e 1.
Sch em e 1
FAB-MS and satisfactory elemental analysis confirmed
the structure of compound 14. The three hydroxyl groups
in 14 were protected as benzyl ethers, and then the
TBDPS group was cleaved with 1 M solution of n-Bu4-
NF in THF (Scheme 2).
of motif E and ethyl 5-O-(â-D-galactofuranosyl)-â-D-
galactofuranoside (7) of motif D (Figure 1).
Resu lts a n d Discu ssion
The installation of third galactofuranose residue at C-6
of 16 was not a straightforward proposition. For instance
Schmidt’s trichloroimidate method,15 the classical Hel-
frich reaction,18 and SEt-mediated glycosidation18 did not
proceed in our hands. The chemistry that worked for the
trisaccharide synthesis was Fraser-Reid’s n-pentenyl-
mediated O-glycosidation approach.19 The synthesis of
pent-4-enyl 2,3,5,6-tetra-O-acetyl-â-D-galactofuranose 18
was accomplished in one step by treating penta-O-acetyl
D-galactofuranose 1720 and 4-penten-1-ol with a catalytic
amount of BF3‚OEt2 and 4 Å molecular sieves in CH2Cl2
at room temperature. The structure of 18 was confirmed
The synthetic strategy utilized for the preparation of
ethyl 5-O-(R-D-arabinofuranosyl)-6-O-(â-D-galactofurano-
syl)-â-D-galactofuranoside (6) was based on stepwise
assembly of three sugar components. The glycosyl donor
10 was synthesized from 1,2,3,5 tetra-O-acetyl D-arabi-
nose 813 by selectively deprotecting the 1-O-acetyl group
by using Bu3SnOEt in refluxing ClCH2CH2Cl. The re-
sulting 2,3,5-tri O-acetyl D-arabinose 9 was then exposed
to CCl3CN-DBU-CH2Cl2 at 0 °C to afford the corre-
sponding arabinosyl trichloroimidate derivative 10. The
aglycone segment 12 found its origin14 from the known
ethyl 2,3-di-O-benzyl-â-D-galactofuranoside 11 by selec-
tive protection of the OH group at C-6 as a sterically
hindered tert-butyldiphenylsilyl (TBDPS) ether linkage
providing the 5-hydroxyl group free for coupling reaction
(Scheme 1).
1
by H and 13C NMR spectral analysis (Scheme 3).
Treatment of 16 and 18 in the presence of N-iodosuc-
cinimide (NIS) and cat. triflic acid in CH2Cl2 at room
temperature gave the crude trisaccharide 19 (contami-
nated with minor quantity of aglycone 16) which after
Ze´mplen deacetylation provided the pure penta-O-ben-
zylated derivative 20. The three anomeric proton signals
in the 1H NMR spectrum of 20 were located at 4.84, 4.96,
The coupling reaction15 between 10 and 12 proceeded
satisfactorily with BF3‚OEt2 as an activator. The result-
ing disaccharide 13 under Ze´mplen conditions16 gave the
triol derivative 14. The structure of 14 was supported
and 5.17 ppm as distinguishable singlets whereas the 13
C
1
by H and 13C NMR data. The characteristic resonances
NMR spectrum revealed21 anomeric carbons at 105.3,
due to anomeric protons H-1 and H-1′ were located at
5.01 and 5.21 ppm as singlets. In the 13C NMR spectrum,
the C-1′ carbon was observed at 104.5 ppm.17 In addition,
106.8, and 107.7 ppm. Finally, compound 20 was sub-
(18) Toshima, K.; Tatsuta, K. Chem. Rev. 1993, 93, 1503.
(19) (a) Mootoo, D. R.; Konradsson, P.; Udodong, U.; Fraser-Reid,
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.
(20) Chittenden, G. J . F. Carbohydr. Res. 1972, 25, 35.
(21) (a) Cyr, N.; Perlin, A. S. Can. J . Chem. 1979, 57, 2504. (b)
Marino, C.; Varela, O.; De Lederkremer, R. M. Carbohydr. Res. 1989,
190, 65.
(13) Kam, B. L.; Barascut, J .-L.; Imbach, J .-L. Carbohydr. Res. 1979,
69, 135.
(14) Thiem, J .; Wessel, H.-P. Tetrahedron Lett. 1980, 21, 3571.
(15) (a) Schmidt, R. R.; Michel, J . Angew. Chem., Int. Ed. Engl. 1980,
19, 731. (b) Schmidt, R. R. Angew. Chem., Int. Ed. Engl. 1986, 25, 212.
(16) McAuliffe, J . C.; Hindsgaul, O. J . Org. Chem. 1997, 62, 1234.
(17) Mizutani, K.; Kasai, R.; Nakamura, M.; Tanaka, O.; Matsuura,
H. Carbohydr. Res. 1989, 185, 27.

Synthesis of Oligosaccharides of Arabinogalactan
J . Org. Chem., Vol. 66, No. 13, 2001 4659
Sch em e 4
Exp er im en ta l Section
The NMR spectra were recorded in CDCl3 with TMS
internal standard or D2O with methanol as standard on AC
200 MHz, MSL 300 MHz, or DRX 500 MHz. Optical rotations
were measured with digital polarimeter. FAB mass spectra
were recorded on an autospec mass spectrometer. Elemental
analyses were done on elemental analyzer model 1108EA.
Column chromatography was carried out with silica gel (60-
120 mesh). TLC was performed on 0.25 mm precoated silica
gel plates (60F-254) with UV or I2 or anisaldehyde reagent in
ethanol. Dry solvents were distilled over CaH2 and stored over
molecular sieves. Light petroleum refers to mixture of hexanes
with bp 60-80 °C.
2,3,5-Tr i-O-a cet yl-r,â-D-a r a b in ofu r a n osyl-t r ich lor o-
a ceta m id a te (10). A solution of per-O-acetyl-D-arabinofura-
nose 8 (4.0 g, 12.57 mmol) and tri-n-butyltin ethoxide (8.4 g,
25.15 mmol) in ClCH2CH2Cl (40 mL) was heated under reflux
for 3 h and concentrated. The residue was purified on silica
gel with light petroleum-ethyl acetate (EtOAc) (3:2) as eluent
to give 9 (2.43 g, 70%). The resulting product was immediately
treated with CCl3CN (6.34 g, 44.02 mmol) in the presence of
DBU (1.47 g) in CH2Cl2 (50 mL) at 0 °C for 2 h. Then solvent
was removed, and the residue was purified by flash chroma-
tography on silica gel with light petroleum-EtOAc (3:1) as
eluent to give 10 (2.35 g, 65%) which was used without delay.
Eth yl 2,3-Di-O-ben zyl-6-O-ter t-bu tyld ip h en ylsilyl-â-D-
ga la ctofu r a n osid e (12). A solution of 11 (3.0 g, 7.75 mmol),
imidazole (1.1 g, 16.28 mmol), and tert-butyldiphenylsilyl
chloride (2.13 g, 7.75 mmol) in CH2Cl2 (50 mL) was stirred at
room temperature for 3 h. The reaction mixture was washed
with brine, dried (Na2SO4), and concentrated. The residue was
purified on silica gel column with light petroleum-EtOAc (10:
1) as eluent to give 12 (4.36 g, 90%) as a syrup: [R]D -53.2 (c
) 1, CHCl3); 1H NMR δ (300 MHz, CDCl3): 1.05 (s, 9 H), 1.20
(t, 3 H, J ) 7.1 Hz), 3.40-3.48 (m, 1 H), 3.65-3.79 (m, 4 H),
3.96 (dd, 1 H, J ) 3.0, 1.3 Hz), 4.03-4.10 (m, 1 H), 4.21 (dd,
1 H, J ) 6.65, 2.7 Hz), 4.44-4.58 (m, 4 H), 5.00 (s, 1 H), 7.20-
7.40 (m, 15 H), 7.60-7.65 (m, 5 H); 13C NMR δ (50 MHz,
CDCl3): 15.0, 19.1, 26.8, 62.9, 64.9, 71.2, 71.8, 72.2, 80.7, 83.4,
87.8, 106.0, 127.5-129.7, 133.0, 134.6-138.0; FAB-MS: 649
(M + 23) Anal. Calcd for C38H46O6Si: C, 72.84; H, 7.34.
Found: C, 73.16; H, 7.55.
jected to exhaustive hydrogenolysis over 10% Pd(OH)2/C
in methanol to provide the trisaccharide 6 (Scheme 3).
1
The structure was supported by high resolution H and
13C NMR spectra. For example, in the 1H NMR spectrum
of 6, resonances typical of anomeric protons (1,2-trans)
were identified at 4.88, 4.92, and 5.10 ppm as three
clearly distinguishable singlets. The chemical shifts of
the anomeric carbons were at 107.6, 108.6, and 109.6
ppm in its 13C NMR spectrum. In addition, FAB-MS and
elemental analysis of 6 were in complete agreement with
assigned structure. Compound 6 constitutes the oligosac-
charide segment of motif E of arabinogalactan segment
of Mycobacterium tuberculosis.
We also undertook the synthesis of ethyl 5-O-(â-D-
galactofuranosyl)-â-D-galactofuranoside 7 representing
motif D of M. tuberculosis. This disaccharide serves as
an anchor that couples arabinan and galactan chains of
AG. The known precursor 11 was converted into a
dibutylstannyl acetal by refluxing with Bu2SnO in tolu-
ene followed by addition of 1.0 equiv of BnBr to give rise22
to the tribenzyl derivative 21 (Scheme 4).
Eth yl 5-O-(r-D-Ar a bin ofu r a n osyl)-2,3-d i-O-ben zyl-6-O-
ter t-bu tyld ip h en ylsilyl-â-D-ga la ctofu r a n osid e (14). To a
stirred solution of 12 (3.0 g, 4.8 mmol), 10 (2.1 g, 5.0 mmol),
and 4 Å molecular sieves (MS) powder (0.5 g) in CH2Cl2 (50
mL) under nitrogen at -20 °C was added BF3‚OEt2 (0.2 mL).
After 2 h at room temperature, solid NaHCO3 (0.2 g) was added
and then filtered. The filtrate was washed with brine, dried
(Na2SO4), and concentrated. The residue was passed through
a short column of silica gel with light petroleum-EtOAc (9:1)
as eluent to obtain the disaccharide 13 (2.96 g) (contaminated
with 12 as a minor impurity) and treated with 0.05 M NaOMe
in methanol (15 mL) at room temperature for 2 h. The reaction
mixture was deionized by the addition of Amberlite IR 120
(H+) resin (pH 6), filtered, and concentrated. The residue was
purified by silica gel column chromatography with light
petroleum-EtOAc (1:1) as eluent to obtain 14 (2.03 g, 56%)
The coupling reaction between 18 and 21 mediated by
NIS and cat. TfOH in CH2Cl2 at room temperature gave
the disaccharide 22 which was then deacetylated under
1
Ze´mplen conditions. The H and 13C NMR spectra of 23
were in complete agreement with the assigned structure.
1
For example, in the H NMR spectrum of 23, signals due
to two anomeric protons (1,2-trans) were observed at 5.00
and 5.34 ppm as singlets. The 13C NMR spectrum showed
anomeric carbons at 104.9 and 106.1 ppm. Finally,
compound 23 was exhaustively debenzylated by hydro-
genolysis over 10% Pd(OH)2/C in methanol to give the
disaccharide 7 (Scheme 4). The structure of 7 was fully
analyzed by 1H and 13C NMR spectral analysis and gave
satisfactory elemental analysis.
1
as a syrup: [R]D -22.4 (c ) 1, CHCl3); H NMR δ (200 MHz,
CDCl3): 1.07 (s, 9 H), 1.20 (t, 3 H, J ) 7.5 Hz), 3.60-3.70 (m,
5 H), 3.90-4.02 (m, 4 H), 4.06-4.17 (m, 4 H), 4.30-4.58 (m, 4
H), 5.01 (s, 1 H), 5.21 (s,1 H), 7.20-7.45 (m,15 H), 7.60-7.75
(m, 5 H); 13C NMR δ (125 MHz, CDCl3): 15.0, 18.0, 25.6, 60.2,
61.7, 62.1, 71.0, 76.8, 77.1, 77.3, 77.4, 79.1, 82.9, 85.8, 86.9,
104.5,106.5,126.1-128.4, 134.0, 136.0; FAB-MS: 781 (M + 23).
Anal. Calcd for C43H54O10Si: C, 68.07; H, 7.12. Found: C, 68.34;
H, 6.93.
Eth yl 5-O-(2′,3′,5′-Tr i-O-ben zyl-r-D-a r a bin ofu r a n osyl)-
2,3-d i-O-b en zyl-6-O-ter t-b u t yld ip h en ylsilyl-â-D-ga la ct o-
fu r a n osid e (15). To a stirred solution of 14 (1.5 g, 1.98 mmol)
and NaH (60% dispersion in paraffin, 0.35 g, 14.8 mmol) in
dry DMF (10 mL) under nitrogen atmosphere was added
benzyl bromide (1.11 g, 6.53 mmol). After 3 h at room
temperature, the reaction was quenched with methanol (2 mL)
Con clu sion
With the di- and trisaccharides reported herein, the
synthesis of all the oligosaccharides present in motifs
A-E of the AG of M. tuberculosis are now completed.
Arabinogalactan of M. tuberculosis has special interest
for two fundamental reasons: (1) it is essential for the
viability and (2) three out of four sugarssaraf, galf, and
rhampsare not found in human beings.
(22) (a) Wagner, D.; Verheyden, J . P. H.; Moffatt, J . G. J . Org. Chem.
1974, 39, 24. (b) Sugawara, F.; Nakayama, H.; Ogawa, T. Agric. Biol.
Chem. 1986, 50, 1557.

4660 J . Org. Chem., Vol. 66, No. 13, 2001
K Gurjar et al.
and partitioned between water and diethyl ether. The ether
layer was washed with brine, dried (Na2SO4), and concentrated
to give a residue which was purified on silica gel with light
petroleum-EtOAc (10:1) as eluent to give 15 (1.77 g, 87%) as
a syrup; [R]D -18.4 (c ) 1.5, CHCl3); 1H NMR δ (500 MHz,
CDCl3): 1.08 (s, 9 H), 1.20 (t, 3 H, J ) 6.25 Hz), 3.45-3.50
(m, 3 H), 3.74-3.80 (m, 2 H), 3.90 (d, 2 H, J ) 5.2 Hz), 3.92-
4.00 (m, 3 H), 4.05-4.10 (m, 3 H), 4.28-4.60 (m, 10 H), 5.09
(s, 1 H), 5.20 (s, 1 H), 7.18-7.37 (m, 30 H), 7.68-7.72 (m, 5
H); 13C NMR δ (125 MHz, CDCl3): 14.8, 18.9, 26.5, 62.5, 63.0,
68.4, 71.4, 71.6, 71.7, 73.0, 76.4, 76.6, 76.9, 79.5, 79.6, 82.8,
83.4, 88.3, 105.1, 106.6, 127.2-129.2, 133.2, 135.3, 137.6; FAB-
MS: 1051 (M + 23). Anal. Calcd for C64H72O10Si: C, 74.70; H,
7.00. Found: C, 74.4; H, 7.14.
H, J ) 7.0 Hz), 3.48-3.72 (m, 8 H), 3.78-4.05 (m, 11 H), 4.88
(s, 1 H), 4.92 (s, 1 H), 5.10 (s, 1 H); 13C NMR δ (125 MHz,
D2O): 15.2, 62.1, 63.8, 65.3, 68.2, 71.8, 77.2, 77.3, 77.4, 77.8,
81.9, 82.0, 82.2, 82.9, 83.9, 84.8, 107.6, 108.6, 109.6; FAB-MS:
525 (M + 23). Anal. Calcd for C19H34O15: C, 45.42, H, 6.77.
Found: C, 46.13, H, 7.0.
E t h yl 2,3,6-Tr i-O-b en zyl-â-D-ga la ct ofu r a n osid e (21).
Compound 11 (1.0 g, 2.1 mmol) and dibutyltin oxide (0.78 g,
3.13 mmol) in dry toluene (20 mL) were heated under reflux
for 6 h with azeotropic removal of water, and then Bu4NBr
(0.10 g) and benzyl bromide (0.35 g, 2.1 mmol) were introduced.
The reaction was heated under reflux for 24 h, solvent
evaporated, and the residue taken up in ethyl acetate. The
organic layer was successively washed with 10% aq KF
solution and brine, dried (Na2SO4), and concentrated. The
residue was purified by silica gel column chromatography with
light petroleum-EtOAc (9:1) as eluent to give 21 (0.87 g, 71%)
as a syrup: [R]D - 48.25 (c ) 1.1, CHCl3); 1H NMR δ (200
MHz, CDCl3): 1.24 (t, 3 H, J ) 7.3 Hz), 3.43-3.56 (m, 3 H),
3.72-3.80 (m,1 H), 4.08-4.14 (m, 1 H), 4.54-4.60 (m, 6 H),
5.05 (s, 1 H), 7.28-7.33 (m, 15 H); 13C NMR δ (50 MHz, CDCl3):
14.8, 62.6, 69.6, 71.3, 71.6, 71.9, 72.9, 80.9, 83.0, 87.6, 105.6,
127.2-128.0, 137.1-137.8. Anal. Calcd for C29H34O6: C, 72.8,
H, 7.11. Found: C, 72.42, H, 7.01.
P en t-4-en yl 2,3,5,6-Tetr a -O-a cetyl-â-D-ga la ctofu r a n o-
sid e (18). To a stirred solution of per-O-acetyl â-D-galacto-
furanose 17 (2.0 g, 5.13 mmol), 4-pentene-1-ol (1.1 mL, 10.25
mmol), and 4 Å MS powder (0.5 g) in CH2Cl2 (20 mL) under
nitrogen was added BF3‚OEt2 (0.1 mL) at 0 °C. After 2 h, solid
NaHCO3 (0.2 g) was added and filtered over Celite. The filtrate
was washed with brine, dried (Na2SO4), concentrated. The
residual syrup was purified by silica gel column chromatog-
raphy by eluting with light petroleum-EtOAc (4:1) to give 18
1
(1.81 g, 85%) as a syrup: [R]D -50.2 (c ) 2, CHCl3); H NMR
δ (200 MHz, CDCl3): 1.64-1.76 (m, 4 H), 2.06-2.14 (4s, 12
H), 3.40-3.51 (m, 1 H), 3.62-3.74 (m, 1 H), 4.16-4.38 (m, 3
H), 4.96-5.08 (m, 5 H), 5.35-5.42 (m, 1 H), 5.71-5.92 (m, 1
H); 13C NMR δ (50 MHz, CDCl3): 20.3, 28.2, 29.7, 62.2, 66.5,
68.9, 76.2, 79.5, 81.0, 105.1, 114.6, 137.6, 169.6. Anal: Calcd
for C19H28O10: C, 54.80, H, 6.73. Found: C, 54.65, H, 6.52.
Eth yl 5-O-(2′,3′,5′-Tr i-O-ben zyl-r-D-a r a bin ofu r a n osyl)-
6-O-(â-D-ga la ct ofu r a n osyl)-2,3-d i-O-b en zyl-â-D-ga la ct o-
fu r a n osid e (20). Compound 15 (1.5 g, 1.46 mmol) and a 1 M
solution of n-Bu4NF (3 mL) in THF (10 mL) were stirred for
30 min at room temperature and then concentrated. The
residue was purified on short silica gel column with light
petroleum-EtOAc (3:1) to provide 16 (1.06 g, 92%) used as
such for subsequent reaction.
To a stirred solution of 16 (1.0 g, 1.26 mmol), 18 (0.63 g,
1.52 mmol), and 4 Å MS powder (0.5 g) in dry CH2Cl2 (20 mL)
was added NIS (0.7 g, 3.16 mmol) followed by TfOH (0.1 mL
in two intervals of 30 min). The reaction was stirred under
dark at room temperature for 48 h and filtered through a
Celite bed. The filtrate was washed with saturated solutions
of NaHSO3 and NaHCO3 followed by brine solution. The
organic layer was dried (Na2SO4) and concentrated. The
residue was subjected column chromatography on silica gel
with light petroleum-EtOAc (3:1) as eluent to obtain the crude
trisaccharide derivative 19 (0.92 g). The crude 19 was treated
with 0.05 M NaOMe in methanol (5 mL) at room temperature
for 2 h. After the usual workup as indicated above for Ze´mplen
reaction, the residue was chromatograghed on silica gel with
light petroleum-EtOAc (2:3) as eluent to give 20 (0.56 g, 47%)
as a syrup: [R]D -22.6 (c ) 0.6, CHCl3); 1H NMR δ (500 MHz,
CDCl3): 1.10 (t, 3 H, J ) 7.5 Hz), 3.36-3.65 (m, 8 H), 3.76-
3.97 (m, 10 H), 4.00-4.05 (m, 1 H), 4.27-4.51 (m, 10 H), 4.84
(s, 1 H), 4.96 (s, 1 H), 5.17 (s, 1 H), 7.12-7.25 (m, 25 H); 13C
NMR δ (125 MHz, CDCl3): 14.8, 62.8, 63.7, 66.6, 69.5, 70.7,
71.5, 71.6, 71.9, 72.0, 73.2, 76.2, 78.3, 78.6, 80.0, 80.6, 83.5,
83.6, 86.9, 87.8, 87.9, 105.3, 106.8, 107.7, 127.4-128.2, 137.1-
137.6. Anal. Calcd for C54H64O15: C, 68.06, H, 6.72. Found: C,
68.35, H, 6.95.
Eth yl 5-O-(â-D-Ga la ctofu r a n osyl)-2,3,6-tr i-O-ben zyl-â-
D-ga la ctofu r a n osid e (23). To a stirred solution of 21 (0.5 g,
1.04 mmol), 18 (0.52 g, 1.25 mmol), 4 Å MS powder (0.5 g),
and NIS (0.58 g, 2.61 mmol) in CH2Cl2 (20 mL) under nitrogen
and in the dark was addded TfOH (0.05 mL). The reaction
mixture was stirred for 24 h at room temperature and filtered
through Celite, and the filtrate washed with saturated solu-
tions of NaHSO3 and NaHCO3 and then with brine, dried (Na2-
SO4), and concentrated. The residual syrup was chromato-
graphed on silica gel with light petroleum-EtOAc (4:1) as
eluent to give the crude disaccharide 22 (0.57 g) which was
treated with 0.05 M NaOMe in methanol (5 mL) at room
temperature. After 2 h and the usual workup as given above
for Ze´mplen reaction, the residue was purified on silica gel
with light petroleum-EtOAc (1:4) as eluent to give 23 (0.32
1
g, 48%) as a syrup: [R]D -84.12 (c ) 1.2, CHCl3); H NMR δ
(200 MHz, CDCl3): 1.18 (t, 3 H, J ) 7.1 Hz), 3.37-3.72 (m, 6
H), 3.85-3.90 (m, 2 H), 3.93-4.10 (m, 6 H), 4.33-4.61 (m, 6
H), 5.00 (s, 1 H), 5.34 (s, 1 H), 7.27-7.35 (m, 15 H); 13C NMR
δ (75 MHz, CDCl3): 14.5, 62.5, 63.5, 69.2, 70.7, 71.5, 71.6, 72.8,
73.0, 78.3, 78.6, 80.2, 83.9, 86.5, 87.0, 104.9, 106.1, 127.1-
128.0, 136.6-137.3. Anal. Calcd for C35H44O11: C, 65.62, H,
6.87. Found: C, 66.32, H, 6.95.
E t h yl 5-O-(â-D-Ga la ct ofu r a n osyl)-â-D-ga la ct ofu r a n o-
sid e (7). A solution of 23 (0.2 g, 0.31 mmol) and 10%
Pd(OH)2/C (50 mg) in methanol (5 mL) was stirred under
hydrogen atmosphere at ntp. After 8 h, the catalyst was
filtered and the filtrate concentrated to give 7 (0.10 g, 88%)
1
as a thick syrup: [R]D -105.22 (c ) 1.1, MeOH); H NMR δ
(500 MHz, D2O): 0.92 (t, 3 H, J ) 6.4 Hz), 3.29-3.32 (m, 1 H),
3.40-3.44 (m, 2 H), 3.46-3.55 (m, 4 H), 3.64 (brs, 1 H), 3.73-
3.77 (m, 4 H), 3.81-3.85 (m, 2 H), 4.92 (s, 2 H); 13C NMR δ
(125 MHz, D2O): 15.1, 61.9, 63.6, 65.1, 71.4, 76.7, 77.0, 77.3,
81.7, 82.1, 82.2, 83.5, 107.5, 107.9. Anal. Calcd for C14H26O11
C, 45.4, H, 7.02. Found: C, 45.75, H, 7.25.
:
Ack n ow led gm en t. L.K.R. and S.H. thank CSIR
(New Delhi) for financial assistance in the form of a
Senior Research Fellowship.
Eth yl 5-O-(r-D-Ar a bin ofu r a n osyl)-6-O-(â-D-ga la ctofu r a -
n osyl)-â-D-ga la ctofu r a n osid e (6). Compound 20 (0.3 g, 0.31
mmol) and 10% Pd(OH)2/C (50 mg) in methanol (7 mL) were
stirred under hydrogen atmosphere at normal temperature
and pressure (ntp) for 8 h. The catalyst was filtered and the
filtrate concentrated to give 6 (0.14 g, 90%) as a syrup: [R]D
-26.8 (c ) 1.1, MeOH); 1H NMR δ (500 MHz, D2O): 1.10 (t, 3
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of 6, 7, 12, 14, 15, 18, 20, 21, 23. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O010180A