Concise synthesis of a pentasaccharide related to the anti-leishmanial triterpenoid saponin isolated from Maesa balansae

Article abstract of DOI:10.1021/jo801171f

(Chemical Equation Presented) Concise synthesis of the glycone part (a pentasaccharide) of the anti-leishmanial triterpenoid saponin isolated from Maesa balansae is reported. A late-stage TEMPO-mediated oxidation of a primary hydroxyl group to carboxylic

Full text of DOI:10.1021/jo801171f

one common feature shared by all saponins is the presence of
a sugar chain attached to the 3-position of the aglycon moiety.
Sugar chains differ substantially from saponin to saponin; they
are often branched and consist of up to five sugar units (usually
selected from glucose, rhamnose, arabinose, xylose, or glucu-
ronic acid).4 The glycosylation step is believed to happen at
the final stage of the saponin biosynthesis, and the saponin
bioactivity largely depends on the glycosylation pattern.5
Therefore, a clear elucidation of the biosynthetic formation of
the glycone chain as well as the enzymes involved in the whole
process is of absolute necessity. Moreover, to explore the
medicinal activities associated with many of the saponins,
chemical synthesis of the saccharide fragments will be useful.
Concise Synthesis of a Pentasaccharide Related to
the Anti-Leishmanial Triterpenoid Saponin
Isolated from Maesa balansae†
Vishal Kumar Rajput and Balaram Mukhopadhyay*
Indian Institute of Science Education and Research-Kolkata
(IISER-K), HC-7, Sector-III, Salt Lake City, Kolkata 700
106, India
sugarnet73@hotmail.com
ReceiVed May 31, 2008
FIGURE 1. Structure of maesabalide I and synthetic target.
Concise synthesis of the glycone part (a pentasaccharide) of
the anti-leishmanial triterpenoid saponin isolated from Maesa
balansae is reported. A late-stage TEMPO-mediated oxida-
tion of a primary hydroxyl group to carboxylic acid has been
achieved under phase-transfer conditions. Glycosylations
were performed either by thioglycoside or glycosyl trichlo-
roacetimidate activation using sulfuric acid immobilized on
silica (H2SO4-silica) in conjunction with N-iodosuccinimide
and alone, respectively. H2SO4-silica was proved to be a
better choice as promoter than conventional Lewis acid
promoters such as TfOH or TMSOTf.
Recently, De Kimpe et al.6 reported the isolation and
characterization of six triterpenoid saponins (maesabalides
I-IV)7 isolated from the Vietnamese medicinal plant Maesa
balansae that showed intense in vitro and in vivo anti-
leishmanial activity against intracellular Leishmania infantum
amastigotes.8 To determine the structure-activity relationship
for the saponins, they have evaluated some semisynthetic
analogs of the same. However, they focused on the modification
of the aglycon part only. As Leishmaniasis is a growing threat
to the public health with about 350 million people living in
endemic areas and an annual incidence of about 2 million cases.9
Inadequate resources are available to tackle this disease, with
treatment options limited to pentavalent antimonials as first-
line chemotherapeutics and to amphotericin and pentamidine
as second-line chemotherapeutics, and novel drug leads are
highly needed10 to combat this deadly disease. In order to exploit
the scopes arisen from the identification of anti-leishmanial
saponins from Maesa balansae, here we report the concise
chemical synthesis of the pentasaccharide side chain (Figure
1), aiming for the elucidation of the biosynthetic pathway of
Saponins, glycosylated secondary metabolites in plants,1 are
synthesized routinely during their normal program of growth
and development. Because of their intense antifungal properties,
it is believed that these molecules act as natural chemical barriers
in plants against fungal attack.2 In addition to their natural
protective activity, many of them are exploited as sources for
drugs, e.g., ginseng and liquorice, or food crops such as legumes
and oats.3 Therefore, this class of compounds is commercially
attractive for diverse reasons. However, the detailed genetic
machinery responsible for the elaboration of this important
family is largely uncharacterized till date. Although a broad
range of architectural diversity is observed in the saponin family,
(4) Hostettmann, K. A.; Marston, A. Saponins. Chemistry and Pharmacology
of Natural Products; Cambridge University Press: Cambridge, UK, 1995.
(5) (a) Paczkowski, C.; Wojciechowski, Z. A. Phytochemistry 1994, 35, 1429–
1434. (b) Wojciechowski, Z. A. Phytochemistry 1975, 14, 1749–1753.
(6) Germonprez, N.; Maes, L.; Van Puyvelde, L.; Van Tri, M.; Tuan, D. A.;
De Kimpe, N. J. Med. Chem. 2005, 48, 32–37.
(7) Germonprez, N.; Van Puyvelde, L.; Maes, L.; De Kimpe, N. Tetrahedron
2004, 60, 219–228.
(8) Maes, L.; Vanden Berghe, D.; Germonprez, N.; Quirijnen, L.; Cos, P.;
De Kimpe, N.; Van Puyvelde, L. Antimicrob. Agents Chemother. 2004, 48, 130–
136.
† Dedicated to Prof. Sushanta Dattagupta, Director, IISER-K on the occasion
of his 60th birthday.
(1) Price, K. R.; Johnson, I. T.; Fenwick, G. R. CRC Crit. ReV. Food Sci.
Nutr. 1987, 26, 27–133.
(2) Papadopoulou, K.; Melton, R. E.; Legget, M.; Daniels, M. J.; Osbourn,
A. E Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12923–12928.
(3) Haralampidis, K.; Trojanowska, M.; Osbourn, A. E. AdV. Biochem. Eng.
Biotechnol. 2002, 75, 31–49.
(9) Desjeux, P Trans. R. Soc. Trop. Med. Hyg. 2001, 95, 239–243.
(10) Guerin, P.; Olliaro, J. P.; Sundar, S.; Boelaert, M.; Croft, S. L.; Desjeux,
P.; Wasunna, M. K.; Bryceson, A. D. Lancet Infect. Dis. 2002, 2, 494–501.
6924 J. Org. Chem. 2008, 73, 6924–6927
10.1021/jo801171f CCC: $40.75 2008 American Chemical Society
Published on Web 08/02/2008

SCHEME 1. Synthesis of Trisaccharide Donor
FIGURE 2. Retrosynthetic analysis for the target oligosaccharides.
the saponin concerned as well as further medicinal use if found
suitable. It is worth noting that all of the six saponins
(maesabalides I-IV) have the same side chain oligosaccharides.
For the synthesis of the target pentasaccharide (1), a 3 + 2
disconnection was planned where the protected trisaccharide
can be coupled to the disaccharide acceptor. The reducing end
glycoside is always an important factor for oligosaccharide
synthesis since this will ultimately determine the possibility of
further conjugate formation. Here we opted for propargyl
glycosides as they can be converted to suitable glycoconjugates
as required through simple multicomponent reactions or cy-
cloaddition reactions.11 Many of these reactions can be done in
aqueous media, and therefore, modifications are possible even
after global deprotection of the oligosaccharide target.12 More-
over, propargyl glycoside can be effectively cleaved to obtain
the corresponding hemiacetal.13,29 The glycosylation strategies
are so chosen that the trisaccharide can be converted to its
corresponding glycosyl trichloroacetimidate for the final gly-
cosylation. For other glycosylations, activation of thioglycosides
were preferred as thioglycosides are stable enough for required
protecting group manipulations and can be activated efficiently
to provide required glycosidic bond. A TEMPO-mediated late-
stage oxidation of the required primary hydroxyl group to
carboxylic acid was planned to afford the target uronic acid
moiety. This 3 + 2 route would provide the tri- and disaccharide
fragments also, which are useful for biological evaluation. The
planned retrosynthetic analysis for the total synthesis of the
pentasaccharide 1 is furnished below (Figure 2).
pyridine15 (isolated yield, 93%) followed by 3,4-isopropylidene
formation using 2,2-dimethoxypropane and 10-camphorsulfonic
acid in acetone16 furnished the acceptor, p-methoxyphenyl 6-O-
tert-butyldiphenylsilyl-3,4-O-isopropylidene-ꢀ-D-glactopyrano-
side (4), in 97% yield. For the synthesis of the rhamnose donor,
known p-tolyl 2,3-O-isopropylidene-1-thio-R-L-rhamnopyrano-
side (5)17 was benzylated using BnBr and NaH in dry DMF18
to afford p-tolyl 4-O-benzyl-2,3-O-isopropylidene-1-thio-R-L-
rhamnopyranoside (6) in 89% yield. Removal of isopropylidene
acetal by 80% AcOH at 80 °C19 (isolated yield, 92%) followed
by selective benzylation through stannylene acetal using Bu2SnO
in toluene and then BnBr in the presence of tetrabutylammonium
iodide20 furnished p-tolyl 3,4-O-benzyl-1-thio-R-L-rhamnopy-
ranoside (8) in 89% yield. Acetylation of compound 8 using
Ac2O in pyridine21 afforded the target donor, p-tolyl 2-O-acetyl-
3,4-O-benzyl-1-thio-R-L-rhamnopyranoside (9), in 97% yield
(Scheme 1).
Glycosylation between acceptor 4 and donor 9 was achieved
through thioglycoside activation by N-iodosuccinimide in the
presence of H2SO4-silica22 to afford the disaccharide 10 in 87%
yield. Use of H2SO4-silica in conjunction with N-iodosuccin-
imide was found to be advantageous as it replaces the use of
Lewis acids such as TfOH23 or TMSOTf.24 The same glyco-
Synthesis of the trisaccharide fragment was commenced with
known p-methoxyphenyl ꢀ-D-glactopyranoside (2).14 Protection
of the primary OH with a TBDPS group using TBDPS-Cl in
(11) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew.Chem., Int. Ed. 2002, 41, 2596–2599. (b) Tornoe, C. W.; Christensen,
C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064. (c) Saxon, E.; Bertozzi,
C. R. Science 2000, 287, 2007–2010.
(12) Mandal, S.; Gauniyal, H. M.; Pramanik, K.; Mukhopadhyay, B. J. Org.
Chem. 2007, 72, 9753–9756.
(15) Limberg, G.; Thiem, J. Carbohydr. Res. 1995, 275, 107–115.
(16) Lipta´k, A.; Imre, J.; Na´na´si, P. Carbohydr. Res. 1981, 92, 154–156.
(17) Mukhopadhyay, B.; Field, R. A. Carbohydr. Res. 2006, 341, 1697–
1701.
(18) Thollas, B.; Biosset, C. Synlett 2007, 11, 1736–1738.
(19) Gent, P. A.; Gigg, R. J. Chem. Soc., Perkin Trans. I 1974, 1446–1455.
(20) Faure, R.; Shiao, T. C.; Damerval, S.; Roy, R. Tettrahedron Lett. 2007,
48, 2385–2388.
(13) Hotha, S.; Kashyap, S. J. Am. Chem. Soc. 2006, 128, 9620–9621.
(29) For corresponding a´-isomer Izumi, M.; Fukase, K.; Kusumoto, S Synlett
2002, 9, 1409–1416.
(14) Ohlsson, J.; Magnusson, G. Carbohydr. Res. 2000, 329, 49–55.
(21) Hudson, C. S.; Dale, J. K. J. Am. Chem. Soc. 1915, 37, 1264–1270.
J. Org. Chem. Vol. 73, No. 17, 2008 6925

SCHEME 2. Synthesis of Disaccharide Acceptor
sylation reaction in the presence of TfOH and TMSOTf
produced 78% and 81% yields, respectively. Moreover, both
TfOH and TMSOTf needed to be diluted with CH2Cl2 to obtain
the required amount, whereas for H2SO4-silica simple weighing
was enough. Zemple´n de-O-acetylation of the disaccharide 10
using NaOMe in methanol afforded the required disaccharide
acceptor 11 in 98% yield. Compound 11 was then coupled with
known p-tolyl 2,3,4-tri-O-acetyl-1-thio-R-L-rhamnopyranoside
(12)25 using the same thioglycoside activation protocol as above
to get the protected trisaccharide 13 in 86% yield. Our
experience with CAN-mediated oxidative cleavage of p-meth-
oxyphenyl group suggests that isopropylidene acetals or silyl
protections are not well tolerated. Therefore, the TBDPS group
was removed using Bu4NF in THF26 to afford the trisaccharide
14 in 81% isolated yield. Further, the isopropylidene acetal was
hydrolyzed using 80% AcOH at 80 °C17 followed by Pd-C
promoted hydrogenolysis of the benzyl protections. The resulting
compound was acetylated using Ac2O in pyridine to afford
trisaccharide 15 in 83% yield. Now the CAN-mediated27
oxidative cleavage of the p-methoxyphenyl group followed by
reaction with trichloroacetonitrile in the presence of DBU28
afforded the glycosyl trichloroacetimidate donor 16 in 79% yield
(Scheme 1).
SCHEME 3. Synthesis of Pentasaccharide, Oxidation, and
Deprotection
For the synthesis of the disaccharide acceptor, propargyl 4,6-
O-benzylidene-ꢀ-D-glucopyranoside (17)29 was selectively pro-
tected at the 3-O-position with p-methoxybenzyl group via its
2,3-stannylene acetal to afford the required 2-ol acceptor 18 in
84% yield. The regioselectivity of the p-methoxybenzyl group
at the 3-position was further confirmed by acetylation of the
free OH group and careful adjudication of 1H and 1H-1H COSY
spectra. Disaccharide acceptor 18 was coupled with known
galactosyl donor 19 using N-iodosuccinimide in the presence
of H2SO4-silica.20 After complete conversion of the acceptor
at -30 °C in 45 min, the temperature of the solution was raised
to room temperature and stirred for an additional hour when
the p-methoxybenzyl group was cleaved completely to furnish
the target disaccharide acceptor 20 in 82% yield (Scheme 2).
Glycosylation of trisaccharide donor 16 and acceptor 20 was
achieved through trichloroacetimidate activation using
H2SO4-silica at -40 °C for 6 h. The reaction proceeded
smoothly to furnish the protected pentasaccharide 21. However,
a trehalose type of compound resulted from the coupling
between donor hemiacetal, and unreacted donor was formed
(<5%) as confirmed by mass spectroscopy that could not be
separated by chromatography. Therefore, the scheme was
continued to the next step without having clean NMR spectra
at this stage. Opening of the benzylidene acetal using 80%
AcOH at 80 °C30 afforded the diol 22, and the trehalose impurity
generated in the previous step was successfully removed.
Required oxidation of the primary-OH group was achieved by
a TEMPO-mediated oxidation under phase transfer conditions
by following a recent protocol developed by Huang et al.31 It
is worth noting that the oxidation was absolutely selective to
the primary-OH without affecting the secondary-OH present
adjacent to it. Use of 2-methyl-2-butene as used in Huang’s
protocol is also found to be optional as 78% yield of the target
uronic acid derivative 23 was achieved without it. Finally, global
deprotection through Zemple´n de-O-acetylation afforded target
pentasaccharide 1 in 86% yield (Scheme 3).
In conclusion, we have accomplished the total synthesis of
the pentasaccharide glycone part of the anti-leishmanial triter-
penoid saponin isolated from Maesa balasae as its propargyl
glycoside. H2SO4-silica has been utilized as the acid source in
conjunction with N-iodosuccinimide for activation of thiogly-
coside and alone for trichloroacetimidate donors and found to
be a better user-friendly alternative of the conventional Lewis
acids. The propargyl glycoside at the reducing end will provide
the scope of making various types of glycoconjugates in future
to asses the biological activity of the oligosaccharide in question.
(22) Preparation of H2SO4-Silica. To a slurry of silica gel (10 g, 200-
400 mesh) in dry diethyl ether (50 mL) was added commercially available
concentrated H2SO4 (1 mL), and the slurry was shaken for 5 min. The solvent
was evaporated under reduced pressure, resulting in free flowing H2SO4-silica,
which was dried at 110 °C for 3 h and used for the reactions. For the use of
H2SO4-silica in various carbohydrate reactions, see: (a) Rajput, V. K.; Roy,
B.; Mukhopadhyay, B. Tetrahedron Lett. 2006, 47, 6987–6991. (b) Rajput, V. K.;
Mukhopadhyay, B. Tetrahedron Lett. 2006, 47, 5939–5941. (c) Mukhopadhyay,
B. Tetrahedron Lett. 2006, 47, 4337–4341. (d) Roy, B.; Mukhopadhyay, B.
Tetrahedron Lett. 2007, 48, 3783–3787.
(23) Veeneman, G. H.; van Leeuwen, S. H.; van Boom, J. H. Tetrahedron
Lett. 1990, 31, 1331–1334.
(24) Terada, T.; Ishida, H.; Kiso, M.; Hasegawa, A. J. Carbohydr. Chem.
1995, 14, 751–768.
Experimental Section
p-Methoxyphenyl 2,3,4-Tri-O-acetyl-r-L-rhamnopyranosyl-
(1f2)-3,4-di-O-acetyl-r-L-rhamnopyranosyl-(1f2)-3,4,6-
tri-O-acetyl-ꢀ-D-galactopyranoside (15). A solution of compound
14 (1.2 g, 1.3 mmol) in AcOH-H2O (9:1, 20 mL) was stirred at
(25) Cumpstey, I.; Fairbanks, A. J.; Redgrave, A. J. Tetrahedron 2004, 60,
9061–9074.
(26) Neumann, K. W.; Tamura, J.-I.; Ogaya, T. Glycoconjugate J. 1996, 13,
933–936.
(27) Classon, B.; Garegg, P. J.; Samuelson, B Acta Chem. Scand. Ser. B.
1984, 38, 419–422.
(28) Kere´kgya´rto´, J.; Szurmai, Z.; Lipta´k, A. Carbohydr. Res. 1993, 245,
65–80.
(30) Medgyes, G.; Jerkovich, G.; Kuszmann, J. Carbohydr. Res. 1989, 186,
225–239.
(31) Huang, L.; Teumelsan, N.; Huang, X. Chem. Eur. J. 2006, 12, 5246–
5252.
6926 J. Org. Chem. Vol. 73, No. 17, 2008

85 °C for 3 h. After the removal of solvents in vacuo, the residue
was dissolved MeOH (1:2, 15 mL), Pd(OH)2 (50 mg) was added,
and the mixture was stirred under H2 atmosphere for 6 h. The
mixture was filtered through a pad of Celite, and the filtrate was
evaporated in vacuo. The residue was dissolved in dry pyridine
(15 mL), Ac2O (10 mL) was added, and the solution was stirred at
room temperature for 3 h. The solvents were evaporated and
coevaporated with toluene to remove traces of pyridine. The residue
was purified by flash chromatography using n-hexane-EtOAc (3:
) 2.4 Hz, 12.6 Hz, H-6a′), 4.45-4.30 (m, 3H, H-2, H-5′, H-6b′),
4.21 (m, 2H, OCH2-Ct CH), 4.10 (m, 2H, H-6a, H-6b), 3.86 (t,
1H, J ) 10.2 Hz, H-3), 3.79 (t, 1H, J ) 10.2 Hz, H-4), 3.48 (m,
1H, H-5), 2.95 (bs, 1H, OH), 2.53 (t, 1H, J ) 2.4 Hz,
OCH2-Ct CH), 2.15, 2.06, 2.04, 1.98 (4s, 12H, 4 × COCH3). 13
C
NMR (CDCl3, 75 MHz) δ: 170.7, 170.2, 170.1, 170.0 (4 ×
COCH3), 136.8, 129.2, 128.2(2), 126.2(2) (ArC), 101.9 (CHPh),
101.7 (C-1′), 100.9 (C-1), 83.5, 79.7, 78.2 (OCH2-Ct CH), 75.6,
72.1, 70.9, 70.7, 69.7, 68.4, 66.9, 66.1, 61.2, 56.9, 20.9, 20.6(2),
20.5 (4 × COCH3). HRMS calcd for C30H36O15Na (M + Na)+
659.1952; found 659.1954.
1) to afford pure compound 15 (1.1 g, 83%) as white foam. [R]25
D
1
+67 (c 1.0, CHCl3). H NMR (CDCl3, 300 MHz) δ: 6.95, 6.76
(2d, 4H, J ) 9.0 Hz, C6H4OCH3), 5.32 (bs, 1H, H-4), 5.24 (dd,
1H, J ) 2.1 Hz, 8.7 Hz, H-3′′), 5.22 (bs, 1H, H-2′′), 5.20-4.94
(m, 5H, H-1′′, H-3, H-3′, H-4′, H-4′′), 4.87 (d, 1H, J ) 7.8 Hz,
H-1), 4.75 (bs, 1H, H-1′), 4.17-4.02 (m, 4H, H-2′, H-5′, H-5′′,
H-6a), 3.95 (m, 3H, H-2, H-5, H-6b), 3.74 (s, 3H, C6H4OCH3),
2.12, 2.10, 2.01(3), 1.99, 1.96(2) (8s, 24H, 8 × COCH3), 1.21 (d,
3H, J ) 6.0 Hz, C-CH3), 1.18 (d, 3H, J ) 6.3 Hz, C-CH3). 13C
NMR (CDCl3, 75 MHz) δ: 169.7, 169.4(2), 169.2(3), 169.1(2) (8
× COCH3), 155.7, 150.7, 118.5 (2), 114.5(2) (ArC), 100.8 (C-1),
99.4 (C-1′′), 99.1 (C-1′), 77.4, 73.4, 73.1, 71.1, 70.8, 70.6, 70.2,
69.8, 68.5, 67.1, 67.0, 66.8, 61.1 (C-6), 55.3 (C6H4OCH3), 20.7(3),
20.6(3), 20.5(2) (8 × COCH3), 17.4, 17.3 (2 × C-CH3). HRMS
calcd for C41H54O23Na (M + Na)+ 937.2954; found 937.2952.
Propargyl 2,3,4,6-Tetra-O-acetyl-ꢀ-D-galactopyranosyl-
(1f2)-4,6-O-benzylidene-ꢀ-D-glucopyranoside (20). A mixture
of compound 18 (2 g, 4.7 mmol), compound 19 (2.6 g, 5.6 mmol),
and MS 4Å (2 g) in dry CH2Cl2 (25 mL) was stirred under nitrogen
for 1 h. NIS (1.5 g, 6.7 mmol) was added, and the mixture was
cooled to -40 °C followed by addition of H2SO4-silica (40 mg).
The mixture was allowed to stir at -40 °C for 6 h when TLC
showed complete consumption of the acceptor 18. At this point
H2SO4-silica (30 mg) was added, the reaction temperature was
raised to room temperature, and the mixture was stirred for an
additional 2 h. The mixture was filtered through a pad of Celite.
The filtrate was diluted with CH2Cl2 (20 mL) and washed
successively with Na2S2O3 (2 × 50 mL), NaHCO3 (2 × 50 mL),
and brine (50 mL). The organic layer was collected, dried (Na2SO4),
and evaporated in vacuo. The residue was purified by flash
Propargyl
r-L-Rhamnopyranosyl-(1f2)-r-L-rhamnopy-
ranosyl-(1f2)-ꢀ-D-galactopyranosyl-(1f3)-2-O-(ꢀ-D-galacto-
pyranosyl)-ꢀ-D-glucopyranosiduronic Acid (1). To a solution of
the protected pentasaccharide 23 (450 mg, 0.33 mmol) in dry MeOH
(5 mL) was added NaOMe (50 mg, 0.7 mmol), and the solution
was stirred at room temperature for 4 h. The solution was
neutralized with DOWEX 50W H+ resin and filtered through a
cotton plug. The filtrate was evaporated and washed with CH2Cl2
(5 mL), removing the TEMPO salt to afford compound 1 (240 mg,
86%) in 99% yield. 1H NMR (D2O, 400 MHz) δ: 5.80 (d, 1H, J )
7.2 Hz, H-1c), 5.18 (bs, 1H, H-1d), 4.98 (d, 1H, J ) 1.6 Hz, H-1e),
4.38 (d, 1H, J ) 7.2 Hz, H-1b), 4.34 (d, 1H, J ) 7.6 Hz, H-1a),
4.06-3.96 (m, 8H, H-2d, H-2e, H-3a, H-4b, H-5e, H-6c, CH2-C≡CH),
3.94-.55 (m, 16H, H-2a, H-2b, H-2c, H-3b, H-3c, H-3d, H-3e, H-4a,
H-4c, H-5a, H-5b, H-5c, H-5d, H-6b, H-6′b, H-6′c), 3.48 (t, 2H, J )
9.6 Hz, H-4d, H-4e), 1.94 (bs, 1H, CH2-C≡CH), 1.37 (d, 3H, J )
6.0 Hz, C-CH3), 1.32 (d, 3H, J ) 6.0 Hz, C-CH3). 13C NMR
(D2O, 100 MHz) δ: 175.4 (COOH), 102.8 (C-1a), 101.9 (C-1b),
100.8 (C-1c), 99.4 (C-1d), 96.0 (C-1e), 79.6, 78.2, 77.5, 76.3, 75.7,
75.0, 74.6(2), 74.5, 73.2, 73.0, 72.3, 72.1, 71.4, 70.8, 70.2(2), 69.8,
69.3, 68.6(2), 61.3, 60.4, 56.3, 17.1, 16.7 (2 × COCH3). HRMS
calcd for C33H52O25Na (M + Na)+ 871.2695; found 871.2698.
Acknowledgment. V.K.R. is thankful to CSIR, New Delhi,
India for a fellowship. The work is funded by DST, New Delhi,
India through SERC Fast-Track Grant SR/FTP/CS-110/2005.
chromatography using n-hexane-EtOAc (3:1) to afford pure
Supporting Information Available: Experimental details and
copies of 1H and 13C spectra of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
compound 20 (2.4 g, 82%). [R]25 +47 (c 1.0, CHCl3). H NMR
D
(CDCl3, 300 MHz) δ: 7.49-7.34 (m, 5H, ArH), 5.53 (s, 1H, CHPh),
5.39 (bd, 1H, J ) 2.7 Hz, H-4′), 5.23 (dd, 1H, J ) 8.1 Hz, 10.5
Hz, H-2′), 5.06 (dd, 1H, J ) 2.7 Hz, 10.5 Hz, H-3′), 4.81 (d, 1H,
J ) 8.1 Hz, H-1′), 4.67 (d, 1H, J ) 7.8 Hz, H-1), 4.47 (dd, 1H, J
JO801171F
J. Org. Chem. Vol. 73, No. 17, 2008 6927