Synthesis of oligosaccharides related to a biodynamic saponin from Calamus Insignis as their propargyl glycosides

Article abstract of DOI:10.1002/ejoc.200800834

The total synthesis of the carbohydrate portion (penta-and tetrasaccharides) of steroidal saponins, isolated from Calamus insignis, is reported. A simple route is followed, starting from commercially available D-glucose and L-rhamnose, through high-yielding protecting group manipulation strategies. Sulfuric acid immobilized on silica (H₂SO ₄/silica) was used as an effective Broensted acid source in conjunction with N-iodosuccinimide for activation of thioglycosides or alone for trichloroacetimidate donors in key glycosylation steps. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800834

FULL PAPER
DOI: 10.1002/ejoc.200800834
Synthesis of Oligosaccharides Related to a Biodynamic Saponin from Calamus
Insignis as Their Propargyl Glycosides
Somnath Dasgupta[a] and Balaram Mukhopadhyay*[a]
Dedicated to Dr. Ganesh Prasad Pandey, National Chemical Laboratory, Pune, India
Keywords: Carbohydrates / Oligosaccharides / Glycosides / Saponins
The total synthesis of the carbohydrate portion (penta-
and tetrasaccharides) of steroidal saponins, isolated from
Calamus insignis, is reported. A simple route is followed,
was used as an effective Brönsted acid source in conjunction
with N-iodosuccinimide for activation of thioglycosides or
alone for trichloroacetimidate donors in key glycosylation
steps.
starting from commercially available D-glucose and L-rham-
nose, through high-yielding protecting group manipulation
strategies. Sulfuric acid immobilized on silica (H2SO4/silica)
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
glucose, arabinose, glucuronic acid, xylose or rhamnose).[5]
An understanding of the glycosylation process, which is be-
lieved to be the terminal stage in the saponin biosynthesis,
is important since the presence of the C-3 sugar chain is
critical for saponin biological activity.[6] As the isolation of
these saponins from natural sources is not easy, in order to
establish the order of events in saponin biosynthesis and
evaluate the medicinal properties associated with the sapo-
nins, synthetic saccharide fragments are needed.[7] Smart
synthetic strategies will provide a large-scale source of these
oligosaccharides so that they can be used as therapeutics,[8]
if they are found suitable for those purposes.
Calamus insignis, a kind of rattan that grows widely in
tropical regions of Southeast Asia, southern China and the
western Pacific, is used as a medicinal agent. During the
search for biodynamic natural products having cell-growth
inhibitory or cell-cycle inhibitory activities, Ishibashi et al.[9]
isolated a group of spirostanol (A–D) saponins and a furos-
tanol (E) saponin from Calamus insignis (Figure 1). The spi-
rostanols (A–D) showed growth-inhibitory activity against
HeLa cells at low concentrations (IC50 Ͻ 10 µ). Moreover,
spirostanol A showed cell-cycle inhibitory activity by arrest-
ing the G2/M phase HeLa cell-cycle progression at concen-
trations of 1.5 µ and 2.9 µ. Recently, Yu et al.[10] clearly
demonstrated the importance of the sugar chain on the hae-
molytic and cytotoxic properties of a saponin. In the con-
tinuation of our constant effort toward the synthesis of
carbohydrate-containing natural products,[11] herein we re-
port a simple route for the synthesis of the tetrasaccharide
(1) and pentasaccharide (2) parts of these natural products
as their propargyl glycosides (Figure 2).
Introduction
With the growing interest in carbohydrate-based drug de-
sign, carbohydrate-containing natural products such as sa-
ponins or flavonoids have gained a great deal of importance
in recent times. Even though numerous plants are used as
folk medicines for various illnesses, the active constituents
have been largely uncharacterized until recently. Steroidal
saponins are glycosylated plant secondary metabolites that
are found in many major food crops,[1] synthesized by
plants as part of their normal programme of growth and
development. Examples include plants that are exploited as
sources of drugs, such as ginseng and liquorice, and also
crop plants, such as legumes and oats.[2] It is assumed that
they act as preformed chemical barriers against fungal at-
tack as many saponins have potent antifungal properties
and are present in healthy plants in high concentration.[3]
Apart from their role in the plant kingdom, saponins often
show promising bioactivities such as cell-growth inhibition
or cell-cycle inhibition, which in turn, opens up a new di-
mension in drug discovery.[4] One common feature of all
saponins is the presence of a sugar chain attached to the
aglycon at the C-3 hydroxy position. The sugar chains differ
substantially between saponins but are often branched and
may consist of up to five sugar units (usually selected from
[a] Indian Institute of Science Education and Research-Kolkata
(IISER-K), HC-VII, Sector-III,
Salt Lake City, Kolkata 700106, India
Fax: +91-33-23348092
E-mail: sugarnet73@hotmail.com
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
5770
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Synthesis of Oligosaccharides Related to a Biodynamic Saponin
Figure 1. Structures of the saponins isolated from Calamus insignis.
Figure 2. Retrosynthetic analysis for the synthesis of the pentasaccharide. -Glc = -glucose, -Rha = -rhamnose and MP = p-meth-
oxyphenyl.
As we desired to evaluate the synthetic oligosaccharides to make different types of conjugates as needed. Interest
for their biological activities in the form of suitable glyco- ingly, many of these reactions can be performed in aqueous
conjugates,[12] the choice of the reducing end appendage media, and therefore, can be exercised after global deprotec-
was very important. We opted for the propargyl glycoside tion of the oligosaccharide. Moreover, the propargyl glyco-
since it is suitable for various types of multicomponent reac- side can be cleaved to form the corresponding hemiacetal
tions[13] and cycloadditions with the azido functionality[14] derivative, if necessary[15] (Figure 1).
Eur. J. Org. Chem. 2008, 5770–5777
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S. Dasgupta, B. Mukhopadhyay
FULL PAPER
Results and Discussion
Our synthesis commenced with the known propargyl β-
-glucopyranoside. The formation of the 4,6-O-benzylidene
acetal derivative (3) with benzaldehyde dimethyl acetal in
the presence of 10-camphorsulfonic acid followed by selec-
tive 3-O-benzoylation with benzoyl cyanide in the presence
of triethylamine[16] afforded the acceptor 4 in 78% yield
over two steps. The acceptor was then glycosylated with the
known rhamnosyl donor 5[17] using N-iodosuccinimide
(NIS) in the presence of H2SO4/silica[18] to furnish the di-
saccharide 6 in 89% yield. To judge the efficiency of H2SO4/
silica,[19] we performed parallel glycosylation reactions with
TfOH[20] and TMSOTf[21] in conjunction with N-iodosucci-
nimide, and the yield of the glycosylations were 83% and
86%, respectively. The results showed that H2SO4/silica was
comparable or even better than TfOH or TMSOTf for NIS-
mediated glycosylation. It is important to note that the pro-
pargyl glycoside was unaffected during the H2SO4/silica-
catalyzed glycosylation reactions. Therefore, we opted for
the H2SO4/silica-promoted reaction. The complete removal
of the benzylidene acetal with 80% AcOH at 80 °C[22] fol-
lowed by the selective protection of the primary hydroxy
with the tert-butyldiphenylsilyl group afforded the disaccha-
ride acceptor 8 in 84% yield. The glycosylation of disaccha-
ride acceptor 8 with the known glucosyl donor 9[23] using
NIS in the presence of H2SO4/silica afforded the protected
trisaccharide 10 in 91% yield. Following the same path as
before, the complete hydrolysis of the benzylidene acetal fol-
lowed by selective primary protection with TBDPS pro-
vided the trisaccharide acceptor 12. We found that the
TBDPS group was the most effective choice for selective
protection of the primary hydroxy group and provided bet-
ter reactivity of the adjacent OH group, which served as the
acceptor site for glycosylation. The regioselective opening
Scheme 1. Synthesis of the tetrasaccharide 2.
of the benzylidene acetal to produce the required 4-OH de- to construct the target pentasaccharide. The trisaccharide
rivative was low-yielding, and the selective protection of the acceptor 12 required no modifications. For the disaccharide
primary OH group with benzoyl cyanide was unsuccessful part, the known p-methoxyphenyl 2,3-O-isopropylidene-α-
due to migration. Therefore, TBDPS was used as the pro- -rhamnopyranoside (15)[25] was glycosylated with the
tecting group, although it is arguably not the best option known thioglucoside 16[26] to afford the disaccharide 17 in
with respect to atom economy. Once the acceptor trisaccha- 86% yield. In order to convert disaccharide 17 into a donor
ride was in hand, glycosylation with the known rhamnosyl for the pentasaccharide formation, the oxidative removal of
donor 5 with NIS and H2SO4/silica resulted in the forma- the p-methoxyphenyl group was necessary. From our pre-
tion of the protected tetrasaccharide 13. However, the com- vious experience, it was evident that isopropylidene acetals
plete purification of tetrasaccharide 13 was unsuccessful at do not withstand CAN-mediated oxidative cleavage.[27]
this stage even after repeated flash chromatography. There- Therefore, the complete removal of the isopropylidene
fore, we proceeded to the next step. The removal of the group and the subsequent protection of the resulting hy-
TBDPS groups with tetrabutylammonium fluoride in droxyls with acetate groups were performed to afford the
THF[24] afforded the corresponding tetrasaccharide diol 14 acetylated disaccharide 18 in 86% yield over two steps. The
in 82% yield over two steps. Finally, global deprotection CAN-mediated oxidative cleavage[28] of the OMP group
was achieved by NaOMe in methanol to afford the target then proceeded smoothly to form the corresponding hemia-
tetrasaccharide 1 in 88% yield (Scheme 1).
cetal, which was subsequently reacted with trichloroaceto-
For the synthesis of the pentasaccharide fragment, a se- nitrile in the presence of DBU[29] to afford the required di-
quential approach from the tetrasaccharide could have saccharide donor 19 in 83% yield over two steps. Due to
reached the target. However, to achieve that, a further pro- the instability of trichloroacetimidate derivatives, 19 was
tecting group manipulation was required in the terminal used for the glycosylation without further characterization.
rhamnose moiety, which was not a wise choice for a large The glycosylation between trisaccharide acceptor 12 and di-
tetrasaccharide molecule. Instead, we took a “3+2“ strategy saccharide donor 19 was successfully achieved by activation
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Synthesis of Oligosaccharides Related to a Biodynamic Saponin
Whatman) with detection by fluorescence and/or by charring fol-
lowing immersion in a 10% ethanolic solution of sulfuric acid. An
orcinol dip, prepared by the careful addition of concentrated sulfu-
ric acid (20 mL) to an ice-cold solution of 3,5-dihydroxytoluene
(360 mg) in EtOH (150 mL) and H2O (10 mL), was used to detect
deprotected compounds by charring. Flash chromatography was
performed with silica gel 230–400 mesh (Qualigens, India). Optical
rotations were measured at the sodium -line at ambient tempera-
ture with a Perkin–Elmer 141 polarimeter. 1H NMR and 13C NMR
spectra were recorded with a Bruker Avance spectrometer at 300
and 75 MHz, respectively, with Me4Si or CH3OH as internal stan-
dards, as appropriate.
with H2SO4/silica to furnish the protected pentasaccharide
20 in 87% yield. Similar to the NIS-mediated glycosyla-
tions, H2SO4/silica proved to be a better promoter than
traditional promoters like TMSOTf. The usual global de-
protection afforded the target pentasaccharide 2 in 77%
yield (Scheme 2).
Propargyl 3-O-benzoyl-4,6-benzylidene-β-D-glucopyranoside (4): To
a suspension of propargyl β--glucopyranoside (3 g, 9.8 mmol) in
dry CH3CN (40 mL) was added benzaldehyde dimethyl acetal
(2.2 mL, 14.7 mmol) followed by 10-CSA (50 mg), and the mixture
was allowed to stir at room temperature for 4 h, at which time the
solution became clear, and TLC (n-hexane/EtOAc, 1:1) showed the
complete conversion of the starting material to a faster-moving
spot. The solution was neutralized with Et3N, and the solvents were
evaporated in vacuo. The semi-solid mass thus obtained was puri-
fied by flash chromatography with a gradient of n-hexane/EtOAc
(2:1 to 1:1) as the eluent. The purified benzylidene acetal was sus-
pended in dry CH3CN (40 mL), and BzCN (1.2 mL, 9.8 mmol) was
added followed by Et3N (20 µL). The solution became clear within
15 min, at which time TLC (n-hexane/EtOAc, 2:1) showed the com-
plete consumption of the starting benzylidene acetal. MeOH
(2 mL) was added to quench the reaction. The solvents were evapo-
rated in vacuo, and the residue was purified by flash chromatog-
raphy with 2:1 n-hexane/EtOAc to afford pure propargyl 2-O-ben-
zoyl-4,6-O-benzylidene-β--glucopyranoside (4, 4.2 g, 78%) as a
white foam. [α]2D5 = +106 (c 1.1, CHCl3). 1H NMR (CDCl3,
300 MHz): δ = 8.12–7.32 (m, 10 H), 5.55 (t, J = 8.7 Hz, 1 H), 5.53
(s, 1 H), 4.80 (d, J = 7.8 Hz, 1 H), 4.46 (m, 2 H), 4.40 (dd, J = 5.1,
10.5 Hz, 1 H), 3.88–3.79 (m, 3 H), 3.66 (m, 1 H), 2.88 (br. s, 1 H),
2.55 (t, J = 2.4 Hz, 1 H) ppm. 13C NMR (CDCl3, 75 MHz): δ =
166.5, 136.8, 133.2, 129.9 (2), 129.6, 129.0, 128.3 (2), 128.2 (2),
126.1 (2), 101.5, 101.3, 78.5, 78.2, 75.8, 74.2, 73.3, 68.6, 66.6, 56.5
ppm. HRMS calcd. for C23H22O7Na [M + Na]+ 433.1263; found
433.1261.
Scheme 2. Synthesis of the pentasaccharide 2.
Propargyl 2,3,4-Tri-O-acetyl-α-
L-rhamnopyranosyl(1Ǟ2)-3-O-ben-
Conclusions
zoyl-4,6-O-benzylidene-β- -glucopyranoside (6): A mixture of 4
D
(2.0 g, 4.9 mmol), 5 (2.3 g, 5.9 mmol) and 4 Å MS (3.0 g) in dry
CH2Cl2 (30 mL) was stirred under nitrogen for 30 min at ambient
temperature. NIS (1.7 g, 7.7 mmol) was added followed by H2SO4/
silica (150 mg), and stirring was continued for another 45 min, at
which time TLC (n-hexane/EtOAc, 2:1) showed the complete con-
sumption of the starting material. The mixture was filtered through
a pad of celite, and the filtrate was washed successively with aque-
ous Na2S2O3 (2ϫ40 mL), aqueous NaHCO3 (2ϫ40 mL) and H2O
(40 mL). The organic phase was separated, dried (Na2SO4), filtered
and concentrated to a syrup in vacuo. The crude product thus ob-
In conclusion, we have accomplished the total synthesis
of a tetra- and a pentasaccharide related to the biologically
significant saponin isolated from Calamus insignis. The tar-
get oligosaccharides were prepared as their propargyl glyco-
sides, which allows for further glycoconjugate formation
with cycloaddition or many other multicomponent strate-
gies. Since these reactions are feasible in aqueous media,
the deprotected oligosaccharides will be used for making
various glycoconjugates. Presently, we are engaged in syn-
thesizing such glycoconjugates, which will, in turn, help us tained was purified by flash chromatography with n-hexane/EtOAc
(3:1) as the eluent to afford pure disaccharide 6 (3.0 g, 89%) as a
colourless foam. [α]2D5 = +113 (c 1.0, CHCl3). 1H NMR (CDCl3,
300 MHz): δ = 8.03–7.25 (m, 10 H), 5.64 (t, J = 9.3 Hz, 1 H), 5.45
(s, 1 H), 5.20 (dd, J = 3.3, 9.6 Hz, 1 H), 4.98 (dd, J = 1.8, 3.3 Hz,
1 H), 4.89 (t, J = 9.6 Hz, 1 H), 4.84 (d, J = 8.7 Hz, 1 H), 4.82 (s,
1 H), 4.41 (m, 2 H), 4.37 (dd, J = 4.5, 10.5 Hz, 1 H), 4.28 (m, 1
H), 3.82 (dd, J = 8.7, 9.3 Hz, 1 H), 3.76 (t, J = 9.3 Hz, 1 H), 3.74
(dd, J = 1.8, 10.5 Hz, 1 H), 3.64 (m, 1 H), 2.54 (t, J = 2.1 Hz, 1
H), 2.00 (s, 3 H), 1.90 (s, 3 H), 1.81 (s, 3 H), 1.17 (d, J = 6.0 Hz,
3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 169.4, 169.1, 168.5,
to evaluate the biological activity of these oligosaccharides
and their potential applications.
Experimental Section
General Methods: All reagents and solvents were dried prior to use
according to standard methods.[30]Commercially available reagents
were used without further purification unless otherwise stated. An-
alytical TLC was performed with silica gel 60-F254 (Merck or
Eur. J. Org. Chem. 2008, 5770–5777
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5773

S. Dasgupta, B. Mukhopadhyay
FULL PAPER
164.7, 136.7, 132.8, 129.9 (2), 129.3, 128.8, 128.1 (2), 128.0 (2),
9.3 Hz, 1 H), 4.90 (d, J = 9.3 Hz, 1 H), 4.42 (dd, J = 4.5, 10.2 Hz,
126.1 (2), 101.3, 99.7, 98.2, 78.4, 78.1, 76.5, 75.9, 74.1, 71.0, 69.5, 1 H), 3.83 (m, 2 H), 3.71 (m, 1 H), 2.35 (s, 3 H) ppm. 13C NMR
68.5, 68.4, 66.7, 66.2, 56.3, 20.6, 20.4, 20.2, 17.1 ppm. HRMS (CDCl3, 75 MHz): δ = 165.4, 164.9, 138.6, 136.8, 134.2 (2), 133.2,
calcd. for C35H38O14Na [M + Na]+ 705.2159; found 705.2156.
133.0, 130.0 (2), 129.7 (2), 129.6 (2), 129.5, 129.0, 128.4 (2), 128.3
(2), 128.1 (2), 127.9 (2), 126.2 (2), 101.6, 87.1, 78.7, 73.4, 71.1, 71.0,
68.6, 21.3 ppm. HRMS calcd. for C34H30O7SNa [M + Na]+
605.1610; found 605.1608.
Propargyl 2,3,4-Tri-O-acetyl-α-L-rhamnopyranosyl(1Ǟ2)-3-O-ben-
zoyl-β- -glucopyranoside (7): A solution of 6 (2.5 g, 3.7 mmol) in
D
80% AcOH (30 mL) was stirred at 80 °C for 2 h, at which time
TLC (n-hexane/EtOAc, 1:1) showed the complete conversion of the
starting material to a slower-moving spot. The solvents were evapo-
rated in vacuo, and the residue thus obtained was purified by flash
chromatography with a gradient of n-hexane/EtOAc (2:1 to 1:1) to
afford pure 7 (2.0 g, 91%) as a white amorphous solid. [α]2D5 = +121
Propargyl 2,3,4-Tri-O-acetyl-α-
di-O-benzoyl-4,6-O-benzylidene-β-
O-(tert-butyldiphenylsilyl)-β- -glucopyranoside (10): A mixture of
L-rhamnopyranosyl-(1Ǟ2)-4-O-(2,3-
D-glucopyranosyl)-3-O-benzoyl-6-
D
disaccharide acceptor 8 (1.8 g, 2.2 mmol), donor 9 (1.7 g,
2.9 mmol) and 4 Å MS (2.0 g) in dry CH2Cl2 (30 mL) was stirred
under nitrogen for 30 min at ambient temperature. NIS (850 mg,
3.8 mmol) was added followed by H2SO4/silica (50 mg), and the
mixture was allowed to stir at ambient temperature for 45 min, at
which time TLC (n-hexane/EtOAc, 2:1) showed the complete con-
sumption of the starting materials. The mixture was filtered
through a pad of celite and washed with CH2Cl2 (10 mL). The
entire filtrate was washed successively with aqueous Na2S2O3
(2ϫ40 mL), saturated aqueous NaHCO3 (2 ϫ40 mL) and H2O
(40 mL). The organic layer was collected, dried (Na2SO4), filtered
and concentrated in vacuo. The crude thus obtained was purified
by flash chromatography with n-hexane/EtOAc (2:1) as the eluent
to afford pure trisaccharide 10 (2.5 g, 91%) as a colourless foam.
[α]2D5 = +136 (c 1.3, CHCl3). 1H NMR (CDCl3, 300 MHz): δ =
8.13–7.15 (m, 30 H), 5.62 (t, J = 9.3 Hz, 1 H), 5.52 (t, J = 9.3 Hz,
1 H), 5.37 (dd, J = 7.8, 9.3 Hz, 1 H), 5.23 (s, 1 H), 5.22 (dd, J =
3.6, 9.9 Hz, 1 H), 5.08 (d, J = 7.8 Hz, 1 H), 5.01 (dd, J = 1.8,
3.6 Hz, 1 H), 4.96 (t, J = 9.9 Hz, 1 H), 4.83 (d, J = 1.8 Hz, 1 H),
4.61 (d, J = 7.5 Hz, 1 H), 4.39–4.18 (m, 4 H), 3.95 (dd, J = 4.8,
10.5 Hz, 1 H), 3.80 (m, 3 H), 3.62 (t, J = 9.3 Hz, 1 H), 3.42 (m, 1
H), 3.25 (bd, 1 H), 2.45 (t, J = 2.1 Hz, 1 H), 2.01 (s, 3 H), 1.90 (s,
3 H), 1.87 (s, 3 H), 1.19 (d, J = 6.6 Hz, 3 H), 1.13 (s, 9 H) ppm.
13C NMR (CDCl3, 75 MHz): δ = 169.6, 169.2, 168.7, 165.2, 164.8,
164.3, 136.6, 136.0 (2), 135.5 (2), 134.1, 133.3, 132.9 (2), 132.4,
130.4, 130.1 (2), 129.7 (2), 129.6 (2), 129.4, 128.9, 128.3, 128.2 (3),
128.1, 128.0 (2), 127.7 (2), 126.2 (2), 101.2, 100.5, 98.8, 98.2, 78.4,
78.3, 77.2, 75.5 (2), 75.3, 74.9, 74.0, 72.3, 72.0, 71.2, 69.8, 68.7,
68.0, 66.6, 66.5, 61.0, 55.7, 26.9 (3), 20.8, 20.6, 20.5, 19.6, 17.2 ppm.
ESIMS: m/z = 1313.3 [M + Na]+.
1
(c 1.2, CHCl3). H NMR (CDCl3, 300 MHz): δ = 8.00–7.98 (m, 5
H), 5.35 (t, J = 9.3 Hz, 1 H), 5.21 (dd, J = 3.6, 10.2 Hz, 1 H), 4.98
(m, 1 H), 4.95 (t, J = 10.2 Hz, 1 H), 4.85 (s, 1 H), 4.75 (d, J =
7.8 Hz, 1 H), 4.42 (m, 2 H), 4.31 (m, 1 H), 3.88 (dd, J = 2.7,
10.5 Hz, 1 H), 3.86–3.76 (m, 4 H), 3.53 (m, 1 H), 2.63 (br. s, 1 H),
2.56 (t, J = 2.4 Hz, 1 H), 2.01 (s, 3 H), 1.88 (s, 3 H), 1.81 (s, 3 H),
1.18 (d, J = 6.0 Hz, 3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ =
170.0, 169.9, 169.1, 166.7, 133.4, 129.8 (2), 128.9, 128.3 (2), 99.4
(C-1), 98.3, 78.7, 78.4, 76.5, 75.7, 75.5, 70.9, 69.4, 69.3, 68.8, 66.7,
61.9, 56.4, 20.7, 20.5, 20.3, 17.0 ppm. HRMS calcd. for
C28H34O14Na [M + Na]+ 617.1846; found 617.1843.
Propargyl 2,3,4-Tri-O-acetyl-α-
L-rhamnopyranosyl-(1Ǟ2)-3-O-ben-
zoyl-6-O-(tert-butyldiphenylsilyl)-β-
D-glucopyranoside (8): To
a
solution of 7 (1.8 g, 3.0 mmol) in dry pyridine (20 mL) was added
TBDPS-Cl (1.0 mL, 3.9 mmol), and the resulting solution was
stirred at ambient temperature for 6 h, at which time TLC (2:1,
n-hexane/EtOAc) showed the complete conversion of the starting
material. The solvents were evaporated in vacuo and co-evaporated
with toluene to remove residual pyridine. The syrupy mass thus
obtained was dissolved in CH2Cl2 (30 mL) and washed successively
with H2O (30 mL). The organic layer was collected, dried
(Na2SO4), filtered and concentrated in vacuo. The crude product
was purified by flash chromatography with n-hexane/EtOAc (3:1)
to yield pure 8 (2.1 g, 84%) as a colourless syrup. [α]2D5 = +102 (c
1.0, CHCl3). 1H NMR (CDCl3, 300 MHz): δ = 8.02–7.21 (m, 15
H), 5.32 (t, J = 9.3 Hz, 1 H), 5.17 (dd, J = 3.6, 10.2 Hz, 1 H), 4.93
(m, 1 H), 4.90 (t, J = 10.2 Hz, 1 H), 4.83 (s, 1 H), 4.69 (d, J =
7.8 Hz, 1 H), 4.35 (dd, J = 2.7, 10.8 Hz, 1 H), 4.31 (dd, J = 1.8,
10.8 Hz, 1 H), 4.28 (m, 1 H), 3.95 (m, 2 H), 3.77 (m, 2 H), 3.48
(m, 1 H), 2.48 (t, J = 2.1 Hz, 1 H), 2.00 (s, 3 H), 1.87 (s, 3 H), 1.83
(s, 3 H), 1.17 (d, J = 6.0 Hz, 1 H), 1.06 (s, 9 H) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 169.4, 169.2, 168.3, 166.3, 135.8 (2), 135.7
(2), 133.3, 133.1, 133.0, 130.0 (2), 129.7 (2), 128.9, 128.2 (2), 127.7
(2), 123.6 (2), 99.1, 98.2, 79.2, 78.5, 76.2, 75.4, 71.1, 69.6, 69.5,
68.7, 66.6, 63.6, 55.9, 26.9 (3), 20.7, 20.5, 20.3, 19.3, 17.7 ppm.
HRMS calcd. for C44H52O14SiNa [M + Na]+ 855.3024; found
855.3026.
Propargyl 2,3,4-Tri-O-acetyl-α-
di-O-benzoyl-β- -glucopyranosyl)-3-O-benzoyl-6-O-(tert-butyldi-
phenylsilyl)-β- -glucopyranoside (11): Trisaccharide 10 (2.0 g,
L-rhamnopyranosyl-(1Ǟ2)-4-O-(2,3-
D
D
1.6 mmol) was suspended in 80% AcOH (20 mL), and the mixture
was allowed to stir at 80 °C for 2 h, at which time TLC (n-hexane/
EtOAc, 1:1) showed the complete conversion of the starting mate-
rial to a slower-running spot. The solvents were evaporated in
vacuo, and the residue thus obtained was purified by flash
chromatography with a gradient of n-hexane/EtOAc (2:1 to 1:1) to
afford pure trisaccharide diol 11 (1.6 g, 87%) as a white amorphous
1
p-Tolyl
2,3-Di-O-benzoyl-4,6-O-benzylidene-1-thio-β-D-glucopyr-
solid. [α]2D5 = +127 (c 1.1, CHCl3). H NMR (CDCl3, 300 MHz): δ
anoside (9): To a mixture of p-tolyl 4,6-O-benzylidene-1-thio-β--
glucopyranoside (2 g, 5.3 mmol) in dry pyridine (20 mL), BzCl
(1.9 mL, 16 mmol) was added dropwise at 0 °C. After complete ad-
dition, the temperature was allowed to rise to room temperature.
After 2 h, TLC (n-hexane/EtOAc, 4:1) showed the complete conver-
sion of the starting material. The solvents were evaporated, and the
residue was diluted with CH2Cl2 (20 mL) and washed with brine
(2ϫ20 mL). The organic layer was collected, dried (Na2SO4) and
concentrated in vacuo. The crude product thus obtained was puri-
fied by flash chromatography with 4:1 n-hexane/EtOAc to afford
pure 9 (2.8 g, 91%). 1H NMR (CDCl3, 300 MHz): δ = 7.96–7.07
(m, 19 H), 5.72 (t, J = 9.3 Hz, 1 H), 5.49 (s, 1 H), 5.37 (t, J =
= 8.12–7.17 (m, 25 H), 5.48 (t, J = 9.3 Hz, 1 H), 5.30 (m, 2 H),
5.21 (dd, 1 H), 5.05 (d, 1 H), 4.98 (m, 1 H), 4.96 (t, J = 10.2 Hz,
1 H), 4.85 (br. s, 1 H), 4.61 (d, J = 8.1 Hz, 1 H), 4.41–4.19 (m, 5
H), 3.89–3.68 (m, 4 H), 3.51 (bd, 1 H), 3.37–3.24 (m, 4 H), 2.46 (t,
J = 1.8 Hz, 1 H), 2.00 (s, 3 H), 1.89 (s, 3 H), 1.82 (s, 3 H), 1.20 (d,
J = 6.0 Hz, 3 H), 1.13 (s, 9 H) ppm. 13C NMR (CDCl3, 75 MHz):
δ = 170.1, 169.7, 169.1, 167.0, 164.8, 164.6, 135.8 (2), 135.4 (3),
133.4, 133.3, 133.2, 133.0, 132.4, 130.2, 129.9 (3), 129.7, 129.5 (3),
128.9, 128.8, 128.5 (2), 128.4 (2), 128.3 (2), 128.1 (2), 127.7 (2),
99.9, 98.7, 98.3, 78.3, 77.1, 75.8, 75.5, 75.3 (2), 74.9, 73.6, 71.4,
71.1, 69.6, 69.0, 68.6, 66.6, 61.5, 60.9, 55.6, 26.9 (3), 20.7, 20.6,
20.4, 19.4, 17.1 ppm. ESIMS: m/z = 1225.3 [M + Na]+.
5774
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 5770–5777

Synthesis of Oligosaccharides Related to a Biodynamic Saponin
Propargyl 2,3,4-Tri-O-acetyl-α-
di-O-benzoyl-6-O-(tert-butyldiphenylsilyl)-β-
benzoyl-6-O-(tert-butyldiphenylsilyl)-β- -glucopyranoside (12): To
L
-rhamnopyranosyl-(1Ǟ2)-4-O-[2,3-
128.4 (2), 128.2 (2), 100.8, 99.2, 98.4, 97.8, 78.2, 75.6 (2), 75.4, 75.3,
75.2, 74.8, 73.9, 73.6, 72.3, 71.1, 70.4, 69.6, 69.5, 68.7, 68.6, 67.0,
66.7, 60.4, 60.0, 56.4, 20.8, 20.7, 20.6 (2), 20.5, 20.3, 17.0, 16.7 ppm.
ESIMS: m/z = 1259.3 [M + Na]+.
D-glucopyranosyl]-3-O-
D
solution of 11 (1.3 g, 1.1 mmol) in dry pyridine (20 mL) was added
TBDPS-Cl (360 µL, 1.4 mmol) and the resulting solution was
stirred at ambient temperature for 6 h, , at which time TLC (n-
hexane/EtOAc, 2:1) showed the complete conversion of the starting
material. The solvents were evaporated in vacuo and co-evaporated
with toluene to remove residual pyridine. The crude product thus
obtained was purified by flash chromatography with 2:1 n-hexane/
EtOAc as the eluent to afford trisaccharide acceptor 12 (1.3 g,
Propargyl α-
L-Rhamnopyranosyl-(1Ǟ4)-β-D-glucopyranosyl-(1Ǟ4)-
2-O-(α- -rhamnopyranosyl)-β-
L
D-glucopyranoside (1): To a solution
of 14 (700 mg, 0.5 mmol) in dry MeOH (10 mL) was added Na-
OMe in MeOH (1 mL, 0.5 ), and the solution was allowed to stir
for 12 h for the complete removal of the O-acetyl and O-benzoyl
groups. The solution was then neutralized with DOWEX 50W H+,
filtered through a cotton plug, and the filtrate was concentrated in
vacuo to afford pure tetrasaccharide 1 (335 mg, 88%) as a white
amorphous solid. [α]2D5 = +58 (c 1.4, H2O). 1H NMR (D2O,
300 MHz): δ = 5.08 (s, 1 H), 4.74 (s, 1 H), 4.49 (d, J = 6.3 Hz, 1
H), 4.47 (d, J = 6.0 Hz, 1 H), 3.98 (m, 5 H), 3.84–3.51 (m, 13 H),
3.45 (m, 3 H), 3.34 (t, J = 9.6 Hz, 1 H), 2.94 (t, J = 1.8 Hz, 1 H),
1.28 (d, 3 H), 1.26 (d, 3 H) ppm. 13C NMR (D2O, 75 MHz): δ =
102.6, 101.4, 100.9, 99.2, 79.2, 78.8, 78.2, 77.2, 76.5, 76.4, 75.2,
74.9, 74.7, 74.2, 72.7, 72.0, 71.9, 70.4, 70.2, 70.1, 69.1, 69.0, 60.2,
60.0, 56.6, 16.6, 16.5 ppm. HRMS calcd. for C27H44O19Na [M +
Na]+ 695.2375; found 695.2373.
1
82%). [α]2D5 = +127 (c 1.1, CHCl3). H NMR (CDCl3, 300 MHz):
δ = 7.95–6.82 (m, 35 H), 5.49 (t, J = 9.6 Hz, 1 H), 5.42 (t, J =
9.9 Hz, 1 H), 5.24 (t, J = 9.9 Hz, 1 H), 5.13 (dd, J = 3.3, 10.2 Hz,
1 H), 5.04 (d, J = 7.8 Hz, 1 H), 4.89 (m, 1 H), 4.85 (t, J = 10.2 Hz,
1 H), 4.67 (s, 1 H), 4.55 (d, J = 7.5 Hz, 1 H), 4.32 (dd, J = 2.1,
15.6 Hz, 1 H), 4.19 (m, 3 H), 3.87–3.67 (m, 6 H), 3.55 (m, 1 H),
3.28 (t, 1 H), 3.19 (bd, 1 H), 2.43 (t, J = 1.8 Hz, 1 H), 1.98 (s, 3
H), 1.87 (s, 3 H), 1.76 (s, 3 H), 1.17 (d, J = 6.0 Hz, 3 H), 1.14 (s,
9 H), 0.99 (s, 9 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 169.6,
169.2, 168.5, 165.9, 164.7, 164.3, 136.0 (2), 135.5 (8), 132.9, 132.6,
132.5, 132.3, 132.2, 132.1, 132.0, 130.3 (2)130.1, 130.0 (2), 129.9,
129.7 (4), 129.0, 128.3 (3), 128.2 (3), 128.19 (4), 127.7 (4), 99.8,
98.9, 98.1, 78.4, 77.2, 75.5 (2), 75.4, 75.0 (2), 73.6, 73.4 (2), 71.5,
71.2, 69.7, 68.7, 66.6, 66.2, 61.0, 55.7, 27.1 (3), 26.8 (3), 20.7, 20.6,
20.4, 19.5, 19.1, 17.2 ppm. ESIMS: m/z = 1463.4 [M + Na]+.
p-Methoxyphenyl 2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyl-(1Ǟ4)-
2,3-isopropylidene-α- -rhamnopyranoside (17): A solution of 15
L
(2.0 g, 6.4 mmol), 16 (3.5 g, 7.7 mmol) and 4 Å MS (3.0 g) in dry
CH2Cl2 (40 mL) was stirred under nitrogen for 30 min at ambient
temperature. NIS (2.25 g, 10 mmol) was then added, followed by
H2SO4/silica (100 mg), and the mixture was allowed to stir for an-
other 1 h, at which time TLC (n-hexane/EtOAc, 3:1) showed the
complete consumption of the starting material. The mixture was
filtered through a pad of celite, and the filtrate was washed success-
ively with aqueous Na2S2O3 (2 ϫ 30 mL), aqueous saturated
NaHCO3 (2ϫ30 mL) and H2O (30 mL). The organic layer was
separated, dried (Na2SO4), filtered and concentrated in vacuo. The
crude product thus obtained was purified by flash chromatography
with a gradient of n-hexane/EtOAc (4:1 to 2:1) to yield the pure
disaccharide 17 (3.6 g, 86%). [α]2D5 = +56 (c 1.0, CHCl3). 1H NMR
(CDCl3, 300 MHz): δ = 6.93 (d, 2 H), 6.77 (d, 2 H), 5.52 (s, 1 H),
5.19 (t, J = 9.3 Hz, 1 H), 5.02 (t, J = 9.3 Hz, 1 H), 4.95 (d, J =
9.0 Hz, 1 H), 4.91 (dd, J = 9.0, 9.3 Hz, 1 H), 4.28–4.17 (m, 3 H),
4.09 (dd, J = 2.1, 12.3 Hz, 1 H), 3.75 (s, 3 H), 3.73 (t, J = 9.9 Hz,
1 H), 3.66 (m, 1 H), 3.57 (dd, J = 7.5, 9.9 Hz, 1 H), 2.07 (s, 3 H),
2.06 (s, 3 H), 2.01 (s, 3 H), 2.00 (s, 3 H), 1.54 (s, 3 H), 1.38 (s, 3
H), 1.19 (d, J = 6.3 Hz, 3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ
= 170.0, 169.8, 169.0 (2), 155.0, 150.2, 117.7 (2), 114.6 (2), 109.4,
99.52, 96.1, 79.4, 78.1, 76.2, 73.0, 71.9, 71.6, 68.7, 64.8, 62.1, 55.4,
28.0, 26.5, 20.7, 20.6, 20.5 (2), 17.5 ppm. HRMS calcd. for
C30H40O15Na [M + Na]+ 663.2265; found 663.2263.
Propargyl 2,3,4-Tri-O-acetyl-α-
benzoyl-β- -glucopyranosyl-(1Ǟ4)-2-O-(2,3,4-tri-O-acetyl-α-
nopyranosyl)-3-O-benzoyl-β- -glucopyranoside (14): A mixture of
L
-rhamnopyranosyl-(1Ǟ4)-2,3-di-O-
D
L-rham-
D
trisaccharide acceptor 12 (1.1 g, 0.8 mmol), known rhamnosyl do-
nor 5 (396 mg, 1.0 mmol) and 4 Å MS (1.0 g) in dry CH2Cl2
(15 mL) was stirred under nitrogen for 30 min at ambient tempera-
ture. NIS (290 g, 1.3 mmol) was added followed by H2SO4/silica
(50 mg), and the mixture was stirred at ambient temperature for
1 h, at which time TLC (n-hexane/EtOAc, 2:1) showed the complete
consumption of the acceptor trisaccharide 12. The mixture was fil-
tered through a pad of celite, and the filtrate was washed success-
ively with aqueous Na2S2O3 (2 ϫ 30 mL), saturated aqueous
NaHCO3 (2ϫ30 mL) and H2O (30 mL). The organic layer was
collected, dried (Na2SO4), filtered and concentrated in vacuo. The
crude product thus obtained was purified by flash chromatography
with n-hexane/EtOAc (2:1) as the eluent. The residue was dissolved
in dry THF (20 mL), AcOH (0.25 mL) was added, followed by
Bu4NF/THF (1.5 mL, 1.6 mmol), and the solution was stirred at
ambient temperature for 6 h, at which time TLC (n-hexane/EtOAc,
2:1) showed the complete conversion of the starting material to a
slower-running spot. The solvents were evaporated in vacuo, and
the residue was purified by flash chromatography with a gradient
of n-hexane/EtOAc (2:1 to 1:1) to yield pure tetrasaccharide diol
14 (775 mg, 82%) as a colourless foam. [α]2D5 = +74 (c 1.1, CHCl3).
p-Methoxyphenyl 2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyl-(1Ǟ4)-
1H NMR (CDCl3, 300 MHz): δ = 8.12–7.28 (m, 15 H), 5.61 (t, J 2,3-di-O-acetyl-α-
L-rhamnopyranoside (18): A solution of 17 (3.0 g,
= 9.3 Hz, 1 H), 5.52 (t, J = 9.3 Hz, 1 H), 5.25 (dd, J = 8.1, 9.3 Hz, 4.7 mmol) in 80% AcOH (30 mL) was stirred at 80 °C for 2 h, at
1 H), 5.19 (dd, J = 3.3, 9.9 Hz, 1 H), 5.03 (m, 1 H), 4.97 (dd, J = which time TLC (n-hexane/EtOAc, 2:1) showed the complete con-
3.3, 9.9 Hz, 1 H), 4.95 (m, 1 H), 4.93 (t, J = 9.9 Hz, 1 H), 4.84 (d, version of the starting material to a slower-moving spot. The sol-
J = 8.1 Hz, 1 H), 4.80 (t, J = 9.9 Hz, 1 H), 4.79 (s, 1 H), 4.76 (s, 1
H), 4.67 (d, J = 7.5 Hz, 1 H), 4.37 (m, 2 H), 4.25 (dd, J = 6.3,
9.9 Hz, 1 H), 4.02 (m, 2 H), 3.77 (m, 2 H), 3.67 (br. s, 2 H), 3.52
vents were evaporated and co-evaporated with toluene to remove
residual AcOH. The semi-solid mass thus obtained was dissolved
in dry pyridine (12 mL) followed by Ac2O (10 mL), and the solu-
(dd, J = 6.3, 9.9 Hz, 1 H), 3.37 (m, 3 H), 3.04 (m, 1 H), 2.50 (t, J tion was stirred at ambient temperature for 3 h until the reaction
= 2.1 Hz, 1 H), 2.04 (s, 3 H), 2.00 (s, 3 H), 1.92 (s, 3 H), 1.90 (s, 3 was complete (by TLC). The solvents were evaporated in vacuo
H), 1.86 (s, 3 H), 1.79 (s, 3 H), 1.16 (d, J = 6.3 Hz, 3 H), 0.54 (d,
J = 6.3 Hz, 3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 170.1,
170.0, 169.9, 169.7, 169.6, 168.9, 165.8, 164.8, 164.5, 133.6, 133.2,
133.1, 129.8 (2), 129.6 (2), 129.5 (2), 129.4, 129.3, 128.9, 128.7 (2),
and co-evaporated with toluene to remove residual pyridine. The
crude acetate derivative thus obtained was purified by flash
chromatography with n-hexane/EtOAc (3:1) to yield pure disaccha-
ride 18 (2.7 g, 86%) as a white foam. [α]2D5 = +73 (c 1.2, CHCl3).
Eur. J. Org. Chem. 2008, 5770–5777
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5775

S. Dasgupta, B. Mukhopadhyay
FULL PAPER
1H NMR (CDCl3, 300 MHz): δ = 6.97 (d, J = 9.0 Hz, 2 H), 6.77
(d, J = 9.0 Hz, 2 H), 5.39 (dd, J = 3.3, 9.6 Hz, 1 H), 5.27 (dd, J =
1.2, 3.6 Hz, 1 H), 5.22 (d, J = 1.2 Hz, 1 H), 5.11 (t, J = 9.3 Hz, 1
H), 5.03 (t, J = 9.3 Hz, 1 H), 4.90 (dd, J = 8.1, 9.3 Hz, 1 H), 4.69
(d, J = 8.1 Hz, 1 H), 4.25 (dd, J = 4.5, 12.0 Hz, 1 H), 4.15 (dd, J
= 2.4, 12.0 Hz, 1 H), 3.91 (m, 1 H), 3.75 (s, 3 H), 3.62 (t, J =
9.6 Hz, 1 H), 2.17 (s, 3 H), 2.11 (s, 3 H), 2.08 (s, 3 H), 2.01 (s, 3
H), 2.00 (s, 3 H), 1.98 (s, 3 H), 1.27 (d, J = 6.0 Hz, 3 H) ppm. 13C
NMR (CDCl3, 75 MHz): δ = 170.0, 169.9, 169.4, 169.2, 168.9 (2),
155.3, 150.0, 117.9 (2), 114.6 (2), 100.8, 96.6, 77.1, 73.0, 71.7, 71.3,
71.1, 70.2, 68.6, 67.4, 61.8, 55.4, 21.0, 20.9, 20.6, 20.5 (2), 20.3, 17.7
ppm. HRMS calcd. for C31H40O16Na [M + Na]+ 691.2214; found
691.2216.
(4), 20.3, 20.2, 19.3, 19.2, 17.1, 16.9 ppm. MALDI-TOF-MS: m/z
= 2023.6 [M + Na]+.
Propargyl β-D-Glucopyranosyl-(1Ǟ4)-α-
L-rhamnopyranosyl-(1Ǟ4)-
β- -glucopyranosyl-(1Ǟ4)-2-O-(α-
D
L-rhamnopyranosyl)-β-D-glucopyr-
anoside (2): Compound 20 (1.5 g, 1.6 mmol) was dissolved in dry
THF (20 mL), AcOH (1 mL) and Bu4NF/THF (1.1 mL, 3.7 mmol)
were added, and the solution was stirred at ambient temperature
for 12 h, at which time TLC (n-hexane/EtOAc, 2:1) showed the
complete conversion of the starting material to a slower-running
spot. The solvents were evaporated in vacuo, and the residue was
purified by flash chromatography with n-hexane/EtOAc (1:1). The
purified product was dissolved in dry MeOH (20 mL). NaOMe in
MeOH (2 mL, 0.5 ) was added, and the solution was stirred over-
night at ambient temperature. The solution was then neutralized
with DOWEX H+ resin, filtered and concentrated to afford pure
pentasaccharide target 2 (320 mg, 77%) as a white amorphous
mass. [α]2D5 = +149 (c 0.8, H2O). 1H NMR (D2O, 300 MHz): δ =
5.09 (s, 1 H), 4.72 (d, J = 7.8 Hz, 1 H), 4.68 (d, J = 7.5 Hz, 1 H),
4.46 (d, J = 7.8 Hz, 1 H), 4.44 (s, 1 H), 2.91 (t, J = 1.8 Hz, 1 H),
1.30 (d, J = 6.0 Hz, 3 H), 1.25 (d, J = 6.0 Hz, 3 H) ppm. 13C NMR
(D2O, 75 MHz): δ = 104.4, 103.6, 102.6, 101.9, 100.3, 82.3, 80.4,
79.9, 79.4, 78.4, 77.6, 77.2, 77.0, 76.3, 76.1, 75.9, 75.4, 75.1, 74.8,
73.2, 71.5, 71.4 (3), 70.7, 70.2, 68.8, 61.8, 61.3, 61.1, 57.8, 50.1,
17.9, 17.8 ppm. HRMS calcd. for C33H54O24Na [M + Na]+
857.2903; found 857.2901.
Propargyl 2,3,4,6-Tetra-O-acetyl-β-
O-acetyl-α- -rhamnopyranosyl-(1Ǟ4)-2,3-di-O-benzoyl-6-O-(tert-
butyldiphenylsilyl)-β- -glucopyranosyl-(1Ǟ4)-2-O-(2,3,4-tri-O-ace-
tyl-α- -rhamnopyranosyl)-3-O-benzoyl-6-O-(tert-butyldiphenylsilyl)-
β- -glucopyranoside (20): To a solution of 18 (2.5 g, 3.65 mmol) in
D-glucopyranosyl-(1Ǟ4)-2,3-di-
L
D
L
D
CH3CN/H2O (9:1, 40 mL) was added CAN (4 g, 7.3 mmol), and
the mixture was stirred at ambient temperature for 45 min, at which
time TLC (n-hexane/EtOAc, 2:1) showed the complete conversion
of the starting material. The solvents were evaporated in vacuo,
and the residue was dissolved in CH2Cl2 (40 mL) and washed with
H2O (2ϫ30 mL). The organic layer was separated, dried (Na2SO4),
filtered and the solvents were evaporated. The crude disaccharide
hemiacetal thus obtained was purified by flash chromatography
with n-hexane/EtOAc (2:1). The syrupy compound obtained after
chromatographic purification was dissolved in dry CH2Cl2
(15 mL), and CCl3CN (1.1 mL, 11 mmol) and DBU (600 µL,
3.65 mmol) were added. The resulting solution was stirred at ambi-
ent temperature for 1 h, until a complete reaction was confirmed
by TLC. The solvents were evaporated in vacuo, and the residue
thus obtained was purified by flash chromatography with n-hexane/
EtOAc (2:1) to afford pure disaccharide trichloroacetimidate (19,
2.2 g, 83%). A mixture of 19 (1 g, 1.4 mmol), trisaccharide acceptor
12 (1.5 g, 1.1 mmol) and molecular sieves (4 Å) in dry CH2Cl2
(15 mL) was then stirred under nitrogen for 30 min. H2SO4/silica
(50 mg) was added, and the mixture was allowed to stir at ambient
temperature for 1 h, at which time TLC (n-hexane/EtOAc, 2:1)
showed the complete consumption of the starting materials. The
mixture was filtered through a pad of celite and washed with
CH2Cl2 (10 mL). The complete filtrate was washed with aqueous
saturated NaHCO3 (2ϫ20 mL) and brine (20 mL). The organic
layer was collected, dried (Na2SO4), filtered and concentrated in
vacuo. The residue was purified by flash chromatography with a
gradient of n-hexane and EtOAc (2:1 to 1:1) to afford pure penta-
saccharide 20 (1.8 g, 87%) as a white foam. [α]2D5 = +101 (c 1.0,
CHCl3). 1H NMR (CDCl3, 300 MHz): δ = 7.72–7.10 (m, 35 H,
ArH), 5.49 (t, J = 9.6 Hz, 1 H), 5.47 (t, J = 9.6 Hz, 1 H), 5.19 (m,
2 H), 5.07 (t, J = 9.3 Hz, 1 H), 5.04–4.91 (m, 7 H), 4.76 (s, 1 H),
4.72 (s, 1 H), 4.54 (m, 2 H), 4.38–4.06 (m, 6 H), 3.87–3.58 (m, 6
H), 3.49–3.31 (m, 4 H), 3.29 (m, 1 H), 2.48 (t, J = 1.8 Hz, 1 H),
2.03 (2s, 6 H), 2.01 (2s, 6 H), 2.00 (s, 3 H), 1.98 (s, 3 H), 1.97 (s, 3
H), 1.93 (s, 3 H), 1.89 (s, 3 H), 1.20 (d, J = 6.3 Hz, 3 H), 1.03 (s,
9 H), 1.01 (s, 9 H), 0.70 (d, J = 6.0 Hz, 3 H) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 170.4, 170.2, 170.0, 169.5, 169.3, 169.2,
169.1, 169.0, 168.8, 165.6, 164.8, 164.6, 135.8 (2), 135.7 (2), 135.6
(2), 135.5 (2), 133.6, 133.2, 133.0, 132.9, 132.8, 132.7 (2), 129.9,
129.8 (4), 129.7 (2), 129.6, 129.5 (2), 129.4, 129.1, 128.9, 128.2 (2),
128.1 (2), 128.0 (2), 127.9 (2), 127.7 (2), 127.6 (4), 100.1, 99.2, 98.9,
98.3, 97.8, 78.4, 77.2, 76.8, 76.2, 75.5, 75.3, 75.2 (2), 74.9, 73.8,
73.7, 72.9, 72.6, 71.2 (2), 71.1, 71.0, 70.6, 70.0, 69.4, 68.8, 68.5,
67.3, 66.5, 63.2, 62.1, 61.9 (2), 55.7, 29.6, 26.9, 20.8, 20.7, 20.6, 20.5
Supporting Information (see also the footnote on the first page of
this article): Copies of 1H NMR and 13C spectra of all compounds.
Acknowledgments
S. D. is thankful to Council of Scientific and Industrial Research
(CSIR), New Delhi, India for a fellowship. The work is funded by
Department of Science and Technology (DST), New Delhi, India
through a SERC Fast-Track Grant SR/FTP/CS-110/2005.
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Received: August 30, 2008
[19] For the use of H2SO4/silica in various carbohydrate reactions,
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Published Online: October 27, 2008
Eur. J. Org. Chem. 2008, 5770–5777
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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