Synthesis of a tetrasaccharide related to the triterpenoid saponin isolated from Schima noronhae

Article abstract of DOI:10.1016/j.tetasy.2010.09.002

A concise synthesis of a tetrasaccharide related to the cell-growth inhibitory triterpenoid saponin isolated from Schima noronhae is reported. A late stage 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-mediated oxidation of a primary hydroxyl group to carboxylic acid has been achieved under phase-transfer conditions. Stereoselective glycosylations were carried out using thioglycoside or glycosyl trichloroacetimidate activation using sulfuric acid immobilized on silica (H₂SO₄-silica) in conjunction with N-iodosuccinimide and alone, respectively.

Full text of DOI:10.1016/j.tetasy.2010.09.002

Tetrahedron: Asymmetry 21 (2010) 2413–2418
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis of a tetrasaccharide related to the triterpenoid saponin isolated
from Schima noronhae
⇑
Priya Verma, Ritu Raj, Bimalendu Roy , Balaram Mukhopadhyay
Indian Institute of Science Education and Research-Kolkata (IISER-K), Mohanpur Campus, PO BCKV Campus Main Office, Mohanpur, Nadia 741252, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A concise synthesis of a tetrasaccharide related to the cell-growth inhibitory triterpenoid saponin iso-
lated from Schima noronhae is reported. A late stage 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-medi-
ated oxidation of a primary hydroxyl group to carboxylic acid has been achieved under phase-transfer
conditions. Stereoselective glycosylations were carried out using thioglycoside or glycosyl trichloroace-
timidate activation using sulfuric acid immobilized on silica (H2SO4–silica) in conjunction with N-iodo-
succinimide and alone, respectively.
Received 4 August 2010
Accepted 16 September 2010
Available online 18 October 2010
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
synthesis of saponins, the chemical synthesis of different sugar
substrates will be useful.
Saponins are glycosylated secondary metabolites synthesized
by plants during their routine programme of growth and develop-
ment.1 They posses intense antifungal properties and are thus be-
lieved to act as the natural chemical barriers in plants against
fungal attack.2 In addition to their antifungal properties, saponins
have been exploited as food crops, for example, legumes and oats
or sources of drugs, for example, ginseng and liquorice making
them commercially attractive for diverse reasons.3 However the
details of the genetic machinery involved in the biosynthesis of
these secondary metabolites have not yet been characterized.
Despite the architectural diversity amongst the saponin structures
isolated from different sources, one common feature that is shared
by all saponins is the presence of a sugar chain attached to the
3-position of the aglycon moiety. These sugar chains are often
branched and usually consist up to five sugar units. Normally the
sugar units are selected from glucose, galactose, arabinose, xylose,
rhamnose and glucuronic acid.4 It is believed that the glycosylation
of the aglycon moiety occurs in the final stage of the saponin
biosynthesis and the biological function of the saponin depends
largely on the glycosylation pattern. Therefore, it is essential to elu-
cidate the exact biosynthesis of the oligosaccharide chain and iden-
tify the enzymes involved in the whole process to realize the
source of saponin bioactivity.5 For a better understanding of the
glycosyltransferases involved and the order of events in the bio-
Schima noronhae Reinw. ex Blume is a commonly grown plant in
Southeast Asia and known as ‘iju’ in Japan.6 The plant is used as a
source of timber and the leaves as fodder crops. They are also used
in traditional medicines in Indonesia and Malaysia, for example,
the astringent corollas are used to treat uterine disorders and hys-
teria and also as an ointment to alleviate the symptoms of small-
pox.7 Recently, Ishibashi et al. reported two new triterpenoid
saponins (Fig. 1) isolated from S. noronhae that showed potent
cell-growth inhibitory activity against human cervical carcinoma
cells (HeLa cells) and human colon carcinoma DLD1 cells.8 Herein
we report the chemical synthesis of the tetrasaccharide side chain
1 (Fig. 1) of the triterpenoid saponins isolated from S. noronhae in
the form of its propargyl glycoside.
2. Results and discussion
Careful consideration of the retrosynthetic analysis suggested
that a 2+2 strategy would be best suited for the construction of
the target tetrasaccharide. Rational protecting group manipula-
tions on the commercially available monosaccharides followed
by stereoselective glycosylations were planned to prepare the
required disaccharide synthons. Further coupling would result in
the formation of the tetrasaccharide framework. A late stage oxida-
tion would furnish the required carboxylic acid moiety (Fig. 2). The
choice of propargyl glycoside at the reducing end opens up the
possibility of further glycoconjugate formation through click
chemistry or other metal catalyzed multi-component reactions.
The synthesis of the disaccharide donor 8 was started from the
⇑
Corresponding author. Tel.: +91 33 64513294; fax: +91 33 25873020.
E-mail address: sugarnet73@hotmail.com (B. Mukhopadhyay).
Present address: Unité de Recherches en Biocatalyse (FRE-CNRS 2230), Faculté
des Sciences et des Techniques, 2, rue de la Houssinière, B. P. 92208, 44322 Nantes
cedex 3, France.
known p-methoxyphenyl b-D
-galactopyranoside 2.9 The formation
of the 4,6-benzylidene acetal using benzaldeylde dimethylacetal in
the presence of catalytic CSA in acetonitrile10 followed by
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.09.002

2414
P. Verma et al. / Tetrahedron: Asymmetry 21 (2010) 2413–2418
O
O
OH
O
R2
Me
OH
HO
HO
O
R1
HO
HO
O
O
O
O
O
O
HO
HO
R
R
1 = R2 = OH
1 = R2 = H
OH
HO
HO
HO
O
OH
O
Me
C
O
A
O
OH
O
HO
HO
HO
HO
O
O
O
HO
O
HO
B
OH
HO
HO
HO
O
OH
D
1
Figure 1. Structure of the triterpenoid and synthetic target.
O
Me
OH
HO
HO
O
HO
HO
O
O
O
O
O
O
HO
HO
OH
HO
HO
HO
O
OH
1
Me
OP
PO
PO
OH
O
O
O
HO
O
O
O
PO
O
PO
OP
PO
PO
O
PO
OP
Me
PO
PO
PO
HO
OP
PO
O
O
O
O
O
O
+
PO
PO
PO
PO
O
O
CCl3
NH
OP
PO
PO
OP
STol
HO
PO
PO
PO
PO
PO
PO
HO
O
Me
PO
O
O
O
STol +
+
O
OP
PO
PO
O
OP
Commercially available L-Rha and D-Gal
Figure 2. Retrosynthetic analysis of the target oligosaccharides.
OMP
Commercially available D-Glc
treatment with benzoyl chloride and pyridine in dichlorometh-
2,3,4-tri-O-acetyl-1-thio-a-L
-rhamnopyranoside 514 using NIS in
ane11 afforded p-methoxyphenyl 4,6-O-benzylidene-2,3 di-O-ben-
the presence of H2SO4–silica to furnish the disaccharide 6 in 85%
yield.15 Our previous experience with the oxidative cleavage of
the p-methoxyphenyl group revealed that the OTBDPS moiety is
not compatible with this transformation. Therefore, the TBDPS
moiety was removed using Bu4NF-THF16 to afford compound 7 in
87% yield and subsequently acetylated using Ac2O in pyridine to af-
ford the disaccharide 8 in 93% yield. Next, CAN-mediated oxidative
zoyl-b-D-galactopyranoside 3 in 81% yield. Hydrolysis of the
benzylidene acetal using 80% AcOH at 80 °C12 followed by selective
protection of the primary hydroxyl group using TBDPS-Cl in pyri-
dine13 afforded the acceptor, p-methoxyphenyl 2,3 di-O-benzoyl
6-O-tert-butyldiphenylsilyl b-
D-galactopyranoside 4 in 87% yield.
Acceptor 4 was then glycosylated with the known donor, p-tolyl

P. Verma et al. / Tetrahedron: Asymmetry 21 (2010) 2413–2418
2415
cleavage17 of the p-methoxyphenyl group followed by reaction
with trichloroacetonitrile in the presence of DBU18 furnished the
required disaccharide donor, 2,3,4-tri-O-acetyl- -rhamnopyrano-
syl-(1?4)-6-O-acetyl-2,3-di-O-benzoyl-b- -galactopyranosyl trichlo-
roacetimidate 9 in 81% yield (Scheme 1).
In a separate experiment, a known propargyl 4,6-O-benzylidene-
3-O-(4-methoxybenzyl)-b-
-glucopyranoside 1019 was coupled
with a known, p-tolyl 2,3,4,6-tetra-O-acetyl-1-thio-b- -glucopyran-
Me
Ph
O
AcO
AcO
AcO
O
O
O
OAc
O
O
O
O
HO
H2SO4-silica
+
a-L
AcO
AcO
BzO
O
D
BzO
O
CCl3
AcO
OAc
9
12
NH
D
D
Me
oside 1114 using NIS in the presence of H2SO4–silica at ꢀ30 °C. Once
the completion of the glycosylation was confirmed by TLC (n-hex-
ane–EtOAc, 3:1), a slight excess of H2SO4–silica was added and the
reaction was allowed to warm up to room temperature. The acid-la-
bile p-methoxybenzyl group was hydrolysed19 to furnish the
required disaccharide acceptor, propargyl 2,3,4,6-tetra-O-acetyl-1-
OAc
AcO
O
Ph
O
O
O
O
O
O
O
O
i. 80% AcOH, Δ
AcO
BzO
AcO
ii. TEMPO
BzO
AcO
O
AcO
AcO
OAc
13
thio-b-D-glucopyranosyl-(1?2)-4,6-O-benzylidene-3-O-(4-
methoxybenzyl)-b-
(Scheme 2).
D
-glucopyranoside 12 in 83% overall yield
O
Me
OAc
O
AcO
AcO
AcO
O
HO
HO
O
O
O
O
NaOMe, MeOH
O
Ph
BzO
BzO
AcO
O
O
HO
OH
O
i. 2,2-DMT, CSA
AcO
O
i. 80% AcOH, Δ
O
AcO
14
OMP
HO
OAc
OMP
ii. BzCl, Py
BzO
STol
ii. TBDPS-Cl, Py
OH
OBz
2
3
O
Me
OH
HO
HO
Me
AcO
O
O
HO
O
O
HO
Me
AcO
O
HO
O
OTBDPS
O
AcO
O
OAc
O
OTBDPS
O
HO
5
O
O
HO
OMP
OH
BzO
AcO
AcO
NIS, H2SO4-silica
OMP
HO
BzO
O
OBz
OBz
HO
4
HO
6
OH
1
Me
AcO
O
OAc
O
i. CAN, CH3CN-H2O
ii. CCl3CN, DBU
i. Bu4NF-THF
ii.Ac2O, Py
Scheme 3. Synthesis of the tetrasaccharide, oxidation and deprotection.
O
AcO
AcO
OMP
BzO
OBz
7
3. Conclusion
Me
AcO
O
In conclusion, we have accomplished the total synthesis of the
tetrasaccharide glycone part of the cell-growth inhibitory triterpe-
noid saponin isolated from S. noronhae as its propargyl glycoside.
The propargyl glycoside at the reducing end will provide scope in
making various types of glycoconjugates to assess the biological
activity of the oligosaccharide in question.
OAc
O
O
AcO
AcO
BzO
BzO
O
CCl3
8
NH
Scheme 1. Synthesis of the disaccharide donor.
4. Experimental
OAc
O
Ph
O
O
4.1. Preparation of H2SO4–silica
O
O
AcO
+
STol
PMBO
AcO
OH
OAc
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 evap-
orated under reduced pressure, resulting in free flowing H2SO4–sil-
ica, which was dried at 110 °C for 3 h and then used for the
reactions.
11
10
Ph
O
O
O
O
O
NIS, H2SO4-silica
-30 oC to rt
HO
AcO
AcO
O
AcO
OAc
4.2. p-Methoxyphenyl 6-O-(tert-butyldiphenylsilyl)-2,3-di-O-
12
benzoyl-b-
D-galactopyranoside 4
Scheme 2. Synthesis of the disaccharide acceptor.
To the mixture of known p-methoxyphenyl-b-
D-galactopyrano-
Final glycosylation between disaccharide donor 9 and the disac-
charide acceptor 12 using H2SO4–silica at 0 °C afforded the tetra-
saccharide 13 in 87% yield. Hydrolysis of the benzylidene acetal
using 80% AcOH at 80 °C followed by TEMPO oxidation20 installed
the required carboxylic acid moiety 14 in 78% overall yield. Finally,
Zemplén de-O-acylation21 using NaOMe in MeOH furnished the
target tetrasaccharide 1 in 89% yield (Scheme 3).
side 2 (3.0 g, 10.4 mmol) in dry acetonitrile (25 mL) was added benz-
aldehydedimethyl acetal (2.4 mL, 15.8 mmol) followed by catalytic
amount of CSA (pH: 3–4) at room temperature and the reaction mix-
ture was allowed to stir for 30 min. After completion of the reaction
as shown by TLC (CH2Cl2–MeOH; 15:1), the solution was neutralized
with Et3N and the solvent was evaporated in vacuo to afford
p-methoxyphenyl 4,6-O-benzylidene-b-D-galactopyranoside as a

2416
P. Verma et al. / Tetrahedron: Asymmetry 21 (2010) 2413–2418
crude product. To a slurry of this product in dry dichloromethane
(30 mL) was added pyridine (3.4 mL, 41 mmol) and the mixture
was placed in an ice bath. To the cold suspension was added benzoyl
chloride (3.65 mL, 31.4 mmol); the exothermic reaction started
immediately; the reaction mixture was allowed to stir at 0 °C for
30 min and then the temperature was raised to room temperature.
The mixture was further allowed to stir for 3 h when TLC (n-hex-
ane–EtOAc; 4:1) showed complete conversion of the starting mate-
rial. The solvents were evaporated in vacuo and the residue was
dissolvedindichloromethane(40 mL)andwashedsuccessivelywith
H2O (2 ꢁ 50 mL). Theorganiclayer wascollected, dried(Na2SO4) and
evaporated to a syrup. The crude compound was purified by flash
chromatography using n-hexane–EtOAc (4:1) to afford pure com-
125 MHz) d: 169.9, 165.7, 164.9 (3 ꢁ COCH3), 165.7, 164.9
(2 ꢁ COPh), 155.3, 151.3, 135.6(2), 135.5(2), 133.4, 133.0, 132.9,
132.8, 130.0(2), 129.9, 129.8, 129.7(2), 129.4, 128.5(2), 128.4,
128.2(2), 127.8(3), 127.7, 118.3(2), 114.4(2), 100.8 (C-1), 99.2 (C-
10), 75.6, 74.2, 73.8, 70.9, 70.3, 70.2, 69.6, 68.8, 67.3, 63.2 (C-6),
55.5 (CH2–C6H5–OCH3), 26.7 [C(CH3)3], 20.9, 20.8, 20.7
(3 ꢁ COCH3), 17.4 (C–CH3). HRMS calcd for C55H60O16SiNa
(M+Na)+: 1027.3548; found 1027.3542.
4.4. p-Methoxyphenyl 2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl-
(1?4)-2,3-di-O-benzoyl-b- -galactopyranoside 7
D
To a solution of compound 6 (1.4 g, 1.4 mmol) in dry THF
(10 mL), Bu4NF in THF (2.8 mL, 2.8 mmol) was added and the solu-
tion was allowed to stir at rt for 6 h. Then the solvents were evap-
orated to dryness. The crude product thus obtained was purified by
flash chromatography using n-hexane–EtOAc (2:1) to afford pure
pound 322 (4.9 g, 81%) as a white solid compound. ½a 2D5
¼ þ107 (c
ꢂ
1.2, CHCl3). HRMS calcd for C34H30O9Na (M+Na)+: 605.1788; found
605.1785.
A solution of compound 3 (3.0 g, 5.1 mmol) in AcOH–H2O (9:1,
30 mL) was stirred at 85 °C for 2 h when TLC showed complete
conversion of the starting material to a slower moving spot. The
solvents were evaporated in vacuo and the residue was purified
by flash chromatography using n-hexane–EtOAc (3:1) to afford a
pure diol (2.4 g, 93%) as a white solid. To a solution of the diol23
(2.4 g, 4.8 mmol) in dry pyridine (25 mL) was added TBDPS-Cl
(1.8 mL, 7.15 mmol) and the solution was stirred for 12 h at room
temperature. Solvents were evaporated in vacuo and the residual
syrup was purified by flash chromatography using n-hexane–
EtOAc (5:1) as eluent to afford pure compound 4 (3.0 g, 87%) as a
compound 7 (930 mg, 87%) as a light yellow syrup. ½a D25
¼ þ85 (c
ꢂ
1.0, CHCl3). 1H NMR (CDCl3, 500 MHz) d: 7.93–7.31 (m, 10H,
0
0
0
0
ArH), 6.93, 6.75 (2d, 4H, ArH), 5.94 (dd, 1H, J3 ,4 8.0 Hz, J4 ,5
10.5 Hz, H-40), 5.48 (dd, 1H, J2 ,3 3.0 Hz, J3 ,4 8.0 Hz, H-30), 5.47
(m, 1H, H-20), 5.38 (dd, 1H, J2,3 9.5 Hz, J3,4 3.5 Hz, H-3), 5.19 (d,
1H, J1,2 8.0 Hz, H-1), 4.97 (t, 1H, J1,2, J2,3 8.5 Hz, H-2), 4.43 (d, 1H,
0
0
0
0
J1 ,2 3.0 Hz, H-10), 4.02–3.97 (m, 3H, H-50, H-6a, H-6b), 3.81 (m,
0
0
0
0
1H, H-5), 2.05, 1.95, 1.94 (3 ꢁ COCH3), 1.20 (d, 3H, J5 ,6 6.5 Hz, C–
CH3). 13C NMR (CDCl3, 125 MHz) d: 170.0, 169.6, 169.3
(3 ꢁ COCH3), 165.7, 165.0 (2 ꢁ COPh), 155.5, 151.0, 133.5, 133.1,
129.9(2), 129.7(2), 129.3, 128.5(2), 128.4, 128.3(2), 118.5(2),
114.5(2) (ArC), 100.9 (C-1), 99.4 (C-10), 74.8, 74.1, 73.6, 70.9,
69.5, 69.4, 68.9, 68.0, 61.2 (C-6), 55.5 (OC6H5OCH3), 20.7, 20.6,
20.5 (3 ꢁ COCH3), 17.3 (C–CH3).
white foam.
½
a 2D5
ꢂ
¼ þ107 (c 1.2, CHCl3). 1H NMR (CDCl3,
300 MHz) d: 8.03 (2d, 4H, J 8.6 Hz, COC6H5), 7.76–7.37 (m, 16H,
ArH), 7.01, 6.76 (2d, 4H, J 8.7 Hz, C6H4OCH3), 6.05 (dd, 1H, J 9.3,
7.9 Hz, H-2), 5.40 (dd, 1H, J 9.3, 3.1 Hz, H-3), 5.15 (d, 1H, J 7.9 Hz,
H-1), 4.51 (br s, 1H, H-4), 4.06–3.88 (m, 3H, H-5, H-6a, H-6b),
3.76 (s, 3H, C6H4OCH3), 2.98 (br s, 1H, OH), 1.11 (s, 9H, C(CH3)3).
13C NMR (CDCl3, 75 MHz) d: 166.0, 165.4 (COC6H5), 155.5, 151.4,
135.7(2), 135.6(2), 133.4, 133.1, 132.8, 132.5, 132.4, 129.9(3),
129.7(2), 129.5, 129.1(2), 128.4(2), 128.3(2), 127.8(3), 118.9(2),
114.4(2) (ArC), 101.3 (C-1), 74.5, 74.4, 69.6, 68.2, 63.6 (C-6), 55.6
(C6H4OCH3), 26.8(3) (C(CH3)3), 19.1 (C(CH3)3). HRMS calcd for
4.5. p-Methoxyphenyl 2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl-
(1?4)-6-O-acetyl-2,3-di-O-benzoyl-b- -galactopyranoside 8
D
Compound 7 (900 mg, 1.2 mmol) was dissolved in pyridine
(8 mL) followed by the addition of Ac2O (4 mL) and the solution
was allowed to stir at rt for 1 h. The solvents were evaporated
and the residue was purified by flash chromatography using n-hex-
ane–EtOAc (3:1) to give pure disaccharide 8 (880 mg, 93%) as a
C
43H49O9SiNa (M+Na)+: 755.2653; found 755.2650.
4.3. p-Methoxyphenyl 2,3,4-tri-O-acetyl- -rhamnopyranosyl-
a
-L
white foam.
½
a 2D5
ꢂ
¼ þ108 (c 1.1, CHCl3). 1H NMR (CDCl3,
(1?4)-2,3-di-O-benzoyl-6-O-tert-butyldiphenylsilyl-b-
D-galac-
500 MHz) d: 7.97–7.34 (m, 10H, ArH), 6.96, 6.78 (2d, J 9.0 Hz,
topyranoside 6
OC6H4OMe), 5.95 (dd, 1H, J1,2 8.0 Hz, J2,3 10.5 Hz, H-2), 5.49 (dd,
1H, J1 ,2 2.5 Hz, J2 ,3 3.0 Hz, H-20), 5.45 (dd, 1H, J2,3 10.5 Hz, J3,4
0
0
0
0
3.0 Hz, H-3), 5.39 (dd, 1H, J2 ,3 3.5 Hz, J3 ,4 8.5 Hz, H-30), 5.14 (d,
0
0
0
0
A mixture of the acceptor 4 (1.3 g, 1.8 mmol), donor 5 (925 mg,
2.3 mmol) and MS 4 Å (2.0 g) in dry CH2Cl2 (25 mL) was stirred un-
der an N2 atmosphere for 30 min. Next, NIS (675 mg, 3.0 mmol)
was added followed by H2SO4–silica (50 mg) and the mixture
was allowed to stir at rt for 45 min when TLC (n-hexane–EtOAc,
3:1) showed complete conversion of the acceptor. The mixture
was filtered through a pad of CeliteÒ and the filtrate was washed
successively with aq Na2S2O3 (2 ꢁ 30 mL), saturated aq NaHCO3
(2 ꢁ 30 mL) and H2O (30 mL). The organic layer was collected,
dried (Na2SO4) and filtered. The solvents were evaporated in vacuo
and the residue was purified by flash chromatography using n-hex-
ane–EtOAc (4:1) as eluent to afford pure disaccharide 6 (1.5 g, 85%)
1H, J1,2 8.0 Hz, H-1), 4.99 (t, 1H, J3 ,4 , J4 ,5 8.5 Hz, H-40), 4.88 (d,
0
0
0
0
1H, J1 ,2 2.5 Hz, H-10), 4.47 (dd, 1H, J5,6a 6.5 Hz, J6a,6b 11.0 Hz, H-
6a), 4.38 (br d, 1H, H-4), 4.26 (dd, 1H, J5,6b 6.0 Hz, J6a,6b 11.5 Hz,
H-6b), 4.07 (bt, 1H, H-5), 4.02 (m, 1H, H-50), 3.75 (s, 3H, C4H4OCH3),
2.11, 2.06, 1.95, 1.90 (4s, 12H, 4 ꢁ COCH3), 1.20 (d, 3H, J5’,6’ 6.0 Hz,
C–CH3). 13C NMR (CDCl3, 125 MHz) d: 170.4, 170.0, 169.5, 169.3
(4 ꢁ COCH3), 165.7, 164.9 (2 ꢁ COC6H5), 155.6, 151.1, 133.6,
133.1, 130(2), 129.7(2), 129.3, 128.6(2), 128.3(2), 118.8(2),
114.4(2) (ArC), 101.1 (C-1), 99.6 (C-10), 74.3, 73.5, 71.9, 70.9,
69.6, 69.3, 68.9, 67.7, 62.5, 55.6 (C6H4OCH3), 20.8, 20.7, 20.6(2)
(4 ꢁ COCH3), 17.3 (C–CH3). HRMS calcd for C41H44O17Na (M+Na)+:
831.2476; found 831.2474.
0
0
as a colourless gel. ½a D25
ꢂ
¼ þ97 (c 1.0, CHCl3). 1H NMR (CDCl3,
500 MHz) d: 7.94 (d, 4H, J 8.0 Hz, ArH), 7.70 (m, 4H, ArH), 7.51–
0
0
7.32 (m, 12H, ArH), 6.96, 6.70 (2d, 4H, ArH), 5.94 (dd, 1H, J3 ,4
4.6. p-Methoxyphenyl 2,3,4-tri-O-acetyl-
a-
L-rhamnopyranosyl-
9.5 Hz, J4 ,5 10.0 Hz, H-40), 5.46 (dd, 1H, J2 ,3 1.5 Hz, J3 ,4 9.5 Hz,
H-30), 5.44 (m, 1H, H-20), 5.35 (dd, 1H, J2,3 9.5 Hz, J3,4 3.5 Hz, H-3),
5.12 (d, 1H, J1,2 9.5 Hz, H-1), 4.89 (t, 1H, J1,2, J2,3 9.5 Hz, H-2), 4.85
(br s, 1H, H-10), 4.35 (br s, 1H, H-4), 4.08 (dd, 1H, J5,6a 8.0 Hz,
J6a,6b 11.5 Hz, H-6a), 3.89–3.46 (m, 2H, H-50, H-6b), 3.74 (m, 1H,
H-5), 3.71 (s, 3H, CH2–C6H5–OCH3), 2.02, 1.98, 1.94 (3s, 9H,
(1?4)-6-O-acetyl-2,3-di-O-benzoyl-b-
chloroacetimidate 9
D
-galactopyranosyl tri-
0
0
0
0
0
0
To a solution of compound 8 (850 mg, 1.1 mmol) in CH3CN–H2O
(9:1, 20 mL), CAN (1.2 g, 2.2 mmol) was added and the solution
was stirred at room temperature for 30 min after which TLC (n-
hexane–EtOAc, 2:1) showed complete conversion of the starting
3 ꢁ COCH3), 0.86 (d, 3H, J5 ,6 6.0 Hz, C–CH3). 13C NMR (CDCl3,
0
0

P. Verma et al. / Tetrahedron: Asymmetry 21 (2010) 2413–2418
2417
1H, J1 ,2 9.0 Hz, H-100), 4.85 (d, 1H, J1
9.0 Hz, H-1000), 4.54 (d,
00 00
000,2000
material to a slower moving spot. Solvents were evaporated in va-
cuo, and the residue was dissolved in CH2Cl2 (20 mL) and washed
with H2O (2 ꢁ 20 mL). The organic layer was separated, dried
(Na2SO4) and evaporated to a syrup. The crude hemiacetal thus ob-
tained was dissolved in dry CH2Cl2 (20 mL), after which CCl3CN
1H, J1,2 7.0 Hz, H-1), 4.33 (m, 2H, OCH2C„CH), 4.31 (br d, 1H, H-
40), 4.30–3.90 (m, 8H, H-3, H-5, H-6a, H-6b, H-6a0, H-6b0, H-6a000,
H-6b000), 3.76 (t, 1H, J3,4, J4,5 9.0 Hz, H-4), 3.72 (m, 1H, H-5000), 3.54
(m, 1H, H-500), 3.42 (m, 1H, H-50), 2.43 (t, 1H, J 1.5 Hz, OCH2C„CH),
2.10, 2.07, 2.06(2), 2.00, 1.98, 1.94, 1.93 (8 ꢁ COCH3), 1.15 (d, 3H, J
6.5 Hz, C–CH3). 13C NMR (CDCl3, 125 MHz) d: 170.8, 170.2, 170.1,
170.0, 169.6, 169.5, 169.4, 169.3 (8 ꢁ COCH3), 165.8, 164.9
(2 ꢁ COPh), 137.1, 133.6, 133.2, 130.1(2), 129.9(2), 129.4, 129.2,
128.5(2), 128.4(2), 128.3(2), 128.2, 126.0(2) (ArC), 101.3 (CHPh),
100.11 (C-1000), 99.8 (C-10), 99.2 (C-1), 98.9 (C-100), 80.8, 79.5,
78.6,77.5, 75.1, 73.9, 73.5, 73.0, 71.9, 71.8, 71.4, 71.0, 70.3, 69.5,
69.1, 68.8, 68.4, 67.7, 65.5, 62.6, 61.8, 60.4, 56.3, 29.7, 21.0, 20.8,
20.7(2), 20.6(2), 20.5, 20.3 (8 ꢁ COCH3), 17.4 (C–CH3). HRMS calcd
for C64H72O30Na (M+Na)+: 1343.4006; found 1343.4002.
(550 lL, mmol) was added followed by DBU (mL, mmol) and the
solution was stirred at room temperature for 2 h when TLC (n-hex-
ane–EtOAc, 3:1) showed complete conversion of the starting mate-
rial to a faster moving spot. The solvents were evaporated in vacuo
and the residue was purified by flash chromatography using 4:1
(n-hexane–EtOAc) as eluent to afford pure trichloroacetimidate
derivative 9 (755 mg, 81%) as a colourless gel. The material was
used directly for further glycosylation.
4.7. Propargyl 2,3,4,6-tetra-O-acetyl-b-
D-glucopyranosyl-(1?2)-
4,6-O-benzylidene-b- -glucopyranoside 12
D
4.9. Propargyl
syl-(1?3)-2-O-(b-
acid 1
a
-
L
-rhamnopyranosyl-(1?4)-b-
D-galactopyrano-
A mixture of acceptor 10 (1.3 g, 3.0 mmol), donor 11 (1.8 g,
3.9 mmol) and MS 4 Å (g) in dry CH2Cl2 (20 mL) was stirred under
nitrogen for 30 min at ꢀ30 °C. Next, NIS (1.1 g, 5.1 mmol) was
added followed by H2SO4–silica (50 mg) and the mixture was stir-
red for another 45 min when TLC (n-hexane–EtOAc, 3:1) showed
complete consumption of the acceptor. Then the temperature
was slowly raised to rt (1 h) to cleave the p-methoxybenzyl group.
The mixture was filtered through a pad of CeliteÒ and the filtrate
was washed successively with aq Na2S2O3 (2 ꢁ 25 mL), saturated
aq NaHCO3 (2 ꢁ 25 mL) and H2O (25 mL). The organic layer was
collected, dried (Na2SO4) and filtered. The solvents were evapo-
rated in vacuo and the residue was purified by flash chromatogra-
phy using n-hexane–EtOAc (3:1) as an eluent to afford pure
D
-glucopyranosyl)-b- -glucopyranosiduronic
D
A solution of compound 13 (850 mg, 0.65 mmol) in 80% aq
AcOH (15 mL) was stirred at 80 °C for 2 h until TLC (n-hexane–
EtOAc, 3:1) showed complete conversion of the starting material
to a slower moving spot. The solvents were evaporated in vacuo
and the syrupy residue was dissolved in 15 mL CH2Cl2. To this solu-
tion, 3 mL of H2O was added followed by aq NaBr (1 M 0.3 mL), aq
tetrabutylammonium bromide (1 M, 0.6 mL), TEMPO (27 mg) and
saturated aq NaHCO3 (1.6 mL) at 0 °C. To the resulting mixture,
aq NaOCl (2.0 mL) was added and the mixture was allowed to stir
for 1.5 h when the temperature was raised to room temperature. At
this point TLC showed complete conversion of the starting material
to a faster moving spot, presumably the corresponding aldehyde
derivative. The mixture was neutralized with 1 M HCl (as required)
to keep the pH of the mixture at 6–7. Then tert-butanol (9 mL),
NaOCl2 (600 mg in 2.3 mL H2O) and NaH2PO4 (750 mg in 6 mL
H2O) were added and the mixture was allowed to stir at room tem-
perature for another 4 h when TLC showed complete conversion.
The mixture was diluted with saturated NaH2PO4 and the product
was extracted with EtOAc. The organic layer was dried (Na2SO4)
and evaporated. The crude product thus obtained was purified by
flash chromatography using n-hexane–EtOAc (1:1) to n-hexane–
EtOAc (1:4) to afford compound 14 (625 mg, 78%). To a solution
of compound 14 (300 mg, 0.24 mmol) in dry MeOH (10 mL),
NaOMe in MeOH (0.5 M, 1 mL) was added and the solution was
stirred at room temperature for 6 h. Excess NaOMe was neutralized
with DOWEX 50 W H+ resin, filtered through a cotton plug and the
filtrate was evaporated to give pure target tetrasaccharide 1
disaccharide 12 (1.6 g, 83%) as a white foam. ½a D25
¼ þ98 (c 1.0,
ꢂ
CHCl3). 1H NMR (CDCl3, 500 MHz) d: 7.46–7.33 (m, 5H, ArH), 5.52
(s, 1H, CHPh), 5.21 (t, 1H, J2 ,3 , J3 ,4 9.5 Hz, H-30), 5.11 (t, 1H, J3 ,4
,
0
0
0
0
0
0
J4 ,5 9.5 Hz, H-40), 5.03 (dd, 1H, J1 ,2 8.0 Hz, J2 ,3 9.5 Hz, H-20), 4.86
0
0
0
0
0
0
(d, 1H, J1 ,2 8.0 Hz, H-10), 4.68 (d, 1H, J1,2 8.0 Hz, H-1), 4.46, 4.34
(2dd, J 2.5 Hz, 15.0 Hz, O–CH2–C„CH), 4.32 (m, 1H, H-6ª), 4.27
0
0
(dd, 1H, J5 ,6a 3.0 Hz, J6a ,6b 11.5 Hz, H-6a0), 4.21 (dd, 1H, J5 ,6b
0
0
0
0
0
0
4.5 Hz, J6a ,6b 11.5 Hz, H-6b0), 3.91 (m, 1H, H-3), 3.78 (m, 1H, H-
50), 3.75 (t, 1H, J3,4, J4,5 9.5 Hz, H-4), 3.53–3.44 (m, 3H, H-2, H-5,
H-6b), 3.14 (br d, 1H, J2,OH 2.5 Hz, OH), 2.51 (t, 1H, J 2.5 Hz, O–
CH2–C„CH), 2.08, 2.04, 2.02, 1.98 (4s, 12H, 4 ꢁ COCH3). 13C NMR
(CDCl3, 125 MHz) d: 170.7, 170.5, 170.2, 169.4 (4 ꢁ COCH3),
136.8, 129.2, 128.2(2), 126.2(2) (ArC), 101.7 (CHPh), 101.3 (C-1),
100.9 (C-10), 83.5, 79.7, 78.4, 75.5, 72.6, 72.0(2), 71.9, 68.5, 68.1,
66.0, 62.0, 53.4, 20.8, 20.7, 20.6, 20.5 (4 ꢁ COCH3). HRMS calcd
for C30H36O15Na (M+Na)+: 659.1952; found 659.1957.
0
0
(150 mg, 89%) as an amorphous white solid. ½a D25
ꢂ
¼ þ63 (c 0.7,
4.8. Propargyl 2,3,4-tri-O-acetyl-
O-acetyl-2,3-di-O-benzoyl-b- -galactopyranosyl-(1?3)-4,6-O-
benzylidene-2-O-(2,3,4,6-tetra-O-acetyl-b- -glucopyranosyl)-b-
-glucopyranoside 13
a-L-rhamnopyranosyl-(1?4)-6-
H2O). 1H NMR (D2O, 500 MHz) d: 5.09 (d, 1H, J1 ,2 1.5 Hz, H-100),
D
00 00
4.74 (d, 1H, J1 ,2 7.5 Hz, H-10), 4.70 (m, 2H, H-1, H-1000), 4.39 (m,
D
0
0
2H, OCH2C„CH), 4.02 (dd, 1H, J1 ,2 1.5 Hz, J2 ,3 3.0 Hz, H-200),
3.95 (br d, 1H, H-40), 3.91–3.36 (m, 16H, H-2, H-20, H-2000, H-3, H-
30, H-300, H-3000, H-4, H-400, H-5, H-500, H-5000, H-6a0, H-6b0, H-6a000, H-
D
00 00
00 00
A mixture of donor 9 (750 mg, 0.9 mmol), acceptor 12 (480 mg,
6b000), 3.34 (t, 1H, J3 , J4
8.5 Hz, H-4000), 3.26 (m, 1H, H-50),
0.75 mmol) and MS 4 Å (1.0 g) in dry CH2Cl2 (15 mL) was stirred
under N2 for 30 min. Next, H2SO4–silica (30 mg) was added and
the solution was stirred for 45 min at 0 °C when TLC (n-hexane–
EtOAc) showed complete conversion of the acceptor. The mixture
was neutralized with Et3N and filtered through a pad of CeliteÒ.
The filtrate was evaporated and the crude product was purified
by flash chromatography using n-hexane–EtOAc (4:1) to afford
000,4000
000,5000
2.85 (t, 1H, J 2.0 Hz, OCH2C„CH), 1.19 (d, 3H, J 6.5 Hz, C–CH3).
13C NMR (D2O, 125 MHz) d: 173.0 (COOH), 102.9 (C-10), 102.4 (C-
1000), 101.8 (C-1), 99.7 (C-100), 83.0, 79.6, 78.5, 76.7, 76.0, 75.7,
75.6, 75.4, 73.6, 73.3, 71.7, 71.0, 70.2, 70.0, 69.5, 69.3, 61.5, 60.6,
56.9, 48.9, 23.1, 16.5 (C–CH3). HRMS calcd for C27H42O21Na
(M+Na)+: 725.2116; found 725.2113.
pure tetrasaccharide 13 (865 mg, 87%) as
a white foam.
½
a 2D5
ꢂ
¼ þ79 (c 0.9, CHCl3). 1H NMR (CDCl3, 500 MHz) d: 8.05–7.32
(m, 15H, ArH), 5.68 (dd, 1H, J3 ,4 9.5 Hz, J4 ,5 10.0 Hz, H-400), 5.59
(s, 1H, CHPh), 5.49 (m, 1H, H-200), 5.32 (m, 3H, H-200, H-30, H-300),
00 00
00 00
Acknowledgements
000,4000
000,5000
P.V. is thankful to the UGC, New Delhi, India, R.R. is thankful to
the IISER-Kolkata and B.R. is thankful to the CSIR, New Delhi, India
5.20 (t, 1H, J3 , J4
9.5 Hz, H-4000), 5.08 (m, 2H, H-2000, H-3000),
4.94 (t, 1H, J1 ,2 , J2 ,3 8.5 Hz, H-20), 4.87 (br s, 1H, H-100), 4.86 (d,
0
0
0
0

2418
P. Verma et al. / Tetrahedron: Asymmetry 21 (2010) 2413–2418
11. Williams, J. M.; Richardson, A. C. Tetrahedron 1967, 23, 1369.
12. Gent, P. A.; Gigg, R. J. Chem. Soc., Perkin Trans. 1 1974, 1446–1455.
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14. Mukhopadhyay, B.; Kartha, K. P. R.; Russell, D. A.; Field, R. A. J. Org. Chem. 2004,
69, 7758–7760.
for fellowship. The work is funded by the DST, New Delhi, India
through grant SR/S1/OC-67/2009.
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