Synthesis of the trisaccharide moiety and a cholesteryl analog of phyteumosides

Article abstract of DOI:10.1039/c3ra47523a

A first synthesis of the trisaccharide moiety of the phyteumosides and its cholesteryl analog is described using easily accessible and regioselectively protected d-glucose, l-rhamnose and d-galactose building blocks via a linear glycosylation approach. Th

Full text of DOI:10.1039/c3ra47523a

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Synthesis of the trisaccharide moiety and a
cholesteryl analog of phyteumosides†
Cite this: RSC Adv., 2014, 4, 7611
*
Sanjoy Adak, Madhu Emmadi and Suvarn S. Kulkarni
A first synthesis of the trisaccharide moiety of the phyteumosides and its cholesteryl analog is described
using easily accessible and regioselectively protected D-glucose, L-rhamnose and D-galactose building
blocks via a linear glycosylation approach. The two glycosyl donors were prepared as thiophenyl (SPh)
glycosides. Trichloroacetimidate (TCAI) coupling was employed for the first glycosylation step while
thioglycoside activation was used for the second glycosylation to assemble the O-allyl trisaccharide.
After removal of the anomeric allyl group, the trisaccharide was converted into a TCAI donor. Using
cholesterol as an acceptor in combination with acetonitrile as a participating solvent to achieve
b-selectivity in glycosylation, a phyteumoside analog was obtained.
Received 11th December 2013
Accepted 7th January 2014
DOI: 10.1039/c3ra47523a
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Cu(OTf)2 (ref. 3) whereas compound 5 (ref. 4) was synthesized
from similarly obtained D-glucosyl thioglycoside by 4,6-O-ben-
zylidenation,5 regioselective silylation at O3,6 and followed by
acetylation at O2. Compound 6 was accessed from the corre-
sponding O-allyl galactoside7 via regioselective 3,4-O-iso-
propylidenation8 and O6 silylation in good yields.
Coupling of donor 5 with acceptor 6 to synthesize disac-
charide 7 was performed next (Scheme 2). However, to our
dismay, no coupling product could be obtained through thio-
glycoside activation under various conditions (NIS/TfOH, NIS/
TMSOTf, conversion to glycosyl bromide and activation with
Introduction
Saponins (glycosides of steroids and terpenes) play key roles as
secondary metabolites in plants and marine organisms.1 In
2011, Potterat and co-workers isolated and characterized two
novel triterpene glycosides phyteumosides A (1a) and B (1b)
from Phyteuma orbiculare (Fig. 1).2 Phyteumosides A and B
comprise a trisaccharide moiety glycosidically b-O-linked to the
unique triterpene aglycons. This trisaccharide is also a part of
the tetrasaccharide glycone of macrophyllicin, a saponin iso-
lated from Primula macrophylla in 1993.2b No synthetic studies
on the trisaccharide moiety or 1a and 1b are reported till date.
Herein, we report a rst synthesis of the trisaccharide moiety
and a cholesteryl analog 1c of the phyteumosides.
AgOTf). So, the thioglycoside
5 was converted to tri-
chloroacetimidate,9 which upon activation with TMSOTf in the
presence of acceptor 6 delivered the desired b-linked disac-
charide 7 in reasonable yields (51% from 5, over 3 steps).
Aer the successful synthesis of disaccharide 7, the 20-O-
acetate was removed using NaOMe in MeOH at RT to afford the
desired disaccharide acceptor 8a in 55% yield, along with a side
Results and discussions
We envisaged that the saponin analog 1c (Scheme 1) could be
assembled by glycosylation of TCAI analog of trisaccharide 2
with cholesterol 3 using solvent effect to ensure the b-selectivity
in lieu of a 2-O- participating group. O-Allyl trisaccharide 2 could
in turn be synthesized by stereoselective coupling of appropri-
ately protected monosaccharide building blocks 4, 5, and 6,
from the reducing end to non-reducing end.
The building blocks 4, 5 and 6 were synthesized following
reported procedures. Thus, compound 4 was obtained from
L-rhamnose by a one-pot per-O-acetylation followed by nulceo-
philic displacement of anomeric acetate with thiophenol using
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai,
400076, India. E-mail: suvarn@chem.iitb.ac.in; Fax: +91-22-2576-7152; Tel: +91-22-
2576-7166
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data for all new compounds, and copies of 1H, 13C and 2D NMR
spectra. See DOI: 10.1039/c3ra47523a
Fig.
orbiculare.
1 Structure of the triterpenoid saponins from Phyteuma
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Scheme 3 Proposed mechanism of silyl group migration.
Scheme 1 Retrosynthesis of phyteumoside analog 1c.
present study, compound 8b is nevertheless an advanced
building block which could be advantageously utilized as a
glycosyl acceptor in the synthesis of related complex carbohy-
drates (Scheme 3).
product 8b (40%) resulting from the concomitant migration of
TBDMS group from O30 to O20 under the prevailing basic
conditions. Although not very common, such silyl group
migrations have been well documented in literature.10,11 We
propose that this O30 to O20 silyl migration probably proceeds in
a concerted manner through an intramolecular attack of the O20
alkoxide II on O30-siliyl group to give a transient ve membered
silyl intermediate III which subsequently opens on the other
side to give alkoxide IV (Scheme 3).10,11e Since alkoxides II and IV
have comparable thermodynamic stability, these species
remain in equilibrium with each other and one obtains a
mixture of 8a and 8b. Compounds 8a and 8b were easily sepa-
rated by column chromatography and their structures were
unambiguously conrmed by analyzing their 2D NMR spectra.
With the desired acceptor 8a in hand we went ahead to
synthesize the trisaccharide moiety of phyteumoside. Acceptor
8a was glycosylated with thioglycoside donor 4 using NIS and
TMSOTf promoter12 system to afford the trisaccharide 2 in 53%
yield (Scheme 4). The modest yield could be attributed to
simultaneous partial activation of the allyl double bond in 8a or
2 by NIS. For the global deprotection, trisaccharide 2 was rst
treated with TBAF,13 followed by 80% AcOH14 to sequentially
remove TBDMS and acetal groups, respectively. The so obtained
polyol was treated with acetic anhydride and pyridine to obtain
the per-O-acetylated trisaccharide 9 in 78% yield over 3 steps.
For the preparation of phyteumoside analog 1c (Scheme 5),
the allyl group in trisaccharide 2 was oxidatively removed by
treatment with OsO4, NMO and NaIO4 (ref. 15) to afford hemi-
acetal 10 (a : b ¼ 0.6 : 1) in 79% yield. Compound 10 was con-
verted to the corresponding imidate by treating with
trichloroacetonitrile and the so formed imidate was coupled
with cholesterol 3, in acetonitrile as participating solvent,16 to
afford exclusively the b-linked phyteumoside analog 11 in 45%
1
The H–1H COSY spectrum of 8b clearly showed correlation of
the OH proton with the C30-H (ESI†).
All attempts to suppress the formation of the migration by-
product 8b were unsuccessful. In order to avoid such migration,
we also tried the reaction on the disaccharide having OBz at 20
position. Despite applying several deacylation reaction condi-
tions (NaOMe in MeOH at rt, NaOMe in MeOH under reux
condition, aqueous NaOH solution) we were not able to cleave
the OBz group. Although, not useful from the point of view of
Scheme 2 Synthesis of disaccharide acceptor 8a.
7612 | RSC Adv., 2014, 4, 7611–7616
Scheme 4 Synthesis of trisaccharide with the linker 9.
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measured at 589 nm (Na) and 25 ꢀC/20 ꢀC. IR spectra were
recorded on an FT-IR spectrometer using CsCl plates.
Allyl 2-O-acetyl-4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-
b-D-glucopyranosyl-(1/2)-6-O-tert-butyldimethylsilyl-3,4-O-
isopropylidene-a-D-galactopyranoside (7)
To a cooled solution of 5 (2.37 g, 4.59 mmol) in THF and water
(40 mL, 4 : 1) at 0 ꢀC, was slowly added NBS (2.45 g,
13.77 mmol). Aer stirring at room temperature for 15 min, the
reaction mixture was diluted with EtOAc (80 mL) and washed
with aq. NaHCO3. Separated organic layer dried over Na2SO4,
ltered and concentrated. The crude product which was
obtained aer solvents removal was dissolved in CH2Cl2
(43 mL). To this clear solution K2CO3 (1.9 g), and NCCCl3 (2.12
mL) were added and stirred at ambient temperature overnight.
The reaction mixture was ltered through celite pad. The ltrate
was concentrated in vacuo and the crude product was used for
the next reaction. To the residue which was obtained aer
Scheme 5 Synthesis of saponin analog 12.
yields over 2 steps. Deprotection of TBDMS and acetal groups
followed by acetylation under similar conditions afforded the
saponin analog 12 as a per-O-acetate derivative in good yields.
Treatment of 12 with NaOMe in MeOH at RT afforded 1c in 86%
yield.
˚
solvents removal was added acceptor 6 (1.03 g, 2.75 mmol), 4 A
MS and CH2Cl2 (30 mL). The resulted turbid was stirred at room
ꢀ
ꢀ
temperature for 30 min and brought to ꢁ15 C. To this cooled
solution TMSOTf (58 mL) was added at ꢁ15 C and stirred for
1.5 h at the same temperature. Aer complete consumption of
starting material reaction mixture was quenched by adding
Et3N. The reaction mixture was ltered through celite and
washed with CH2Cl2. The ltrate was concentrated to give a
residue that was puried by silica gel column chromatography
(petroleum ether : EtOAc ¼ 8 : 1) to afford 7 (1.81 g, 51% over
three steps from 5) as a white foam: [a]2D5 20.3 (c 1.0, CHCl3); 1H
NMR (400 MHz, CDCl3) d 7.48–7.41 (m, 2H, ArH), 7.36–7.32 (m,
3H, ArH), 5.93–5.83 (m, 1H, HC]CH2), 5.51 (s, 1H, benzyli-
dene), 5.30 (dd, J ¼ 17.2, 1.6 Hz, 1H, HC]CH2), 5.18 (dd, J ¼
10.4, 1.6 Hz, 1H, HC]CH2), 4.96 (t, J ¼ 8.6 Hz, 1H, H-20), 4.87 (d,
J ¼ 3.4 Hz, 1H, H-1), 4.76 (d, J ¼ 8.6 Hz, 1H, H-10), 4.28-4.22 (m,
2H, H-3 & H-6a0), 4.19–4.11 (m, 2H, H-6a & H2C]CH2), 4.05–
3.97 (m, 2H, H-6b & H2C]CH2), 3.90–3.83 (m, 2H, H-30 & H-5),
3.79–3.73 (m, 3H, H-2, H-4, H-6b0), 3.52 (t, J ¼ 8.6, 1H, H-40),
3.41–3.35 (m, 1H, H-50), 2.08 (s, 3H, CH3), 1.49 (s, 3H, CH3), 1.31
(s, 3H, CH3), 0.89 (s, 9H, (CH3)3C), 0.81 (s, 9H, (CH3)3C), 0.07 (s,
6H, 2CH3), 0.02 (s, 3H, CH3), ꢁ0.02 (s, 3H, CH3); 13C NMR (100
MHz, CDCl3) d 169.6, 137.2, 133.9, 129.1, 128.3, 126.3, 117.5,
109.0, 102.0, 101.9, 97.1, 81.6, 78.1, 76.8, 75.5, 74.6, 73.5, 72.7,
68.8, 68.6, 68.2, 66.4, 62.4, 29.8, 28.5, 26.7, 25.9, 25.7, 21.2, 18.4,
18.1, ꢁ4.0, ꢁ4.8, ꢁ5.1, ꢁ5.2. HRMS calcd for C39H64NaO12Si2 [M
+ Na]+ 803.3829, found 803.3837.
All the new compounds were thoroughly characterized using
spectral means. A typical NMR characterization sequence
involved 1H, 13C, HMQC and 1H–1H COSY analysis to unam-
biguously assign all the sugar protons (see ESI†).
Conclusions
In summary, we have synthesized the trisaccharide moiety and
a cholesteryl analog of phyteumosides for the rst time, by
using easily accessible monosaccharide building blocks. The
trisaccharide derivative 10 can be coupled with a variety of
aglycons to prepare phyteumoside A and B and various other
analogs for biological studies.
Experimental
All reactions were conducted under a dry nitrogen atmosphere.
Solvents (CH2Cl2 >99%, THF 99.5%, acetonitrile 99.8%, DMF
99.5%) were purchased in capped bottles and dried under
sodium or CaH2. All other solvents and reagents were used
without further purication. All glassware was oven-dried
before use. TLC was performed on precoated aluminum plates
of silica gel 60 F254 (0.25 mm, E. Merck). Developed TLC plates
were visualized under a short-wave UV lamp and by heating
plates that were dipped in ammonium molybdate/cerium(IV)
sulfate solution. Silica gel column chromatography was per-
Allyl 4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-b-D-
glucopyranosyl-(1/2)-6-O-tert-butyldimethylsilyl-3,4-O-
isopropylidene-a-D-galactopyranoside (8a)
formed using silica gel (100–200 mesh) and employed a solvent To a stirred solution of disaccharide 7 (1.5 g, 1.92 mmol) in dry
polarity correlated with TLC mobility. NMR experiments were MeOH (9 ml) was added NaOMe (0.24 g) and the mixture was
conducted on 400 and 500 MHz instrument using CDCl3 (D, stirred at room temperature for 2 h. Aer complete consump-
99.8%) CD3OD (D, 99.8%) as solvents. Chemical shis are tion of starting material, it was neutralized by addition of Dowex
relative to the deuterated solvent peaks and are in parts per resin. Then the reaction mixture was ltered and concentrated
million (ppm). Mass spectra were acquired in the ESI mode under reduced pressure. The residue was puried by column
using Q-TOF analyzer. Specic rotation experiments were chromatography (petroleum ether : EtOAc ¼ 6 : 1) to afford
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compound 8a as a white foam (0.77 g, 55% yield): [a]D25 20.8 (c Hz, 1H, HC]CH2), 5.05 (t, J ¼ 10.0 Hz, 1H, H-400), 4.93 (d, J ¼
1.0, CHCl3); 1H NMR (400 MHz, CDCl3) d 7.49–7.42 (m, 2H, 3.3 Hz, 1H, H-1), 4.74 (d, J ¼ 7.8 Hz, 1H, H-10), 4.61–4.58 (m, 1H,
ArH), 7.37–7.31 (m, 3H, ArH), 5.95–5.85 (m, 1H, HC/CH2), 5.50 H-500), 4.3–4.12 (m, 4H, H-6a, H-6b0, H-6b & CH2), 4.03–3.96 (m,
(s, 1H, benzylidene), 5.31 (dd, J ¼ 17.2, 1.5 Hz, 1H, HC/CH2), 3H, H-3, H-30 & CH2), 3.88–3.66 (m, 5H, H-20, H-2, H-4, H-5 & H-
5.20 (dd, J ¼ 10.3, 1.5 Hz, 1H, HC/CH2), 4.94 (d, J ¼ 3.4 Hz, 1H, 6a0), 3.43–3.34 (m, 2H, H-40 & H-50), 2.10 (s, 3H, CH3), 2.01 (s, 3H,
H-1), 4.65 (d, J ¼ 7.8 Hz, 1H, H-10), 4.40 (dd, J ¼ 8.2, 5.2 Hz, 1H, CH3), 1.95 (s, 3H, CH3), 1.54 (s, 3H, CH3), 1.31 (s, 3H, CH3), 1.18
H-6a0), 4.28–4.15 (m, 3H, H-3, H-6a & CH2), 4.06–4.00 (m, 2H, (d, J ¼ 6.2 Hz, 3H, CH3), 0.89 (s, 9H, (CH3)3C), 0.72 (s, 9H,
CH2 & H-6b), 3.88–3.65 (m, 5H, H-30, H-2, H-4, H-5 & H-6b0), (CH3)3C), 0.07 (s, 6H, 2 CH3), ꢁ0.07 (s, 3H, CH3), ꢁ0.09 (s, 3H,
3.55–3.44 (m, 2H, H-20 & H-40), 3.40–3.34 (m, 1H, H-50), 1.51 (s, CH3); 13C NMR (100 MHz, CDCl3) d 170.1, 170.0, 169.8, 137.0,
3H, CH3), 1.33 (s, 3H, CH3), 0.90 (s, 9H, (CH3)3C), 0.87 (s, 9H, 134.0, 129.4, 128.3, 126.6, 117.5, 109.2, 103.0, 102.6, 98.1, 97.5,
(CH3)3C), 0.10 (s, 3H, CH3), 0.08 (s, 6H, 2CH3), 0.04 (s, 3H, CH3); 81.4, 78.9, 77.3, 76.4, 74.9, 73.7, 71.2, 69.7, 68.8, 68.4, 68.3, 66.5,
13C NMR (100 MHz, CDCl3) d 137.4, 133.8, 129.1, 128.3, 126.3, 66.0, 62.5, 28.5, 26.5, 26.0, 25.9, 21.0, 20.98, 20.93, 18.4, ꢁ3.7,
118.0, 109.3, 104.2, 101.7, 96.9, 81.5, 77.9, 76.9, 75.5, 75.4, 74.4, ꢁ4.3, ꢁ5.2, ꢁ5.3; HRMS calcd for C49H78NaO18Si2 [M + Na]+
73.6, 68.9, 68.4, 68.3, 66.6, 62.4, 28.5, 26.6, 26.1, 25.9, 18.5, 18.4, 1033.4619, found 1033.4619.
ꢁ4.2, ꢁ4.5, ꢁ5.1, ꢁ5.3; HRMS calcd for C37H62NaO11Si2 [M +
Na]+ 761.3723, found 761.3721.
Allyl 2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl (1/2)-3,4,6-tri-O-
acetyl-b-D-glucopyranosyl-(1/2)-3,4,6-tri-O-acetyl-a-D-
Allyl 4,6-O-benzylidene-2-O-tert-butyldimethylsilyl-b-D-
glucopyranosyl-(1/2)-6-O-tert-butyldimethylsilyl-3,4-O-
isopropylidene-a-D-galactopyranoside (8b)
galactopyranoside (9)
TBAF (370 mL of 1 M solution in THF) was added to a stirred
solution of the trisaccharide 2 (0.14 g, 0.013 mmol) in THF
(0.56 g, 40% yield): [a]2D0 + 11.6 (c 2.0, CHCl3); 1H NMR (2.5 mL). The mixture was stirred at rt for 4 h, then evaporated
(400 MHz, CDCl3) d 7.49–7.47 (m, 2H, ArH), 7.37–7.34 (m, 3H, to dryness under reduced pressure. Aer complete removal of
ArH), 5.94–5.85 (m, 1H, HC ]CH2), 5.51 (s, 1H, benzylidene), solvents, crude product was dissolved in 80% AcOH (13 mL),
5.29 (dd, J ¼ 17.2, 1.5 Hz, 1H, HC]CH2), 5.23 (dd, J ¼ 10.3, and the solution was heated at 80 ꢀC for 3 h. The reaction
1.5 Hz, 1H, HC ¼ CH2), 4.39 (dd, J ¼ 8.4, 5.2 Hz, 1H, H-6a), 4.29– mixture was then concentrated, and the resulting compound
4.12 (m, 3H, H-3, H-6a0 & CH2), 4.07–3.93 (m, 4H, H-2, H-5, H-6b was dissolved in 2 : 1 pyridine : Ac2O (12 mL). Aer stirring for
& CH2), 3.90–3.71 (m, 3H, H-30, H-6b0 & H-5), 3.57–3.47(m, 3H, 16 h, the reaction mixture was concentrated and the product
H-20, H-40 & H-50), 2.51 (brs, 1H, 30–OH) 1.49 (s, 3H, CH3), 1.33 (s, was puried by silica gel chromatography (40% ethyl acetate in
3H, CH3), 0.93 (s, 9H, (CH3)3C), 0.91 (s, 9H, (CH3)3C), 0.19 (s, pet ether) to give compound 9 (97 mg, 78% Over 3 steps): [a]D25
1
3H, CH3), 0.15 (s, 3H, CH3), 0.09 (s, 6H, 2CH3); 13C NMR 26.2 (c 1.0, CHCl3); H NMR (400 MHz, CDCl3) d 5.88–5.79 (m,
(100 MHz, CDCl3) d 137.2, 134.0, 129.2, 128.4, 126.3, 117.4, 1H, HC]CH2), 5.47 (d, J ¼ 1.5 Hz, 1H, H-4), 5.29 (d, J ¼ 17.2 Hz,
108.9, 102.0, 101.8, 97.5, 80.7, 76.0, 74.8, 74.6, 73.6, 68.9, 68.6, 1H, HC]CH2), 5.20 (t, J ¼ 10.0 Hz, 1H, H-30), 5.14 (d, J ¼
68.2, 66.1, 62.4, 28.3, 26.7, 26.1, 25.9, 18.4, 18.3, ꢁ4.0, ꢁ4.6, 12.0 Hz, 1H, HC]CH2), 5.09–5.04 (m, 2H, H-3, H-300), 4.99–4.89
ꢁ5.2, ꢁ5.3; HRMS calcd for C37H62NaO11Si2 [M + Na]+ 761.3723, (m, 4H, H-1, H-200, H-40 & H-400), 4.83 (s, 1H, H-100), 4.56 (d, J ¼
found 761.3729.
7.6 Hz, 1H, H-10), 4.22–4.10 (m, 4H, H-5, H-6a0, H-6b0, CH2),
4.07–4.0 (m, 3H, H-6a, H-6b & CH2), 3.96–3.87 (m, 2H, H-2 & H-
500), 3.70–3.63 (m, 2H, H-20 & H-50), 2.11 (s, 3H, CH3), 2.07 (s, 6H,
2CH3), 2.03 (s, 9H, 3 CH3), 2.02 (s, 3H, CH3), 1.98 (s, 3H, CH3),
1.97 (s, 3H, CH3), 1.11 (d, J ¼ 6.0 Hz, 3H, CH3); 13C NMR
(100 MHz, CDCl3) d 170.6, 170.5, 170.4, 170.3, 170.2, 170.1,
169.8, 169.7, 133.6, 117.3, 101.7, 98.4, 98.1, 76.6, 74.4, 73.2, 71.7,
69.3, 69.0, 68.9, 68.3, 68.2, 66.9, 66.2, 61.8, 61.7, 20.9, 20.87,
20.81, 20.78, 20.74, 20.71, 20.6, 16.8; HRMS calcd for C39H54O24
[M + Na]+ 929.2897, found 929.2886.
Allyl 2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl (1/2)-4,6-O-
benzylidene-3-O-tert-butyldimethylsilyl-b-D-glucopyranosyl-
(1/2)-6-O-tert-butyldimethylsilyl-3,4-O-isopropylidene-a-D-
galactopyranoside (2)
Acceptor 8a (21 mg, 0.028 mmol) and donor 4 (13 mg,
0.035 mmol) were combined and co-evaporated with toluene
(3 ꢂ 5 ml) and dried under high vacuum for 2 h. The resulting
residue was dissolved in anhydrous CH2Cl2 (1.0 mL) and stirred
over activated molecular sieves for 30 min. The anhydrous
mixture was cooled to 0 ꢀC, NIS (10 mg, 1.23 mmol) and TMSOTf
(2 mL) were added and the mixture was stirred at 0 ꢀC for 90 min.
Aer complete consumption of starting material, it was
quenched by adding triethylamine. The mixture was ltered
2,3,4-Tri-O-acetyl-a-L-rhamnopyranosyl (1/2)-4,6-O-
benzylidene-3-O-tert-butyldimethylsilyl-b-D-glucopyranosyl
(1/2)-6-O-tert-butyldimethylsilyl-3,4-O-isopropylidene-a/b-D-
galactopyranose (10)
and washed with CH2Cl2. The ltrate was concentrated to give a To a solution of 2 (0.36 g, 0.35 mmol) in dioxane (4 ml) and
residue that was puried by silica gel column chromatography water (0.4 ml) was added a mixture of 4-methylmorpholine
(petroleum ether : EtOAc ¼ 8 : 1) to afford 2 as white foam N-oxide (124 mg, 1.0 mmol) and OsO4 (0.1 mL, 100 mg mLꢁ1
(15 mg, 53%): [a]2D5 ꢁ8.6 (c 1.0, CHCl3); 1H NMR (400 MHz, solution in t-BuOH) followed by addition of a suspension of
CDCl3) d 7.44–7.40 (m, 2H, ArH), 7.36–7.30 (m, 3H, ArH), 5.91– NaIO4 (227 mg, 1.0 mmol) in water (1 mL). The mixture was
5.83 (m, 1H, HC]CH2), 5.41–5.39 (m, 2H, H-100 & benzylidene), allowed to stir at 60 ꢀC for 16 h and then diluted with brine and
5.32–5.25 (m, 3H, H-200, H-300 & HC]CH2), 5.15 (dd, J ¼ 10.0, 1.3 extracted with CH2Cl2. Separated organic layer dried over
7614 | RSC Adv., 2014, 4, 7611–7616
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Na2SO4, ltered and concentrated. The obtained residue was
puried by column chromatography on silica gel with petro-
leum ether–ethyl acetate (4 : 1) to give the compound 10 as a
foam (0.28 g 79%): [a]D25 ꢁ27.8 (c 1.0, CHCl3); 1H NMR (400 MHz,
CDCl3) d 7.43–7.40 (m, 2H, ArH), 7.36–7.26 (m, 3H, ArH), 5.40–
5.39 (m, 2H), 5.34–5.28 (m, 2H), 5.08–5.02 (m, 1H), 4.74 (d, J ¼
7.8 Hz, 1H), 4.54–4.15 (m, 5H), 4.03–3.95 (m, 1H), 3.89–3.64 (m,
6H), 3.46–3.41 (m, 2H), 2.11 (s, 3H, CH3), 2.02 (s, 3H, CH3), 1.95
(s, 3H, CH3), 1.54 (s, 3H, CH3), 1.31 (s, 3H, CH3), 1.18 (d, J ¼
6.2 Hz, 3H, CH3), 0.89 (s, 9H, (CH3)3C), 0.72 (s, 9H, (CH3)3C),
0.07 (s, 6H, 2CH3), ꢁ0.07 (s, 3H, CH3), ꢁ0.09 (s, 3H, CH3); 13C
NMR (100 MHz, CDCl3) d 170.1, 170.0, 169.8, 137.0, 134.0, 129.4,
128.3, 126.6, 117.5, 109.2, 103.0, 102.6, 98.1, 97.5, 81.4, 78.9,
76.8, 76.4, 74.9, 73.7, 71.2, 69.7, 68.8, 68.4, 68.3, 66.5, 66.0, 62.5,
28.5, 26.5, 26.0, 25.9, 21.0, 20.98, 20.93, 18.4, ꢁ3.8, ꢁ4.5, ꢁ5.3,
ꢁ5.4; HRMS calcd for C46H74O18Si2 [M + Na]+ 993.4306, found
993.4283.
Cholesteryl 2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl-(1/2)-
3,4,6-tri-O-acetyl-b-D-glucopyranosyl-(1/2)-3,4,6-tri-O-acetyl-
b-D-galactopyranoside (12)
TBAF (75 mL of 1 M solution in THF) was added to a stirred
solution of 11 (37 mg, 0.027 mmol) in THF (0.5 mL). The
mixture was stirred at room temperature for 4 h, then evapo-
rated to dryness under reduced pressure. Aer that, the crude
product was dissolved in 80% AcOH in H2O (3 mL), and the
solution was heated at 80 ꢀC for 3 h. Then the reaction mixture
was concentrated, and the resulting white solid was dissolved
in 2 : 1 pyridine : Ac2O (3 mL). Aer stirring for 16 h, the
reaction mixture was concentrated and the product was puri-
ed by silica gel chromatography (40% ethyl acetate in pet
ether) to give compound 12 as a white solid (26 mg, 76%): [a]D25
1
ꢁ5.1 (c 1.0, CHCl3); H NMR (400 MHz, CDCl3) d 5.42 (d, J ¼
2.8 Hz, 1H, H-4), 5.36 (m, 1H, HC]), 5.20 (t, J ¼ 9.2 Hz, 1H, H-
30), 5.10 (dd, J ¼ 10.0, 3.4 Hz, 1H, H-300), 5.05–4.97 (m, 3H, H-
200, H-40 & H-400), 4.90 (d, J ¼ 1.3 Hz, 1H, H-100), 4.77 (dd, J ¼
10.3, 3.2 Hz, 1H, H-3), 4.65 (d, J ¼ 7.4 Hz, 1H, H-10), 4.44 (d, J ¼
7.6 Hz, 1H, H-1), 4.38 (dd, J ¼ 12.3, 3.8 Hz, 1H, H-6a), 4.18–4.05
(m, 3H, H-6a0, H-6b0 & H-6b), 3.98–3.93 (m, 2H, H-2), 3.87 (t, J ¼
6.8 Hz, 1H, H-5), 3.72–3.67 (m, 1H, H-50) 3.62 (t, J ¼ 9.4 Hz, 1H,
H-20) 3.53–3.45 (m, 1H, H-500), 2.35–2.29 (m, 2H), 2.11 (s, 3H,
CH3), 2.07 (s, 6H, 2CH3), 2.03 (s, 9H, 3CH3), 2.02 (s, 3H, CH3),
1.98 (s, 3H, CH3), 1.97 (s, 3H, CH3), 1.89–1.82 (m, 3H), 1.68–
1.42 (m, 8H), 1.39–1.31 (m, 9H), 1.11 (d, J ¼ 6.0 Hz, 3H, CH3),
1.18–1.07 (m, 8H), 1.01 (s, 3H), 0.94–0.93 (m, 9H), 0.68 (m, 3H);
13C NMR (100 MHz, CDCl3) d 170.6, 170.5, 170.4, 170.3, 170.2,
170.1, 169.8, 169.7, 133.6, 117.3, 101.7, 98.4, 98.1, 76.6, 74.4,
73.2, 71.7, 69.3, 69.0, 68.9, 68.3, 68.2, 66.9, 66.2, 61.8, 61.7,
20.95, 20.90, 20.84, 20.83, 20.78, 20.74, 19.5, 18.8, 17.5, 12.0;
HRMS calcd for C63H94O24 [M + Na]+ 1257.6027, found
1257.6035.
Cholesteryl 2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl (1/2)-4,6-
O-benzylidene-3-O-tert-butyldimethylsilyl-b-D-glucopyranosyl
(1/2)-6-O-tert-butyldimethylsilyl-3,4-O-isopropylidene-b-D-
galactopyranoside (11)
A suspension of 10 (0.11 g, 0.12 mmol), K2CO3 (50 mg), and
NCCCl3 (56 mL) in CH2Cl2 (2 mL) was allowed to stir at
ambient temperature overnight. The mixture was ltered
through celite and washed with CH2Cl2. Filtrate was concen-
trate under reduced pressure. The suspension of residue
which was obtained aer solvent removal, cholesterol (68 mg,
˚
0.175 mmol) and 4 A molecular sieves in CH2Cl2 : CH3CN
(2 mL, 1 : 1) were stirred at room temperature for 30 min.
ꢀ
Then, the solution was cooled 0 C and then TMSOTf (2 mL,
0.012 mmol) was added dropwise. Aer 30 min, the mixture
was diluted with CH2Cl2, ltered through celite and concen-
trated. The residue was puried by silica gel chromatography
(15% ethyl acetate in pet ether) to give the desired product 11
Cholesteryl a-L-rhamnopyranosyl-(1/2)-b-D-glucopyranosyl-
(1/2)-b-D-galactopyranoside (1c)
1
as a foam (60 mg, 45%): [a]2D5 ꢁ5.4 (c 0.12, CHCl3); H NMR
(400 MHz, CDCl3) d 7.46–7.43 (m, 2H, ArH), 7.36–7.29 (m, 3H, NaOMe (40 mg) was added to a clear solution of 12 (35 mg) in
ArH), 5.42 (s, 1H, benzylidene), 5.36–5.28 (m, 4H, H-100, H-200, CH2Cl2 : MeOH (4 mL, 1 : 1) and the reaction mixture was kept
H-300 & HC]), 5.05 (t, J ¼ 10.0 Hz, 1H, H-400), 4.99 (d, J ¼ for stirring at rt for 8 h. Aer complete consumption of starting
7.8 Hz, 1H, H-10), 4.38 (d, J ¼ 7.8 Hz, 1H, H-1), 4.27–4.19 (m, material, reaction mixture was neutralized with amberlite
3H, H-5, H-500 & H-4), 3.98 (t, J ¼ 10.0 Hz, 1H, H-30), 3.86–68 (acidic resin) and ltered. The ltrate was concentrated and
(m, 5H, H-2, H-6a, H-6b, H-6a0 & H-6b0), 3.57–3.52 (m, 4H, H- chromatographed on silica gel (15% MeOH in EtOAc) to give
20, H-3, H-40 & H-50), 2.30–2.28 (m, 2H), 2.12 (s, 3H, CH3), 2.03 the desired product 1c as white solid (20 mg, 86%): [a]D20 ꢁ17.2
(s, 3H, CH3), 1.96 (s, 3H, CH3), 1.87–1.79 (m, 3H), 1.68 (s, 3H), (c 0.57, MeOH); 1H NMR (500 MHz, MeOD) d 5.39 (s, 1H), 5.25
1.51–1.45 (m, 6H), 1.48 (s, 3H, CH3), 1.33 (s, 3H, CH3), 1.29- (s, 1H), 4.89 (1H under MeOD), 4.43 (d, J ¼ 7.8 Hz, 1H), 4.17–
1.21 (m, 21H), 1.11 (d, J ¼ 6.0 Hz, 3H, CH3), 0.91–0.83 (m, 4.09 (m, 1H), 4.06–3.57 (m, 11H), 3.54–3.36 (m, 4H), 3.28–3.22
16H), 0.75 (s, 9H), 0.67 (s, 3H), 0.06 (s, 6H), ꢁ0.04 (s, 3H), (m, 2H), 2.48–2.46 (m, 1H), 2.29–2.27 (m, 1H), 2.08–2.01 (m,
ꢁ0.07 (s, 3H); 13C NMR (100 MHz, CDCl3) d 170.2, 170.0, 1H), 2.01–1.90 (m, 4H), 1.89–1.45 (m, 8H), 1.49–1.24 (m, 13H),
169.9, 140.8, 137.2, 129.3, 128.3, 126.6, 121.9, 110.2, 109.1, 1.21–1.10 (m, 7H), 1.09–1.0.99 (m, 5H), 0.96 (d, J ¼ 6.0 Hz, 3H),
102.5, 100.2, 99.3, 97.7, 81.4, 79.4, 79.2, 76.3, 73.6, 73.4, 71.6, 0.92–0.82 (m, 6H), 0.72 (s, 3H); 13C NMR (125 MHz, MeOD) d
69.7, 69.3, 69.1, 66.4, 66.2, 62.4, 56.9, 56.3, 50.3, 42.5, 39.9, 142.1, 122.9, 102.2, 102.1, 81.4, 79.45, 79.40, 77.9, 77.3, 76.7,
39.6, 39.0, 37.5, 36.9, 36.3, 35.9, 32.1, 32.0, 29.9, 28.4, 28.2, 76.0, 74.2, 72.4, 72.3, 70.9, 69.8, 69.3, 63.4, 62.6, 61.7, 58.3, 57.7,
27.8, 26.3, 26.0, 25.9, 24.2, 23.9, 23.0, 22.7, 21.2, 21.1, 21.0, 43.6, 41.3, 40.8, 40.2, 38.6, 38.0, 37.5, 37.2, 33.4, 33.2, 30.9,
20.9, 19.5, 18.8, 18.4, 18.3, 17.7, 12.0, ꢁ3.6, ꢁ4.4, ꢁ5.1, ꢁ5.2, 30.87, 30.82, 30.62, 30.60, 29.4, 29.2, 25.4, 25.0, 23.8, 23.3, 23.0,
ꢁ5.3; HRMS calcd for C73H118O18Si2 [M + Na]+ 1361.7749, 22.3; HRMS calcd for C45H76O15 [M + Na]+879.5076, found
found 1361.7788.
879.5071.
This journal is © The Royal Society of Chemistry 2014
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This work was supported by the Department of Science and
Technology (grant no. SR/S1/OC-40/2009) and Council of
Scientic and Industrial Research (grant no. 01(2376)/10/EMR-
II). ME thanks CSIR-New Delhi for a fellowship.
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