Synthesis of a pentasaccharide derivative corresponding to a triterpenoid saponin isolated from Spergularia ramosa

Article abstract of DOI:10.1039/b205106k

A highly convergent and regioselective glycosylation strategy has been employed for the synthesis of a hetero-pentasaccharide that corresponds to the major sugar chain of a saponin isolated from herbaceous plant, Spergularia ramosa.

Full text of DOI:10.1039/b205106k

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Synthesis of a pentasaccharide derivative corresponding to a
triterpenoid saponin isolated from Spergularia ramosa
Guofeng Gu and Yuguo Du*
1
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,
P.O. Box 2871, Beijing 100085, China
Received (in Cambridge, UK) 27th May 2002, Accepted 25th July 2002
First published as an Advance Article on the web 21st August 2002
A highly convergent and regioselective glycosylation strategy has been employed for the synthesis of a
hetero-pentasaccharide that corresponds to the major sugar chain of a saponin isolated from herbaceous plant,
Spergularia ramosa.
was acetylated with acetic anhydride in pyridine (
5), fol-
Introduction
lowed by cleavage of acetonide in 90% aqueous trifluoroacetic
acid (TFA) giving 3,4-diol 6 in 94% yield (Scheme 2). Direct
condensation of 6 and fucopyranosyl donor 78 in anhydrous
CH2Cl2 in the presence of TMSOTf gave the undesired β-
(1 4) disaccharide 8 in 65% yield. The selectivity did not
change significantly when the reaction was carried out at Ϫ40
ЊC. The 1 4 linkage in 8 was confirmed by a downfield shift of
Saponins, the glycosides of steroids or triterpenes, are widely
distributed in plants and animals.1 Many studies reveal that
saponins show excellent physiological and pharmacological
activities, such as anti-cancer, anti-inflammatory, cardio-
vascular, and cytotoxic activities.1,2 They also exhibit important
roles in the cell-mediated immune response and in inter- and
intracellular communication processes.3 The oligosaccharides
have a very important role in the bioactivity of saponins. For
example, when the ester-linked tetrasaccharide was removed
from julibrosides, its cytotoxicity dramatically decreased.4 We
also have demonstrated that a saponin analog, containing the
same julibroside tetrasaccharide, shows mild activity against
mouse leukaemia P388.5
1
H-3 (δ 5.47 ppm, J2,3 10.8 Hz, J3,4 3.5 Hz) in the H NMR
spectra of its benzoylated derivative, 9. The attempted regio-
selective benzoylation of 6, with benzoyl chloride in pyridine at
Ϫ10 ЊC–0 ЊC, unexpectedly afforded 10, as a major product
(∼70% yield). When 6 was treated with tert-butylchloro-
dimethylsilane and imidazole in pyridine, the 3-O-silylated 11
was obtained in 91% yield. Benzoylation of 11 with benzoyl
Spergularia ramosa is a herbaceous plant from Perù.6 The
leaves of this species are used to feed sheep and, in the form of a
decoction, it is used as a remedy for respiratory ailments, tuber-
culosis and rickets. Recent phytochemical investigation of the
methanol extract from the aerial parts of Spergularia ramosa
uncovered six new oleanene saponins, which possess gysogenin
or quillaic acid as the aglycon. Interestingly, the oligosaccharide
chains linked to C-28 of the aglycons are made up of five differ-
ent saccharide residues as -glucose, -xylose, -rhamnose,
-fucose and -arabinose. Attracted by its complex structure
and curious of the structure–activity relationship, we syn-
thesized this highly branched oligosaccharide of Spergularia
ramosa through a convergent regio- and stereoselective strategy.
chloride in pyridine (
12), followed by desilylation in 90%
TFA, gave 13 in an overall yield of 90%.9 Glycosylation of 13
and 7 in anhydrous CH2Cl2 with TMSOTf as catalyst afforded
β-(1 3)-linked disaccharide 14. Deacetylation of 14 with
acetyl chloride in CH2Cl2–MeOH10 gave acceptor 2 in a total
yield of 81% (from 13).
In our previous synthesis of julibroside5 and rhamnose-
containing oligosaccharides,11 we showed that regioselective
glycosylation on 3-OH of rhamnose derivatives could be
achieved by a two-step procedure, orthoester formation and
rearrangement, using sugar bromides as donors. We also
successfully synthesized -arabinofuranosyl octamer using
trichloroacetimidates as glycosyl donors and unprotected or
partially protected arabinofuranosides as glycosyl acceptors.12
We thus decided to undertake the direct glycosylation of tri-
chloroacetimidate donor and rhamnose acceptor to produce
the β-(1 3)-linked disaccharide in a one-pot reaction. Coup-
ling of trichloroacetimidate 16 and 3,4-diol 15 in the presence
of catalytic amounts of TMSOTf at Ϫ42 ЊC 0 ЊC gave desired
product 17 in 63% yield. The presence of the β-(1 3) glycosyl
Results and discussion
The retrosynthetic analysis of the target molecule 1 leads to the
disaccharide acceptor 2 and trisaccharide trichloroacetimidate
donor 3 (Scheme 1). The synthesis started with the preparation
of β-(1 3)-linked disaccharide 2. To this end, compound 47
Scheme 1 Retrosynthetic analysis.
DOI: 10.1039/b205106k
J. Chem. Soc., Perkin Trans. 1, 2002, 2075–2079
This journal is © The Royal Society of Chemistry 2002
2075

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Scheme 2 Reagents and conditions (yields): a) Ac2O, Pyr (100% for 5, 100% for 18); b) 90% TFA (94% for 6, 94% for 13, 88% for 21); c) TMSOTf,
CH2Cl2 (65% for 8, 87% for 14, 63% for 17, 81% for 20, 83% for 22); d) BzCl, Pyr (93% for 9, 70% for 10, 95% for 12); e) TBSCl, Im, Pyr (91%);
f ) MeOH, AcCl (94%); g) NH3, MeOH–THF (3 : 7), then CCl3CN, DBU, CH2Cl2 (84% for two steps).
bond formation in 17 was supported by NMR analysis of the
acetylated derivative 18. The chemical shifts of H-4 (d 4.99
were measured using MALTI-TOF-MS with α-cyano-4-
hydroxycinnamic acid (CCA) as matrix. Thin-layer chrom-
atography (TLC) was performed on silica gel HF254 with detec-
tion by charring with 30% (v/v) H2SO4 in MeOH or in some
cases by a UV detector. Column chromatography was con-
ducted by elution of a silica gel column with EtOAc–petroleum
ether (bp 60–90 ЊC) as the eluent. Solutions were concentrated
at <60 ЊC under diminished pressure.
ppm) and H-1Ј (d 4.77 ppm, J 8.0 Hz) in 18 confirmed 1
3
linkage in 17. Condensation of 17 and 19 in CH2Cl2 at 0 ЊC,
using TMSOTf as catalyst, generated 20 as a R,S-mixture
in high yield.13 Trisaccharide donor 3 was prepared in 73%
overall yield through protection group manipulation5 of 20,
i.e., de-ethylidenation in 90% TFA, acetylation with acetic
anhydride in pyridine (
21), deacetylation of the anomeric
carbon in ammonia saturated THF : MeOH (7 : 3) followed
by trichloroacetimidate formation.14 Finally, coupling of di-
saccharide acceptor 2 and trisaccharide donor 3 proceeded
smoothly in anhydrous dichloromethane in the presence of
TMSOTf completing the synthesis of pentasaccharide 22 in
83.4% isolated yield.
In conclusion, an efficient and convergent synthesis of the
triterpenoid pentasaccharide was achieved in a regio- and
stereoselective manner. Pentasaccharide 22, having only acyl
protecting groups, could be used for the total synthesis of
saponin of Spergularia ramosa. This strategy also provides
an entry to the preparation of oligosaccharides with other
structures than those isolated from Spergularia ramosa.
Allyl 2-O-acetyl-ꢀ-L-arabinopyranoside 6
Compound 4 (1.1 g, 4.78 mmol) was acetylated in pyridine
(10 mL) with acetic anhydride (3 mL) at room temperature for
4 h, then co-evaporated with toluene 3 times (3 × 30 mL). The
residue was dissolved in 90% TFA (20 mL), stirred at room
temperature for 30 min and concentrated with the help of tolu-
ene in vacuo. Purification of the residue by silica gel column
chromatography using petroleum ether–EtOAc 1 : 1 as eluent
gave crystalline 6 (1.04 g, 94%); [α]2D0 ϩ117 (c 5, CHCl3); mp: 67–
69 ЊC; 1H NMR (CDCl3): δ 2.13 (s, 3 H, CH3CO), 3.94 (dd, 1 H,
J5a,5b 12.6, J5a,4 1.8 Hz, H-5a), 3.91 (dd, 1 H, J5b,41.2 Hz, H-5b),
3.98–4.04 (m, 2 H, H-4, one proton of CH ᎐CH–CH -), 4.08
᎐
2
2
(dd, 1 H, J3,4 3.5, J3,2 9.7 Hz, H-3), 4.17–4.20 (m, 1 H, one
proton of CH ᎐CH–CH -), 4.99–5.05 (m, 2 H, H-1, H-2), 5.19–
᎐
2
2
5.35 (m, 2 H, CH ᎐CH–CH -), 5.84–5.92 (m, 1 H, CH ᎐CH–
CH2-) (Calc. for C10H16O6: C, 51.72; H, 6.94. Found: C, 51.53;
H, 7.04%).
᎐
᎐
2
2
2
Experimental
General methods
Optical rotations were determined at 20 ЊC with a Perkin–Elmer
Model 241-Mc automatic polarimeter. Melting points were
determined with a “Mel-Temp” apparatus. 1H NMR, 13C NMR
Allyl 2,3,4-tri-O-benzoyl-ꢀ-D-fucopyranosyl-(1 4)-2-O-acetyl-
3-O-benzoyl-ꢀ-L-arabinopyranoside 9
1
1
and H–1H, H–13C COSY spectra were recorded with Bruker
ARX 400 spectrometers for solutions in CDCl3. Chemical shifts
are given in ppm downfield from internal Me4Si. Mass spectra
To a solution of 7 (281 mg, 0.454 mmol) and 6 (100 mg, 0.431
mmol) in anhydrous CH2Cl2 (4 mL) was added TMSOTf
(8.2 µL, 0.045 mmol) under a N2 atmosphere at 0 ЊC. The
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View Article Online
mixture was stirred at these conditions for 1.5 h, at the end of
which time TLC (3 : 1 petroleum ether–EtOAc) indicated the
completion of the reaction. The reaction mixture was neutral-
ized with triethylamine, and concentrated. The residue was
purified on a silica gel column using 3 : 1 petroleum ether–
EtOAc as eluent to give syrupy 8. To the above residue in
CH2Cl2 (3 mL) was added pyridine (3 mL) and benzoyl chloride
(100 µL). The mixture was stirred at room temperature for 4 h
and flash column separation gave 9 as a syrup (60.5% for two
H-5a), 3.98–4.06 (m, 2 H, J5b,41.2 Hz, H-5b, one proton of
CH ᎐CH–CH -), 4.18–4.24 (m, 1 H, one proton of CH ᎐CH–
᎐
᎐
2
2
2
CH2-), 4.33 (dd, 1 H, J3,4 3.7 Hz, H-3), 5.13 (d, 1 H, J1,2 3.6 Hz,
H-1), 5.19 (dd, 1 H, J2,3 10.3 Hz, H-2), 5.21–5.37 (m, 2 H, CH ᎐
᎐
2
CH–CH -), 5.44 (ddd, 1 H, H-4), 5.86–5.93 (m, 1 H, CH ᎐CH–
᎐
2
2
CH2-), 7.44–8.11 (m, 5 H, Ph) (Calc. for C17H20O7: C, 60.71; H,
5.99. Found: C, 60.79; H, 6.05%).
Allyl 2,3,4-tri-O-benzoyl-ꢀ-D-fucopyranosyl-(1 3)-2-O-acetyl-
4-O-benzoyl-ꢀ-L-arabinopyranoside 14
steps); [α]2D0 ϩ168 (c 1, CHCl3); H NMR (CDCl3): δ 1.20 (d,
1
3 H, J 6.4 Hz, H-6Ј), 1.89 (s, 3 H, CH3CO), 3.92 (br q, 1 H,
To a solution of 13 (300 mg, 0.893 mmol) and 7 (582 mg,
0.937 mmol) in anhydrous CH2Cl2 (5 mL) was added TMSOTf
(18 µL, 0.10 mmol) under a N2 atmosphere at 0 ЊC. The mixture
was stirred under these conditions for 1.5 h, at the end of which
time TLC (3 : 1 petroleum ether–EtOAc) indicated the comple-
tion of the reaction. The reaction mixture was neutralized with
triethylamine, and concentrated. The residue was purified on a
silica gel column using 3 : 1 petroleum ether–EtOAc as eluent to
H-5Ј), 3.99–4.05 (m, 3 H, H-5a,5b and one proton of CH ᎐CH–
᎐
2
CH -), 4.18–4.23 (m, 1 H, one proton of CH ᎐CH–CH -), 4.46
᎐
2
2
2
(br s, 1 H, H-4), 4.88 (d, 1 H, J 8.0 Hz, H-1Ј), 5.15 (d, 1 H, J 3.4
Hz, H-1), 5.16–5.33 (m, 3 H, H-2, CH ᎐CH–CH -), 5.45 (dd,
᎐
2
2
1 H, J 3.5, 10.4 Hz, H-3Ј), 5.47 (dd, 1 H, J 3.5, 10.8 Hz, H-3),
5.62 (d, 1 H, J 3.5 Hz, H-4Ј), 5.76 (dd, 1 H, J 10.4, 8.0 Hz,
H-2Ј), 5.82–5.93 (m, 1 H, CH ᎐CH–CH -), 7.20–8.12 (m, 20 H,
᎐
2
2
Ph) [Calc. for C44H42NaO14 (M ϩ Na)ϩ: 817.26. Found: m/z,
817.3 (M ϩ Na)ϩ].
give syrupy 14 (615 mg, 86.7%); [α]2D0 ϩ195 (c 1, CHCl3); H
1
NMR (CDCl3): δ 1.18 (d, J6Ј,5Ј 6.4 Hz, H-6Ј), 1.60 (s, 3 H,
CH CO), 3.89–3.96 (m, 2 H, H-5a, one proton of CH ᎐CH–
᎐
2
3
Allyl 2-O-acetyl-3-O-tert-butyldimethylsilyl-ꢀ-L-arabino-
pyranoside 11
CH2-), 3.98–4.18 (m, 2 H, H-5b, H-5Ј), 4.13–4.18 (m, 1 H, one
proton of CH ᎐CH–CH -), 4.35 (dd, 1 H, J 3.7, J3,2 9.9 Hz,
᎐
2
2
3,4
H-3), 4.96 (d, 1 H, J1,2 7.8 Hz, H-1Ј), 5.06 (d, 1 H, J1,2 3.5 Hz,
To a solution of compound 6 (336 mg, 1.45 mmol) in pyrid-
ine (5 mL) at 0 ЊC was added tert-butyldimethylsilyl chloride
(229 mg, 1.52 mmol) and imidazole (100 mg, 1.5 mmol). The
mixture was stirred at room temperature for about 4 h and
quenched by one drop of methanol, and then co-evaporated
with toluene under diminished pressure to dryness. The residue
was purified by column chromatography using 5 : 1 petroleum
ether–EtOAc as eluent to give syrupy 11 (458 mg, 91.4%); [α]2D0
ϩ156 (c 3.8, CHCl3); 1H NMR (CDCl3): δ 0.12, 0.13 (2 s, 6 H,
2 Si(CH3)2), 0.90 (s, 9 H, C(CH3)3), 2.09 (s, 3 H, CH3CO), 3.76
(dd, 1 H, J5a,5b12.4, J5a,41.8 Hz, H-5a), 3.82 (dd, 1 H, J5b,41.4 Hz,
H-5b), 3.84–3.87 (m, 1 H, H-4), 3.96–4.02 (m, 1 H, one proton of
H-1), 5.17–5.28 (m, 3 H, H-2, CH ᎐CH–CH -), 5.50 (dd, 1 H,
᎐
2
2
J3,4 3.4, J3,2 10.4 Hz, H-3Ј), 5.57 (ddd, 1 H, H-4), 5.61–5.67 (m,
2 H, H-2Ј, H-4Ј), 5.76–5.86 (m, 1 H, CH ᎐CH–CH -), 7.21–8.14
᎐
2
2
(m, 20 H, Ph) (Calc. for C44H42O14: C, 66.49; H, 5.33. Found: C,
66.56; H, 5.41%).
Allyl 2,3,4-tri-O-benzoyl-ꢀ-D-fucopyranosyl-(1 3)-4-O-
benzoyl-ꢀ-L-arabinopyranoside 2
To a solution of 14 (580 mg, 0.730 mmol) in MeOH–CH2Cl2
(v/v 1 : 1, 30 mL) was added acetyl chloride (2 mL) in 10 min at
0 ЊC. The reaction mixture was stirred at room temperature
overnight, then neutralized with saturated aqueous NaHCO3,
extracted with dichloromethane (2 × 25 mL). The organic
phases were combined, dried with anhydrous Na2SO4, concen-
trated under reduced pressure to dryness, then passed through a
silica gel column with 2 : 1 petroleum ether–EtOAc as eluent to
afford the disaccharide acceptor 2 (515 mg, 93.8%) as a syrup;
CH ᎐CH–CH -), 4.11–4.20 (m, 2 H, H-3, one proton of CH ᎐
᎐
᎐
2
2
2
CH–CH2-), 4.99 (dd, 1 H, J2,3 9.3 Hz, H-2), 5.03 (d, 1 H, J1,2
3.6 Hz, H-1), 5.17–5.32 (m, 2 H, CH ᎐CH–CH -), 5.84–5.91
᎐
2
2
(m, 1 H, CH ᎐CH–CH -) (Calc. for C H O Si: C, 55.46; H,
᎐
2
2
16 30
6
8.73. Found: C, 55.73; H, 8.65%).
[α]2D0 ϩ180 (c 1, CHCl3); H NMR (CDCl3): δ 1.20 (d, 3 H,
Allyl 2-O-acetyl-3-O-tert-butyldimethylsilyl-4-O-benzoyl-ꢀ-L-
arabinopyranoside 12
1
J6,5 6.4 Hz, H-6Ј), 3.89 (dd, 1 H, J5a,5b11.4, J5a,4 2.6 Hz, H-5a),
3.94–4.06 (m, 3 H, H-2, H-5b, one proton of CH ᎐CH–CH -),
᎐
To a solution of compound 11 (430 mg, 1.24 mmol) in pyridine
(5 mL) was added benzoyl chloride (1.72 mmol, 0.2 mL) at 0 ЊC.
The mixture was stirred at room temperature for 10 h, then
co-evaporated with toluene under reduced pressure. Column
chromatography (9 : 1 petroleum ether–EtOAc) of the residue
gave 12 (531 mg, 95.2%) as a syrup; [α]2D0 ϩ158 (c 2, CHCl3); 1H
NMR (CDCl3): δ 0.20, 0.23 (2 s, 6 H, 2 Si(CH3)2), 0.90 (s, 9 H,
C(CH3)3), 2.26 (s, 3 H, CH3CO), 3.95 (dd, 1 H, J5a,5b12.8, J5a,4
2
2
4.09–4.15 (m, 1 H, H-5Ј), 4.16–4.24 (m, 2 H, H-3, one proton of
CH ᎐CH–CH -), 4.92 (d, 1 H, J 3.4 Hz, H-1), 5.10 (d, 1 H,
᎐
2
2
1,2
J1,2 7.8 Hz, H-1Ј), 5.20–5.32 (m, 2 H, CH ᎐CH–CH -), 5.48 (br
᎐
2
2
s, 1 H, H-4), 5.52 (dd, 1 H, J3,4 3.4, J3,2 10.4 Hz, H-3Ј), 5.63–5.69
(m, 2 H, H-2Ј, H-4Ј), 5.84–5.94 (m, 1 H, CH ᎐CH–CH -), 7.21–
᎐
2
2
8.06 (m, 20 H, Ph) (Calc. for C42H40O13: C, 67.01; H, 5.36.
Found: C, 66.92; H, 5.40%).
2.3 Hz, H-5a), 4.10–4.19 (m, 2 H, H-5b, one proton of CH ᎐
᎐
2
2,3,4,6-Tetra-O-acetyl-ꢀ-D-glucopyranosyl-(1 3)-1,2-O-
ethylidene-ꢀ-L-rhamnopyranose 17
CH–CH -), 4.32–4.38 (m, 1 H, one proton of CH ᎐CH–CH -),
᎐
2
2
2
4.44 (dd, 1 H, J3,43.6, J3,2 9.2 Hz, H-3), 5.25–5.29 (m, 2 H, H-1,
H-2), 5.34–5.48 (m, 2 H, CH ᎐CH–CH -), 5.51 (ddd, 1 H, H-4),
The reaction mixture of 15 (270 mg, 1.42 mmol) and 16
(698 mg, 1.42 mmol) were dissolved in anhydrous CH2Cl2
(5 mL). TMSOTf (7.5 µL, 0.04 mmol) was added dropwise at
Ϫ42 ЊC under N2 protection. The mixture was stirred at this
temperature for 2 h, then stirred at 0 ЊC for 16 h, neutralized
with Et3N and concentrated under reduced pressure to dryness.
Purification of the residue by column chromatography (2 : 1
petroleum ether–EtOAc) gave 17 (R,S mixture, 465 mg, 63%) as
a syrup.11
᎐
2
2
5.99–6.05 (m, 1 H, CH ᎐CH–CH -), 7.56–8.23 (m, 5 H, Ph)
᎐
2
2
(Calc. for C23H34O7Si: C, 61.31; H, 7.61. Found: C, 61.55; H,
7.52%).
Allyl 2-O-acetyl-4-O-benzoyl-ꢀ-L-arabinopyranoside 13
A solution of 12 (466 mg, 1.04 mmol) in 90% aqueous TFA
(4 mL) was stirred at room temperature for 2 h, then neutralized
with saturated aqueous sodium bicarbonate and extracted with
dichloromethane (3 × 15 mL). The organic phases were com-
bined, dried over anhydrous sodium sulfate and concentrated.
Purification of the residue on a silica gel column using 3 : 1
petroleum ether–EtOAc as eluent afforded 13 (330 mg, 94.3%)
2,3,4,6-Tetra-O-acetyl-ꢀ-D-glucopyranosyl-(1 3)-[2,3,4-tri-O-
acetyl-ꢀ-D-xylopyranosyl-(1 4)]-1,2-O-ethylidene-ꢀ-L-rhamno-
pyranose 20
as white solid; [α]2D0 ϩ212 (c 1.3, CHCl3); H NMR (CDCl3):
Compound 17 (584 mg, 1.12 mmol) and 19 (481 mg, 1.14
mmol) were pre-dried in one flask under vacuum at 60 ЊC for
1
δ 2.16 (s, 3 H, CH3CO), 3.88 (dd, 1 H, J5a,5b13.1, J5a,41.9 Hz,
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3 h. The mixture was then dissolved in anhydrous CH2Cl2
(5 mL). To the solution was added TMSOTf (15 µL, 0.083
mmol) under a N2 atmosphere at 0 ЊC. The reaction mixture
was stirred at this temperature for 1.5 h, then neutralized with
triethylamine, concentrated under reduced pressure and puri-
fied on a silica gel column with 2 : 1 petroleum ether–EtOAc as
eluent to give a syrupy R,S-mixture of 20 (710 mg, 81.4%); 1H
NMR (CDCl3): δ 1.29 (d, 1.2 H, J6,5 6.0 Hz, S–H-6I), 1.30 (d,
1.8 H, J6,5 6.0 Hz, R–H-6I), 1.36 (d, 1.2 H, S-CHCH3), 1.51 (d,
1.8 H, R-CHCH3), 2.01 (br s, 3 H, CH3CO), 2.04 (br s, 6 H,
2 CH3CO), 2.05 (br s, 3 H, CH3CO), 2.08 (s, 1.8 H, CH3CO),
2.09 (s, 1.2 H, CH3CO), 2.15 (s, 1.2 H, CH3CO), 2.17 (s, 1.8 H,
CH3CO), 2.18 (br s, 3 H, CH3CO), 3.31–3.43 (m, 1.8 H, H-5I, S-
H-5aIII and S-H-5bIII), 3.69 (m, 1 H, H-5II), 3.80–3.94 (m, 1.6
H, S-H-3I, R-H-5aIII and R-H-5bIII), 4.06–4.13 (m, 1.2 H, R-H-
3I and H-4I), 4.15–4.24 (m, 3 H, R-H-2I, S-H-4I, H-6aII, H-6bII),
4.31 (br t, 0.4 H, S-H-2I), 4.77 (d, 1 H, J1,2 7.4 Hz, H-1III),
4.83–4.93 (m, 3.2 H, R-H-2II, R-H-2III and H-4II, H-4III), 5.06–
5.14 (m, 3 H, H-1II, S-H-2II, S-H-2III, R-H-3II and R-H-3III),
5.15–5.19 (m, 1.4 H, R-H-1I, S-H-3III and S-H-3II), 5.24–5.28
(m, 1 H, S-H-1I, R–CHCH3), 5.62 (q, 0.4 H, S–CHCH3)
[MALDI TOF-MS calc. for C33H46O21: 778.25; Found: 801.2
[M ϩ Na]ϩ] (Calc. for C33H46O21ؒH2O: C, 49.70; H, 6.02.
Found: C, 49.48; H, 6.10%).
H-6bII), 4.73 (d, 1 H, J1,2 7.7 Hz, H-1III), 4.83–4.94 (m, 3 H,
H-1II, H-4II, H-4III), 5.02 (dd, 1 H, J2,1 7.8, J2,3 9.3 Hz,
H-2II/III), 5.08–5.20 (m, 3 H, H-2III/II, H-3II, H-3III), 5.35 (dd,
1 H, H-2I), 6.13 (d, 1 H, J1,2 1.9 Hz, H-1I), 8.71 (s, 1 H, ᎐NH)
᎐
(Calc. for C35H46Cl3NO22: C, 44.76; H, 4.94. Found: C, 44.48;
H, 4.85%).
Allyl 2,3,4,6-tetra-O-acetyl-ꢀ-D-glucopyranosyl-(1 3)-[2,3,4-
tri-O-acetyl-ꢀ-D-xylopyranosyl-(1 4)]-2-O-acetyl-ꢁ-L-
rhamnopyranosyl-(1 2)-[2,3,4-tri-O-benzoyl-ꢀ-D-fuco-
pyranosyl-(1 3)]-4-O-benzoyl-ꢀ-L-arabinopyranoside 22
To a solution of 2 (400 mg, 0.532 mmol) and 3 (524 mg, 0.558
mmol) in anhydrous dichloromethane (5 mL) was added
Me3SiOTf (15 µL, 0.083 mmol) at 0 ЊC with a N2 atmosphere.
The reaction mixture was stirred at this condition for 1.5 h, at
the end of which time TLC (1 : 1 petroleum ether–EtOAc) indi-
cated that the reaction was complete. The reaction mixture was
neutralized with triethylamine and then concentrated. The resi-
due was purified on a silica gel column using 1 : 1 petroleum
ether–EtOAc as eluent to give the pentasaccharide 22 (678 mg,
83.4%) as a syrup; [α]2D0 ϩ42 (c 1.4, CHCl3); 1H NMR (CDCl3):
δ 1.18 (d, 3 H, J6,5 6.8 Hz, H-6IV), 1.20 (d, 3 H, J6,5 6.2 Hz,
H-6II), 2.00, 2.01, 2.02, 2.04, 2.05, 2.08, 2.12, 2.19 (8 s, 8 × 3 H,
8 CH3CO), 3.37 (dd, 1 H, J5a,5b 11.8, J5a,4 9.1 Hz, H-5aV), 3.68–
3.88 (m, 5 H, H-4II, H-5aI, H-5II, H-5III, one proton of CH ᎐
᎐
2
2,3,4,6-Tetra-O-acetyl-ꢀ-D-glucopyranosyl-(1 3)-[2,3,4-tri-O-
acetyl-ꢀ-D-xylopyranosyl-(1 4)]-1,2-di-O-acetyl-ꢁ-L-rhamno-
pyranose 21
CH–CH2-), 3.91–4.02 (m, 3 H, H-2I, H-3II, H-5bI), 4.05–4.17
(m, 4 H, H-5IV, H-5bV, H-6aIII, one proton of CH ᎐CH–CH -),
᎐
2
2
4.27 (dd, 1 H, J6b,6a11.4, J6b,5 5.1 Hz, H-6bIII), 4.48 (dd, 1 H,
J3,2 3.6, J3,4 9.1 Hz, H-3I), 4.71–4.84 (m, 4 H, H-1I, H-1III, H-1V,
H-2V), 4.86–4.97 (m, 2 H, H-1II, H-4V), 4.99–5.23 (m, 7 H,
A solution of compound 20 (662 mg, 0.85 mmol) in aqueous
90% TFA (5 mL) was stirred at room temperature for 0.5 h,
then co-evaporated with toluene under diminished pressure to
dryness. The above product was then dissolved in pyridine
(4 mL). To the above mixture was added acetic anhydride
(3 mL). The reaction mixture was stirred at room temperature
overnight, then evaporated under reduced pressure to dryness.
The residue was purified by column chromatography using 1 : 1
petroleum ether–EtOAc to afford 21 (623 mg, 87.6% for two
steps) as a white solid; [α]2D0 ϩ65 (c 1, CHCl3); 1H NMR
(CDCl3): δ 1.29 (d, 3 H, J6,5 6.1 Hz, H-6I), 2.01, 2.03, 2.04, 2.06,
2.08, 2.13, 2.15, 2.16, 2.19 (9 s, 9 × 3 H, 9 CH3CO), 3.43 (dd,
1 H, J5a,5b12.0, J5a,47.8 Hz, H-5aIII), 3.65–3.74 (m, 2 H, H-5I,
H-5II), 3.82 (t, 1 H, J4,3 = J4,5 9.3 Hz, H-4I), 4.05 (dd, 1 H, J4,3
3.6 Hz, H-3I), 4.10–4.17 (m, 2 H, H-5bIII, H-6aII), 4.20 (dd, 1 H,
J6a,6b12.3, J6b,5 2.2 Hz, H-6bII), 4.73 (d, 1 H, J1,2 7.7 Hz, H-1III),
4.83–4.93 (m, 3 H, H-1II, H-4II, H-4III), 4.98–5.19 (m, 5 H,
H-2I, H-2II, H-2III, H-3II, H-3III), 5.94 (d, 1 H, J1,21.8 Hz, H-1I)
(Calc. for C35H48O23: C, 50.24; H, 5.78. Found: C, 50.41; H,
5.83%).
H-1IV, H-3III, H-2III, H-4III, H-3V, CH ᎐CH–CH -), 5.32 (dd,
᎐
2
2
1 H, J 1.9, 3.8 Hz, H-2II), 5.49 (br t, 1 H, H-4I), 5.59 (dd, 1 H,
J3,210.4, J3,4 3.5 Hz, H-3IV), 5.60–5.72 (m, 3 H, H-2IV, H-4IV,
CH ᎐CH–CH -), 7.23–8.13 (m, 20 H, Ph); 13C NMR (100
᎐
2
2
MHz, CDCl3): 16.06 (C-6IV), 17.82 (C-6I), 20.58, 20.67, 20.80,
20.97, 21.04 (8 C, CH3CO, some overlapped), 60.85 (C-5I),
61.12 (C-6III), 61.75 (C-5V), 66.80 (C-5III), 67.89 (C-4III), 68.91
(CH ᎐CH-CH -), 68.96 (C-4V), 69.38 (C-4I), 70.05 (C-5IV),
᎐
2
2
71.00 (C-2IV), 71.14 (2 C, C-2II, C-4IV), 71.24 (C-3V), 71.53
(C-5II), 71.68 (C-2V), 71.96 (C-3IV), 72.07 (C-2III), 72.17 (C-3I),
73.12 (C-3III), 73.85 (C-4II), 74.65 (C-2I), 78.86 (C-3II), 97.10
(C-1I), 98.04 (C-1II), 98.67 (C-1V), 99.38 (C-1IV), 100.12 (C-1III),
118.18 (CH ᎐CH–CH -), 132.90 (CH ᎐CH–CH -), 165.28,
᎐
᎐
2
2
2
2
165.39, 165.98, 166.01 (4 C, PhCO), 169.19, 169.36, 169.81,
169.90, 169.93, 170.02, 170.13, 170.58 (8 C, CH3CO) [Calc. for
C75H84O34: 1528.48; Found: 1551.5 [M ϩ Na]ϩ and 1567.5
[M ϩ K]ϩ] (Calc. for C75H84O34: C, 58.90; H, 5.54. Found: C,
58.99; H, 5.47%).
2,3,4,6-Tetra-O-acetyl-ꢀ-D-glucopyranosyl-(1 3)-[2,3,4-tri-O-
acetyl-ꢀ-D-xylopyranosyl-(1 4)]-2-O-acetyl-ꢁ-L-rhamno-
pyranosyl trichloroacetimidate 3
Acknowledgements
This work was supported by NNSF of China (Projects
29972053, 39970179) and RCEES of Chinese Academy of
Sciences.
Into a solution of 21 (583 mg, 0.697 mmol) in MeOH–THF
(v/v 3 : 7, 10 mL) ammonia was bubbled at room temperature
for 15 min, and then stirred at this temperature for 1.5 h, at the
end of which time TLC indicated the completion of the
reaction. The reaction mixture was evaporated under reduced
pressure to dryness. The above residue was then dissolved in
anhydrous CH2Cl2 (6 mL), and trichloroacetonitrile (0.21 mL,
2.1 mmol) and DBU (0.05 mL, 0.33 mmol) were added sequen-
tially. The reaction mixture was stirred at room temperature for
2 h, and then concentrated. Purification of the residue on a
silica gel column with 3 : 2 petroleum ether–EtOAc as eluent
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2079