Synthesis of saponins using partially protected glycosyl donors

Article abstract of DOI:10.1021/ol035353s

(Matrix Presented) A new class of glycosyl donors having unprotected 2- and 2,4-hydroxyl groups were investigated under the standard glycosylation conditions. This approach was shown to be generally effective for the synthesis of alkyl and steroidal glyco

Full text of DOI:10.1021/ol035353s

ORGANIC
LETTERS
2003
Vol. 5, No. 20
3627-3630
Synthesis of Saponins Using Partially
Protected Glycosyl Donors
Yuguo Du,*,† Guofeng Gu,† Guohua Wei,† Yuxia Hua,† and Robert J. Linhardt*,‡
Research Center for Eco-EnVironmental Sciences, Chinese Academy of Sciences,
Beijing 100085, China, and Departments of Chemistry, Biology, and Chemical and
Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180
duyuguo@mail.rcees.ac.cn; linhar@rpi.edu
Received July 21, 2003
ABSTRACT
A new class of glycosyl donors having unprotected 2- and 2,4-hydroxyl groups were investigated under the standard glycosylation conditions.
This approach was shown to be generally effective for the synthesis of alkyl and steroidal glycosides. A natural saponin, containing 2,4-
branched oligosaccharide, was prepared in 35% overall yield in four straightforward sequential reactions by taking advantage of these partially
protected donors.
Recent investigations in glycobiology have revealed impor-
tant roles for many glycoconjugates in the immune response,
viral and bacterial infection, cell regulation, differentiation,
development, inflammation, cell adhesion, and many other
inter- and intracellular communication processes.1 Saponins,
a structurally and biologically diverse class of glycosides of
steroids and triterpenes, are major components in traditional
Chinese medicines and represent important examples of
glycoconjugates with promising pharmaceutical and biologi-
cal activities.2 Dioscin (diosgenyl 2,4-di-O-R-L-rhamnopy-
ranosyl-â-D-glucopyranoside), polyphyllin D (diosgenyl R-L-
rhamno-pyranosyl-(1f2)-[R-L-arabinofuranosyl-(1f4)]-â-
D-glucopyranoside), and balanitin 7 (diosgenyl R-L-rham-
nopyranosyl-(1f2)-[â-D-xylopyranosyl-(1f3)-â-D-gluco-
pyranosyl-(1f4)]-â-D-gluco-pyranoside) display cardiovas-
cular, antifungal, and antitumor activities.3 Moreover, the
rhamnose moiety of solamargine (solasodinyl 2,4-di-O-R-
L-rhamnopyranosyl-â-D-glucopyranoside) plays a crucial role
in triggering cell death by apoptosis.4 These bioactive
saponins are comprised of a 2,4-branched oligosaccharide
moiety, as are N-linked oligosaccharides and many plant
polysaccharides.5 Thus, because of their biological functions
and also their unique 2,4-dibranched chain structures, the
efficient synthesis of these steroidal glycosides deserves
extensive exploration.
Substantial effort has been devoted to the development of
novel glycosylation reactions as strategies to access natural
glycoconjugate structures.6 Saponins, having 2,4-branched
oligosaccharides, have been traditionally assembled in three
ways.7 In the first approach, the reducing end sugar unit is
coupled to the C-3 of a steroid or triterpene, next protection
group manipulation is performed on the sugar residue, usually
(3) (a) Nakano, K.; Murakami, K.; Takaishi, Y.; Tomimatsu, T.; Nohara,
T. Chem. Pharm. Bull. 1989, 37, 116. (b) Hufford, C. D.; Liu, S.; Clark,
A. M. J. Nat. Prod. 1988, 51, 94. (c) Liu, C.; Chen, Y. Acta Pharm. Sinica
1984, 19, 799. (d) Namba, T.; Huang, X.; Shu, Y.; Huang, S.; Hattoti, M.;
Kakiuchi, N.; Wang, Q.; Xu, G. Planta Med. 1989, 55, 501. (e) Zhou, J.
Pure Appl. Chem. 1989, 61, 457.
† Chinese Academy of Sciences.
‡ Rensselaer Polytechnic Institute.
(1) For reviews see: (a) Varki, A. Glycobiology 1993, 3, 97. (b) Lee,
Y. C.; Lee, R. T. Acc. Chem. Res. 1995, 28, 321.
(2) Hostettmann, K.; Marston, A. Saponins; Cambridge University
Press: New York, 1995.
(4) Chang, L.-C.; Tsai, T.-R.; Wang, J.-J.; Lin, C.-N.; Kuo, K.-W.
Biochem. Biophys. Res. Commun. 1998, 242, 21.
10.1021/ol035353s CCC: $25.00
© 2003 American Chemical Society
Published on Web 08/29/2003

4,6-benzylidination followed by blocking the 3-hydroxyl
group to give the glycosyl acceptor containing a free
2-hydroxyl group. Glycosylation at this 2-hydroxyl group,
followed by selective opening of the 4,6-benzylidene, affords
a free 4-hydroxyl group, which is further glycosylated to
furnish a 2,4-branched saponin. The disadvantages of this
method are that it involves a lengthy and low-efficiency
synthesis. This is especially problematic when the aglycone
is expensive or available only in limited quantities. In the
second approach, a more highly convergent synthesis of
saponin is carried out using a suitably modified monosac-
charide donor with a participatory C-2 acyl protecting group
to ensure the â-bond formation. However, the subsequent
removal of C-2 acyl protecting groups from saponin deriva-
tives, to expose the free 2-hydroxyl group for glycosylation,
can be difficult.7d In the third approach, the 2,4-branched
oligosaccharide is first prepared and then condensed with
aglycone in the final step. Unfortunately, glycosylation with
such an oligosaccharide results in decreased neighboring
group participation and often generates R,â-mixtures.7g With
these difficulties in mind, we speculated that a partially
protected glycosyl donor could be used to shorten the total
synthesis of saponins.8
Table 2. Saponin Synthesis Using Partially Protected Glycosyl
Donors
Model studies were first carried out on the preparation of
alkyl glycosides using partially protected sugar donors and
alkyl alcohol acceptors (Table 1) in CH2Cl2 at -42 °C under
a An additional 6% of the R-isomer was isolated. b R: â ) 1:2. c R: â )
2:3. d An additional 14% yield of the disaccharide saponin derivative was
also isolated.
Table 1. Alkyl Glycoside Synthesis Using Partially Protected
Glycosyl Donor
ranosyl thioglycoside donor 4, containing 2,4-dihydroxyl
groups, was subjected to similar reaction conditions, octyl
â-D-galactopyranoside 6 was obtained in a yield of 83% (R:
â ) 10:1). Furthermore, 2,3-dihydroxyl donor 7 afforded a
modest (42%) yield of galactopyranoside 9 as a 1:3 R/â
mixture.9
Encouraged by these preliminary results, we next turned
our attention to saponin synthesis (Table 2). Condensation
of donor 4 with diosgenin 10 in CH2Cl2 at -42 °C under
NIS-TMSOTf promotion afforded a 54% isolated yield of
â-glycoside 11. A doublet at 4.53 ppm (J ) 7.7 Hz) in H
NMR spectrum clearly demonstrated the pure â-configuration
a A 7% yield of R isomer was also isolated. b Including 32% of the
â-isomer and 10% of the R-isomer.
1
promotion with N-iodosuccinimide (NIS) and trimethylsilyl
trifluoromethanesulfate (TMSOTf). We were pleased to
discover that mannopyranosyl thioglycoside 1, containing an
unprotected hydroxyl group on C-2, still acted as an excellent
glycosyl donor to afford R-glycoside 3 in high yield (80%).
No trace of self-condensed disaccharide was detected in
our experiments. More impressively, when the galactopy-
(7) (a) Lahmann, M.; Gyba¨ck, H.; Garegg, P. J.; Oscarson, S.; Suhr, R.;
Thiem, J. Carbohydr. Res. 2002, 337, 2153. (b) Yu, H.; Yu, B.; Wu, X.;
Hui, Y.; Han, X. J. Chem. Soc., Perkin Trans. 1 2000, 1445. (c) Cheng,
M.; Wang, Q.; Tian, Q.; Song, H.; Liu, Y.; Li, Q.; Xu, X.; Miao, H.; Yao,
X.; Yang, Z. J. Org. Chem. 2003, 68, 3658. (d) Deng, S.; Yu, B.; Hui, Y.;
Yu, B.; Han, X. Carbohydr. Res. 1999, 317, 53. (e) Deng, S.; Yu, B.; Hui,
Y. Tetrahedron Lett. 1998, 39, 6511. (f) Li, B.; Yu, B.; Hui, Y.; Li, M.;
Han, X.; Fung, K.-P. Carbohydr. Res. 2001, 331, 1. (g) Ikeda, T.; Miyashita,
H.; Kajimoto, T.; Nohara, T. Tetrahedron Lett. 2001, 42, 2353.
(5) (a) Iorizzi, M.; De Marino, S.; Zollo, F. Curr. Org. Chem. 2001, 5,
951. (b) Bedir, E.; Khan, I. A. J. Nat. Prod. 2000, 63, 1699. (c) Yin, J.;
Kouda, K.; Tezuka, Y.; Le Tran, Q.; Miyahara, T.; Chen, Y.; Kadota, S. J.
Nat. Prod. 2003, 66, 646. (d) Nakamura, T.; Komori, C.; Lee, Y.-Y.;
Hashimoto, F.; Yahara, S.; Nohara, T.; Ejima, A. Biol. Pharm. Bull. 1996,
19, 564. (e) Miyamura, M.; Nakano, K.; Nohara, T.; Tomimatsu, T.;
Kawasaki, T. Chem. Pharm. Bull. 1982, 30, 712. (f) Akhov, L. S.; Musienko,
M. M.; Piacente, S.; Pizza, C.; Oleszek, W. J. Agric. Food Chem. 1999,
47, 3193. (g) Dwek, R. A. Chem. ReV. 1996, 96, 683.
(8) Plante, O.; Palmacci, E. R.; Andrade, R. B.; Seeberger, P. H. J. Am.
Chem. Soc. 2001, 123, 9545.
(9) General Procedure. To a mixture of thioglycosyl donor (1 mmol)
and ROH (1 mmol) in anhydrous dichloromethane (2 mL) at -42 °C were
added 1.1 mmol of NIS and catalytic amount of TMSOTf (0.1 equiv) with
N2 protection. The reaction mixture was stirred under these conditions for
45 min, at which time TLC indicated the completion of the reaction. The
mixture was then neutralized with Et3N and concentrated to dryness. The
residue was subjected to column chromatography on silica gel with
petroleum ether/EtOAc (6/1-3/1) as the eluent to give the desired product.
(6) For reviews, see: (a) Toshima, K.; Tasuta, K. Chem. ReV. 1993, 93,
1503. (b) Schmidt, R. R.; Kinzy, W. AdV. Carbohydr. Chem. Biochem. 1994,
50, 21. (c) Garegg, P. J. AdV. Carbohydr. Chem. Biochem. 1997, 52, 179.
3628
Org. Lett., Vol. 5, No. 20, 2003

of 11. The R-isomer might also be generated in this reaction,
as the presence of inseparable contaminants did not permit
us to rule out its formation. The 3,6-disilylated donor 12
gave an easily separable â-glycoside 13 (59%), together with
a 6% yield of the R-product under the same reaction
conditions. The 4,6-benzylidenated donor 7 produced a
complex product mixture. In contrast, the corresponding C-3
protected donors 14 and 20 provided 15 and 22, respectively,
in better yields and excellent regioselectivities, indicating
that appropriate protection of the 3-hydroxyl group is critical
for the effective application of this type of glycosyl donor.
In parallel experiments, the stereochemical outcomes of 3,4-
isopropylidenated donors were greatly influenced by the
substitution on C-6. For example, 6-silylated donor 23 gave
cholesterol saponin 24 as an R,â-mixture with low stereo-
selectivity, but 6-deoxy donor 25 afforded 45% yield of
â-glycoside 26. When 2,4-unprotected glucosyl donor 16 was
coupled with diosgenin 10 under the same reaction condi-
tions, only a 36% yield of â-glycoside 17 was obtained, much
lower than that obtained with the corresponding galactosyl
donor 4. Interestingly, using a partially benzylated glucosyl
donor 18 significantly improved the yield (80%) and gave
â-glycoside 19 as the sole product.
The formation of the predominantly â-product can be
rationalized as shown in Scheme 1.10 When glycosylation
Scheme 1
with thioglycoside A was promoted using NIS and TMSOTf,
two reactive intermediates B and C could be generated. The
1,2-anhydrosugar intermediate C is formed through the
intermolecular ring closure of B. Stereoselective opening of
intermediate C would afford â-product E, while the R,â-
mixture D would be obtained through R- and â-attack on
oxocarbenium B.
Coupling reactions between sugar residues were also
investigated using thioglycoside donors with unprotected
2-hydroxyl or 2,4-hydroxyl groups (Table 3). When glucosyl
To ascertain the efficiency of our new synthetic method,
we next applied it to the synthesis of diosgenyl R-L-
rhamnopyranosyl-(1f2)-[â-D-glucopyranosyl-(1f4)]-â-D-
galactopyranoside,5f a potent drug candidate used to decrease
the cholesterol level in serum.
Compound 4, containing unprotected 2,4-hydroxyl groups,
was prepared from commercially available IPTG 35 accord-
ing to the method described by Chan (Scheme 2).11 This
Table 3. Oligosaccharide Synthesis Using Partial Protected
Glycosyl Donor
Scheme 2 a
a An R: â ) 1:2 mixture with additional trisaccharide (15%) was isolated.
b An R: â ) 2:3 mixture.
a Reaction conditions: (a) BzCl, Pyr, -10 °C, 70%. (b) 10, NIS,
TMSOTf, CH2Cl2, -42 °C, 54%. (c) 36, TMSOTf, CH2Cl2, -42
°C; then 37, TMSOTf, CH2Cl2, 0 °C, 68.7% for one-pot reaction.
(d) Aqueous 1 N NaOH, MeOH, 95%.
donor 18 was reacted with 1,2:3,4-di-O-isopropylidene-R-
D-galactopyranose (27) in CH2Cl2 at -42 °C in the presence
of NIS-TMSOTf, a 77% yield of disaccharide 28 was
isolated. However, a similar reaction between 4 and 27 gave
only 50% yield of the R,â-mixture 29. An inseparable R,â-
mixture of glycoside 31 (â: R ) 3:2) was also obtained in
57% yield on glycosylation of 30 with 23. When sugar
acceptors containing secondary hydroxyl groups (32 and 34)
were examined, low yields or complex product mixtures were
obtained (Table 3, entries 4 and 5).
donor was condensed with diosgenin 10 in CH2Cl2 at -42
°C in the presence of NIS-TMSOTf, to afford the desired
(10) (a) Leeuwenburgh, M. A.; Timmers, C. M.; van der Marel, G. A.;
van Boom, J. H.; Mallet, J.-M.; Sinay¨, P. G. Tetrahedron Lett. 1997, 38,
6251. (b) Du, Y.; Kong, F. J. Carbohydr. Chem. 1995, 14, 341.
Org. Lett., Vol. 5, No. 20, 2003
3629

â-glycoside 11 in 54% isolated yield. Compound 11 was
glycosylated in one-pot with rhamnopyranosyl trichloroace-
timidate 36 at -42 °C in the presence of TMSOTf, followed
by glucopyranosyl imidate 37 at 0 °C, to afford the protected
trisaccharide saponin derivative 38 (68.7% from 11). Natural
saponin 39 was then readily obtained by deacylation with
aqueous 1 N NaOH in MeOH (95%). Remarkably, this
complex natural saponin was prepared in four simple steps
and in 35% overall yield.
steroidal glycoside synthesis. More importantly, the applica-
tion of this method with combinatorial chemistry might be
useful as an efficient entry into libraries of more complex
glycoconjugates.12
Acknowledgment. This work was supported by NNSF
of China (29972053), RCEES of CAS, and NIH of the U.S.
(HL62244).
Supporting Information Available: Preparation and
physical data for compounds 3, 6, 9, 11, 13, 15, 17, 19, 22,
26, 28, 29, 33, 38, and 39. This material is available free of
charge via the Internet at http://pubs.acs.org.
In conclusion, an efficient and practical method has been
developed for the preparation of saponins having 2,4-
branched oligosaccharide moieties. The key to this chemistry
is the use of partially unprotected thioglycosides as glycosyl
donors. This results in significantly simplified protecting
group manipulation and oligosaccharide assembly. The
approach described is general and effective for alkyl and
OL035353S
(12) Marcaurelle, L. A.; Seeberger, P. H. Curr. Opin. Chem. Biol. 2002,
6, 289.
(11) Jiang, L.; Chan, T.-H. J. Org. Chem. 1998, 63, 6035.
3630
Org. Lett., Vol. 5, No. 20, 2003