Synthesis of oleanolic disaccharide derivatives

Article abstract of DOI:10.1016/j.cclet.2012.01.022

Oleanolic acid 1 exists widely in food, medicinal herbs and other plants and possesses many attractive biological activities. Recently, some oleanolic derivatives were reported to have interesting anti-tumor activities. Herein, using association principle

Full text of DOI:10.1016/j.cclet.2012.01.022

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Chinese Chemical Letters 23 (2012) 403–406
www.elsevier.com/locate/cclet
Synthesis of oleanolic disaccharide derivatives
Shu Chen Pei a, Jian Bo Wu a, Fan Lei a, Shan Qian b, Li Hai a, Yong Wu a,
*
a Key Laboratory of Drug Targeting and Drug Delivery System of the Education Ministry, Department of Medicinal Chemistry,
West China School of Pharmacy, Sichuan University, Chengdu 610041, China
b Bioengineering College, Xihua University, Chengdu 610039, China
Received 25 October 2011
Available online 4 March 2012
Abstract
Oleanolic acid 1 exists widely in food, medicinal herbs and other plants and possesses many attractive biological activities.
Recently, some oleanolic derivatives were reported to have interesting anti-tumor activities. Herein, using association principle of
drug design, OA disaccharide 17 was designed and synthesized by coupling a-L-rhamnopyranosyl-(1!2)-a-L-arabinopyranosyl
disaccharide moiety, which has proved to be a unique sugar sequence with strongly antitumor activity, and modified OA with a
nitrile at C-17.
# 2012 Yong Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Oleanolic acid; Rhamnopyranosyl; Arabinopyranosyl; Anti-tumor; Association principle
Oleanolic acid (3b-hydroxy-olea-12-en-28-oic acid, OA, 1), firstly separated from Oleaeurepaea, exists widely in
food, medicinal herbs and other plants [1]. In recent years, it was found that OA had marked anti-tumor effects and
exhibited cytotoxic activity toward many cancer cell lines [2,3], while these potencies are relatively weak. Therefore,
it is necessary to further study the structure–activity relationship based on chemical modification of oleanane
triterpenoids.
In addition, the a-L-rhamnopyranosyl-(1!2)-a-L-arabinopyranosyl disaccharide moiety has proved to be an
unique sugar sequence which strongly induces antitumor activity of oleanane-type glycosides [4–7] and has attracted
more and more attention of researchers. Moreover, it was reported that modified OA with a nitrile at C-17 was much
more potent than that with a carboxyl group [8]. In this context, using association principle, OA disaccharide 17 was
designed and synthesized by coupling this disaccharide moiety to the 3-position of modified OA with a nitrile at C-17,
expecting to obtained a more promising anti-tumor agent.
Nitrile 6 was synthesized from OA by the method [8] shown in Scheme 1. OA 1 was introduced acetyl in C-3 with
acetic anhydride in pyridine to obtain ester 2. Addition of oxalyl chloride to the above ester gave acyl chloride 3 in
quantitative yield, which reacted with ammonia gas to afford amide 4, then dehydrated with thionyl chloride gave 5.
Cyano derivative 5 was hydrolyzed under the effectiveness of NaOH to afford nitrile 6.
The usual strategy of preparing the oleanane-type glycosides bears the a-L-rhamnopyranosyl-(1!2)-a-L-
arabinopyranosyl disaccharide moiety followed by coupling with the aglycon. As we reported previously [9],
* Corresponding author.
E-mail address: wyong@scu.edu.cn (Y. Wu).
1001-8417/$ – see front matter # 2012 Yong Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2012.01.022

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S.C. Pei et al. / Chinese Chemical Letters 23 (2012) 403–406
Scheme 1. Synthesis of 6. Reagents and conditions: (a) Ac2O, pyridine, rt, 95.6%; (b) (COCl)2, CH2Cl2, rt; (c) NH3, CH3OH, rt, 76% yield over
steps b and c; (d) POCl3, Et3N, CH2Cl2, rt, 73%; (e) 4 mol/L NaOH aq, THF/MeOH, rt, 95%.
trichloroacetimidate 10 was successfully carried out from arabinose via seven steps. However, it was confirmed so
difficult to obtain an a-anomer merely in many repeated experiments, furthermore separation of the two anomers
completely was not achieved even by HPLC. The existence of the two anomers was proved by the distinction of the
coupling constant of the glycosyl hydrogen. This could be ascribed to lack of a neighboring participating group on C-2
of arabinose residue which leaded the reaction to afford single a-anomers selectively (Scheme 2) [10]. So, this strategy
is not adoptable.
Finally, stepwise glycosylation was adopted in our work and this strategy provided a versatile approach for
preparing glycoside family by altering monosaccharide donors. Trichloroacetimidates 12 and 14 were prepared by the
method we reported previously [9]. Firstly, coupling of nitrile 6 and 12 promoted by trimethylsilyl
trifluoromethanesulfinate (TMSOTf) gave benzoyl protected monoglycoside 13a. Then Et3N was added to neutralize
the Lewis acid after glycosylation finished and an additional step of deprotection was performed to provide
monoglycoside 13b. 13c was prepared successfully by selective shelter of hydroxyl groups at C-3 and C-4 of arabinose
residue on intermediate using 2, 2-dimethoxypropane. Coupling of monoglycoside 13c and 14 promoted by TMSOTf
gave diglycoside with protection group 15, followed by removal of protecting groups to afford benzylated diglycoside
16. Finally, removal of benzoyl group through NaOMe afforded diglycoside 17 (Scheme 3), which was verified by
elemental analyses, IR, NMR and ESIMS data analysis [11]. The 1H NMR data displayed an olefinic hydrogen at dH
5.35 and glycosyl hydrogen at dH 4–5, and the coupling constants between the vicinal protons at positions 1 and 2 of
Scheme 2. Synthetic method 1 of diglycoside. Reagents and conditions: (a) BnOH, BF3ꢀEt2O, 80 8C; (b) (CH3O)2C(CH3)2, TsOH, acetone, rt, 56%
˚
yield over steps a and b; (c) pentabenzoyl-a-L-rhamnopyranosyl, TMSOTf, 4 A MS, CH2Cl2, ꢁ40 8C, 71%; (d) TsOH, CH2Cl2/CH3OH, rt, 86%; (e)
BzCl, pyridine, rt, 95%; (f) H2, 10% Pd/C, MeOH, rt, 93%; (g) CCl3CN, DBU, CH2Cl2, rt, 34%.

S.C. Pei et al. / Chinese Chemical Letters 23 (2012) 403–406
405
NH
CCl3
O
CN
O
OBz
OR
1
BzO
O
CCl3
NH
BzO
14
OBz
O
O
R O
1
BzO
a
O
OBz
OR
2
12
13a: R =OBz, R =OBz
1
2
b
c
13b: R =OH, R =OH
1
2
d
13c: R =C(CH ) , R =OH
1
3 2
2
CN
CN
O
OH
O
O
O
HO
O
O
e
O
O
O
O
16
BzO
15
BzO
BzO
OBz
BzO
OBz
f
CN
OH
O
O
HO
HO
O
O
17
HO
OH
˚
Scheme 3. Synthesis of 17. Reagents and conditions: (a) 6, TMSOTf, 4 A MS, CH2Cl2, ꢁ40 8C; (b) NaOMe, CH2Cl2–MeOH, rt, 26% yield over
˚
steps a and b; (c) (CH3O)2C(CH3)2, TsOH, acetone, rt, 90%. (d) TMSOTf, 4 A MS, CH2Cl2, ꢁ40 8C ! ꢁ20 8C, yield: 93%; (e) TsOH, CH2Cl2/
CH3OH, rt, 86%; (f) NaOMe, CH2Cl2–MeOH, rt, 51% yield over steps e and f.
the arabinose was 5.5, which dedicated the diglycoside was a single a-anomer. The IR spectrum showed an absorption
band at 2230 cmꢁ1, suggesting the formation of the nitrile moiety, and an absorption band at 1640 cmꢁ1, indicating the
C-12, C-13 double bond. Elemental and ESIMS analysis confirmed the structure of diglycoside 17.
Preliminary result of the bioactivity evaluation showed 17 had good inhibitory activity against HepG2 cells in vitro.
Further biological evaluation of 17 is also in progress. Base on these results, further design and biological evaluation of
OA derivatives as promising drugs are ongoing, and the results will be reported in due course.
Acknowledgments
This work was supported by the National Natural Sciences Foundation of China (No. 81072532) and Scientific
Research Fund of Sichuan Provincial Education Department (No. 11205023).
References
[1] N. Zhu, S. Sheng, S. Sang, J. Agric. Food Chem. 50 (2002) 865.
[2] L. Chen, Y.H. Zhang, X.W. Kong, J. Med. Chem. 51 (2008) 4834.
[3] D. Huang, Y. Ding, Y. Li, Cancer Lett. 233 (2006) 289.
[4] Y. Mimaki, M. Kuroda, T. Asano, J. Nat. Prod. 62 (1999) 1279.
[5] C. Barthomeuf, E. Debiton, V. Mshvidadze, Planta Med. 68 (2002) 672.
[6] H.J. Jung, C.O. Lee, K.T. Lee, J. Biol. Pharm. Bull. 27 (2004) 744.
[7] H.J. Park, S.H. Kwon, J.H. Lee, Planta Med. 67 (2001) 118.

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S.C. Pei et al. / Chinese Chemical Letters 23 (2012) 403–406
[8] T. Honda, Y. Honda, F.G. Favaloro, Bioorg. Med. Chem. Lett. 12 (2002) 1027.
[9] S. Qian, J.H. Li, Y.W. Zhang, J. Asian Nat. Prod. Res. 12 (2010) 20.
[10] M.A. Leeuwenburgh, C.M. Timmers, G.A. Van der Marel, Tetrahedron Lett. 38 (1997) 6251.
[11] Physical and spectral data of the key intermediate and the target compound. 6: Mp > 250 8C; [a]20D +80 (c 0.5, CH2Cl2); 1H NMR (CDCl3,
400 MHz): d 5.36 (s, 1H), 3.21 (dd, 1H, J = 10.8, 4.0 Hz), 2.57 (dd, 1H, J = 13.6, 4.8 Hz), 1.14, 1.04, 1.00, 0.96, 0.91, 0.88, 0.80 (s each, 21H,
each 3H). 17: Mp: 190–192 8C; [a]20 ꢁ0.6 (c 0.5, CH2Cl2); IR (KBr): ymax 3421, 2940, 2230, 1640, 1462, 1387, 1259, 1058 cmꢁ1; 1H NMR
D
(CDCl3, 400 MHz): d 5.35 (t, 1H, J = 3.6 Hz, H-12), 5.02 (d, 1H, J = 5.5 Hz, Ara H-1), 4.59 (brs, 1H, –OH), 4.04 (brs, 1H, Rha H-1), 3.70–3.90
(m, 6H), 3.40–3.60 (m, 3H), 2.60–3.90 (brs, 4H, –OHꢂ4), 3.13 (brd, 1H, J = 6.4 Hz, H-3), 2.57 (d, 1H, J = 12 Hz, H-18), 2.07 (d, 1H,
J = 6.8 Hz, H-11), 1.25 (s, 3H, H-26), 1.13 (s, 3H, H-27), 1.02 (s, 3H, H-25), 0.98 (s, 3H, H-23), 0.94 (s, 3H, H-24), 0.91 (s, 3H, H-29), 0.81 (s,
3H, H-30); ESI-MS m/z: 716 [M+H]+; Anal. Calcd. for C41H65NO9: C, 68.78; H, 9.1; N, 1.96. Found: C, 68.67; H, 9.06; N, 1.89.