Synthesis of oleanolic acid dimers as inhibitors of glycogen phosphorylase

Article abstract of DOI:10.1002/cbdv.200900086

Recently, oleanolic acid was found to be an inhibitor of glycogen phosphorylase. For further structural modification, we have synthesized several dimers of oleanolic acid by using amide, ester, or triazole linkage with click chemistry. The click chemistry

Full text of DOI:10.1002/cbdv.200900086

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
Synthesis of Oleanolic Acid Dimers as Inhibitors of Glycogen Phosphorylase
by Keguang Chenga)b), Jun Liuc), Hongbin Sun*b), and Juan Xie*a)
a) PPSM, ENS Cachan, CNRS, UniverSud, 61 av President Wilson, F-94230 Cachan
(phone: þ33147405586; fax: þ33147402454; e-mail: joanne.xie@ens-cachan.fr)
b) Center for Drug Discovery, College of Pharmacy, China Pharmaceutical University, 24 Tongjiaxiang,
Nanjing 210009, P. R. China (phone: þ862585327950; fax: þ862583271335;
e-mail: hbsun2000@yahoo.com)
c) Jiangsu Center for Drug Screening, China Pharmaceutical University, 1 Shennonglu, Nanjing 210038,
P. R. China
Recently, oleanolic acid was found to be an inhibitor of glycogen phosphorylase. For further
structural modification, we have synthesized several dimers of oleanolic acid by using amide, ester, or
triazole linkage with click chemistry. The click chemistry was shown to be the most efficient method for
the dimer synthesis. Nearly quantitative yield of triazole-linked dimers was obtained. Biological
evaluation of the synthesized dimers as inhibitors of glycogen phosphorylase has been described. Four of
six dimers exhibited inhibitory activity against rabbit muscle glycogen phosphorylase a (RMGPa), with
compounds 2 and 7 as the most potent inhibitors, which displayed an IC50 value (ca. 3 mm) lower than that
of oleanolic acid (IC50 ¼14 mm).
Introduction. – Oleanolic acid (1; OA) is a naturally existing pentacyclic triterpene.
It possesses some attractive pharmacological activities including protection of the liver
against toxic injury, anti-inflammation, anti-HIV, antitumor, antioxidation, anti-
hyperglycemia, and cardiovascular activities, and has been in active clinical use as an
antihepatitis drug in China for over 20 years [1]. Recently, we have found that oleanolic
acid and related pentacyclic triterpenes displayed also inhibitory activity against
glycogen phosphorylases (GP) [2]. As a safe nonprescription drug for the treatment of
hepatitis, OA may find its new clinical uses in treating diseases caused by disorders in
glycogen metabolism. Since OA is widely distributed in the plant kingdom, and very
cheap and easily available in large bulk, lead modification based on OA would be in a
good position for further mechanistic studies and drug development. Various
derivatives of OA have been synthesized as antitumor [3], antibacterial [4], or anti-
osteoporosis agents [5], as percutaneous transport promoters [6], or as inhibitors of
nitric oxide production [7].
Concerning glycogen phosphorylases, several structural classes of inhibitors have
been reported [8], whose binding sites identified in GP include the catalytic site (which
binds glucose-1-P, glycogen, glucose, and glucose analogues), the Ser14-phosphate
recognition site, the purine inhibitory site (which binds purine derivatives as well as
pentacyclic triterpenes [2]), the allosteric site (which binds the activator AMP and the
inhibitor glucose-6-P), the glycogen storage site, a novel allosteric inhibitor site, and
the newly discovered benzimidazole-binding site [9]. The catalytic site, a deep cavity
ꢀ 2010 Verlag Helvetica Chimica Acta AG, Zꢁrich

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located at the center of the molecule, 15 ꢂ from the protein surface, and close to the
essential cofactor pyridoxal 5’-phosphate (PLP), has been probed with glucose and
glucose analogue inhibitors [8]. Since there are several binding sites in GP, we have
then designed the dimeric compounds in order to increase the binding specificity and
strength by additional binding at allosteric sites. Appending a second pentacyclic
triterpene may also increase the local concentration of the pentacyclic triterpene near
the enzymeꢃs binding sites (statistical rebinding mechanism) [10]. It is noteworthy that
several symmetrical compounds have been screened as GP inhibitors, some of them
bound at the dimer interface site of GP [11], others at the allosteric site of the enzyme
[12]. It is to be noted that in medicinal chemistry, multivalent ligands have been used as
inhibitors of several biological targets (e.g., glycosidase inhibitors) [13].
In this article, we report the synthesis of OA dimers (Fig.) linked at C(3) or C(28)
position via ester, amide, or triazole groups, as well as their inhibitory activities against
glycogen phosphorylases. Our results showed that dimeric compounds 2 and 7
displayed IC50 values lower than that of the monomer 1.
Results and Discussion. – Synthesis. For the comparison of the nature and the length
of the chain linker between two oleanolic acids, we have synthesized compounds 2 to 4
linked at C(28) (Scheme 1). Compound 8 [11] (2 equiv.) was first treated with oxalyl
chloride to give the corresponding acyl chloride, which was then reacted with
diethylenetriamine (1 equiv.) in the presence of Et3N, to afford the dimer 2 in 66%
yield. Esterification of 1 (2 equiv.) with 1,6-dibromohexane (1 equiv.) gave directly the
dimer 3 (57%). Compound 4 was prepared by an esterification reaction between the
known 6-bromohexyl oleanolate 9 [14] and l-malic acid in the presence of K2CO3.
We have also synthesized the dimers 5 to 7 (Scheme 2) by the CuI-catalyzed
Huisgen [2þ3] dipolar cycloaddition reaction (ꢄclick chemistryꢃ) [15]. Reaction of
propargyl oleanolate 10 [14] with 1,6-diazidohexane in the presence of catalytic
amounts of sodium ascorbate and CuSO4 ·5 H2O in CH2Cl2/H2O afforded the dimer 5
in a very good yield (97%). Similarly, click reaction between benzyl 3-O-propargyl
oleanolate 11 [14] with 1,6-diazidohexane led to the dimer 6, which was debenzylated
over Pd/C (10%) in THF/MeOH at room temperature to afford compound 7.
Compounds 2 to 7 have been fully characterized by 1H- and 13C-NMR spectroscopy and
by mass spectrometry.
Bioactivity. The above synthesized derivatives were evaluated in the enzyme
inhibition assay against rabbit muscle glycogen phosphorylase a (RMGPa) (Table)
which shared considerable sequence similarity with human liver GPa. As described
previously [16], the activity of rabbit muscle GPa was measured through detecting the
release of phosphate from glucose-1-phosphate in the direction of glycogen synthesis.
The assay results show that compounds 2 (IC50 ¼3.25 mm), 4 (IC50 ¼12.3 mm), 5
(IC50 ¼20.7 mm), and 7 (IC50 ¼2.59 mm) were inhibitors of GPa. Among the oleanolic
dimers linked at 28-position, the amide-linked dimer 2 displayed the most efficient
inhibition, with a lower IC50 value than oleanolic acid 1 (IC50 ¼14 mm). Compounds
with a longer linker diminished the inhibitory potency (compounds 4 and 5 vs. 2).
Previous results showed that the esterified monomers 9 and 10 inhibited GPa with IC50
values of 32.7 and 405 mm, respectively [14]. However, the ester-linked dimer 3 was not
active. For dimers linked at 3-position (compounds 6 and 7), only the dimer 7 with a

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
Figure. Structure of oleanolic acid (1) and dimers 2–7

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693
Scheme 1. Synthesis of Oleanolic Acid Dimers 2–4
Scheme 2. Synthesis of Oleanolic Acid Dimers 5–7
free carboxylic function inhibited GPa in spite of the inhibitory activity of the starting
monomer 11 [14].
Conclusions. – Oleanolic acid dimers could be easily synthesized using amidifica-
tion, esterification, and click chemistry. Nearly quantitative yield has been obtained for
triazole-linked dimers. This result demonstrated once again the efficiency of this very
popular reaction. The results of the GP inhibition assay indicate that efficient inhibitors

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Table. IC50 Values [mm] for RMGPa Inhibition Assay Results
a
Compound
GPa IC50
14ꢂ1.9
)
1 [2]
2
3
3.25ꢂ0.27
nab)
9 [14]
4
10 [14]
5
11 [14]
6
32.7ꢂ2.9
12.3ꢂ1.4
405ꢂ15.6
20.7ꢂ2.1
26.5ꢂ1.5
nab)
7
2.59ꢂ1.5
Caffeinec)
102.3ꢂ6.7
a) Values are means of three experiments. b) na¼No activity. c) Positive control.
can be obtained by introducing an appropriate linker at either C(3) or C(28) of
oleanolic acid, with N-containing dimers 2 and 7 as the most potent inhibitors in this
series of compounds. Further research and drug development on pentacyclic
triterpenes as promising modulators of glycogen metabolism are ongoing in our
laboratories, and the results will be reported in due course.
K. C. thanks the French Embassy in China for a co-tutor Doctorate fellowship. This work is
supported by a grant from Region Ile-de-France. H. S. thanks financial support by National Natural
Science Foundation of China (grants 30672523 and 90713037), research grants from Chinese Ministry of
Education (grants 706030 and 20050316008) and program for New Century Excellent Talents in
University (NCET-05-0495).
Experimental Part
General. All commercially available solvents and reagents were used without further purification.
Column chromatography (CC): Merck silica gel 60 (SiO2; 230–400 mesh) or SiO2 (200–300 mesh,
Qingdao Ocean Chemical Company, P. R. China). M.p.: RY-1 melting point apparatus. IR Spectra:
Shimadzu FTIR-8400S spectrometer. 1H- and 13C-NMR spectra: Bruker AV-300 spectrometer; chemical
shifts are reported as values from an internal TMS standard. MS: Agilent 1100 LC/DAD/MSD or Q-Tof
Micro MS/MS spectrometers.
Iminobis[ethane-2,1-diylimino(3b)-28-oxoolean-12-ene-28,3-diyl] Diacetate (2). To a soln. of 8
(0.5 g, 1 mmol) in dry CH2Cl2 (0.5 ml) was added oxalyl chloride (1.5 ml, 0.017 mol) at 08. After stirring at
258 for 5 h, the mixture was evaporated, and co-evaporated with CH2Cl2 (3ꢀ1 ml). The residue was
dissolved in dry CH2Cl2 (1 ml), and Et3N (1.4 ml, 10 mmol) and H2N(CH2)2NH(CH2)2NH2 (0.1 ml,
1 mmol) were added at 08. After stirring at r.t. for 24 h, the solvent was evaporated. The residue was
taken up in 2m HCl (30 ml) and extracted with AcOEt (3ꢀ15 ml). The combined org. layers were
washed with H2O and brine, dried (MgSO4), filtered, and concentrated to give a crude product.
Purification by flash CC (CH2Cl2/MeOH 20 :1) gave 2 as a white solid (0.39 g, 66%). M.p. 198–2008. Rf
(CH2Cl2/MeOH, 7:1) 0.49. IR (KBr): 3415, 2946, 2876, 1735, 1659, 1518, 1465, 1368, 1246, 1027. 1H-NMR
(300 MHz, CDCl3): 0.76, 0.82, 0.86, 0.92, 1.16 (5s, 10 Me); 0.87 (s, 4 Me); 0.76–1.99 (m, 44 H); 2.05 (s,
2 MeCO); 2.52–2.57 (m, HꢁC(18), HꢁC(18’)); 2.78–2.80 (m, 2 CH2N); 3.16–3.19 (m, CONCH2);
3.43–3.49 (m, CONCH2), 4.49 (t, J¼7.5, HꢁC(3), HꢁC(3’)); 5.39 (s, HꢁC(12), HꢁC(12’)). 13C-NMR
(75 MHz, CDCl3): 15.9; 17.1; 17.4; 18.6; 21.7; 23.9; 24.2; 26.2; 27.8; 28.4; 31.1; 32.7; 33.0; 33.4; 34.5; 37.3;

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38.1; 38.5; 38.6; 39.9; 42.3; 42.4; 46.8; 47.0; 47.9; 49.0; 55.7; 81.2; 123.6; 144.8; 171.4; 180.8. ESI-MS: 1064
([MþH]þ ), 1086 ([MþNa]þ ).
Hexane-1,6-diyl (3b,3’b)-Bis(3-hydroxyolean-12-en-28-oate) (3). K2CO3 (1.4 g, 9.9 mmol) and 1,6-
dibromohexane (0.51 ml, 3.3 mmol) were added to a soln. of oleanolic acid (3 g, 6.6 mmol) in DMF
(20 ml). After stirring at r.t. for 12 h, the mixture was diluted with H2O (80 ml) and extracted with
AcOEt (3ꢀ40 ml). The combined org. layers were washed successively with 1n HCl, H2O, sat. NaHCO3,
and brine, dried (Na2SO4), filtered, and concentrated to give a crude oil, which was purified by flash CC
(AcOEt/petroleum ether (PE) 1:2) to obtain 3 as a white solid (1.87 g, 57%). M.p. 217–2198. Rf (AcOEt/
PE 1:3) 0.45. IR (KBr): 3516, 2944, 2864, 1720, 1463, 1386, 1364, 1261, 1236, 1215, 1176, 1162, 1030, 1010,
1
995, 761, 667. H-NMR (300 MHz, CDCl3): 0.73, 0.78, 0.92, 0.99, 1.13 (5s, 10 Me); 0.90 (s, 4 Me); 0.90–
2.04 (m, 52 H); 2.84–2.90 (dd, J¼3.0, 13.3, HꢁC(18), HꢁC(18’)); 3.19–3.24 (dd, J¼4.2, 10.2, HꢁC(3),
HꢁC(3’)); 4.01 (t, J¼6.3, 2 CH2CO2); 5.28 (s, HꢁC(12), HꢁC(12’)). 13C-NMR (75 MHz, CDCl3): 15.4;
15.6; 17.1; 18.4; 23.1; 23.5; 23.7; 25.8; 25.9; 27.3; 27.8; 28.2; 28.7; 30.7; 32.6; 32.9; 33.1; 34.0; 37.1; 38.6; 38.8;
39.5; 41.5; 41.8; 46.0; 46.8; 47.7; 55.4; 64.1; 79.1; 122.4; 143.9; 177.7. ESI-MS: 1017 ([MþNa]þ ).
Bis(6-{[(3b)-3-hydroxy-28-oxoolean-12-en-28-yl]oxy}hexyl) (2R)-2-Hydroxybutanedioate (4).
K2CO3 (0.65 g, 4.7 mmol) was added to a soln. of 9 (0.42 g, 0.68 mmol) and l-malic acid (0.45 g,
3.4 mmol) in DMF (5 ml). After stirring at 508 for 48 h, the mixture was filtered, diluted with H2O
(40 ml), and extracted with CH2Cl2 (3ꢀ25 ml). The combined org. layers were washed with H2O and
brine, dried (Na2SO4), filtered, and concentrated to give a crude product, which was purified by flash CC
(AcOEt/PE 1:6) to obtain 4 as a white solid (96 mg, 23%). M.p. 81–828. Rf (AcOEt/PE 1:3) 0.53. IR
(KBr): 3449, 2942, 2867, 1730. 1H-NMR (300 MHz, CDCl3): 0.66, 0.71, 0.85, 0.92, 1.06 (5s, 10 Me); 0.83 (s,
4 Me); 0.66–1.82 (m, 60 H); 2.72–2.82 (m, CH2O, HꢁC(18), HꢁC(18’)); 3.12–3.17 (dd, J ¼ 4.5, 10.3,
HꢁC(3), HꢁC(3’)); 3.94 (t, J¼6.3, 2 CH2CO2); 4.04 (t, J¼6.5, CH2OCO); 4.14 (t, J¼6.6, CH2OCO);
4.42 (br. s, CH); 5.21 (br. s, HꢁC(12), HꢁC(12’)). 13C-NMR (75 MHz, CDCl3): 15.3; 15.6; 17.0; 18.3;
23.0; 23.4; 23.6; 25.4; 25.5; 25.66; 25.7; 25.9; 27.2; 27.7; 28.1; 28.4; 28.5; 29.7; 30.7; 32.5; 32.8; 33.1; 33.9;
37.0; 38.5; 38.7; 39.4; 41.3; 41.7; 45.9; 46.7; 47.6; 55.2; 63.9; 64.0; 65.0; 65.9; 67.3; 79.0; 122.3; 143.8; 170.5;
173.3; 177.7. ESI-MS: 1234 ([MþNa]þ ).
Hexane-1,6-diylbis(1H-1,2,3-triazole-1,4-diylmethanediyl) (3b,3’b)-Bis(3-hydroxyolean-12-en-28-
oate) (5). CuSO4 ·5 H2O (0.042 g, 0.168 mmol) and sodium ascorbate (0.068 g, 0.34 mmol) were added
to a soln. of 10 (0.21 g, 0.42 mmol) and 1,6-diazidohexane (0.035 g, 0.21 mmol) in CH2Cl2 (2 ml) and H2O
(2 ml). After stirring at r.t. for 12 h, the mixture was extracted three times with CH2Cl2. The combined
org. layers were dried (MgSO4), filtered, and concentrated to give 5 as a white solid (0.263 g, 97%). M.p.
134–1368. Rf (AcOEt/PE 1:1) 0.12. IR (KBr): 3437, 2945, 2846, 1723, 1717, 1463, 666. 1H-NMR
(300 MHz, CDCl3): 0.52, 0.74, 0.95, 1.08 (4s, 8 Me); 0.86 (d, J¼2.6, 6 Me); 0.52–1.92 (m, 52 H); 2.84 (d,
J¼12.9, HꢁC(18), HꢁC(18’)); 3.15–3.20 (m, HꢁC(3), HꢁC(3’)); 4.28 (t, J¼6.8, 2 CH2N); 5.14 (br. s,
2 CH2CO2); 5.24 (br. s, HꢁC(12), HꢁC(12’)); 7.55 (br. s, 2 CHN). 13C-NMR (75 MHz, CDCl3): 15.3;
15.6; 16.7; 18.3; 22.9; 23.3; 23.5; 25.7; 25.8; 27.1; 27.6; 28.0; 29.6; 29.9; 30.6; 32.3; 32.6; 33.0; 33.7; 36.9;
38.4; 38.7; 39.2; 41.2; 41.6; 45.8; 46.6; 47.5; 50.0; 55.1; 57.4; 78.8; 122.4; 143.5; 177.6. ESI-MS: 1157 ([Mþ
H]þ ), 1179 ([MþNa]þ ).
Dibenzyl (3b,3’b)-3,3’-[Hexane-1,6-diylbis(1H-1,2,3-triazole-1,4-diylmethanediyloxy)]bisolean-12-
en-28-oate (6). CuSO4 ·5 H2O (0.075 g, 0.3 mmol) and sodium ascorbate (0.12 g, 0.6 mmol) were added
to a soln. of 11 (0.288 g, 0.49 mmol) and 1,6-diazidohexane (0.042 g, 0.25 mmol) in CH2Cl2 (2 ml) and
H2O (2 ml). After stirring at r.t. for 12 h, the mixture was extracted three times with CH2Cl2. The
combined org. layers were dried (MgSO4), filtered, and concentrated to give a crude pink solid.
Purification by CC (AcOEt/PE 2 :1) afforded 6 as a white solid (0.29 g, 88%). M.p. 122–1248. Rf
(AcOEt/PE 1:1) 0.27. IR (KBr): 2944, 2864, 1725, 1462, 1158, 1074, 736, 697. 1H-NMR (300 MHz,
CDCl3): 0.63, 0.81, 0.92, 0.95, 1.15 (5s, 10 Me); 0.93 (s, 4 Me); 0.63–2.08 (m, 52 H); 2.92–3.03 (m,
HꢁC(18), HꢁC(18’), HꢁC(3), HꢁC(3’)); 4.36 (t, J¼6.8, 2 CH2N); 4.61, 4.83 (d, J¼12.5, 2 CH2O);
5.09, 5.11 (d, J¼12.5, 2 CH2Ph); 5.32 (s, HꢁC(12), HꢁC(12’)); 7.36 (m, 10 arom. H); 7.53 (s, 2 CHN).
13C-NMR (75 MHz, CDCl3): 15.3; 16.4; 16.9; 18.2; 22.7; 23.0; 23.4; 23.6; 25.7; 25.8; 27.6; 28.1; 30.0; 30.7;
32.4; 32.7; 33.1; 33.8; 36.9; 38.3; 38.7; 39.3; 41.4; 41.7; 45.8; 46.7; 47.5; 49.9; 55.6; 63.3; 65.9; 86.5; 121.9;
122.5; 127.8; 127.9; 128.4; 136.4; 143.6; 146.5; 177.4. ESI-MS: 1338 ([MþH]þ ), 1360 ([MþNa]þ ).

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(3b,3’b)-3,3’-[Hexane-1,6-diylbis(1H-1,2,3-triazole-1,4-diylmethanediyloxy)]bisolean-12-en-28-oic
Acid (7). 10% Pd/C (0.009 g) was added to a soln. of 6 (0.045 g, 0.034 mmol) in THF (1.5 ml) and MeOH
(1 ml). After stirring at r.t. under H2 at atmospheric pressure for 12 h, the mixture was filtered through
Celite. The filtrate was concentrated to give a crude product, which was purified by flash CC (AcOEt/PE
1
4 :1) to obtain 7 as a white solid (0.034 g, 87%). M.p. 203–2048. Rf (AcOEt/PE 3 :1) 0.09. H-NMR
(300 MHz, CDCl3): 0.69, 0.75, 0.90, 0.93, 0.95, 0.96, 1.10 (7s, 14 Me); 0.59–2.04 (m, 52 H); 2.82–2.85 (m,
HꢁC(18), HꢁC(18’)); 2.95–2.99 (m, HꢁC(3), HꢁC(3’)); 4.26 (t, J¼7.0, CH2N); 4.40 (t, J¼7.0, CH2N);
4.42 (d, J¼11.8, CH2O); 4.84 (d, J¼11.8, CH2O); 5.27 (s, HꢁC(12), HꢁC(12’)); 7.47 (s, 2 CHN).
13C-NMR (75 MHz, CDCl3): 15.4; 17.1; 17.8; 18.2; 22.4; 22.8; 23.3; 23.6; 26.0; 26.2; 27.7; 28.5; 29.7; 30.2;
30.7; 32.3; 32.7; 33.1; 33.7; 37.2; 37.8; 38.8; 39.2; 40.8; 41.2; 45.7; 46.5; 47.3; 50.3; 55.4; 63.9; 77.2; 86.5;
121.3; 122.8; 128.0; 128.4; 143.6; 147.2; 185.1. ESI-MS: 1157 ([MþH]þ ), 1179 ([MþNa]þ ).
Enzyme Assay. The inhibitory activity of the test compounds against rabbit muscle glycogen
phosphorylase a (RMGPa) was monitored using microplate reader (BIO-RAD) based on the published
method [16]. In brief, GPa activity was measured in the direction of glycogen synthesis by the release of
phosphate from glucose-1-phosphate. Each test compound was dissolved in DMSO and diluted at
different concentrations for IC50 determination. The enzyme was added into 100 ml of buffer containing
50 mm HEPES (pH¼7.2), 100 mm KCl, 2.5 mm MgCl2, 0.5 mm glucose-1-phosphate, 1 mg/ml glycogen,
and the test compound in 96-well microplates (Costar). After the addition of 150 ml of 1m HCl containing
10 mg/ml ammonium molybdate and 0.38 mg/ml malachite green, reactions were run at 228 for 25 min,
and then the phosphate absorbance was measured at 655 nm. The IC50 values were estimated by fitting
the inhibition data to a dose-dependent curve using a logistic derivative equation.
REFERENCES
[1] J. Liu, J. Ethnopharmacol. 1995, 49, 57; P. Dzubak, M. Hajduch, D. Vydra, A. Hustova, M. Kvasnica,
D. Biedermann, L. Markova, M. Urban, J. Sarek, Nat. Prod. Rep. 2006, 23, 394.
[2] X. Wen, H. Sun, J. Liu, K. Cheng, P. Zhang, L. Zhang, J. Hao, L. Zhang, P. Ni, S. E. Zographos, D. D.
Leonidas, K.-M. Alexacou, T. Gimisis, J. M. Hayes, N. G. Oikonomakos, J. Med. Chem. 2008, 51,
3540; J. Chen, J. Liu, L. Zhang, G. Wu, W. Hua, X. Wu, H. Sun, Bioorg. Med. Chem. Lett. 2006, 16,
2915; J. Hao, P. Zhang, X. Wen, H. Sun, J. Org. Chem. 2008, 73, 7405; K. Cheng, P. Zhang, J. Liu, J.
Xie, H. Sun, J. Nat. Prod. 2008, 71, 1877.
[3] H.-J. Jeong, H.-B. Chai, S.-Y. Park, D. S. H. L. Kim, Bioorg. Med. Chem. Lett. 1999, 9, 1201.
[4] F. Hichri, H. B. Jannet, J. Cheriaa, S. Jegham, Z. Mighri, C. R. Chim. 2003, 6, 473.
[5] Y. Zhang, J.-X. Li, J. Zhao, S.-Z. Wang, Y. Pan, K. Tanaka, S. Kadota, Bioorg. Med. Chem. Lett. 2005,
15, 1629.
[6] L. Zaprutko, D. Partyka, B. Bednarczyk-Cwynar, Bioorg. Med. Chem. Lett. 2004, 14, 4723.
[7] T. Honda, B. V. Rounds, L. Bore, H. J. Finlay, F. G. Favaloro Jr., N. Suh, Y. Wang, M. B. Sporn, G. W.
Gribble, J. Med. Chem. 2000, 43, 4233.
´
[8] L. Agius, Best Pract. Res. Clin. Endocrin. Metab. 2007, 21, 587; N. G. Oikonomakos, L. Somsak, Curr.
´
´
´
´
Opin. Invest. Drugs 2008, 9, 379; L. Somsak, K. Czifrak, M. Toth, E. Bokor, E. D. Chrysina, K.-M.
Alexacou, J. M. Hayes, C. Tiraidis, E. Lazoura, D. D. Leonidas, S. E. Zographos, N. G. Oikonoma-
kos, Curr. Med. Chem. 2008, 15, 2933; D. J. Baker, P. L. Greenhaff, J. A. Timmons, Expert Opin.
Ther. Pat. 2006, 16, 459; N. G. Oikonomakos, Curr. Protein Pept. Sci. 2002, 3, 561.
[9] E. D. Chrysina, M. N. Kosmopoulou, C. Tiraidis, R. Kardakaris, N. Bischler, D. D. Leonidas, Z.
´
Hadady, L. Somsak, T. Docsa, P. Gergely N. G. Oikonomakos, Protein Sci. 2005, 14, 873.
[10] R. J. Pieters, Org. Biomol. Chem. 2009, 7, 2013.
[11] K. Onda, R. Shiraki, Y. Yonetoku, K. Momose, N. Katayama, M. Orita, T. Yamaguchi, M. Ohta, S.
Tsukamoto, Bioorg. Med. Chem. 2008, 16, 8627; V. L. Rath, M. Ammirati, D. E. Danley, J. L.
Ekstrom, E. M. Gibbs, T. R. Hynes, A. M. Mathiowetz, R. K. McPherson, T. V. Olson, J. L.
Treadway, D. J. Hoover, J. Chem. Biol. 2000, 7, 677.
[12] C. Tiraidis, K.-M. Alexacou, S. E. Zographos, D. D. Leonidas, T. Gimisis, N. G. Oikonomakos,
Protein Sci. 2007, 16, 1773.

CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
697
[13] J. E. Gestwicki, C. W. Cairo, L. E. Strong, K. A. Oetjen, L. L. Kiessling, J. Am. Chem. Soc. 2002, 124,
14922; D. Fournier, D. Poirier, Bioorg. Med. Chem. Lett. 2009, 19, 693; A. L. LaFrate, K. E. Carlson,
J. A. Katzenellenbogen, Bioorg. Med. Chem. 2009, 17, 3528; T. Wennekes, R. J. B. H. N. van den
Berg, K. M. Bonger, W. E. Donker-Koopman, A. Ghisaidoobe, G. A. van der Marel, A. Strijland,
J. M. F. G. Aerts, H. S. Overkleeft, Tetrahedron: Asymmetry 2009, 20, 836.
[14] K. Cheng, J. Liu, X. Liu, H. Li, H. Sun, J. Xie, Carbohydr. Res. 2009, 344, 841.
[15] H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem., Int. Ed. 2001, 40, 2004; C. W. Tornøe, C.
Christensen, M. Meldal, J. Org. Chem. 2002, 67, 3057; H. C. Kolb, K. B. Sharpless, Drug Discovery
Today 2003, 8, 1128.
[16] W. H. Martin, D. J. Hoover, S. J. Armento, I. A. Stock, R. K. McPherson, D. E. Danley, R. W.
Stevenson, E. J. Barrett, J. L. Treadway, Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 1776.
Received March 23, 2009