C-7 modified flavonoids as novel tyrosyl-tRNA synthetase inhibitors

Article abstract of DOI:10.1039/c6ra28061g

Twenty C-7 modified flavonoids were designed and synthesized. Biological evaluation in vitro indicated that compounds generated by SYBYL-X with high scores also showed good inhibitory activities against TyrRS. Compounds containing the nargenin core exhibi

Full text of DOI:10.1039/c6ra28061g

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C-7 modified flavonoids as novel tyrosyl-tRNA
synthetase inhibitors†
Cite this: RSC Adv., 2017, 7, 6193
Zhu-Ping Xiao, Wei Wei,a Qi Liu,a Peng-Fei Wang,b Xing Luo,a Fang-Yuan Chen,a
a
*
a
a
a
ab
*
Yang Cao, Hong-Xia Huang, Mi-Mi Liu and Hai-Liang Zhu
Twenty C-7 modified flavonoids were designed and synthesized. Biological evaluation in vitro indicated that
compounds generated by SYBYL-X with high scores also showed good inhibitory activities against TyrRS.
Compounds containing the nargenin core exhibit better enzyme inhibitory activities than other flavonoid
cores, with (S)-5-hydroxy-40-hydroxy-7-(2-morpholino-2-oxoethoxy)-2,3-dihydroflavone (b1) being the
most active (IC50 ¼ 0.10 ꢀ 0.01 mM) in all assayed compounds. All compounds were also assayed for
antimicrobial activities against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa,
and b1 also displayed excellent activity, showing 6-fold more potent than the marketed antibiotic
ciprofloxacin. In comparison with Gram-positive organism, all these derivatives exhibited better activity
against Gram-negative organism, and did not displayed significant differences between the two assayed
Gram-negative strains (E. coli ATCC 8739 and P. aeruginosa ATCC 9027).
Received 12th December 2016
Accepted 6th January 2017
DOI: 10.1039/c6ra28061g
www.rsc.org/advances
our efforts have focused on the development of novel TyrRS
inhibitors based on modications of 3-arylfuran-2(5H)-one
core.20,22,23 4-Hydroxyphenyl, a moiety of the substance of TyrRS,
1. Introduction
Since the rst therapeutic use of penicillin in 1941, each newly
marketed antibiotic has invariably resulted in the emergence of
drug-resistant pathogens,1–3 which has rendered marketed
antibiotics less effective.4–6 One particularly troubling event is
the widespread emergence of MRSA (methicillin resistant
Staphylococcus aureus) strains, which are resistant to many
major classes of drugs.7,8 Therefore, new classes of antibacterial
agents are urgently needed to combat drug-resistant bacteria.
Flavonoids, natural products of the benzopyran class
heterocycles, ubiquitously present in fruits and vegetables.9 In
the past decades, a wide range of important biological activities
have also been described, such as antioxidant, antibacterial,
selective anticancer, anti-inammatory and antiallergic prop-
erties.10–16 However, most avonoids were not developed as
clinical drugs because of poor bioavailability (less than 5%) and
induction or inhibition of some metabolic enzymes.17 There-
fore, great interest has been attracted in the eld of structural
modications of the avonoid skeleton for improving
bioavailability and toxicity proles.18
binds at the bottom of the active pocket,24 which is oen
observed in avonoid molecules, such as naringenin from
pomelo, apigenin from celery and daidzein from soybean
(Fig. 1). Based on this consideration, modications of avonoid
skeleton were here performed to mimic the substance of TyrRS
(Tyrosyl-tRNA Synthetase), tyrosyl adenylate, as possible TyrRS
inhibitors. Therefore, a series of novel avonoid derivatives were
designed and synthesized, and were subsequently evaluated for
biological activities against a representative Gram-positive
AaRSs (aminoacyl tRNA synthetases) play a crucial role in the
process of protein synthesis, and are believed to be novel targets
of antibacterial, antifungal and antiparasitic drugs.19–21 Recently,
aCollege of Chemistry and Chemical Engineering, Hunan Engineering Laboratory for
Analyse and Drugs Development of Ethnomedicine in Wuling Mountains, Jishou
University, Jishou 416000, P. R. China. E-mail: xiaozhuping2005@163.com
bState Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing
210093, P. R. China. E-mail: zhuhl@nju.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra28061g
Fig. 1 Structures of representative flavonoids.
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organism (Staphylococcus aureus ATCC 6538) and two Gram- modules. Compounds were selected according to synthesis
negative organisms (Escherichia coli ATCC 8739; Pseudomonas possibility and values of Total_Score and Cscore, and the ob-
aeruginosa ATCC 9027). The results indicated that some of the tained top 20 compounds (b1–b20) were listed in Table 1.
synthesized compounds show very good antibacterial activities.
2.2 Synthesis and characterization
The synthetic route illustrated in Schemes 1 and 2 was devel-
2. Results and discussion
oped according to the reported methods with some modica-
tion,28–31 and the prepared compounds are depicted in Table 1.
2.1 Design and virtual screen
In general, deprotonation and alkylation of the 5-hydroxyl
Based on scaffold hopping drug design, our laboratory recently
developed several novel TyrRS inhibitors.25,26 As a continuation
of this drug discovery program, scaffold hopping of tyrosyl
adenylate was performed by Surex Sim suite of SYBYL-X 2.1.1
(ref. 27) according to the hypothesis mentioned above. In
details, tyrosyl adenylate were selected as the template for
superposing with a series of typical O-gra avonoids based on
shape matching method (Fig. 2). Results revealed that 7-O-gra
avonoid was able to be the scaffold hopping structure of
tyrosyl adenylate. To further determine the possibility of
developing TyrRS inhibitors from this scaffold, compound
library with C-7 side chain modications were generated using
the Library Design soware module within the SYBYL-X envi-
ronment. Subsequently, virtual screening was performed using
docking mode of Surex-Dock soware module, where 20
possible conformations (poses) for each docked ligand were
produced. The docked conformations of ligands were evaluated
and ranked using Total_Score and Cscore (Concensus score)
group is hardly since the hydrogen atom of the 5-hydroxyl group
is attracted to the oxygen atom of the 4-carbonyl group forming
intramolecular hydrogen bond.32 Chrysin therefore shows
excellent O-alkylation selectivity at 7-OH in an alkaline envi-
ronment, which followed by the production of compounds b16–
b20. This was supported the signal at about d 12.8 in 1H NMRs
of b16–b20, which was assigned to the hydrogen atom of 5-OH.
Similarly, 5-OH in b1–b6 leaves intact in alkylation of their
precursors naringenin and genistein. In comparison with 40-
OH, 7-OH shows 2 orders of magnitude decrease in pKa values
of naringenin and genistein,32–34 and alkylation of the hydroxyl
groups occurs following a specic sequential position order: 7 >
40.35,36 The signals at about d 12.8 and 9.6 in b1–b6 were assigned
to 5-OH and 40-OH, respectively.37–39 These facts suggested the
selective O-alkylation of 7-OH.
2.3 Inhibitory activities of avonoid derivatives against
TyrRS
Compounds b1–b20 were synthesized (Schemes 1 and 2) to test
for inhibitory activity against TyrRS from E. coli, and IC50s of
these compounds are presented in Table 1. The main avonoids
involving in modications are naringenin, genistein, 40-O-meth-
yldaidzein and chrysin. In these four series, derivatives from
naringenin were determined to be the most potent, and
compound b1 with a morpholine ring in the 7-side chain dis-
played IC50 of 0.10 ꢀ 0.01 mM against TyrRS, being the most
active. Substitution of the genistein core (b3) for naringenin core
(b1) led to a slight decrease in potency. It was changed into
daidzein or chrysin core, however, resulting in a signicant
decrease of potency with IC50 values of 6- to 70-fold increase (b1
vs. b12 or b19, b2 vs. b9 or b18). It can be seen that R4 at the C-7
side chain is also a major inuence of molecule potency, and
variation of the group at this position may result in large changes
of IC50 values. N-Containing heterocyclic compounds were more
active than alkyl or aromatic amino containing analogues, with
evidences of anilino and benzylamino containing compounds
(b7 and b8) being completely loss of activity against this enzyme.
Replacement of morpholino with piperidino or N-methylpiper-
azino caused 2 to 5-fold decreases in potency, while a dramatic
decrease was observed when it was changed into pyrrolidino
group (b1 vs. b2, b12 vs. b9 and b19 vs. b18).
2.4 Antibacterial activity
All compounds were tested against representative Gram-
positive organism (S. aureus ATCC 6538) and two Gram-
negative organisms (E. coli ATCC 8739 and P. aeruginosa ATCC
9027), which are presented in Table 2. The results revealed that
Fig. 2 Scaffold hopping and library design. Superpositions of tyrosyl
adenylate (white) with a 7-O-graft flavonoid derivative (light purple)
and 5-O-graft flavonoid derivative (light brown) generated by Surflex
Sim suite of SYBYL-X 2.1.1.
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Table 1 Scores of the obtained top 20 compounds from virtual screen Table 1 (Contd.)
and their inhibitory activities against TyrRS
Entry
R1
H
R2
R3
H
R4
Score
8.62
IC50 (mM)
2.0 ꢀ 0.1
Entry
R1
R2
H
R3
R4
Score
10.20
IC50 (mM)
b12
b1
OH
0.10 ꢀ 0.01
b13
b14
H
H
H
H
7.42
7.21
34 ꢀ 1.8
41 ꢀ 2.2
b2
b3
b4
b5
b6
b7
b8
b9
b10
b11
H
OH
OH
OH
OH
OH
H
8.65
9.26
7.74
9.15
9.26
7.21
6.99
7.99
8.82
8.54
2.7 ꢀ 0.1
0.20 ꢀ 0.01
17 ꢀ 0.9
0.60 ꢀ 0.02
1.10 ꢀ 0.07
>100
H
H
H
H
H
H
H
H
H
b15
b16
H
H
8.22
8.34
6.7 ꢀ 0.3
20 ꢀ 1.1
H
OH
b17
b18
b19
H
H
H
OH
OH
OH
8.57
7.56
8.66
13 ꢀ 0.8
86 ꢀ 6.2
6.5 ꢀ 0.4
H
>100
b20
H
OH
7.74
28 ꢀ 1.6
H
16 ꢀ 1.0
7.7 ꢀ 0.3
5.2 ꢀ 0.2
H
compounds with outstanding inhibition against TyrRS also
exhibit strong antibacterial activities, which shows that their
inhibition of TyrRS may lead to the antibacterial activity. In
comparison with Gram-positive organism, all these derivatives
exhibited better activity against Gram-negative organism, and
signicant differences were not observed between the two
assayed Gram-negative strains. Only one compound b1 was
proved to exhibit activity comparable to that of the marketed
antibiotic penicillin G against S. aureus ATCC 6538. While three
H
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Table 2 Inhibitory activity (MIC50) of the synthesized compounds
against pathogensa
MIC50 (mg mLꢁ1
)
Entry
A
B
C
b1
b2
b3
b4
b5
b6
b7
b8
0.11
1.8
0.29
2.7
0.55
56.1
0.71
1.2
>100
>100
42.4
2.2
0.59
14.8
1.6
47.8
9.3
10.9
>100
>100
35.5
12.9
6.8
0.37
44.3
0.55
0.67
>100
>100
30.2
1.0
b9
b10
b11
2.7
3.0
b12
b13
b14
b15
b16
b17
b18
b19
15.3
>100
>100
10.2
70.4
18.5
>100
6.7
18.7
>100
>100
14.2
94.9
22.7
>100
8.5
29.4
>100
>100
25.6
82.7
31.3
>100
17.8
98
Scheme 1 Synthetic route of compounds b1–b2.
b20
Ciprooxacin
Penicillin G
>100
0.67
—
>100
0.81
—
—
0.54
a
(A) E. coli ATCC 8739; (B) P. aeruginosa ATCC 9027; (C) S. aureus ATCC
6538.
Table 3 Cytotoxicity data of selected compounds
Entry
EC50 (mM)
b1
b3
b5
b6
b10
>100
>100
>100
>100
>100
Scheme 2 Synthetic route of compounds b3–b20.
compounds, b1, b3 and b5, showed better activities than mar-
keted antibiotic ciprooxacin against Gram-negative organ-
isms, together with two compounds (b6 and b10) being
compared to ciprooxacin. Out of these compounds, b1 was the
most active with MIC50 of 0.11 mg mLꢁ1, showing 6-fold more
potent than ciprooxacin.
evidence for the potency of inhibition. Initially sketched b1 was
subjected to energy minimization (10 000 iterations) by the
Powell minimization standard method. The nal b1 confor-
mational coordinates were stored as a mol2 le. Accuracy
docking was performed using the Surex-Dock GeomX soware
module within the SYBYL-X 2.1.1 environment. Molecular
docking results revealed that b1 docks favorably into the active
site of TyrRS, and adopts a binding pose as that of co-
crystallized ligand Tyr-AMS (5-O-[N-(L-tyrosyl)sulfamoyl]adeno-
sine) (Fig. 3). Along the active site, the morpholino moiety packs
in the upper cle of the binding pocket, and forms hydrophobic
interactions with Val 53, Leu 227 and Ile 228. Oxygen atom at C-
7 as a hydrogen bond acceptor receives two strong hydrogen
bond interactions from Gly 197 and Asp 200 with H/O
2.5 In vitro toxicity
As these derivatives were inhibitors against TyrRS, we studied
the toxicity of active compounds measuring cytotoxicity (EC50
)
against mammalian cells (HeLa). None of the derivatives
showed signicant toxicity up to 100 mM concentration (Table
3), which indicates assayed compounds showed selectivity >100.
˚
distances of 2.185 and 2.868 A, while Asp 200 in turn as
2.6 Docking studies
a acceptor builds another O–H/O hydrogen bond with 5-OH.
On the contrary, the B-ring of the naringenin core drops
Molecular dockings of the most active compound b1 were per-
formed to illustrate the binding mode and to provide an
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carried out with silica gel 60 (200–300 mesh ASTM, E. Merck).
The quantity of silica gel used was 30–70 times the weight
charged on the column. Then, the eluates were monitored using
TLC (thin-layer chromatography). Melting points (uncorrected)
were determined on a XT4 MP apparatus (Taike Corp., Beijing,
China). ESI mass spectra were obtained on a Mariner System
5304 mass spectrometer, and 1H NMR and 13C NMR spectra
ꢂ
were recorded on Bruker AV-300 or 400 spectrometer at 25 C
with TMS and solvent signals allotted as internal standards.
Chemical shis were reported in ppm (d). Elemental analyses
were performed on a CHN-O-Rapid instrument and were within
ꢀ0.4% of the theoretical values.
3.1.1 General procedure for preparation of compounds a1–
a4.40 A selected avone (or isoavone) (10.0 mmol) and K2CO3
(10.0 mmol) in DMF (100 mL) were stirred for 30 min. Aer
addition of ethyl bromoacetate (15 mmol), the obtained mixture
was stirred at 60–70 ꢂC for 3–5 h (monitored by TLC), which
were poured into 30 mL aqueous solution to dissolve K2CO3.
Subsequently, pH value was adjusted to the range of 5.0–6.0
with HCl, and the resulted mixture was extracted twice with 200
mL of EtOAc. The organic layer was dried over MgSO4 followed
by removal of the solvent under reduced pressure. The residue
was dissolved in acetone (100 mL), and the pH value was
adjusted to the rangeꢂof 9.0–10.0 with 0.1 M NaOH. The mixture
was stirred at 50–70 C for 1–2 h (monitored by TLC). The pH
value was adjusted to the range of 5.0–6.0 with 0.1 M HCl. The
precipitate was collected, which was puried by column chro-
matography on silica gel, eluting with CHCl3/CH3OH (v/v, from
10/1 to 50/1, containing 0.3% of acetic acid) to give compound
a1–a4 (Schemes 1 and 2) in yields of 75–90%.
Fig. 3 Binding pose of compound b1 with TyrRS (shown as gray) and
the original binding pose of co-crystal ligand Tyr-AMS (shown as red).
3.1.2 General procedure for preparation of compounds b1–
b21.41 Compound a (3.0 mmol) was dissolved in 60 mL of dry
CH2Cl2, followed by addition of 2-(1H-benzotriazole-1-yl)-
1,1,3,3-tetramethyluronium tetrauoroborate (TBTU, 6.0
mmol), an appropriate substituted amine (4.5 mmol) and trie-
thylamine (9.0 mmol). The mixture was then stirred for 5–7 h at
Fig. 4 Binding mode of compound b1 with TyrRS.
ꢂ
60 C (monitored by TLC). Aer the solvent was removed, the
residue was partitioned between EtOAc and water. The organic
layer was then dried over MgSO4 and concentrated under
reduced pressure. Flash chromatography (EtOAc/petroleum
ether, from 5/2 to 2/3) furnished compound b1–b20 (Schemes
1 and 2) in good yields.
3.1.2.1 (S)-5-Hydroxy-40-hydroxy-7-(2-morpholino-2-oxoethoxy)-
2,3-dihydroavone (b1). Light yellow powder, 57.4%, mp 161–
162 ꢂC; 1H NMR (DMSO): 2.60 (dd, J ¼ 17.2 Hz, J ¼ 3.0 Hz, 1H);
3.29–3.32 (m, 1H); 3.42 (t, J ¼ 6.8 Hz, 4H); 4.33 (t, J ¼ 7.2 Hz, 2H);
4.82 (s, 2H); 5.49 (dd, J ¼ 13.4 Hz, J ¼ 2.9 Hz, 1H); 6.07 (dd, J ¼
7.8 Hz, J ¼ 2.3 Hz, 2H); 6.79 (s, 1H); 6.80 (s, 1H); 7.33 (d, J ¼
8.5 Hz, 2H); 9.62 (s, 1H); 12.07 (s, 1H); EIMS m/z 399 (M+). Anal.
calcd for C21H21NO7: C, 63.15; H, 5.30; N, 3.51; found: C, 63.09; H,
5.31; N, 3.51.
completely into the bottom of the binding pocket, with OH as
the hydrogen bond acceptor as well as donor forming two
hydrogen bonds (O–H/O) with Tyr 37 and Asp 182, respec-
tively. In addition, three non-classical hydrogen bonds (C–H/
O) were observed between B-ring and the enzyme backbone with
˚
H/O distances in the range of 2.3 to 3.2 A. Hydrogen bonds
built between 4-C]O and Arg 89, 3-CH2 and Tyr 175 also
furnish important contributions to binding affinities (Fig. 4).
The docking studies demonstrate that b1 strongly binds with
TyrRS, powerfully supporting its excellent inhibitory potency in
enzyme assays.
3.1.2.2 (S)-5-Hydroxy-40-hydroxy-7-(2-(pyrrolidin-1-yl)-2-oxoe-
thoxy)-2,3-dihydroa-vone (b2). Light yellow powder, 53.9%, mp
260–262 ꢂC; 1H NMR (DMSO): 1.72–1.79 (m, 2H); 1.84–1.91 (m,
3. Experimental
3.1 Chemistry
All chemicals (reagent grade) used were purchased from Aldrich 2H); 2.71 (dd, J ¼ 17.1 Hz, J ¼ 3.0 Hz, 1H); 3.33–3.43 (m, 5H);
(U.S.A) and Sinopharm Chemical Reagent Co., Ltd (China). 4.81 (s, 2H); 5.48 (dd, J ¼ 11.1 Hz, J ¼ 2.9 Hz, 1H); 6.07 (dd, J ¼
Separation of the compounds by column chromatography was 7.8 Hz, J ¼ 2.3 Hz, 2H); 6.79 (s, 1H); 6.81 (s, 1H); 7.33 (d, J ¼
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8.5 Hz, 2H); 9.61 (s, 1H); 12.08 (s, 1H); EIMS m/z 383 (M+). Anal. 379 (M+). Anal. calcd for C22H21NO5: C, 69.64; H, 5.58; N, 3.69;
calcd for C21H21NO6: C, 65.79; H, 5.52; N, 3.65; found: C, 65.83; found: C, 69.68; H, 5.57; N, 3.69.
H, 5.51; N, 3.65.
3.1.2.10 40-Methoxy-7-(2-(4-methylpiperazino)-2-oxoethoxy)
3.1.2.3 5-Hydroxy-40-hydroxy-7-(2-morpholino-2-oxoethoxy) isoavone (b10). White powder, 76.8%, mp 178–180 ꢂC; 1H NMR
isoavone (b3). White powder, 82.5%, mp 235–237 ꢂC; 1H NMR (DMSO): 2.23 (s, 3H); 2.27–2.34 (m, 2H); 2.37–2.43 (m, 2H); 3.45–
(DMSO): 3.46 (t, J ¼ 4.7 Hz, 4H); 3.59 (t, J ¼ 4.8 Hz, 2H); 3.64 (t, J 3.52 (m, 4H); 3.80 (s, 3H); 5.05 (s, 2H); 7.00 (d, J ¼ 8.8 Hz, 2H);
¼ 4.8 Hz, 2H); 5.03 (s, 2H); 6.44 (d, J ¼ 2.5 Hz, 1H); 6.67 (d, J ¼ 7.10 (dd, J ¼ 8.9 Hz, J ¼ 2.4 Hz, 1H); 7.15 (d, J ¼ 2.4 Hz, 1H); 7.54
2.3 Hz, 1H); 6.84 (d, J ¼ 8.4 Hz, 2H); 7.40 (d, J ¼ 8.3 Hz, 2H); 8.42 (d, J ¼ 8.7 Hz, 2H); 8.04 (d, J ¼ 8.9 Hz, 1H); 8.43 (s, 1H); 13C NMR
(s, 1H); 9.63 (s, 1H); 12.96 (s, 1H); EIMS m/z 397 (M+). Anal. calcd (DMSO): 44.26, 46.01, 54.94, 55.60, 66.56, 102.02, 114.07,
for C21H19NO7: C, 63.47; H, 4.82; N, 3.52; found: C, 63.50; H, 115.49, 118.23, 123.79, 124.51, 127.30, 130.53; EIMS m/z 408
4.81; N, 3.52.
(M+). Anal. calcd for C23H24N2O5: C, 67.63; H, 5.92; N, 6.86;
3.1.2.4 5-Hydroxy-40-hydroxy-7-(2-butylamino-2-oxoethoxy) found: C, 67.67; H, 5.91; N, 6.85.
isoavone (b4). White powder, 64.8%, mp 203–205 ꢂC; 1H NMR
3.1.2.11 40-Methoxy-7-(2-piperidino-2-oxoethoxy)isoavone (b11).
(DMSO): 0.85–0.897 (t, J ¼ 7.3 Hz, 3H); 1.23–1.32 (m, 2H); 1.34– White powder, 59.8%, mp 146–148 ꢂC; 1H NMR (DMSO): 1.41–1.47
1.55 (m, 2H); 3.14 (q, J ¼ 6.7 Hz, 2H); 4.62 (s, 2H); 6.46 (d, J ¼ (m, 2H); 1.51–1.65 (m, 4H); 3.38–3.46 (m, 4H); 3.80 (s, 3H); 5.04 (s,
2.3 Hz, 1H); 6.66 (d, J ¼ 2.3 Hz, 1H); 6.84 (d, J ¼ 8.6 Hz, 2H); 7.40 2H); 7.01 (d, J ¼ 8.7 Hz, 2H); 7.10 (dd, J ¼ 8.9 Hz, J ¼ 2.4 Hz, 1H);
(d, J ¼ 8.6 Hz, 2H); 8.17 (t, J ¼ 5.8 Hz, 1H); 8.43 (s, 1H); 9.63 (s, 7.14 (d, J ¼ 2.3 Hz, 1H); 7.54 (d, J ¼ 8.7 Hz, 2H); 8.04 (d, J ¼ 8.8 Hz,
1H); 12.96 (s, 1H); EIMS m/z 383 (M+). Anal. calcd for 1H); 8.44 (s, 1H); EIMS m/z 393 (M+). Anal. calcd for C23H23NO5: C,
C
21H21NO6: C, 65.79; H, 5.52; N, 3.65; found: C, 65.74; H, 5.53; 70.21; H, 5.89; N, 3.56; found: C, 70.17; H, 5.90; N, 3.56.
N, 3.65.
3.1.2.12 40-Methoxy-7-(2-morpholino-2-oxoethoxy)isoavone
3.1.2.5 5-Hydroxy-40-hydroxy-7-(2-piperidino-2-oxoethoxy) (b12). White powder, 84.6%, mp 186–188 ꢂC; 1H NMR (DMSO):
isoavone (b5). White powder, 65.5%, mp 235–237 ꢂC; 1H NMR 3.45–3.51 (m, 4H); 3.57–3.64 (m, 2H); 3.64–3.68 (m, 2H); 3.80 (s,
(DMSO): 1.44–1.47 (m, 2H); 1.52–1.77 (m, 4H); 3.38–3.45 (m, 3H); 5.07 (s, 2H); 7.01 (d, J ¼ 8.7 Hz, 2H); 7.11 (dd, J ¼ 8.9 Hz, J ¼
4H); 4.99 (s, 2H); 6.42 (d, J ¼ 2.2 Hz, 1H); 6.65 (d, J ¼ 2.2 Hz, 1H); 2.4 Hz, 1H); 7.17 (d, J ¼ 2.4 Hz, 1H); 7.54 (d, J ¼ 8.7 Hz, 2H); 8.04
6.83 (d, J ¼ 8.6 Hz, 2H); 7.40 (d, J ¼ 8.5 Hz, 2H); 8.42 (s, 1H); 9.63 (d, J ¼ 8.9 Hz, 1H); 8.43 (s, 1H); EIMS m/z 395 (M+). Anal. calcd
(s, 1H); 12.96 (s, 1H); EIMS m/z 395 (M+). Anal. calcd for for C22H21NO6: C, 66.83; H, 5.35; N, 3.54; found: C, 66.88; H,
C
22H21NO6: C, 66.83; H, 5.35; N, 3.54; found: C, 66.87; H, 5.31; 5.31; N, 3.54.
N, 3.54.
3.1.2.13 40-Methoxy-7-(2-butylamino-2-oxoethoxy)isoavone
1
3.1.2.6 5-Hydroxy-40-hydroxy-7-(2-(4-methylpiperazino)-2- (b13). White crystal, 60.5%, mp 148–152 C; H NMR (DMSO):
oxoethoxy)isoavone (b6). White powder, 63.2%, mp 193–195 ꢂC; 0.86 (t, J ¼ 7.3 Hz, 3H); 1.28 (p, J ¼ 7.3 Hz, 2H); 1.42 (p, J ¼
1H NMR (DMSO): 3.51 (s, 3H); 3.85–3.94 (m, 4H); 4.08–4.23 (m, 7.1 Hz, 2H); 3.14 (q, J ¼ 6.7 Hz, 2H); 3.79 (s, 3H); 4.66 (s, 2H);
4H); 4.62 (s, 2H); 4.64 (d, J ¼ 2.2 Hz, 1H); 6.66 (d, J ¼ 2.3 Hz, 1H); 7.00 (d, J ¼ 8.7 Hz, 2H); 7.14 (d, J ¼ 1.9 Hz, 1H); 7.14 (dd, J ¼
6.84 (d, J ¼ 8.6 Hz, 2H); 7.40 (d, J ¼ 8.6 Hz, 2H); 8.44 (s, 1H); 9.64 8.7 Hz, J ¼ 2.0 Hz, 1H); 7.53 (d, J ¼ 8.7 Hz, 2H); 8.06 (d, J ¼
(s, 1H); 12.96 (s, 1H); EIMS m/z 410 (M+). Anal. calcd for 8.6 Hz, 1H); 8.21 (t, J ¼ 5.9 Hz, 1H); 8.44 (s, 1H); EIMS m/z 381
ꢂ
C
22H22N2O6: C, 64.38; H, 5.40; N, 6.83; found: C, 64.34; H, 5.41; (M+). Anal. calcd for C22H23NO5: C, 69.28; H, 6.08; N, 3.67;
N, 6.84.
3.1.2.7 40-Methoxy-7-(2-(3,5-dichlorophenylamino)-2-oxoethoxy)-
found: C, 69.23; H, 6.09; N, 3.67.
3.1.2.14 40-Methoxy-7-(2-diethylamino-2-oxoethoxy)isoavone
1
1
ꢂ
ꢂ
isoavone (b7). Light yellow powder, 82.4%, mp 208–210 C; H (b14). Pink powder, 71.3%, mp 124–126 C; H NMR (DMSO):
NMR (DMSO): 3.79 (s, 3H); 4.95 (s, 2H); 7.00 (d, J ¼ 8.8 Hz, 2H); 1.05 (t, J ¼ 7.1 Hz, 6H); 3.30 (q, J ¼ 6.9 Hz, 4H); 3.79 (s, 3H); 5.01
7.14–7.30 (m, 2H); 7.35 (t, J ¼ 1.9 Hz, 1H); 7.53 (d, J ¼ 8.8 Hz, 2H); (s, 2H); 7.00 (d, J ¼ 8.8 Hz, 2H); 7.09 (dd, J ¼ 8.4 Hz, J ¼ 2.4 Hz,
7.75 (d, J ¼ 1.9 Hz, 2H); 8.09 (d, J ¼ 8.7 Hz, 1H); 8.45 (s, 1H); 10.51 1H); 7.10 (d, J ¼ 2.4 Hz, 1H); 7.53 (d, J ¼ 8.7 Hz, 2H); 8.04 (d, J ¼
(s, 1H); EIMS m/z 469 (M+). Anal. calcd for C24H17Cl2NO5: C, 61.29; 9.1 Hz, 1H); 8.43 (s, 1H); EIMS m/z 381 (M+). Anal. calcd for
H, 3.64; Cl, 15.08; N, 2.98; found: C, 61.23; H, 3.64; Cl, 15.09; N,
2.98.
C
22H23NO5: C, 69.28; H, 6.08; N, 3.67; found: C, 69.33; H, 6.07;
N, 3.67.
3.1.2.15 40-Methoxy-7-(2-(2-morpholino)ethylamino-2-oxoethoxy)
3.1.2.8 40-Methoxy-7-(2-benzylamino-2-oxoethoxy)isoavone
1
1
ꢂ
(b8). White powder, 57.8%, mp 195–197 C; H NMR (DMSO): isoavone (b15). White powder, 75.5%, mp 178–180 ꢂC; H NMR
3.79 (s, 3H); 4.37 (d, J ¼ 6.1 Hz, 2H); 4.76 (s, 2H); 7.01 (d, J ¼ (DMSO): 2.30–2.44 (m, 6H); 3.28 (q, J ¼ 6.5 Hz, 2H); 3.55 (t, J ¼
8.8 Hz, 2H); 7.13–7.37 (m, 7H); 7.54 (d, J ¼ 8.7 Hz, 2H); 8.07 (d, J 4.6 Hz, 4H); 3.80 (s, 3H); 4.69 (s, 2H); 7.01 (d, J ¼ 8.7 Hz, 2H);
¼ 9.6 Hz, 1H); 8.44 (s, 1H); 8.78 (t, J ¼ 6.1 Hz, 1H); EIMS m/z 415 7.15 (d, J ¼ 2.3 Hz, 1H); 7.16 (dd, J ¼ 8.6 Hz, J ¼ 2.4 Hz, 1H); 7.54
(M+). Anal. calcd for C25H21NO5: C, 72.28; H, 5.10; N, 3.37; (d, J ¼ 8.8 Hz, 2H); 8.08 (d, J ¼ 9.6 Hz, 1H); 8.12 (t, J ¼ 5.7 Hz,
found: C, 72.23; H, 5.11; N, 3.37.
1H); 8.44 (s, 1H); EIMS m/z 438 (M+). Anal. calcd for C24H26N2O6:
3.1.2.9 40-Methoxy-7-(2-pyrrolidino-2-oxoethoxy)isoavone (b9). C, 65.74; H, 5.98; N, 6.39; found: C, 65.71; H, 5.99; N, 6.40.
1
ꢂ
Light yellow powder, 70.3%, mp 139–140 C; H NMR (DMSO):
3.1.2.16 5-Hydroxy-7-(2-(4-methyl)piperazino-2-oxoethoxy)
1
ꢂ
1.74–1.81 (m, 2H); 1.88–1.94 (m, 2H); 3.32 (t, J ¼ 6.9 Hz, 2H); 3.45 avone (b16). Light yellow powder, 77.8%, mp 144–146 C; H
(t, J ¼ 6.8 Hz, 2H); 3.73 (s, 3H); 4.85 (s, 2H); 6.45 (d, J ¼ 2.5 Hz, 1H); NMR (DMSO): 2.85 (s, 3H); 2.95–3.20 (m, 4H); 3.35–3.60 (m, 4H);
6.54 (dd, J ¼ 9.0 Hz, J ¼ 2.5 Hz, 1H); 6.89 (d, J ¼ 8.6 Hz, 2H); 7.22 5.09 (s, 2H); 6.47 (d, J ¼ 2.2 Hz, 1H); 6.85 (d, J ¼ 2.2 Hz, 1H); 7.06
(d, J ¼ 8.6 Hz, 2H); 8.01 (d, J ¼ 9.0 Hz, 1H); 8.45 (s, 1H); EIMS m/z (s, 1H); 7.58–7.67 (m, 3H); 8.10 (d, J ¼ 8.2 Hz, 2H); 12.85 (s, 1H);
6198 | RSC Adv., 2017, 7, 6193–6201
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EIMS m/z 394 (M+). Anal. calcd for C22H22N2O5: C, 66.99; H, 5.62;
N, 7.10; found: C, 66.94; H, 5.61; N, 7.11.
3.3 Antimicrobial activity
The antibacterial activities of the synthesized compounds were
tested against Gram-positive bacterial strain (Staphylococcus
aureus ATCC 6538, penicillin G sodium as positive control) and
two Gram-negative bacterial strains (Escherichia coli ATCC 8739
and Pseudomonas aeruginosa ATCC 9027, ciprooxacin as posi-
tive control). The MTT (methylthiazolyldiphenyl-tetrazolium
bromide) proliferation assay was used to measure the MIC50s
of the test compounds by determining the absorbance of the
cells in culture.44 A stock solution of the synthesized compound
(1000 mg mLꢁ1) in DMSO was prepared with different concen-
trations using sterilized liquid medium (50% (v/v) of DMSO in
PBS (phosphate-buffered saline)). A specied quantity of the
medium containing the test compound was added into 96-well
plates, which was replaced by the sterilized liquid medium as
blank control. Suspension of the microorganism was prepared
to contain approximate 105 cfu mLꢁ1 and applied to 96-well
plates with serially diluted compounds (or blank control) to be
tested and incubated at 37 ꢂC. In the case of fungi, plates were
incubated at 28 ꢂC. Fiy mL of PBS containing 3 mg of MTT per
mL was added to each well. Incubation was continued for 4–5 h,
then 100 mL of 10% sodium dodecyl sulfate containing 5%
isopropanol and 10 mol mLꢁ1 HCl was added to extract the dye.
Aer 8–10 h of incubation, the control OD (optical density)
value (at 570 nm) should be controlled under 1.0–2.0 to achieve
reliable results. The observed MIC50s were presented in Table 2.
3.1.2.17 5-Hydroxy-7-(2-piperidino-2-oxoethoxy)avone (b17).
Yellow powder, 61.7%, mp 181–183 ꢂC; 1H NMR (DMSO): 1.40–
1.47 (m, 2H); 1.57–1.62 (m, 4H); 3.39–3.46 (m, 4H); 5.00 (s, 2H);
6.42 (d, J ¼ 2.2 Hz, 1H); 6.82 (d, J ¼ 2.2 Hz, 1H); 7.06 (s, 1H);
7.57–7.66 (m, 3H); 8.11 (d, J ¼ 6.6 Hz, 2H); 12.82 (s, 1H); EIMS m/
z 379 (M+). Anal. calcd for C22H21NO5: C, 69.64; H, 5.58; N, 3.69;
found: C, 69.59; H, 5.59; N, 3.69.
3.1.2.18 5-Hydroxy-7-(2-pyrrolidino-2-oxoethoxy)avone (b18).
1
ꢂ
Light yellow powder, 80.6%, mp 159–161 C; H NMR (DMSO):
1.79 (p, J ¼ 6.8 Hz, 2H); 1.92 (p, J ¼ 6.8 Hz, 2H); 3.34 (t, J ¼ 6.8 Hz,
2H, overlapping with signals of water); 3.48 (t, J ¼ 6.8 Hz, 2H);
4.92 (s, 2H); 6.42 (d, J ¼ 2.2 Hz, 1H); 6.82 (d, J ¼ 2.2 Hz, 1H); 7.06
(s, 1H); 7.57–7.66 (m, 3H); 8.11 (d, J ¼ 6.6 Hz, 2H); 12.82 (s, 1H);
EIMS m/z 365 (M+). Anal. calcd for C21H19NO5: C, 69.03; H, 5.24;
N, 3.83; found: C, 69.08; H, 5.23; N, 3.83.
3.1.2.19 5-Hydroxy-7-(2-morpholino-2-oxoethoxy)avone (b19).
1
ꢂ
Yellow powder, 58.7%, mp 212–213 C; H NMR (DMSO): 3.42–
3.49 (m, 4H); 3.53–3.61 (m, 2H); 3.61–3.68 (m, 2H); 5.03 (s, 2H);
6.44 (d, J ¼ 2.3 Hz, 1H); 6.82 (d, J ¼ 2.3 Hz, 1H); 7.06 (s, 1H); 7.57–
7.66 (m, 3H); 8.10 (d, J ¼ 6.6 Hz, 2H); 12.82 (s, 1H); EIMS m/z 381
(M+). Anal. calcd for C21H19NO6: C, 66.13; H, 5.02; N, 3.67; found:
C, 66.09; H, 5.03; N, 3.67.
3.1.2.20 5-Hydroxy-7-(2-(2-morpholino)ethylamino-2-oxoethoxy)
1
avone (b20).42 Yellow powder, 63.4%, mp 216–217 C; H NMR
ꢂ
(DMSO): 2.30–2.42 (m, 4H); 3.20–3.33 (m, 4H); 3.45–3.60 (m, 4H);
4.67 (s, 2H); 6.46 (d, J ¼ 2.2 Hz, 1H); 6.85 (d, J ¼ 2.3 Hz, 1H); 7.08
(s, 1H); 7.58–7.67 (m, 3H); 8.11 (d, J ¼ 6.6 Hz, 2H); 8.85 (bs, 1H);
12.83 (s, 1H); EIMS m/z 424 (M+). Anal. calcd for C23H24N2O6: C,
65.08; H, 5.70; N, 6.60; found: C, 65.11; H, 5.69; N, 6.59.
3.4 Cytotoxicity assay45
Cytotoxicity of selected compounds was assessed against an
HeLa cell line. The cells were grown in a 96-well plate in DMEM
media supplemented with 10% fetal bovine serum and 5%
penicillin–streptomycin until they reached 70–80% conuency.
The cells were then treated with serially diluted compounds.
Two controls were made, one containing no cells (blank control)
and the other control containing cells with no compounds
(control). Cells were incubated for 24 h at 37 ꢂC under 5% CO2
atmosphere. Aer 24 h, the supernatant was carefully removed
and 100 mL of 5 mg mLꢁ1 concentration MTT solution was
added to each well. The plate was incubated for 3 h at 37 ꢂC
under 5% CO2 atmosphere. The cells were then treated with 100
mL of DMSO to solubilize formazan crystals. The plate was then
read at 570 nm. The quantity of viable cells aer treatment with
each compound was expressed as a percentage of the control.
3.2 Extraction of the TyrRS and enzyme assay
TyrRS was over-expressed in E. coli and puried to near homo-
geneity (ꢃ98% as judged by SDS-PAGE (sodium dodecyl sulfate
polyacrylamide gel electrophoresis)) using standard purica-
tion procedures. TyrRS activity was measured by amino-
acylation using modications to previously described
methods.43 The assays were performed at 37 ꢂC in a mixture
containing (nal concentrations) 100 mM Tris/Cl pH 7.9,
50 mM KCl, 16 mM MgCl2, 5 mM ATP (adenosine triphosphate),
3 mM DTT (DL-dithiothreitol), 4 mg mLꢁ1 E. coli MRE600 tRNA
(Roche) and 10 mM L-tyrosine (0.3 mM L-[ring-3,5-3H] tyrosine
(PerkinElmer, specic activity: 1.48–2.22 TBq mmolꢁ1), 10 mM
carrier). TyrRS (0.2 nM) was preincubated with a range of
inhibitor concentrations for 10 min at room temperature fol-
3.5 Protocol of docking study
lowed by the addition of pre-warmed mixture at 37 ꢂC. Aer The X-ray structure of TyrRS from Bacillus stearothermophilus
specic intervals, the reaction was terminated by adding was downloaded from the Protein Data Bank (PDB code: 3ts1)24
aliquots of the reaction mix into ice-cold 7% trichloroacetic acid and was modied by adding hydrogen atoms and removing
and harvesting onto 0.45 mm hydrophilic Durapore lters water as well as cocrystallized substrate (tyrosinyl adenylate)
(Millipore Multiscreen 96-well plates) and counted by liquid using SYBYL-X version 2.1.1 soware suite (Tripos, Inc., St.
scintillation. The rate of reaction in the experiments was linear Louis, MO).27 The active site was dened as all the amino acid
˚
with respect to protein and time with less than 50% total tRNA residues conned within a 5 A radius sphere centered about
acylation. IC50s correspond to the concentration at which half of tyrosinyl adenylate, and the composite structure without orig-
the enzyme activity is inhibited by the compound. The results inal ligand was utilized as the in silico model for docking
are presented in Table 1.
studies. Default parameters and values within the minimization
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dialogue were used except where otherwise mentioned. The
docked conformations of ligands were evaluated and ranked
using Surex-Dock and four scoring functions implemented in
the Cscore soware module within the SYBYL-X environment.
The Cscore module allowed consensus scoring that integrated
multiple well-known scoring functions such as ChemScore, D-
Score, G-Score and PMF-Score to evaluate docked ligand
conformations.46
A. R. McKenzie, J. V. Newman, L. J. Otterson and
G. Martinez-Botella, J. Med. Chem., 2014, 57(11), 4584–4597.
7 E. Klein, D. L. Smith and R. Laxminarayan, Emerging Infect.
Dis., 2007, 13(12), 1840–1846.
8 T. Lam, M. T. Hilgers, M. L. Cunningham, B. P. Kwan,
K. J. Nelson, V. Brow-Driver, V. Ong, M. Trzoss, G. Hough,
K. G. Shaw and J. Finn, J. Med. Chem., 2014, 57(3), 651–668.
9 A. K. Verma and R. Pratap, Tetrahedron, 2012, 68(41), 8523–
8538.
10 R. Landol, R. L. Mower and M. Steiner, Biochem.
Pharmacol., 1984, 33(9), 1525–1530.
4. Conclusions
11 P. A. Berg and P. T. Daniel, Prog. Clin. Biol. Res., 1988, 280,
157–171.
12 S. Gupta, F. Afaq and H. Mukhtar, Biochem. Biophys. Res.
Commun., 2001, 287(4), 914–920.
13 L. C. Chiang, L. T. Ng, I. C. Lin, P. L. Kuo and C. C. Lin,
Cancer Lett., 2006, 237(2), 207–214.
14 P. W. Snijman, E. Joubert, D. Ferreira, X. C. Li, Y. Q. Ding,
I. R. Green and W. C. A. Gelderblom, J. Agric. Food Chem.,
2009, 57(15), 6678–6684.
A series of avonoid derivatives was designed as TyrRS inhibi-
tors based on scaffold hopping drug design. They were gener-
ated by Library Design soware module within the SYBYL-X
environment. Twenty compounds with top scores were ob-
tained, and nineteen of them were reported for the rst time.
Enzyme assays identied three compounds showing excellent
inhibitory activities against TyrRS with IC50 lower than 1 mM. In
general, all the assayed compounds exhibited better activity
against Gram-negative organism than Gram-positive organism.
Furthermore, compounds with good inhibition against TyrRS
also exhibit good antibacterial activities. Out of these
compounds, b1 is the most active, which structurally containing
a nargenin core with MIC50 of 0.11 mg mLꢁ1 against E. coli ATCC
8739 shows 6-fold more potent than marketed antibiotic
ciprooxacin, and further investigations are being carried out in
our laboratory. These ndings together with molecular docking
studies revealed that antibacterial activities of these avonoid
based amides may be caused by their inhibition of TyrRS, and
compounds with a naringenin core are worth to be modied for
discovery of novel antibacterial agents.
˜
´
´
´
15 M. Duenas, S. Gonzalez-Manzano, A. Gonzalez-Paramas and
C. Santos-Buelga, J. Pharm. Biomed. Anal., 2010, 51(2), 443–
449.
16 H. Gao, H. Wang and J. Peng, Cell Biochem. Biophys., 2014,
69(1), 27–34.
17 L. Tang, Q. Feng, J. Zhao, L. N. Dong, W. Liu, C. H. Yang and
Z. Q. Liu, Food Chem. Toxicol., 2012, 50(5), 1460–1467.
18 M. Singh, M. Kaur and O. Silakari, Eur. J. Med. Chem., 2014,
84, 206–239.
19 T. J. Bullwinkle and M. Ibba, Proc. Natl. Acad. Sci. U. S. A.,
2016, 113(8), 2252–2257.
20 Z. P. Xiao, W. Wei, P. F Wang, W. K. Shi, N. Zhu, M. Q. Xie,
Y. W. Sun, L. X. Li, Y. X. Xie, L. S. Zhu, N. Tang, H. OuYang,
X. H. Li, G. C. Wang and H. L. Zhu, Eur. J. Med. Chem., 2015,
102, 631–638.
21 P. V. S. Mohana, R. Sankaranarayanan, M. Udayakumar and
S. Thamotharan, Res. J. Pharm., Biol. Chem. Sci., 2015, 6(3),
1737–1743.
Conflict of interest
The authors declare no conict of interest.
Acknowledgements
22 X. D. Wang, R. C. Deng, J. J. Dong, Z. Y. Peng, X. M. Gao,
S. T. Li, W. Q. Lin, C. L. Lu, Z. P. Xiao and H. L. Zhu,
Bioorg. Med. Chem., 2013, 21(17), 4914–4922.
23 X. D. Wang, W. Wei, P. F. Wang, Y. T. Tang, R. C. Deng, B. Li,
S. S. Zhou, J. W. Zhang, L. Zhang, Z. P. Xiao, H. Ou-Yang and
H. L. Zhu, Bioorg. Med. Chem., 2014, 22(14), 3620–3628.
24 P. Brick, T. N. Bhat and D. M. Blow, J. Mol. Biol., 1989, 208(1),
83–98.
The work was nanced by grants from National Natural Science
Foundation of China (grant No. 21262013 and 81273382), by
a Project supported by Hunan Provincial Natural Science
Foundation of China (grant No. 2015JJ2116).
Notes and references
1 S. B. Levy and B. Marshall, Nat. Med., 2004, 10, S122–S129.
2 P. Fernandes, Nat. Biotechnol., 2006, 24(12), 1497–1503.
25 Z. P. Xiao, X. B. He, Z. Y. Peng, T. J. Xiong, J. Peng, L. H. Chen
and H. L. Zhu, Bioorg. Med. Chem., 2011, 19(5), 1571–1579.
3 F. Fontaine, A. Hequet, A. S. Voisin-Chiret, B. Alexandre, 26 W. Wei, W. K. Shi, P. F. Wang, X. T. Zeng, P. Li, J. R. Zhang,
´
L. Aurelien, C. Thierry, J. Claude and R. Sylvain, J. Med.
Q. Li, Z. P. Tang, J. Peng, L. Z. Wu, M. Q. Xie, C. Liu, X. H. Li,
Y. C. Wang, Z. P. Xiao and H. L. Zhu, Bioorg. Med. Chem.,
2015, 23(20), 6602–6611.
Chem., 2014, 57(6), 2536–2548.
4 D. J. Payne, M. N. Gwynn, D. J. Holmes and D. L. Pompliano,
Nat. Rev. Drug Discovery, 2007, 6(1), 29–40.
5 M. A. Fischbach and C. T. Walsh, Science, 2009, 325(5944),
1089–1093.
6 S. P. Kawatkar, T. A. Keating, N. B. Olivier, J. N. Breen,
O. M. Green, S. Y. Guler, M. F. Hentemann, J. T. Loch,
27 SYBYL Molecular Modeling Soware, version SYBYL-X 2.1,
Tripos Inc., St. Louis, MO, 2015.
28 E. R. Lee, Y. J. Kang, H. Y. Choi, G. H. Kang, J. H. Kim,
B. W. Kim, Y. S. Han, S. Y. Nah and H. D. Paik, Biol.
Pharm. Bull., 2007, 30, 2394–2398.
6200 | RSC Adv., 2017, 7, 6193–6201
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
29 M. Pal, R. Dakarapu, K. Parasuraman, V. Subramanian and 40 S.-T. Chung, Y.-T. Huang, H.-Y. Hsiung, W.-H. Huang,
K. R. Yeleswarapu, J. Org. Chem., 2005, 70, 7179–7187.
C.-W. Yao and A.-R. Lee, Chem. Biodiversity, 2015, 12(4),
685–696.
´
30 K. Dahlen, M. Grøtli and K. Luthman, Synlett, 2006, 897–900.
31 M. Pal, V. Subramanian, K. Parasuraman and 41 J. D. Williams, M. C. Torhan, V. R. Neelagiri, C. Brown,
K. R. Yeleswarapu, Tetrahedron, 2003, 59, 9563–9570.
32 G. J. Nan, J. Shi, Y. R. Huang, J. Sun, J. H. Lv, G. D. Yang and
Y. P. Li, J. Chem. Eng. Data, 2014, 59, 1304–1311.
N. O. Bowlin, M. Di, C. T. McCarthy, D. Aiello, N. P. Peet,
T. L. Bowlin and D. T. Moir, Bioorg. Med. Chem., 2015,
23(5), 1027–1043.
´
´
´
33 I. Kron, Z. Pudychova-Chovanova, B. Velika, J. Guzy and 42 H. Choe, J. Kim and S. Hong, Bioorg. Med. Chem. Lett., 2013,
´
P. Perjesi, Monatsh. Chem., 2012, 143, 13–17.
34 A. Dhainaut, G. Lewin, E. Canet, M. Lonchampt and 43 G. P. Sang, P. Schimmel and S. Kim, Proc. Natl. Acad. Sci. U. S.
Y. Rolland, US 5889003, 1999. A., 2008, 105, 11043–11049.
35 M. Bouktaib, S. Lebrun, A. Atmani and C. Rolando, 44 X.-D. Wang, R.-C. Deng, Y. Liu, B. Li, S. Huang, H. Ouyang
Tetrahedron, 2002, 58, 10001–10009. and Z.-P. Xiao, Asian J. Chem., 2014, 26, 8015–8018.
36 Q. G. Mei, C. Wang, W. C. Yuan and G. L. Zhang, Beilstein J. 45 H. Mohammad, A. S. Mayhoub, A. Ghafoor, M. Soo,
23, 4324–4327.
Org. Chem., 2015, 11, 288–293.
37 N. Ohta, G. Kuwata, H. Akahori and T. Watanabe, Agric. Biol.
Chem., 1979, 43(7), 1415–1419.
38 J. M. Hastings, M. K. Hadden and B. S. J. Blagg, J. Org. Chem.,
2008, 73, 369–373.
39 M. F. Oldeld, L. Chen and N. P. Botting, J. Labelled Compd.
Radiopharm., 2007, 50, 1266–1271.
R. A. Alajlouni, M. Cushman and M. N. Seleem, J. Med.
Chem., 2014, 57, 1609–1615.
46 M. Hassani, W. Cai, K. H. Koelsch, D. C. Holley, A. S. Rose,
F. Olang, J. P. Lineswala, W. G. Holloway, J. M. Gerdes,
M. Behforouz and H. D. Beall, J. Med. Chem., 2008, 51,
3104–3115.
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