Rational design and synthesis of novel diphenyl ether derivatives as antitubercular agents

Article abstract of DOI:10.2147/DDDT.S104037

A series of triclosan mimic diphenyl ether derivatives have been synthesized and evaluated for their in vitro antitubercular activity against Mycobacterium tuberculosis H37Rv. The binding mode of the compounds at the active site of enoyl-acyl carrier protein reductase of M. tuberculosis has been explored. Among them, compound 10b was found to possess antitubercular activity (minimum inhibitory concentration =12.5 μg/mL) comparable to triclosan. All the synthesized compounds exhibited low levels of cytotoxicity against Vero and HepG2 cell lines, and three compounds 10a, 10b, and 10c had a selectivity index more than 10. Compound 10b was also evaluated for log P, pKa, human liver microsomal stability, and % protein binding, in order to probe its druglikeness. Based on the antitubercular activity and druglikeness profile, it may be concluded that compound 10b could be a lead for future development of antitubercular drugs.

Full text of DOI:10.2147/DDDT.S104037

Drug Design, Development and erapy
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O r i g i n a l r e s e a r c h
rational design and synthesis of novel diphenyl
ether derivatives as antitubercular agents
This article was published in the following Dove Press journal:
Drug Design, Development andTherapy
18 July 2016
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sidhartha s Kar1
Varadaraj Bhat g1
Praveen Pn rao2
Vishnu P shenoy3
indira Bairy4
Abstract: A series of triclosan mimic diphenyl ether derivatives have been synthesized
and evaluated for their in vitro antitubercular activity against Mycobacterium tuberculosis
H37Rv. The binding mode of the compounds at the active site of enoyl-acyl carrier protein
reductase of M. tuberculosis has been explored. Among them, compound 10b was found to
possess antitubercular activity (minimum inhibitory concentration =12.5 µg/mL) comparable
to triclosan. All the synthesized compounds exhibited low levels of cytotoxicity against Vero
and HepG2 cell lines, and three compounds 10a, 10b, and 10c had a selectivity index more
than 10. Compound 10b was also evaluated for log P, pKa, human liver microsomal stability,
and % protein binding, in order to probe its druglikeness. Based on the antitubercular activity
and druglikeness profile, it may be concluded that compound 10b could be a lead for future
development of antitubercular drugs.
g gautham shenoy1
1Department of Pharmaceutical
chemistry, Manipal college of
Pharmaceutical sciences, Manipal
University, Manipal, india; 2school of
Pharmacy, health sciences campus,
University of Waterloo,Waterloo, On,
canada; 3Department of Microbiology,
Kasturba Medical college, Manipal,
4Melaka-Manipal Medical college,
Manipal University, Manipal, india
Keywords: diphenyl ether, tuberculosis, cytotoxicity, druglikeness
Introduction
Tuberculosis (TB), caused by Mycobacterium tuberculosis, is the leading cause of
mortality worldwide, as indicated by the World Health Organization report.1 Currently,
there is a need for antitubercular (anti-TB) agents effective against the drug-resistant
strains of M. tuberculosis.2,3
Synthesis of mycolic acid, which is a central constituent of the mycobacterial
cell wall, is carried out by fatty acid synthase (FAS) systems, namely, FAS-I and -II.
Inhibition of type II fatty acid biosynthesis (FAS-II) pathway is a promising strategy
in devising novel anti-TB agents.4,5
The final reaction in the FAS-II pathway of M. tuberculosis is catalyzed by the
enoyl-acyl carrier protein reductase (ENR), which mediates the nicotinamide adenine
dinucleotide (reduced form)-dependent reduction of trans-2-enoyl-acyl carrier protein
to acyl-acyl carrier protein, and this biosynthetic pathway is an excellent target for
antimicrobial agents.
Triclosan (1), a diphenyl ether derivative (Figure 1), exhibits a minimum inhibitory
concentration (MIC) value of 12.5 µg/mL against M. tuberculosis H37Rv. Triclosan is
an inhibitor of mycobacterial ENR enzyme.6 Unlike isoniazid, triclosan does not require
prior activation to bind with ENR.7 Several investigators have attempted to develop
diphenyl ether-based anti-TB agents, which chemically resemble triclosan (1).8–12
Triclosan has drawbacks including poor solubility13 and suboptimal bioavailability.14
Encouraged with the outcome of our previous work on the anti-TB activity of
diphenyl ethers,15,16 we decided to further explore the diphenyl ether scaffold. In the pres-
ent study, we report the design of triclosan mimic diphenyl ether derivatives as anti-TB
correspondence: g gautham shenoy
Department of Pharmaceutical
chemistry, Manipal college of
Pharmaceutical sciences, Manipal
University, Manipal 576 104, india
Tel +91 820 292 2482
Fax +91 820 257 1998
email gautham.gs@manipal.edu
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http://dx.doi.org/10.2147/DDDT.S104037

Kar et al
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Figure 1 Design of triclosan (1) mimic diphenyl ether derivatives (5a–f and 10a–c).
agents using computational techniques simultaneously of reaction (72 hours), the reaction mixture was diluted
applying Lipinski’s criteria. The aim of the present study with dichloromethane (100 mL) and filtered under reduced
was to develop triclosan mimic diphenyl ether derivatives as pressure. The filtrate was washed with dilute hydrochloric
potential anti-TB agents with improved druglikeness.
acid (2 M, 75 mL), followed by water (75 mL), dried over
anhydrous MgSO4, and evaporated under reduced pressure.
The crude compound was purified by column chromatogra-
phy over silica 100–200 mesh with hexane:ethyl acetate (9:1)
as the mobile phase to obtain the target compound.
Materials and methods
Materials
Column chromatography was carried out on 100–200 mesh
silica gel. Progress of the reactions was monitored by thin
layer chromatography (TLC) using aluminum-backed
sheets of silica gel 60 F24 (EMD Millipore, Billerica, MA,
USA). Melting points were recorded with a laboratory
Yield =2.25 g (48%); Rf=0.95 (hexane:ethyl acetate =
9:1); λmax=301 nm (MeOH); 1H NMR (400 MHz, (DMSO)-d6)
δ ppm: 7.77–7.72 (m, 1H), 7.55–7.51 (m, 1H), 7.47–7.46 (m,
1H), 7.43–7.39 (m, 2H), 7.28–7.26 (ddd, J=8.0, 2.8, and 0.8
Hz, 1H), 7.20–7.16 (m, 1H), 7.06–7.03 (m, 2H), 2.55 (s, 3H);
calculated for C14H12O2 [M+]: 212.24, found GC–MS (EI,
m/z): 212 (M)+, 197 (M-CH3)+, 169 (M-COCH3)+.
1
melting point apparatus as uncorrected values. H NMR
and 13C NMR spectra were recorded on an NMR spectrom-
eter (AV400 – 400 MHz High-Resolution Multinuclear
FT-NMR Spectrometer; Bruker India Scientific Pvt. Ltd.,
Bangalore, India) using dimethyl sulfoxide (DMSO)-d6 as
the solvent. Mass spectrometry (MS) data were obtained
general method for the synthesis of
3-(3-phenoxyphenyl)-1-aryl prop-2-en-1-
on liquid chromatography (LC)–MS (Agilent 6520 series, one (3a–f)
Q-TOF LC/MS; Agilent Technologies, Santa Clara, CA, To a solution of compound 2 (1 g, 4.711 mmol) and aryl
USA) and gas chromatography (GC)–MS (Shimadzu acetophenones (4.711 mmol) in absolute alcohol (25 mL),
GC-17A, GCMS-QP5050A; Shimadzu Analytical (India) ethanolic solution of KOH (0.527 g, 9.423 mmol) was added
Pvt. Ltd., Mumbai, India). The purity of the final compounds at 25°C–27°C. The reaction mixture was stirred at ambient
was checked by reverse-phase high-performance liquid temperature. Progress of the reaction was monitored by
chromatography (HPLC) (ultra fast liquid chromatograph TLC using hexane:ethyl acetate (8:2). After the completion
[UFLC], Shimadzu) using C-18 column in isocratic mode of reaction (14 hours), the reaction mixture was poured into
solvent systems (methanol and buffer, pH =7.4) and was ice-cold water (100 mL) and the residue was extracted with
found to be $95%.
ethyl acetate (3×50 mL). The combined organic layers were
washed with water, brine, dried over anhydrous MgSO4, and
evaporated under reduced pressure. The crude compound
was purified by column chromatography over silica 100–200
mesh with hexane:ethyl acetate (8:2) as the mobile phase to
obtain the target compound.
synthesis of 1-(3-phenoxy-phenyl)-
ethanone (2)
To the stirred solution of 3-hydroxy acetophenone (3 g,
22.02 mmol) in anhydrous dichloromethane (120 mL),
activated molecular sieves (4 Å, 3 g), phenylboronic acid
(4.02 g, 33.18 mmol), copper (II) acetate (7.98 g, 44.04 mmol), 1-(3-Phenoxyphenyl)-3-phenylprop-2-en-1-one (3a)
and anhydrous pyridine (3.48 g, 44.04 mmol, 3.51 mL) were Yield =1.1 g (78%); mp =72°C–74°C; Rf=0.66 (hexane:ethyl
added successively. The resulting suspension was stirred acetate =8:2); λmax=309.80 nm (MeOH); 1H NMR (400 MHz,
at 25°C–27°C. Progress of the reaction was monitored by (DMSO)-d6) δppm: 7.96–7.94 (dd, J=8.0and1.2Hz,2H),7.83
TLC using hexane:ethyl acetate (9:1). After the completion (d, J=15.6 Hz, 1H), 7.77–7.72 (m, 3H), 7.65 (d, J=8.0 Hz, 2H),
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Diphenyl ether derivatives as anti-TB agents
3-(4-chlorophenyl)-1-(3-phenoxyphenyl)prop-2-en-
1-one (3f)
7.48–7.45 (m, 2H), 7.37 (d, J=7.8 Hz, 1H), 7.32–7.25
(m, 1H), 7.23 (t, J=7.6 Hz, 1H), 7.16–7.14 (m, 2H), 7.12–7.08
(m, 1H); calculated for C21H16O2 [M+]: 300.35, found LC–MS
(+ESI, m/z): 301.1069 (M+H)+.
Yield =1.12 g (72%); mp =77°C–79°C; Rf=0.84 (hexane:ethyl
acetate =8:2); λmax=305.40 nm (MeOH); 1H NMR (400 MHz,
(DMSO)-d6) δ ppm: 7.89 (d, J=15.6 Hz, 1H), 7.76–7.73
(m, 2H), 7.52 (t, J=8.0 Hz, 1H), 7.49–7.48 (m, 2H), 7.43–7.39
(m, 3H), 7.27–7.26 (dd, J=2.8 and 1.2 Hz, 1H), 7.20–7.15
(m, 2H), 7.05–7.02 (m, 3H); calculated for C21H15ClO2 [M+]:
334.80, found LC–MS (+ESI, m/z): 335.1613 (M+H)+.
1-(3-Phenoxyphenyl)-3-p-tolylprop-2-en-1-one (3b)
Yield =1.0 g (68%); mp =58°C–60°C; Rf=0.8 (hexane:ethyl
acetate =8:2); λmax=321.40 nm (MeOH); 1H NMR (400 MHz,
(DMSO)-d6) δ ppm: 7.96–7.93 (td, J=8.0, 1.2, and 1.2 Hz,
1H), 7.85 (d, J=15.6 Hz, 1H), 7.77 (d, J=8.4 Hz, 2H), 7.72
(s, 1H), 7.69–7.68 (m, 1H), 7.57 (t, J=8.0 Hz, 1H), 7.44–7.39
(m, 2H), 7.307–7.301 (dd, J=2.4 and 0.8 Hz, 1H), 7.28–7.25
(m, 2H), 7.20–7.16 (m, 1H), 7.08–7.06 (m, 2H), 2.34 (s, 3H);
calculated for C22H18O2 [M+]: 314.38, found LC–MS (+ESI,
m/z): 315.1473 (M+H)+.
general method for the synthesis
of 4-oxo-4-(3-phenoxyphenyl)-2-
arylbutanenitrile (4a–f)
Cyanohydrin acetone (2.32 mmol), tributyl methyl ammonium
hydroxide (1.16 mmol), and K2CO3 (2.32 mmol) were added
successively to a solution of chalcone (3a–f, 1.16 mmol) in
acetone (15 mL) and water (1 mL). The resulting reaction
mixture was refluxed at 57°C–58°C for 14 hours. Progress of
the reaction was monitored by TLC using hexane:ethyl acetate
(8:2) as the mobile phase. After the completion of reaction
(14 hours), the solvent was evaporated under reduced pres-
sure. The residue was treated with ice-cold water and extracted
with ethyl acetate (3×25 mL). The organic layers were pooled,
washed with water, brine, dried over anhydrous MgSO4, and
evaporated under reduced pressure. The crude compound was
purified by column chromatography over silica 100–200 mesh
with hexane:ethyl acetate (8:2) as the mobile phase to obtain
the target compound.
3-(4-Methoxyphenyl)-1-(3-phenoxyphenyl)prop-2-
en-1-one (3c)
Yield =1.2 g (77%); mp =80°C–82°C; Rf=0.6 (hexane:ethyl
acetate =8:2); λmax=343.20 nm (MeOH); 1H NMR (400 MHz,
(DMSO)-d6) δ ppm: 7.87 (d, J=15.6 Hz, 1H), 7.76–7.73 (m,
2H), 7.54–7.50 (dt, J=8.0 and 1.2 Hz, 1H), 7.47–7.46 (m,
1H), 7.43–7.39 (m, 2H), 7.27–7.25 (ddd, J=8.4, 2.8, and 1.2
Hz, 1H), 7.20–7.14 (m, 3H), 7.05–7.02 (m, 2H), 6.82–6.80
(dd, J=8.4 and 1.2 Hz, 2H), 3.70 (s, 3H); calculated for
C22H18O3 [M+]: 330.38, found LC–MS (+ESI, m/z): 331.2903
(M+H)+.
3-(2-Methoxyphenyl)-1-(3-phenoxyphenyl)prop-2-
en-1-one (3d)
4-Oxo-4-(3-phenoxyphenyl)-2-phenylbutanenitrile
(4a)
Yield =1.2 g (77%); mp =55°C–57°C; Rf=0.71 (hexane:ethyl
acetate =8:2); λmax=316.40 nm (MeOH); 1H NMR (400 MHz,
(DMSO)-d6) δ ppm: 7.76–7.74 (m, 2H), 7.52 (t, J=7.8 Hz,
1H), 7.48 (t, J=2.0 Hz, 1H), 7.43–7.39 (m, 2H), 7.28–7.25
(ddd, J=8.4, 2.8, and 1.2 Hz, 2H), 7.19–7.14 (m, 2H), 7.05–
7.02 (m, 2H), 6.81 (d, J=7.2 Hz, 2H), 6.74–6.71 (m, 1H),
3.70 (s, 3H); calculated for C22H18O3 [M+]: 330.38, found
LC–MS (+ESI, m/z): 331.2903 (M+H)+.
Yield =0.32 g (84%); mp =62°C–64°C; Rf=0.47 (hexane:ethyl
acetate =8:2); λmax=224.4 nm (MeOH); 1H NMR (400 MHz,
(DMSO)-d6) δ ppm: 8.02–7.98 (td, J=9.2, 2.2, and 2.4 Hz,
2H), 7.61–7.57 (td, J=9.2, 2.2, and 2.4 Hz, 2H), 7.42–7.36 (m,
3H), 7.28 (d, J=7.6 Hz, 1H), 7.16 (t, J=1.0 Hz, 1H), 7.17–7.12
(m, 2H), 7.02–6.94 (m, 2H), 6.92–6.91 (dd, J=2.4 and 1.2
Hz, 1H), 4.62–4.59 (dd, J=8.8 and 5.2 Hz, 1H), 4.01–3.95
(dd, J=18.4 and 8.8 Hz, 1H), 3.74–3.68 (dd, J=18.4 and
4.8 Hz, 1H); calculated for C22H17NO2 [M+]: 327.38, found
LC–MS (+ESI, m/z): 328.1341 (M)+.
3-(4-Fluorophenyl)-1-(3-phenoxyphenyl)prop-2-en-1-
one (3e)
Yield =1.1 g (67%); mp =68°C–70°C; Rf=0.71 (hexane:ethyl
acetate =8:2); λmax=310.20 nm (MeOH); 1H NMR (400 MHz, 4-Oxo-4-(3-phenoxyphenyl)-2-p-tolylbutanenitrile
(DMSO)-d6) δ ppm: 7.94 (d, J=15.6 Hz, 1H), 7.76–7.74 (4b)
(td, J=7.6 and 1.2 Hz, 2H), 7.52 (t, J=8.0 Hz, 1H), 7.48 (t, Yield =0.365 g (93%); Rf=0.62 (hexane:ethyl acetate =8:2);
1
J=2.0 Hz, 1H), 7.43–7.38 (m, 3H), 7.30–7.25 (m, 4H), 7.11– λmax=300 nm (MeOH); H NMR (400 MHz, (DMSO)-d6)
6.99 (m, 3H), calculated for C21H15FO2 [M+]: 318.34, found δ ppm: 7.39–7.35 (m, 2H), 7.31 (t, J=8.0 Hz, 1H), 7.13
LC–MS (+ESI, m/z): 319.1206 (M+H)+.
(d, J=7.2 Hz, 1H), 7.08–7.06 (m, 1H), 7.02–7.01 (m, 4H),
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6.99 (t, J=1.6 Hz, 2H), 6.98–6.95 (m, 1H), 6.86–6.83 (ddd, 7.25–7.21 (m, 2H), 7.13–7.08 (m, 2H), 7.08–7.05 (m, 2H),
J=8.0, 2.4, and 0.8 Hz, 1H), 4.51–4.46 (dd, J=8.0 and 5.4 7.00–6.96 (m, 3H), 6.88–6.84 (m, 1H), 4.56–4.5 (m, 1H),
Hz, 1H), 4.10–4.05 (dd, J=14.4 and 8.0 Hz, 1H), 3.51–3.46 3.98–3.93 (m, 1H), 3.38–3.33 (m, 1H); calculated for
(dd, J=14.6 and 8.8 Hz, 1H), 2.23 (s, 3H); calculated for C22H16ClNO2 [M+]: 361.82, found LC–MS (+ESI, m/z):
C23H19NO2 [M+]: 341.40, found LC–MS (+ESI, m/z): 362.1623 (M+H)+.
342.1509 (M+H)+.
general method for the synthesis
of 4-oxo-2-(3-phenoxyphenyl)-4-
arylbutanamide (5a–f)
2-(4-Methoxyphenyl)-4-oxo-4-(3-phenoxyphenyl)
butanenitrile (4c)
Yield =0.348 g (84%); mp =52°C–54°C; Rf=0.70 To the solution of 4-oxo-4-(3-phenoxy-phenyl)-2-aryl-
(hexane:ethyl acetate =8:2); λmax=254.4 nm (MeOH); butyronitrile (4a–f) (0.76 mmol) in DMSO (8 mL), anhydrous
1H NMR (400 MHz, (DMSO)-d6) δ ppm: 7.42–7.40 (dd, K2CO3 (0.209 g, 1.52 mmol) was added. Hydrogen peroxide
J=7.6 and 2 Hz, 2H), 7.38 (t, J=4.4 Hz, 1H), 7.16 (t, J=1.2 solution (28%, 1.52 mmol) was added drop wise at 5°C–10°C.
Hz, 1H), 7.14–7.07 (m, 3H), 7.02–6.98 (m, 3H), 6.88–6.88 The reaction mixture was stirred at 25°C–27°C. Progress of
(dd, J=2.4 and 0.8 Hz, 1H), 6.86–6.81 (m, 2H), 4.53–4.48 the reaction was monitored by TLC using hexane:ethyl ace-
(dd, J=10.8 and 6.0 Hz, 1H), 4.13–4.07 (dd, J=18.8 and 10 tate (8:2) as the mobile phase. After the completion of reaction
Hz, 1H), 3.71 (s, 3H), 3.60–3.55 (dd, J=18.4 and 6.0 Hz, (2 hours), the reaction mixture was poured into ice-cold water
1H); calculated for C23H19NO3 [M+]: 357.40, found LC–MS and the precipitate obtained was extracted with ethyl acetate
(+ESI, m/z): 358.1012 (M+H))+.
(3×25 mL). The organic layers were separated, pooled,
washed with water, brine, dried over anhydrous MgSO4, and
evaporated under reduced pressure. The crude compound was
purified by column chromatography over silica 100–200 mesh
2-(2-Methoxyphenyl)-4-oxo-4-(3-phenoxyphenyl)
butanenitrile (4d)
Yield =0.33 g (80%); mp =58°C–60°C; Rf=0.71 (hexane:ethyl with hexane:ethyl acetate (5:5) as the mobile phase to obtain
1
acetate =8:2); λmax=271 nm (MeOH); H NMR (400 MHz, the target compound with good yield.
(DMSO)-d6) δ ppm: 7.39–7.34 (m, 2H), 7.31 (t, J=7.8 Hz,
1H), 7.16–7.11 (m, 2H), 7.10–7.07 (m, 2H), 7.00–6.95 4-Oxo-4-(3-phenoxyphenyl)-2-phenylbutanamide (5a)
(m, 3H), 6.86–6.83 (ddd, J=8, 2.4, and 0.8 Hz, 1H), 6.73–6.69 Yield =0.165 g (70%); mp =165°C–167°C; Rf=0.21
1
(m, 3H), 4.52–4.48 (dd, J=11.2 and 6.0 Hz, 1H), 4.12–4.07 (hexane:ethyl acetate =6:4); λmax=295.8 nm (MeOH); H
(dd, J=14.2 and 6 Hz, 1H), 3.70 (s, 3H), 3.56–3.50 (dd, NMR (400 MHz, (DMSO)-d6) δ ppm: 7.78 (d, J=7.6 Hz,
J=14.2 and 5.6 Hz, 1H); calculated for C23H19NO3 [M+]: 1H), 7.52 (t, J=8.0 Hz, 1H), 7.48–7.47 (m, 2H), 7.43–7.39
357.40, found LC–MS (+ESI, m/z): 358.1012 (M)+.
(m, 2H), 7.38–7.36 (m, 2H), 7.30–7.25 (m, 3H), 7.21–7.19
(m, 2H), 7.17–7.15 (m, 2H), 6.79 (s, 1H), 4.06–4.02 (dd,
J=10.0 and 4.4 Hz, 1H), 3.86–3.79 (dd, J=17.6 and 10.0 Hz,
1H), 3.18–3.13 (dd, J=17.6 and 4.0 Hz, 1H); 13C NMR
2-(4-Fluorophenyl)-4-oxo-4-(3-phenoxyphenyl)
butanenitrile (4e)
Yield =0.342 g (85%); mp =60°C–62°C; Rf=0.84 (100.64 MHz, (DMSO)-d6) δ ppm: 198.08, 174.34, 157.61,
(hexane:ethyl acetate =8:2); λmax=303 nm (MeOH); 1H NMR 156.61, 140.96, 138.85, 131.02, 130.70, 128.76, 128.24,
(400 MHz, (DMSO)-d6) δ ppm: 7.41–7.37 (m, 2H), 7.32–7.29 127.16, 124.45, 123.70, 119.48, 117.36, 42.18; calculated
(m, 1H), 7.26 (t, J=7.8 Hz, 1H), 7.20–7.15 (m, 2H), 7.10–7.08 for C22H19NO3 [M+]: 345.39, found LC–MS (+ESI, m/z):
(m, 2H), 7.05–7.04 (m, 3H), 7.00–6.96 (dd, J=2.8 and 1.2 Hz, 346.1485 (M+H)+.
1H), 6.88–6.84 (m, 1H), 4.56–4.50 (m, 1H), 3.98–3.93 (m,
1H), 3.38–3.33 (m, 1H); calculated for C22H16FNO2 [M+]: 4-Oxo-4-(3-phenoxyphenyl)-2-p-tolylbutanamide (5b)
345.37, found LC–MS (+ESI, m/z): 345.1254 (M)+.
Yield =0.2 g (73%); mp =139°C–141°C; Rf=0.25
(hexane:ethyl acetate =6:4); λmax=299 nm (MeOH); 1H NMR
(400 MHz, (DMSO)-d6) δ ppm: 7.78–7.76 (dd, J=6.4 and
1.2 Hz, 1H), 7.52 (t, J=8.0 Hz, 1H), 7.46 (t, J=2.0 Hz, 1H),
2-(4-chlorophenyl)-4-oxo-4-(3-phenoxyphenyl)
butanenitrile (4f)
Yield =0.35 g (83%); mp =69°C–71°C; Rf=0.86 (hexane:ethyl 7.43–7.39 (m, 3H), 7.28–7.27 (dd, J=2.4 and 0.8 Hz, 1H),
acetate =8:2); λmax=295.4 nm (MeOH); 1H NMR (400 MHz, 7.26–7.23 (m, 2H), 7.19–7.15 (m, 1H), 7.09 (d, J=8.0 Hz,
(DMSO)-d6) δ ppm: 7.39–7.35 (m, 2H), 7.34–7.29 (m, 1H), 2H), 7.05–7.03 (m, 2H), 6.75 (s, 1H), 4.01–3.97 (dd, J=10.0
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Diphenyl ether derivatives as anti-TB agents
and 4.4 Hz, 1H), 3.83–3.76 (dd, J=18.0 and 10.0 Hz, 1H), 1H), 4.07–4.03 (dd, J=10.8 and 4.0 Hz, 1H), 3.85–3.78 (dd,
3.14–3.09 (dd, J=18.0 and 4.4 Hz, 1H), 2.24 (s, 3H); 13C J=18.0 and 10.0 Hz, 1H), 3.19–3.14 (dd, J=17.6 and 4.4 Hz,
NMR (100.64 MHz, (DMSO)-d6) δ ppm: 198.12, 174.49, 1H); 13C NMR (100.64 MHz, (DMSO)-d6) δ ppm: 196.80,
157.62, 156.60, 137.94, 131.02, 130.70, 129.29, 128.08, 174.9, 159.00, 157.66, 156.5, 133.60, 132.2, 131.00, 130.5,
124.46, 123.67, 119.49, 117.32, 46.29, 42.57, 42.19, 21.08; 128.87, 124.5, 123.16, 118.5, 117.2, 114.00, 43.83, 41.25;
calculated for C23H21NO3 [M+]: 359.42, found LC–MS calculated for C22H18FNO3 [M+]: 363.38, found LC–MS
(+ESI, m/z): 360.1588 (M+H)+.
(+ESI, m/z): 364.134 (M+H)+.
2-(4-Methoxyphenyl)-4-oxo-4-(3-phenoxyphenyl)
butanamide (5c)
2-(4-chlorophenyl)-4-oxo-4-(3-phenoxyphenyl)
butanamide (5f)
Yield =0.2 g (77%); mp =102°C–104°C; Rf=0.21(hexane:ethyl
acetate =6:4); λmax=273.8 nm (MeOH); 1H NMR (400 MHz,
(DMSO)-d6) δ ppm: 7.77 (d, J=7.6 Hz, 1H), 7.52 (t, J=8.0
Hz, 1H), 7.46 (s, 1H), 7.41 (t, J=8.0 Hz, 3H), 7.27 (d, J=8.4
Hz, 3H), 7.17 (t, J=7.4 Hz, 1H), 7.04 (d, J=8.0 Hz, 2H), 6.85
(d, J=8.8 Hz, 2H), 6.74 (s, 1H), 3.98 (d, J=6.0 Hz, 1H), 3.82–
3.75 (dd, J=17.6 and 9.2 Hz, 1H), 3.70 (s, 3H), 3.16–3.12 (dd,
J=18.0 and 4.0 Hz, 1H); 13C NMR (100.64 MHz, (DMSO)-d6)
δ ppm: 198.17, 174.69, 158.56, 157.61, 156.59, 138.89,
132.91, 131.03, 130.70, 129.21, 124.47, 123.67, 119.49,
117.31, 114.16, 55.52, 42.57; calculated for C23H21NO4 [M+]:
375.42, found LC–MS (+ESI, m/z): 376.1043 (M+H)+.
Yield =0.205 g (71%); mp =105°C–107°C; Rf=0.29
(hexane:ethyl acetate =6:4); λmax=274.5 nm (MeOH); H
1
NMR (400 MHz, (DMSO)-d6) δ ppm: 7.75 (d, J=8.0 Hz,
1H), 7.64 (d, J=8.0 Hz, 1H), 7.49–7.46 (m, 2H), 7.43–7.39
(m, 4H), 7.24–7.21 (dd, J=8.4 and 2.0 Hz, 1H), 7.14–7.11
(m, 2H), 7.07 (t, J=7.6 Hz, 1H), 7.04–7.00 (m, 2H), 6.87
(s, 1H), 4.10–4.06 (dd, J=10.4 and 5.6 Hz, 1H), 3.92–3.86
(m, 1H), 3.18–3.14 (dd, J=15.6 and 5.6 Hz, 1H); 13C NMR
(100.64 MHz, (DMSO)-d6) δ ppm: 199.20, 177.65, 159.69,
158.24, 156.5, 134.00, 132.43, 131.65, 130.15, 129.8, 124.5,
121.44, 118.54, 116.82, 115.12, 42.5, 41.2; calculated for
C22H18ClNO3 [M+]: 379.84, found LC–MS (+ESI, m/z):
380.1416 (M+H)+.
2-(2-Methoxyphenyl)-4-oxo-4-(3-phenoxyphenyl)
butanamide (5d)
synthesis of 1-(3-methoxy-4-
Yield =0.18 g (70%); mp =153°C–155°C; Rf=0.24 phenoxyphenyl)ethanone (6)
1
(hexane:ethyl acetate =6:4); λmax=272 nm (MeOH); H To the stirred solution of 4-hydroxy-3-methoxy-acetophe-
NMR (400 MHz, (DMSO)-d6) δ ppm: 7.76 (d, J=8.0 Hz, none (3 g, 18.05 mmol) in anhydrous dichloromethane
1H), 7.51 (t, J=7.8 Hz, 1H), 7.46 (s, 1H), 7.39 (t, J=8.8 (90 mL), activated molecular sieves (4 Å, 3 g), phenylbo-
Hz, 2H), 7.28–7.25 (dd, J=8.4 and 1.6 Hz, 1H), 7.23–7.17 ronic acid (4.49 g, 36.82 mmol), copper (II) acetate (7.36 g,
(m, 3H), 7.16–7.12 (m, 1H), 7.04 (d, J=7.6 Hz, 2H), 6.98 40.58 mmol), and anhydrous pyridine (5.70 g, 72.15 mmol,
(d, J=8.0 Hz, 1H), 6.89 (t, J=7.6 Hz, 2H), 6.82 (s, 1H), 5.82 mL) were added successively. The resulting suspen-
4.43–4.39 (dd, J=10.0 and 3.6 Hz, 1H), 3.78 (s, 3H), sion was stirred at 25°C–27°C. Progress of the reaction was
3.76–3.66 (m, 1H), 3.01–2.95 (m, 1H); 13C NMR (100.64 monitored by TLC using hexane:ethyl acetate (8:2) as the
MHz, (DMSO)-d6) δ ppm: 198.00, 174.69, 158.50, 157.61, mobile phase. After the completion of reaction (72 hours), the
156.59, 135.00, 132.32, 131.05, 130.00, 129.2, 124.5, reaction mixture was diluted with dichloromethane (40 mL)
124.11, 119.95, 118.12, 114.34, 56.12, 40.5; calculated and filtered under reduced pressure. The filtrate was washed
for C23H21NO4 [M+]: 375.42, found LC–MS (+ESI, m/z): with dilute aqueous hydrochloric acid solution (2 M, 50 mL),
376.1043 (M+H)+.
followed by water (50 mL), dried over anhydrous MgSO4,
and evaporated under reduced pressure. The crude compound
was purified by column chromatography over silica 100–200
mesh with hexane:ethyl acetate (8:2) as the mobile phase to
2-(4-Fluorophenyl)-4-oxo-4-(3-phenoxyphenyl)
butanamide (5e)
Yield =0.19 g (70%); mp =130°C–132°C; Rf=0.27 obtain the target compound as a white crystalline solid.
(hexane:ethyl acetate =6:4); λmax=271 nm (MeOH); 1H NMR
Yield =4.12 g (94%); mp =82°C–84°C; Rf=0.9
1
(400 MHz, (DMSO)-d6) δ ppm: 7.78 (d, J=7.6 Hz, 1H), 7.53 (hexane:ethyl acetate =8:2); λmax=269.6 nm (MeOH); H
(d, J=8 Hz, 1H), 7.49 (d, J=8.8 Hz, 2H), 7.43–7.39 (m, 4H), NMR (400 MHz, (DMSO)-d6) δ ppm: 7.60 (d, J=8.0 Hz, 2H),
7.28–7.26 (dd, J=8.0 and 2.0 Hz, 1H), 7.17 (t, J=7.2 Hz, 7.36 (t, J=7.8 Hz, 2H), 7.12 (t, J=7.2 Hz, 1H), 7.00 (d, J=8.0
2H), 7.11 (t, J=7.6 Hz, 1H), 7.04 (d, J=7.6 Hz, 2H), 6.81 (s, Hz, 1H), 6.94 (d, J=8.0 Hz, 2H), 3.83 (s, 3H), 2.56 (s, 3H);
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calculated for C15H14O3 [M+]: 242.27, found LC–MS (+ESI, NMR (400 MHz, (DMSO)-d6) δ ppm: 9.91 (s, 1H), 7.81
m/z): 243.1032 (M+H)+.
(s, 1H), 7.76 (t, J=7.4 Hz, 2H), 7.70–7.67 (dd, J=8.4 and
2.4 Hz, 2H), 7.66 (d, J=2.4 Hz, 1H), 7.38–7.34 (td, J=7.0
and 2.0 Hz, 2H), 7.26 (d, J=8.0 Hz, 2H), 7.12–7.08 (m, 1H),
7.00–6.95 (m, 3H), 2.34 (s, 3H); calculated for C22H18O3 [M+]:
synthesis of 1-(3-hydroxy-4-
phenoxyphenyl)ethanone (7)
To a solution of 1-(3-methoxy-4-phenoxyphenyl) ethanone 330.38, found LC–MS (+ESI, m/z): 331.1348 (M+H)+.
(6) (4 g, 16.52 mmol) in anhydrous dichloromethane (60 mL)
maintained at -78°C under nitrogen atmosphere, BBr3 (1 M, 1-(3-hydroxy-4-phenoxyphenyl)-3-(4-methoxyphenyl)
8.28 g, 33.04 mmol) was added. The reaction mixture was prop-2-en-1-one (8c)
stirred for 2 hours at -78°C; then its temperature was allowed Yield=0.94g(62%);mp=138°C–140°C;Rf=0.70(hexane:ethyl
to increase up to 10°C and it was stirred continuously. Prog- acetate =6:4); λmax=352.4 nm (MeOH); 1H NMR (400 MHz,
ress of the reaction was monitored by TLC using hexane:ethyl (DMSO)-d6) δ ppm: 9.89 (s, 1H), 7.83–7.80 (dd, J=8.8 and 1.6
acetate (6:4). After the completion of reaction (6 hours), Hz, 2H), 7.70 (d, J=3.6 Hz, 2H), 7.66–7.65 (m, 2H), 7.38–7.34
the reaction was quenched by pouring the reaction mixture (m, 2H), 7.12–7.08 (m, 1H), 7.01–6.97 (m, 3H), 6.96–6.94
into ice-cold aqueous sodium bicarbonate with continuous (m, 2H), 3.81 (s, 3H); calculated for C22H18O4 [M+]: 346.38,
stirring. The organic layer was separated, washed with found LC–MS (+ESI, m/z): 347.1256 (M+H)+.
water, brine, dried over anhydrous MgSO4; and evaporated
general method for the synthesis of
under reduced pressure. The crude compound was purified
4-(3-hydroxy-4-phenoxyphenyl)-4-oxo-2-
by column chromatography over silica 100–200 mesh with
hexane:ethyl acetate (6:4) as the mobile phase to obtain the aryl butanenitrile (9a–c)
target compound.
Compounds 9a–c were synthesized by following the method
Yield =3.6 g (96%); mp =87°C–89°C; Rf=0.63 adopted for synthesis of compounds 4a–f.
1
(hexane:ethyl acetate =6:4); λmax=269.4 nm (MeOH); H
NMR (400 MHz, (DMSO)-d6) δ ppm: 9.89 (s, 1H), 7.51 4-(3-hydroxy-4-phenoxyphenyl)-4-oxo-2-phenyl
(d, J=2.0 Hz, 1H), 7.45–7.43 (dd, J=8.4 and 2.4 Hz, 1H), butanenitrile (9a)
7.37–7.33 (m, 2H), 7.11–7.07 (m, 1H), 6.96 (s, 1H), 6.94– Yield =0.3 g (80%); mp =81°C–82°C; Rf=0.28 (hexane:ethyl
1
6.92 (m, 2H), 2.50 (s, 3H); calculated for C14H12O3 [M+]: acetate =6:4); λmax=275.4 nm (MeOH); H NMR (400
228.24, found LC–MS (+ESI, m/z): 229.085 (M+H)+.
MHz, (DMSO)-d6) δ ppm: 9.94 (s, 1H), 7.54–7.47 (m, 4H),
7.42–7.33 (m, 5H), 7.20 (d, J=8.0 Hz, 1H), 7.10 (t, J=7.4 Hz,
1H), 6.94 (d, J=8.0 Hz, 2H), 4.59–4.49 (m, 1H), 3.92–3.81
(m, 1H), 3.65–3.56 (m, 1H); calculated for C22H17NO3 [M+]:
343.38, found LC–MS (+ESI, m/z): 344.1294 (M+H)+.
general method for the synthesis of
1-(3-hydroxy-4-phenoxyaryl)ethanone
(8a–c)
Compounds 8a–c were synthesized by following the method
adopted for the synthesis of compounds 3a–f.
4-(3-hydroxy-4-phenoxyphenyl)-4-oxo-2-p-
tolylbutanenitrile (9b)
1-(3-hydroxy-4-phenoxyphenyl)-3-phenylprop-2-en-
1-one (8a)
Yield =0.33 g (83%); mp =60°C–62°C; Rf=0.32 (hexane:ethyl
acetate =6:4); λmax=274.8 nm (MeOH); 1H NMR (400 MHz,
Yield =1 g (72%); mp =94°C–96°C; Rf=0.61 (hexane:ethyl (DMSO)-d6) δppm: 9.97 (s, 1H), 7.52 (d, J=2.4 Hz, 1H), 7.49–
acetate =6:4); λmax=322.2 nm (MeOH); 1H NMR (400 MHz, 7.46 (dd, J=8.4 and 2.4 Hz, 1H), 7.38–7.33 (m, 4H), 7.19
(DMSO)-d6) δ ppm: 9.98 (s, 1H), 7.87–7.84 (m, 3H), 7.71 (d, (d, J=7.6 Hz, 2H), 7.12–7.07 (dt, J=7.4 and 1.0 Hz, 1H),
J=7.6 Hz, 1H), 7.68 (t, J=2.4 Hz, 2H), 7.44 (t, J=2.6 Hz, 3H), 6.95–6.92 (m, 3H), 4.52–4.49 (dd, J=8.8 and 5.2 Hz, 1H),
7.36 (t, J=7.8 Hz, 2H), 7.10 (t, J=7.4 Hz, 1H), 7.01–6.94 (m, 3.87–3.80 (dd, J=18.8 and 9.2 Hz, 1H), 3.61–3.55 (dd, J=18.0
3H); calculated for C21H16O3 [M+]: 316.35, found LC–MS and 5.6 Hz, 1H), 2.28 (s, 3H); calculated for C23H19NO3 [M+]:
(+ESI, m/z): 317.1157 (M+H)+.
357.40, found LC–MS (+ESI, m/z): 358.1448 (M+H)+.
1-(3-hydroxy-4-phenoxyphenyl)-3-p-tolylprop-2-en-
1-one (8b)
4-(3-hydroxy-4-phenoxyphenyl)-2-(4-
methoxyphenyl)-4-oxobutanenitrile (9c)
Yield =0.8 g (55%); mp =152°C–153°C; Rf=0.82 Yield =0.305 g (81%); mp =52°C–54°C; Rf=0.31
1
(hexane:ethyl acetate =6:4); λmax=321.8 nm (MeOH); H (hexane:ethyl acetate =6:4); λmax=274.8 nm (MeOH); 1H NMR
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Diphenyl ether derivatives as anti-TB agents
(400 MHz, (DMSO)-d6) δ ppm: 9.95 (s, 1H), 7.76 (d, J=2.4 1H NMR (400 MHz, (DMSO)-d6) δ ppm: 9.92 (s, 1H),
Hz, 1H), 7.50–7.45 (m, 2H), 7.41–7.38 (m, 3H), 7.26 (d, J=8.0 7.62 (d, J=2.4 Hz, 1H), 7.54–7.51 (m, 2H), 7.48 (s, 1H),
Hz, 2H), 7.17–7.13 (m, 2H), 7.08–7.02 (m, 2H), 4.57–4.54 7.38–7.35 (m, 2H), 7.24 (d, J=8 Hz, 2H), 7.18 (t, J=7.6
(dd, J=8.4 and 5.4 Hz, 1H), 3.89–3.80 (m, 1H), 3.67–3.58 Hz, 1H), 7.05–6.99 (m, 2H), 6.84 (s, 1H), 4.00–3.96 (dd,
(m, 1H), 2.28 (s, 3H); calculated for C23H19NO4 [M+]: 373.40, J=10.2 and 5.4 Hz, 1H), 3.91–3.86 (dd, J=16.6 and 10.2
found LC–MS (+ESI, m/z): 374.1396 (M+H)+.
Hz, 1H), 3.71 (s, 3H), 3.14–3.09 (m, 1H); 13C NMR (100.64
MHz, (DMSO)-d6) δ ppm: 197.39, 174.78, 158.56, 157.20,
149.24, 148.07, 133.79, 133.09, 130.32, 129.17, 123.49,
120.89, 120.63, 117.88, 116.66, 114.18, 55.52, 45.86, 42.58,
41.99; calculated for C23H21NO5 [M+]: 391.42, found LC–MS
general method for the synthesis of
4-(3-hydroxy-4-phenoxyphenyl)-4-oxo-2-
aryl butanamide (10a–c)
Compounds 10a–c were synthesized by following the method (+ESI, m/z): 392.135 (M+H)+.
adopted for synthesis of compounds 5a–f.
Results and discussion
4-(3-hydroxy-4-phenoxyphenyl)-4-oxo-2-
phenylbutanamide (10a)
The detailed methodology for anti-TB screening, pKa
determination, log P determination, cytotoxicity studies, and
Yield =0.215 g (82%); mp =143°C–145°C; Rf=0.09 liver microsomal stability studies is given in Supplementary
1
(hexane:ethyl acetate =5:5); λmax=269.4 nm (MeOH); H material.
NMR (400 MHz, (DMSO)-d6) δ ppm: 9.87 (s, 1H), 7.51
Triclosan mimic novel diphenyl ether derivatives were
(t, J=5.0 Hz, 1H), 7.50–7.46 (m, 2H), 7.43 (s, 1H), 7.38– designed (Figure 1) with functional diversifications at the
7.34 (m, 3H), 7.33–7.30 (m, 2H), 7.28–7.20 (m, 1H), 7.09 C-3 position of ring-A. With the intention of improving the
(t, J=7.2 Hz, 1H), 6.95–6.92 (dd, J=8.4 and 5.2 Hz, 3H), hydrogen bond interactions at the catalytic sites of InhA
6.80–6.76 (m, 1H), 4.07–4.04 (dd, J=10.0 and 4.0 Hz, 1H), enzyme and decreasing the lipophilicity of diphenyl ether
3.81–3.74 (m, 1H), 3.13–3.07 (dd, J=18.0 and 4.4 Hz, 1H); moiety, hydrogen bond donors/acceptors were introduced
13C NMR (100.64 MHz, (DMSO)-d6) δ ppm: 197.28, 174.41, in the substituted alkyl chain. Sivaraman et al17 reported
157.20, 149.25, 148.08, 141.13, 133.75, 130.33, 129.30, that the chloro substitutions in ring-B of triclosan (1) are
128.78, 128.20, 128.05, 127.14, 123.49, 120.90, 120.62, involved in unfavorable steric interactions with the enzyme,
117.88, 116.65, 46.73, 41.89; calculated for C22H19NO4 [M+]: and their deletion from the scaffold increases the affinity
361.39, found LC–MS (+ESI, m/z): 362.1562 (M+H)+.
sevenfold. Hence, diphenyl ether derivatives were designed
without chloro substitutions in ring-B. Removal of halogens
also helps in decreasing the lipophilicity of the designed
molecules.
4-(3-hydroxy-4-phenoxyphenyl)-4-oxo-2-p-
tolylbutanamide (10b)
Yield =0.235 g (86%); mp =135°C–137°C; Rf=0.12
Compounds 5a–f were prepared to probe the effect of
1
(hexane:ethyl acetate =5:5); λmax=269.6 nm (MeOH); H presence of electron-withdrawing carbonyl group adjacent
NMR (400 MHz, (DMSO)-d6) δ ppm: 9.87 (s, 1H), 7.52 to ring-A of diphenyl ether ring (Figure 2). To begin with,
(d, J=2.0 Hz, 1H), 7.48–7.46 (dd, J=8.4 and 2.0 Hz, 1H), 3-phenoxy acetophenone (2) was prepared through the
7.43 (s, 1H), 7.36–7.32 (m, 2H), 7.24 (d, J=8 Hz, 2H), 7.10 Chan–Lam coupling reaction using Cu(OAc)2.18 Chalcones
(d, J=7.2 Hz, 3H), 6.95–6.92 (m, 3H), 6.76 (s, 1H), 4.02–3.99 (3a–f) (Figure 2) were synthesized by reacting 3-phenoxy
(dd, J=10.0 and 4.4 Hz, 1H), 3.78–3.71 (dd, J=17.6 and 11.2 acetophenone (2) with different aryl aldehydes.19 Primary
Hz, 1H), 3.09–3.03 (dd, J=17.6 and 4.8 Hz, 1H), 2.25 (s, 3H); amide functionality was incorporated at the β-carbon of
13C NMR (100.64 MHz, (DMSO)-d6) δ ppm: 197.34, 174.59, the chalcones through base-catalyzed selective hydration of
157.19, 149.24, 148.07, 138.11, 136.17, 133.77, 130.33, the hydrocyanated chalcones20 in the presence of hydrogen
129.31, 128.05, 123.49, 120.89, 120.63, 117.88, 116.64, peroxide to obtain the corresponding amides (5a–f).21
46.32, 42.57, 41.89, 21.09; calculated for C23H21NO4 [M+]:
375.42, found LC–MS (+ESI, m/z): 376.1511 (M+H)+.
Phenolic –OH of the A-ring of triclosan plays a crucial
role in binding to the catalytic site of enoyl-acyl carrier
protein reductase of M. tuberculosis. In addition to this,
diphenyl ethers substituted with long alkyl chains at the C-4
position of ring-A have been reported to display better affinity
4-(3-hydroxy-4-phenoxyphenyl)-2-(4-
methoxyphenyl)-4-oxobutanamide (10c)
Yield =0.23 g (81%); mp =123°C–125°C; Rf=0.11 than triclosan toward enoyl-acyl carrier protein reductase of
(hexane:ethyl acetate =6:4); λmax=270.6 nm (MeOH); M. tuberculosis.6 However, the lipophilicity of the reported
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2
2
2
2
+2
2
0H
0H
E
D
5
ꢁD±I
ꢀ
ꢁDꢂꢀ5ꢀ ꢀ+ꢁꢀꢁEꢂꢀ5ꢀ ꢀꢂꢃ0HꢁꢀꢁFꢂꢀ5ꢀ ꢀꢂꢃ20Hꢁ
ꢁGꢂꢀ5ꢀ ꢀꢄꢃ20HꢁꢀꢁHꢂꢀ5ꢀ ꢀꢂꢃ)ꢁꢀꢁIꢂꢀ5ꢀ ꢀꢂꢃ&O
F
2
&1
1+ꢄ
2
2
2
2
G
5
5
ꢀD±I
ꢃD±I
ꢁDꢂꢀ5ꢀ ꢀ+ꢁꢀꢁEꢂꢀ5ꢀ ꢀꢂꢃ0HꢁꢀꢁFꢂꢀ5ꢀ ꢀꢂꢃ20Hꢁ
ꢁGꢂꢀ5ꢀ ꢀꢄꢃ20HꢁꢀꢁHꢂꢀ5ꢀ ꢀꢂꢃ)ꢁꢀꢁIꢂꢀ5ꢀ ꢀꢂꢃ&O
ꢃDꢂꢀ5ꢀ ꢀ+ꢁꢀꢃEꢂꢄ5ꢀ ꢀꢂꢃ0HꢁꢀꢃFꢂꢀ5ꢀ ꢀꢂꢃ20Hꢁ
ꢃGꢂꢀ5ꢀ ꢀꢄꢃ20HꢁꢀꢃHꢂꢀ5ꢀ ꢀꢂꢃ)ꢁꢀꢃIꢂꢀ5ꢀ ꢀꢂꢃ&O
Figure 2 The general synthetic route for the synthesis of compounds 5a–f.
Notes: reagents and conditions: (a) PhB(Oh)2, cu(Oac)2, c5h5n, ch2cl2, 25°c–27°c, 72 hours, 48%; (b) archO, KOh, etOh, 25°c–27°c, 24 hours, 67%–78%;
(c) cyanohydrin acetone, Bu3(Me)n+Oh-, K2cO3, Me2cO, h2O, 55°c–27°c, 14 hours, 80%–93%; and (d) h2O2, K2cO3, DMsO, from 5°c to 25°c–27°c, 2 hours, 70%–77%.
Abbreviation: DMsO, dimethyl sulfoxide.
diphenyl ethers is high. Therefore, it was decided to synthe- time, it was ensured that the newly designed molecules
size diphenyl ether derivatives with hydrophilic substituents achieved similar orientation and interactions as those of
at the fourth position and –OH group at the C-2 position of triclosan at the target binding site. Synthesis of compounds
ring-A (Figure 1). These changes were made to decrease the 10a–c is described in Figure 3. Acetovanillone was condensed
lipophilicity of the diphenyl ether derivatives. At the same with phenyl boronic acid by Chan–Lam O-arylation reaction.
2+
20H
20H
2
2
+2
D
E
0H
0H
0H
ꢁ
ꢀ
2
2
2
F
2+
2+
2
2
G
5
5
2
&1
2
ꢀD±F
ꢀD±F
ꢁDꢂꢀ5ꢀ ꢀ+ꢁꢀꢁEꢂꢀ5ꢀ ꢀꢂꢃ0HꢁꢀꢁFꢂꢀ5ꢀ ꢀꢂꢃ20Hꢃ
ꢀDꢂꢃ5ꢀ ꢀ+ꢁꢃꢀEꢂꢀ5ꢀ ꢀꢂꢃ0HꢄꢃꢀFꢂꢀ5ꢀ ꢀꢂꢃ20H
H
2+
2
5
2
1+ꢄ
2
ꢁꢂD±F
ꢁꢂDꢃꢀ5ꢀ ꢀ+ꢁꢀꢁꢂEꢃꢀ5ꢀ ꢀꢂꢃ0HꢁꢀꢁꢂFꢃꢀ5ꢀ ꢀꢂꢃ20H
Figure 3 The general synthetic route for the synthesis of compounds 10a–c.
Notes: reagents and conditions: (a) PhB(Oh)2, cu(Oac)2, c5h5n, ch2cl2, 25°c–27°c, 72 hours, 94%; (b) BBr3 (1 M, ch2cl2), ch2cl2, from -78°c to 25°c–27°c, 3 hours,
96%; (c) archO, KOh, etOh, 25°c–27°c, 24 hours, 55%–72%; (d) cyanohydrin acetone, Bu3(Me)n+Oh-, K2cO3, Me2cO, h2O, 55°c–57°c, 14 hours, 80%–83%; and
(e) h2O2, K2cO3, DMsO, from 0°c to 25°c–27°c, 5 hours, 81%–86%.
Abbreviation: DMsO, dimethyl sulfoxide.
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Diphenyl ether derivatives as anti-TB agents
Table 1 in vitro antitubercular activity (Mic), cytotoxicity (cc50), si, clog P, and Psa of compounds 5a–f and 10a–c
Compound
R
MICa
(µg/mL)
SIc
Clog Pd
PSAe
(Å2)
CC50b (µg/mL)
Vero
HepG2
5a
-h
–
–
–
–
–
–
4.01
4.51
3.93
3.93
4.16
4.73
3.64
4.14
3.56
5.52
–
69.39
69.39
78.62
78.62
69.39
69.39
89.62
89.62
98.85
29.46
–
.100
.100
.100
50
.300
.300
.300
.300
.300
.300
.300
.300
.300
.300
–
.300
.300
.300
.300
.300
.300
.300
.300
260.6
5b
4-Me
4-OMe
2-OMe
4-F
5c
5d
5e
50
5f
4-cl
-h
4-Me
4-OMe
–
.100
20
10a
.10
.10
.10
.10
–
10b
12.5
25
10c
Triclosan
Isoniazid
12.5
0.125
.300
–
–
Notes: aMic, minimal drug concentration required to stop the growth of Mycobacterium tuberculosis h37rv; bcc50, minimal drug concentration required for 50% death of
viable cells; csi, cc50/Mic; dclog P predicted from chemDraw Ultra-2008; ePsa predicted from chemDraw Ultra-2008.
Abbreviations: cc50, half-maximal cytotoxicity concentration; Mic, minimum inhibitory concentration; Psa, polar surface area; si, selectivity index.
Compound 6 that was obtained was subjected to BBr3-assisted
Compounds 5a–f were screened for their anti-TB
demethylation reaction as described by Gillmore et al22 to potential to study the effect of the presence carbonyl group at
obtain compound 7. Compounds 8a–c, 9a–c, and 10a–c were the proximity of ring-A on the anti-TB activity. This series of
prepared by using the same chemistry shown in Figure 2.
compounds possessed very weak activity (MIC $50 µg/mL).
The synthesized diphenyl ether derivatives were screened This indicates that structural modifications at C-3 of ring-A in
in vitro for anti-TB activity against M. tuberculosis H37Rv diphenyl ether are unfavorable for anti-TB activity. Potential
strain using microplate Alamar Blue assay method.23,24 toxicity of the synthesized diphenyl ether derivatives was
Known InhA inhibitors, triclosan and isoniazid, were used evaluated on mammalian Vero cell lines and HepG2 cell lines
as standard drugs for comparison.
using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
Figure 4 Molecular docking interaction of compound 10b (green) with Mtb enr (PDB 1P45).
Note: Dotted yellow line shows the hydrogen bonding interaction.
Abbreviations: Mtb enr, enoyl-acyl carrier protein reductase of Mycobacterium tuberculosis; naD, nicotinamide adenine dinucleotide.
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Table 2 Molecular docking results of compounds 5a–f and 10a–c
Compound
Docking
score
Hydrophobic
surface (1.4 Å)
Hydrogen bonding
NAD-300
Distance (Å)
NAD-300
Tyr-158
Tyr-158
5a
5b
5c
5d
5e
5f
10a
10b
10c
Triclosan
186.74
182.64
163.99
179.07
172.84
164.43
180.91
174.96
156.04
117.36
√
X
√
√
√
√
√
√
√
√
X
X
X
X
X
X
X
X
X
√
1.75
2.14
1.42
1.91
1.51
1.43
2.03
1.93
1.82
1.63
2.74
2.41
2.57
2.60
2.24
2.58
1.99
2.03
2.52
1.76
-9.64
-8.05
-5.83
-7.39
-6.51
-6.41
-8.53
-8.65
-7.64
-8.48
Notes: √, presence of interaction; X, absence of interaction.
Abbreviations: naD, nicotinamide adenine dinucleotide; Tyr, tyrosine.
bromide assay technique. Data showed that compounds Although the most promising compound 10b did not pos-
5a–f are not cytotoxic against (half-maximal cytotoxicity sess the hydrogen bonding interaction pattern of triclosan,
concentration .300 µg/mL) against Vero and HepG2 cells it fitted better inside the active site of the ENR to show
(Table 1).25–27
Compounds 10a–c were screened for their anti-TB
superior docking score (Table 2).
In-depth analysis of docking results and their correla-
activity. Compound 10b showed MIC value of 12.5 µg/mL, tion with the anti-TB activity of these series of compounds
which is equal to the MIC of triclosan. The Clog P and showed that docking orientations, van der Waals interactions,
polar surface area of compound 10b (Table 1) showed that and hydrogen bonding interactions with NAD play a decisive
it is comparatively more polar than triclosan and obeys the role in improving the anti-TB activity.
Lipinski’s rule for druglikeness.28 All the compounds of this
series possessed acceptable safety profile against Vero as anti-TB activity, experimental log P of the most promising
well as HepG2 cells. compound 10b was determined by reverse-phase HPLC
In order to understand the effect of lipophilicity on
To understand the interaction of the proposed diphenyl method31,32 and is shown in Table 3. The log P obtained
ether derivatives with mycobacterial ENR, molecular was compared with its calculated Clog P using ChemDraw
docking study was conducted in Schrodinger-201029 using Ultra-2008 software (Cambridge, MA, USA) (Table 1). Com-
PDB 1P45 as the target protein.30 The molecular docking pound 10b demonstrated moderate log P and was within the
study of compounds 5a–f showed that the amide group limit of Lipinski’s28 suggested parameters for oral bioavail-
at β-carbon was oriented toward the pyrophosphate part ability. In-depth analysis of predicted Clog P of compounds
of nicotinamide adenine dinucleotide (NAD) forming indicated that there is no relationship between lipophilicity
hydrogen bond with it. Although this series of compounds and the observed anti-TB activity.
showed apparently good docking scores, their interactions
pKa of the representative compound 10b was deter-
with the pyrophosphate moiety of NAD proved unfavorable mined using the reverse-phase HPLC method (Table 3).33
for anti-TB activity. Molecular docking study of com- Compound 10b was found to be weakly acidic (pKa =7.59).
pound 10b revealed that the prochiral carbonyl group was The weak acidity of the molecule may be due to the effect
oriented opposite to the plane of NAD, and lost hydrogen of the carbonyl group on the acidity of ionizable phenolic
bonding interactions with both 2′ OH of NAD and Tyr- OH group at C-2 position of ring-A. pKa of compound 10b
158 (Figure 4). However, the flexible amide group of 10b showed that it would have remained in the ionized form
formed hydrogen bond with the carbonyl oxygen of nico- to a certain extent at pH 7.4 during the in vitro whole-cell
tinamide of NAD. Compound 10c oriented in a different
Table 3 evaluation of druglikeness of compound 10b
manner and displayed hydrogen bonding interaction with
the pyrophosphate region of NAD. This hydrogen bond-
ing interaction was noticed to be unfavorable for anti-TB
activity. Although these three compounds lost hydrogen
bonding interactions at the catalytic site of ENR, they
displayed strong van der Waals interaction with NAD.
log Pa
pKaa
% protein
bindinga
4.059
7.59
90.5
Note: alog P, pKa, and % protein binding were estimated from reverse-phase hPlc
experiment.
Abbreviation: hPlc, high-performance liquid chromatography.
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Diphenyl ether derivatives as anti-TB agents
Table 4 human microsome stability study of compound 10b
4. Payne DJ, Warren PV, Holmes DJ, Ji Y, Lonsdale JT. Bacterial
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discovery. Drug Discov Today. 2001;6:537–544.
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Trends Mol Med. 2001;17:25–33.
Percentage remaininga
15 minutes
30 minutes
1 hour
2 hours
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57.49
50.68
40.35
6. Sullivan TJ, Truglio JJ, Boyne ME, et al. High affinity InhA inhibitors
with activity against drug-resistant strains of Mycobacterium
tuberculosis. ACS Chem Biol. 2006;1:43–53.
Note: aPercentage of compound that remained at different time points during
incubation with hlM.
Abbreviation: hlM, human liver microsomes.
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antimicrobial biocides target the FabI component of fatty acid
synthesis. J Biol Chem. 1998;273:30316–30320.
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aryl ether inhibitors of the Bacillus anthracis enoyl-ACP reductase.
ChemMedChem. 2008;3:1260–1268.
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2-pyridones as novel inhibitors of the Bacillus anthracis enoyl–ACP
reductase. Bioorg Med Chem Lett. 2008;18:3565–3569.
10. Takahata S, Iida M, Osaki Y, et al. AG205, a novel agent directed against
FabKofStreptococcuspneumoniae.AntimicrobAgentsChemother.2006;
50:2869–2871.
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tives as specific inhibitors of enoyl-acyl carrier protein reductase (FabI)
with antibacterial activity against Staphylococcus aureus. J Antibiot.
2007;60:123–128.
12. Kitagawa H, Ozawa T, Takahata S, Iida M, Saito J, Yamada M. Phenyl
imidazole derivatives of 4-pyridone as dual inhibitors of bacterial enoyl-
acyl carrier protein reductases FabI and FabK. J Med Chem. 2007;
50:4710–4720.
13. McLeod R, Muench SP, Rafferty JB, et al. Triclosan inhibits the growth
of Plasmodium falciparum and Toxoplasma gondii by inhibition of
apicomplexan Fab I. Int J Parasitol. 2001;31:109–113.
14. Wang LQ, Falany CN, James MO. Triclosan as a substrate and inhibitor
of 3′-phosphoadenosine-5′-phosphosulfate-sulfotransferase and UDP-
glucuronosyl transferase in human liver fractions. Drug Metab Dispos.
2004;32:1162–1169.
15. Thomas CA, Kar SS, Bairy I, Bhat VG, Shenoy VP, Shenoy GG.
Synthesis and evaluation of antitubercular activity of novel diphenyl
ether derivatives. Ind Glob J Pharm Sci. 2015;5:19–25.
16. Thomas CA, Kar SS, Bairy I, Bhat VG, Shenoy VP, Shenoy GG.
Design, synthesis and evaluation of antitubercular activity of triclosan
analogues. Arabian J Chem. 2015 (accepted). Available from: http://
dx.doi.org/10.1016/j.arabjc.2015.09.003
17. Sivaraman S, Sullivan TJ, Johnson F, et al. Inhibition of the bacterial
enoyl reductase FabI by triclosan: a structure-reactivity analysis of FabI
inhibition by triclosan analogues. J Med Chem. 2004;47:509–518.
18. Evans DA, Katz JL, West TR. Synthesis of diaryl ethers through
the copper-promoted arylation of phenols with arylboronic acids.
An expedient synthesis of thyroxine. Tetrahedron Lett. 1998;39:
2937–2940.
19. AbdelHafezOM, AhmedMK, HaggagEE. Synthesisofsomepotentially
bioactive compounds from visnaginone. Molecules. 2001;6:396–405.
20. Ellis JE, Davis EM, Brower PL. A novel and practical method for
hydrocyanation of chalcones. Org Pro Res Dev. 1997;1:250–252.
21. Katritzky AR, Pilarski B, Urogdi L. Efficient conversion of nitriles to
amides with basic hydrogen peroxide in dimethyl sulfoxide. Synthesis.
1989;12:949–952.
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for puetuberosanol and approaches to the natural products marshrin
and phebalosin. Tetrahedron. 2003;59:4363–4375.
23. Leonard B, Coronel J, Siedner M. Inter- and intra-assay reproducibil-
ity of microplate alamar blue assay results for isoniazid, rifampicin,
ethambutol, streptomycin, ciprofloxacin, and capreomycin drug sus-
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2008;46:3526–3529.
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Chemother. 1997;41:1004–1009.
assay (microplate Alamar Blue assay). Hence, insignificant
penetration into the mycobacterial cell wall may be the sole
reason for its moderate anti-TB activity.
The protein binding of compound 10b was evaluated by
reverse-phase HPLC method using human serum albumin
column (Table 3).34,35 Compound 10b was observed to be
highly susceptible to protein binding (90.5%), whereas
plasma protein binding of triclosan is known to be even
higher than compound 10b.36 Human liver microsomal sta-
bility (%) of compound 10b was evaluated at different time
points (15 minutes, 30 minutes, 1 hour, and 2 hours).37,38
After 2 hours of incubation, 50.68% of 10b was found to be
intact (Table 4).
Conclusion
A series of triclosan mimic diphenyl ether derivatives were
prepared and evaluated for their anti-TB activity against M.
tuberculosis H37Rv strain. The synthesized compounds were
found to be less lipophilic than triclosan. Among the synthe-
sized molecules, compound 10b had anti-TB activity equal
to that of triclosan and was less lipophilic than triclosan. 10b
had good microsomal stability and low cytotoxicity. Docking
studies indicate that there is scope for further optimization
of diphenyl ether derivatives in order to gain access to new
lead molecules as anti-TB agents.
Acknowledgments
This research was funded by the Indian Council of Medical
Research, New Delhi. The authors thank Schrodinger Inc.
for providing software to carry out molecular modeling
studies. The authors are grateful to AICTE, New Delhi,
DST, New Delhi, and Manipal University, Manipal, for pro-
viding necessary facilities to carry out the research work.
Disclosure
The authors report no conflicts of interest in this work.
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