[Et3NH][HSO4] catalyzed efficient synthesis of 5-arylidene-rhodanine conjugates and their antitubercular activity

Article abstract of DOI:10.1007/s11164-016-2484-0

Abstract: We have described a highly efficient, safer protocol for the synthesis of 5-arylidene-rhodanine conjugates catalyzed by Bronsted acidic ionic liquid [Et₃NH][HSO₄] in excellent yields. The protocol offers cost-effective, environmentally benign, solvent-free conditions and recycle–reuse of the catalyst. The synthesized 5-arylidene-rhodanine conjugates were characterized on the basis of ¹H NMR, ¹³C NMR and HRMS spectral data. A series of 5-arylidene-rhodanine derivatives 3a–h, 4a–h were synthesized and evaluated for their in vitro antitubercular activity against dormant Mycobacterium tuberculosis H37Ra and M. bovis BCG strains. Moreover, compounds 3a, 3b, 3e, 3f, 3g, 3h and 4f exhibited good antitubercular activity and were also evaluated for anti-proliferative activity against MCF-7, A549 and HCT116 cell lines using modified MTT assay and found to be noncytotoxic. Compounds 3a–h and 4f were further screened for their antibacterial activity against four bacteria strains to assess their selectivity towards M. tuberculosis. Furthermore, in silico ADME prediction of all the tested compounds followed the criteria for orally active drug and, therefore, these compounds may have a good potential for eventual development as oral agents. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2484-0

Res Chem Intermed
DOI 10.1007/s11164-016-2484-0
[Et3NH][HSO4] catalyzed efficient synthesis
of 5-arylidene-rhodanine conjugates and their
antitubercular activity
Dnyaneshwar D. Subhedar1 Mubarak H. Shaikh1
•
•
Laxman Nawale2 Amar Yeware2 Dhiman Sarkar2
Bapurao B. Shingate1
•
•
•
Received: 5 November 2015 / Accepted: 16 February 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract We have described a highly efficient, safer protocol for the synthesis
of 5-arylidene-rhodanine conjugates catalyzed by Bronsted acidic ionic liquid
[Et3NH][HSO4] in excellent yields. The protocol offers cost-effective, environmentally
benign, solvent-free conditions and recycle–reuse of the catalyst. The synthesized
5-arylidene-rhodanine conjugates were characterized on the basis of 1H NMR, 13C NMR
and HRMS spectral data. A series of 5-arylidene-rhodanine derivatives 3a–h, 4a–h were
synthesized and evaluated for their in vitro antitubercular activity against dormant
Mycobacterium tuberculosis H37Ra and M. bovis BCG strains. Moreover, compounds
3a, 3b, 3e, 3f, 3g, 3h and 4f exhibited good antitubercular activity and were also
evaluated for anti-proliferative activity against MCF-7, A549 and HCT116 cell lines
using modified MTT assay and found to be noncytotoxic. Compounds 3a–h and 4f were
further screened for their antibacterial activity against four bacteria strains to assess their
selectivity towards M. tuberculosis. Furthermore, in silico ADME prediction of all the
tested compounds followed the criteria for orally active drug and, therefore, these
compounds may have a good potential for eventual development as oral agents.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2484-0)
contains supplementary material, which is available to authorized users.
&
Bapurao B. Shingate
bapushingate@gmail.com
1
2
UGC-SAP and DST-FIST Sponsored, Department of Chemistry, Dr. Babasaheb Ambedkar
Marathwada University, Aurangabad 431 004, India
Combichem Bioresource Center, Organic Chemistry Division, CSIR-National Chemical
Laboratory, Pune 411 008, India
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D. D. Subhedar et al.
Graphical Abstract
Keywords Ionic liquid Á Antitubercular activity Á Cytotoxicity Á Rhodanine and
Knoevenagel condensation
Introduction
Almost 9 million new cases of human tuberculosis (TB) are diagnosed and 1.5
million deaths are reported worldwide [1]. Mycobacterium tuberculosis (MTB) is
among the most challenging bacterial infections, requiring daily combination
therapy of up to 4 drugs for at least 6 months to treat uncomplicated drug-sensitive
infections [2]. Nowadays, synergy with HIV and the appearance of MTB strains,
which are multi-drug resistant or hyper-virulent, poses further threats [3–5]. Thus,
the discovery of novel anti-TB drug targets is of increasing importance in combating
this disease. In addition, M. bovis BCG vaccination is also among the most
commonly administered vaccines worldwide [6]. It is usually the first vaccine
administered to newborns in developing countries and is a part of the expanded
programme of immunization of the World Health Organization WHO [7].
Rhodanine (2-thioxothiazolid-4-one) and their analogues have been known as
privileged structures in pharmacological research which display a broad spectrum of
biological activities [8]. In particular, 5-arylidene rhodanines display antifungal [9]
and antibacterial properties [10, 11], are DNA polymerase k inhibitors [12, 13], and
anti-inflammatory [14, 15], while ZTB24, ZTB28R and ZTB29R are antitubercular
agents [16, 17]. In addition, they are active against dengue virus protease [18],
MurD ligase [19], chikungunya virus [20], tau aggregation [21, 22], Alzheimer’s
disease [23], GTPase activity [24], HIV [25] and histone acetyltransferases [26],
while epalrestat is an aldose reductase inhibitor [27], and also against HCV NS5b
[28] and AF6 PDZ domain [29]. Representive structures of the reported 5-arylidene-
rhodanine derivatives exhibiting various biological activities are shown in Fig. 1.
Ionic liquids (ILs) have created interest as environmentally benign media for
catalytic applications due to their unique chemical and physical properties, such as
low vapor pressure, high thermal and chemical stability, ease of recyclability and
controlled miscibility [30, 31]. Thus, ILs are considered to be a safer alternative to
original organic solvents as they are cleaner and safer to use [32, 33].
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[Et3NH][HSO4] catalyzed efficient synthesis of…
Fig. 1 Some biologically active agents bearing rhodanine scaffolds
Owing to the potential importance of rhodanine as a key moiety in life sciences
and pharmaceuticals, worldwide efforts have made in the last few decades by
researchers, and several protocols have been developed for their synthesis using
various catalysts such as pyridine ethanol [34, 35], piperidine ethanol/acetic acid
[36], CH3COONa/AcOH [37–39], NH4OAc/AcOH [40, 41], NH4OAc [42],
NH4OAc/toluene [43, 44], NH4Cl/NH4OH-EtOH [45] and [bmim][OH] [46].
However, the reported methods suffer from one or more drawbacks, like the use of
excess or stoichiometric quantities of reactants, or of toxic, corrosive and non-
recoverable catalysts, as well as tedious separation procedures, long reaction times,
high temperatures and low yields of the desired products. Therefore, it seems that a
major task of current research is to replace the less efficient and traditional catalysis
procedures with more acceptable methods using improved, stable and recoverable
catalysts. Thus, Bronsted acidic ionic liquids (ABILs), e.g., triethylamine hydrogen
sulfate [Et3NH][HSO4], have not yet been explored for the synthesis of 5-arylidene-
rhodanines. They are prepared via a simple and atom economic acid–base
neutralization reaction from amine and acid [47]. Moreover, ABILs are of special
importance as they simultaneously possess the proton acidity and the characteristic
properties of an ionic liquid [48, 49]. Recently, we have utilized [Et3NH][HSO4] for
the synthesis of a-amino-phosphonates in excellent yields [50]. Keeping in mind the
biological significance of 5-arylidene-rhodanine conjugates and the utility of ionic
liquids for organic transformation, and in continuation of our study on the synthesis
and bioevaluation of various compounds [51–59], we report here that
[Et3NH][HSO4] IL catalyzed a solvent-free [60] reaction of aryl aldehydes and
2-thioxothiazolidin-4-one/2-(4-oxo-2-thioxothiazolidin-3-yl)acetic acid to yield a
series of 5-arylidene-rhodanines. The synthesized compounds have also been
evaluated for their antitubercular property with cytotoxic activities for the first time.
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D. D. Subhedar et al.
Experimental
All the solvents and reagents were purchased from the commercial suppliers,
Spectrochem, Rankem and Sigma Aldrich and are used without further purification.
Reaction time and purity of the products were monitored by thin layer chromatog-
raphy (TLC) aluminum sheets, silica gel 60-F254 precoated (Merck, Germany),
locating the spots using UV light or Iodine vapors as the visualizing agent. All the
melting points were determined by the open capillary method and are uncorrected.
1H NMR spectra were recorded on a Bruker Advance III 400 MHz and 13C NMR on
a 100 MHz spectrometer using the residual solvent as internal standard (DMSO-d6).
The chemical shifts were reported in ppm. High-resolution mass spectra (HRMS)
were recorded on an Agilent 6520 (QTOF) ESI-HRMS instrument.
Synthesis of triethylammonium hydrogen sulfate [Et3NH][HSO4]
The synthesis of ionic liquid was carried out in a 100-mL round-bottom flask, which
was immersed in a recirculating heated water-bath and fitted with a reflux
condenser. Sulfuric acid, 98 % (1.96 g, 0.02 mol), was added dropwise into
triethylamine (2.02 g, 0.02 mol) at 60 °C for 1 h. After the addition, the reaction
mixture was stirred for an additional period of 1 h at 70 °C to ensure the reaction
was proceeded for completion. The water molecule was removed by heating the
residue at 80–90 °C under a high vacuum until the weight of the residue remained
1
constant. The obtained yield of [Et3NH] [HSO4] was 95 %. H NMR (400 MHz,
DMSO-d6): dH ppm = 1.16–1.23 (t, 9H, 3 9 CH3), 3.04–3.15 (q, 6H, 3 9 CH2)
and 8.85 (s, 1H, NH); 13C NMR (100 MHz, DMSO-d6): dH ppm = 11.03 (CH3) and
52.6 (CH2); FT-IR cm-1 3024 (for N–H stretch), 2815.74 (for C–H stretch), 1231.96
(for C–N stretch).
General procedure for synthesis of (Z)-5-benzylidene-2-thioxothiazolidin-4-
one (3a-h) and (Z)-2-(5-benzylidene-4-oxo-2-thioxothiazolidin-3-yl)acetic
acid (4a-h)
A mixture of aldehyde (1 mmol), rhodanine/rhodanine acetic acid (1 mmol) and
ionic liquid [Et3NH][HSO4] (20 mol%) was heated at 80 °C for 20 min. The
progress of the reaction was monitored by thin layer chromatography (n-Hexane/
EtOAc 8:2). The reaction mixture was allowed to cool at room temperature. Then
ice-cold water (10 mL) was added to it and stirred for 10 min and filtered. The
obtained solid was filtered and washed with cold water to remove the ionic liquid.
The obtained crude compounds were recrystallized using ethanol.
(Z)-5-benzylidene-2-thioxothiazolidin-4-one (3a)
The compound 3a was obtained via a Knoevenagel condensation reaction between
benzaldehyde 1a and rhodanine 2a in 20 min as a yellow solid: 1H NMR (400 MHz,
DMSO-d6): dH ppm = 7.67 (s, 1H, =CH) and 7.35–7.39 (m, 5H, Ar–H); 13C NMR
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
(100 MHz, DMSO-d6): dc ppm = 124.4, 130.1, 130.8, 132.2, 133.2, 167.4 and
195.4; HRMS (ESI-qTOF): Calcd for C10H8NOS2 [M ? H]?, 222.0423: found:
222.0820.
(Z)-5-(4-methylbenzylidene)-2-thioxothiazolidin-4-one (3b)
The compound 3b was obtained via a Knoevenagel condensation reaction between
1
4-methylbenzaldehyde 1b and rhodanine 2a in 20 min as a yellow solid: H NMR
(400 MHz, DMSO-d6): dH ppm = 7.91 (d, 2H, Ar–H), 7.66 (s, 1H, =CH), 7.02 (d,
2H, Ar–H) and 2.25 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6): dc ppm = 22.6,
128.6, 129.1, 129.3, 129.9, 130.7, 131.3, 165.7 and 191.3; HRMS (ESI-qTOF):
Calcd for C11H9NOS2Na [M ? Na]?, 258.0460: found: 258.0444.
(Z)-5-(4-methoxybenzylidene)-2-thioxothiazolidin-4-one (3c)
The compound 3c was obtained via a Knoevenagel condensation reaction between
4-methoxybenzaldehyde 1c and rhodanine 2a in 20 min as a yellow solid: 1H NMR
(400 MHz, DMSO-d6): dH ppm = 7.65 (s, 1H, =CH), 7.38 (d, 2H, Ar–H), 6.92
(d, 2H, Ar–H) and 3.79 (s, 3H, OCH3); 13C NMR (100 MHz, DMSO-d6):
dc ppm = 56.0, 130.2, 130.9, 131.3, 132.4, 136.0, 162.6 and 192.8. HRMS (ESI-
qTOF): Calcd for C11H9NO2S2Na [M ? Na]?, 274.0760 found: 274.0798.
(Z)-5-(4-bromobenzylidene)-2-thioxothiazolidin-4-one (3d)
The compound 3d was obtained via a Knoevenagel condensation reaction between
1
4-bromobenzaldehyde 1d and rhodanine 2a in 20 min as a yellow solid: H NMR
(400 MHz, DMSO-d6): dH ppm = 7.98 (s, 1H, =CH), 7.39 (d, 2H, Ar–H) and 7.29
(d, 2H, Ar–H); 13C NMR (100 MHz, DMSO-d6): dc ppm = 130.8, 132.2, 132.8,
133.2, 135.8, 165.6 and 191.4; Calcd for C10H6BrNOS2Na [M ? Na]?, 321.0620
found: 321.725.
(Z)-5-(4-(dimethylamino)benzylidene)-2-thioxothiazolidin-4-one (3e)
The compound 3e was obtained via a Knoevenagel condensation reaction between
4-(dimethylamino)benzaldehyde 1e and rhodanine 2a in 20 min as an orange solid:
1H NMR (400 MHz, DMSO-d6): dH ppm = 11.71 (s, 1H, NH), 7.85 (s, 1H, =CH),
7.54 (d, 2H, Ar–H) and 7.25 (d, 2H, Ar–H), 3.00 (s, 6H, CH3); 13C NMR (100 MHz,
DMSO-d6): dc ppm = 37.25, 130.0, 130.8, 130.9, 131.5, 132.3, 135.9, 167.6 and
190.6; Calcd for C12H13N2OS2 [M ? H]?, 265.0240 found: 265.0227.
(Z)-5-(4-chlorobenzylidene)-2-thioxothiazolidin-4-one (3f)
The compound 3f was obtained via a Knoevenagel condensation reaction between
1
4-chlorobenzaldehyde 1f and rhodanine 2a in 20 min as a yellow solid: H NMR
(400 MHz, DMSO-d6): dH ppm = 7.93 (s, 1H, =CH) and 7.25–7.36 (m, 4H, Ar–H);
13C NMR (100 MHz, DMSO-d6): dc ppm = 129.2, 130.0, 130.2, 130.6, 131.2,
123

D. D. Subhedar et al.
133.3, 165.6 and 191.1; Calcd for C10H6ClNOS2Na [M ? Na]?, 273.2079 found:
273.2057.
(Z)-5-(4-fluorobenzylidene)-2-thioxothiazolidin-4-one (3g)
The compound 3g was obtained via a Knoevenagel condensation reaction between
1
4-fluorobenzaldehyde 1g and rhodanine 2a in 20 min as a yellow solid: H NMR
(400 MHz, DMSO-d6): dH ppm = 12.15 (s, 1H, NH), 7.64 (s, 1H, =CH), 7.40
(d, 2H, Ar–H) and 7.11 (d, 2H, Ar–H); 13C NMR (100 MHz, DMSO-d6):
dc ppm = 128.0, 128.3, 128.8, 131.5, 132.6, 132.8, 162.1 and 192.7; Calcd for
C10H6FNOS2Na [M ? Na]?, 261.5690 found: 261.5673.
(Z)-5-(4-nitrobenzylidene)-2-thioxothiazolidin-4-one (3h)
The compound 3h was obtained via a Knoevenagel condensation reaction between
1
4-nitrobenzaldehyde 1h and rhodanine 2a in 20 min as a yellow solid: H NMR
(400 MHz, DMSO-d6): dH ppm = 7.86 (d, 2H, Ar–H), 7.75 (s, 1H, =CH) and 7.53
(d, 2H, Ar–H); 13C NMR (100 MHz, DMSO-d6): dc ppm = 129.3, 130.1, 130.8,
130.9, 132.7, 135.8, 162.0 and 194.0; Calcd for C10H6N2O3S2K [M ? K]?,
304.1031 found: 304.0902.
(Z)-2-(5-benzylidene-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4a)
The compound 4a was obtained via a Knoevenagel condensation reaction between
benzaldehyde 1a and rhodanine acetic acid 2b in 20 min as a yellow solid: 1H NMR
(400 MHz, DMSO-d6): dH ppm = 7.91 (s, 1H, =CH), 7.25–7.33 (m, 5H, Ar–H) and
4.85. (s, 2H, CH2); 13C NMR (100 MHz, DMSO-d6): dc ppm = 46.6, 124.4, 129.9,
130.8, 132.0, 132.2, 133.9, 165.7, 167.5 and 196.0; Calcd for C12H10NO3S2
[M ? H]?, 280.0470 found: 280.0491.
(Z)-2-(5-(4-methylbenzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4b)
The compound 4b was obtained via a Knoevenagel condensation reaction between
4-methoxybenzaldehyde 1b and rhodanine acetic acid 2b in 20 min as a yellow
1
solid: H NMR (400 MHz, DMSO-d6): dH ppm = 7.93 (s, 1H, =CH), 7.66 (d, 2H,
Ar–H), 7.53 (d, 2H, Ar–H), 4.67 (s, 2H, -CH2) and 2.21 (s, 3H, CH3); 13C NMR
(100 MHz, DMSO-d6): dc ppm = 24.0, 52.5, 124.1, 125.1, 129.4, 129.6, 130.8,
130.9, 160.3, 161.8 and 190.4; Calcd for C13H11NO3S2K [M ? K]?, 332.0968
found: 332.0980.
(Z)-2-(5-(4-methoxybenzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4c)
The compound 4c was obtained via a Knoevenagel condensation reaction between
4-methoxybenzaldehyde 1c and rhodanine acetic acid 2b in 20 min as a yellow
1
solid: H NMR (400 MHz, DMSO-d6): dH ppm = 7.65 (s, 1H, =CH), 7.36 (d, 2H,
Ar–H), 6.93 (d, 2H, Ar–H), 4.83 (s, 2H, CH2) and 3.80 (s, 3H, OCH3); 13C NMR
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
(100 MHz, DMSO-d6): dc ppm = 52.4, 54.9, 124.1, 124.4, 125.3, 127.5, 129.4,
129.6, 160.3, 161.8 and 194.4; Calcd for C13H11NO4S2Na [M ? Na]?, 332.0987
found: 332.0967.
(Z)-2-(5-(4-bromobenzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4d)
The compound 4d was obtained via a Knoevenagel condensation reaction between
4-bromobenzaldehyde 1d and rhodanine acetic acid 2b in 20 min as a yellow solid:
1H NMR (400 MHz, DMSO-d6): dH ppm = 7.92 (s, 1H, =CH), 7.49–7.58 (m, 4H,
Ar–H) and 4.71 (s, 2H, CH2); 13C NMR (100 MHz, DMSO-d6): dc ppm = 52.6,
125.4, 126.3, 129.4, 130.7, 132.2, 135.9, 160.3, 161.9 and 193.4; Calcd for
C12H8BrNO3S2Na [M ? Na]?, 379.9935 found: 379.9948.
(Z)-2-(5-(4-(dimethylamino)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid
(4e)
The compound 4e was obtained via a Knoevenagel condensation reaction between
4-(dimethylamino)benzaldehyde 1e and rhodanine acetic acid 2b in 20 min as a
1
yellow solid: H NMR (400 MHz, DMSO-d6): dH ppm = 7.93 (s, 1H, =CH), 7.53
(d, 2H, Ar–H) and 7.22 (d, 2H, Ar–H), 4.85 (s, 2H, CH2), 3.03 (s, 6H, CH3);
13C NMR (100 MHz, DMSO-d6): dc ppm = 36.3, 52.5, 124.4, 126.0, 129.6, 132.2,
163.3, 163.8 and 193.6;Calcd for C14H14N2O3S2Na [M ? Na]?, 345.0763 found:
345.0751.
(Z)-2-(5-(4-chlorobenzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4f)
The compound 4f was obtained via a Knoevenagel condensation reaction between
4-chlorobenzaldehyde 1f and rhodanine acetic acid 2b in 20 min as a yellow solid:
1H NMR (400 MHz, DMSO-d6): dH ppm = 7.93 (s, 1H, =CH), 7.35 (d, 4H, Ar–H),
7.22 (d, 4H, Ar–H) and 4.66 (s, 2H, CH2); 13C NMR (100 MHz, DMSO-d6):
dc ppm = 52.4, 124.0, 124.4, 126.0, 127.5, 129.3, 129.6, 160.2, 162.0 and 193.5;
Calcd for C12H8ClNO3S2Na [M ? Na]?, 335.0540 found: 335.0504.
(Z)-2-(5-(4-fluorobenzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4g)
The compound 4g was obtained via a Knoevenagel condensation reaction between
4-fluorobenzalaldehyde 1g and rhodanine acetic acid 2b in 20 min as a yellow solid:
1H NMR (400 MHz, DMSO-d6): dH ppm = 7.67 (s, 1H, =CH), 7.46 (d, 2H, Ar–H),
7.11 (d, 2H, Ar–H) and 4.72 (s, 2H, CH2); 13C NMR (100 MHz, DMSO-d6):
dc ppm = 52.3, 123.3, 124.0, 125.9, 130.9, 131.4, 135.3, 163.3, 163.8 and 194.7;
Calcd for C12H8FNO3S2K [M ? Na]?, 335.1457 found: 335.1463.
(Z)-2-(5-(4-nitrobenzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (4h)
The compound 4h was obtained via a Knoevenagel condensation reaction between
4-fluorobenzalaldehyde 1h and rhodanine acetic acid 2b in 20 min as an orange-
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D. D. Subhedar et al.
1
yellow solid: H NMR (400 MHz, DMSO-d6): dH ppm = 7.66 (s, 1H, =CH), 7.36
(d, 2H, Ar–H), 6.91 (d, 2H, Ar–H) and 4.79 (s, 2H, CH2); 13C NMR (100 MHz,
DMSO-d6): dc ppm = 53.4, 125.6, 128.5, 129.0, 129.3, 132.1, 136.4, 163.3, 163.9
and 190.7; Calcd for C12H8N2O5S2Na [M ? Na]?, 346.0388 found: 346.0372.
Experimental protocol for biological activity
Antitubercular testing using the XRMA protocol
Compounds were tested for their in vitro effects against MTB H37Ra (ATCC
25177) which is susceptible to the control drug rifampicin. Compounds were
screened for their inhibitory effect on MTB in vitro according to the standard XTT
Reduction Menadione Assay (XRMA) protocol as described previously [68–70].
Primary screening was done against MTB at 30, 10 and 3 lg/mL concentrations of
the compound.
The in vitro effects of compounds against M. bovis BCG (ATCC35743) was
carried out according to the standard NR assay protocol as described previously
[68–70]. Briefly, in the NR assay, take 80 lL of culture from an incubated 96-well
plate into another 96-well plate, then add 80 lL of 1 % sulfanilic acid in 20 % of
conc. HCl, incubate it for 10 min at room temperature, then add 80 lL of 0.1 %
NEDD solution in D/W. Finally, the optical density of the suspension was measured
at 470 nm by using a microplate reader. All experiments were carried out in
triplicate and the quantitative value was expressed as the average standard
deviation, and IC50 values were calculated from their dose–response curves.
The % inhibition of bacilli was measured by the following formula,
% Inhibition ¼ ðA1 À BlankÞ=ðA2 À BlankÞ Â 100
where, A1 is the culture absorbance at 470 nm in the presence of the compound after
the addition of menadione, A2 is the ulture absorbance at 470 nm (DMSO solvent
control) after the addition of menadione, and blank is the culture absorbance at
470 nm of the respective data points before the addition of XTT/menadione.
Antibacterial activity
To check the specificity of the compound for mycobacteria, MIC and IC50 of these
compounds against two Gram-positive (Staphylococcus aureus and Bacillus
subtilis) and two Gram-negative (Escherichia coli and Pseudomonas flurescense)
bacteria were also estimated (Table 4). Antibacterial assay was done by using
96-well broth dilution methods. For this assay, an OD adjusted (OD620 = 1) culture
was inoculated into LB broth (0.1 %). In a 96-well microtiter plate, 2.5 lL of
compound and 247.5 lL ofculture were dispensed and incubated for 18 h at 37 °C
before reading the absorbance at 620 nm. Ampicillin and Kanamycin were taken as
positive control. Wells for growth and sterility control were also included. MIC and
IC50 were calculated from the dose–response curve [73].
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[Et3NH][HSO4] catalyzed efficient synthesis of…
Cytotoxic activity [74–76]
The effect of compounds 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h and 4f on cell growth was
determined in primary human cancer cell lines (MCF-7, A549 and HCT116)
obtained from the European Collection of Cell Cultures (ECACC). In vitro
cytotoxicity of compounds 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h and 4f against the above-
mentioned cells as determined using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide] as described previously for measuring cellular
proliferation. Each concentration was tested in triplicate in a single experiment.
The viability and growth in the presence of test material is calculated by using the
following formula: % cytotoxicity = (average of control-average of compound)/
(average of control-average of blank) 9 100), where control is the culture medium
with cells and DMSO and blank are the culture medium without cells. IC50 and MIC
values were calculated by plotting the percentage survival versus concentrations
using Origin Pro software.
In silico ADME prediction
The success of a drug is determined not only by good efficacy but also by an
acceptable ADME (absorption, distribution, metabolism and excretion) profile. In
the present study, we have calculated molecular volume (MV), molecular weight
(MW), logarithm of partition coefficient (miLog P), number of hydrogen bond
acceptors (n-ON), number of hydrogen bonds donors (n-OHNH), topological polar
surface area (TPSA), number of rotatable bonds (n-ROTB) and Lipinski’s rule of
five [77] using the Molinspiration online property calculation toolkit [78].
Absorption (% ABS) was calculated by: % ABS = 109 - (0.345 9 TPSA) [79].
Drug-likeness model score (a collective property of physic-chemical properties,
pharmacokinetics and pharmacodynamics of a compound is represented by a
numerical value) was computed by Mol Softsoftware [80].
Results and discussion
5-Arylidene-rhodanine conjugates 3a–h and 4a–h were synthesized using [Et3-
NH][HSO4] IL via Knoevenagel condensation of various aromatic aldehydes 1a–h
with rhodanines 2a–b in good to excellent yields (Scheme 2). Rhodanines 2a–b [61]
have been prepared by the reaction of respective amines with carbon disulfide in
ammonia followed by sodium chloroacetate at 80 °C in good yield.
In preliminary experiments, we investigated the optimum reaction conditions
with respect to solvent, temperature and amount of catalyst. For this, benzaldehyde
1a and 2-thioxothiazolidin-4-one 2a were chosen as model substrates for the
synthesis of representative compound 3a as shown in Scheme 1.
Firstly, the model reaction was performed using 20 mol% of catalyst under reflux
condition in different solvents (Table 1). The model reaction carried out in MeOH
and EtOH (Table 1, entries 1 and 2) was completed in 20 min with the yield of 65
and 57 %, respectively, whereas, in tert-BuOH (Table 1, entry 3), a better yield
123

D. D. Subhedar et al.
Scheme 1 Standard model reaction
Table 1 Optimizations of the
Entry
Solvent
Temp (°C)
Yieldb (%)
reaction conditions for the
synthesis of 5-arylidene-
rhodaninesa
1
MeOH
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
80
65
57
70
42
41
52
48
59
94
53
80
94
93
2
EtOH
3
Tert-BuOH
H2O
4
5
THF
6
Toluene
CH3CN
7
8
DMF
9
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
a
Reaction conditions:
10
11
12
13
60
benzaldehyde (1 mmol) and
2-thioxothiazolidin-4-one
(1 mmol), 20 mol% catalyst for
20 min
70
90
100
b
Isolated yields
(70 %) was obtained in 20 min. In H2O and THF, decreased yields (42 and 41 %) of
the product 3a were obtained (Table 1, entries 4 and 5). Conducting the reaction in
toluene, CH3CN and DMF (Table 1, entries 6–8) do not improve the yield of the
product. However, when the model reaction was carried out under a solvent-free
condition with 20 mol% [Et3NH][HSO4], a significant increase in the yield was
observed (Table 1). Therefore, it proved that the solvent-free condition is best suited
for the transformation. Predictably, solvent decreases the reaction rate (Table 1,
entries 1–8) due to the decrease in the solute concentration which resulted in less
solute–solute interaction and under the solvent-free conditions the solute–solute
interaction increases with the rate of reaction (Table 1, entry 9). In the next, the
effect of temperature was studied on the model reaction at different temperatures,
60, 70, 90 and 100 °C, under solvent-free conditions (Table 1, entries 10–13) and
the best results were obtained at 80 °C (Table 1, entry 9). An increase in the
temperature does not improve the yield of 3a (Table 1, entries 12 and 13).
To examine the catalyst efficiency, the model reaction was tested with 5, 10, 15,
20 and 25 mol% of the catalyst at 80 °C (Supporting information, Table S1). The
maximum yield (94 %) of the reaction was obtained in 20 min with 20 mol% of the
catalyst.
Furthermore, the increase in the amount of catalyst does not affect the reaction
yield (Supporting information, Table S1, entry 5). According to these observations,
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
a solvent-free condition at 80 °C with 20 mol% of the catalyst is the best suited
condition for this reaction (Supporting information, Table S1).
The recyclability of the ionic liquid [Et3NH][HSO4] was also studied and the
results are summarized in the Supporting information, Table S2. After completion
of the reaction, the reaction mixture was poured into ice-cold water and the product
was separated by filtration. The filtrate was subjected to evaporation of the water to
get a viscous liquid, which on cooling afforded IL. The IL was reused for five
consecutive cycles without considerable loss in its catalytic activity.
The structural elucidation of the synthesized compound 3a has been confirmed by
1
1
the H NMR, 13C NMR and HRMS spectral analysis. H NMR spectrum of 3a
exhibited a singlet at d 7.67 ppm due to methylidine proton. The 13C NMR
spectrum of 3a displayed two characteristic signals exhibited at d 167.4 and
195.4 ppm due to the –N–C=O and –S–C=S groups, respectively. In addition, the
formation of compound 3a has been confirmed by HRMS spectrum. The calculated
[M?H]? is 222.0423 and observed at 222.0820.
The scope and efficiency of this method was explored under the optimized
conditions. For this, a broad range of structurally diverse aromatic aldehydes were
successfully condensed with 2-thioxothiazolidin-4-one 2a and 2-(4-oxo-2-thioxoth-
iazolidin-3-yl)acetic acid 2b using [Et3NH][HSO4] as a catalyst under solvent-free
conditions at 80 °C (Scheme 2) and the obtained results are displayed in Table 2.
The results clearly reveal that the condensation reactions using the [Et3NH][HSO4]
catalyst shows a remarkable and excellent performance irrespective of the presence
Scheme 2 Synthesis of 5-arylidene-rhodanine conjugates 3a–h and 4a–h
123

D. D. Subhedar et al.
Table 2 Synthesis of 5-arylidene-rhodanine conjugatesa 3a–h and 4a–h
Compound
Structure
Yields (%)
94
Mp °C (observed)
Mp °C (reported)
O
3a
198–200
204–206 [62]
NH
S
S
O
3b
3c
3d
3e
3f
94
90
87
86
94
90
89
92
90
89
89
84
232–234
258–260
232
235 [63]
NH
S
S
S
O
260–261 [64]
230–232 [65]
289 [63]
NH
e
M
O
S
O
NH
S
r
B
S
O
284–286
226–228
224–226
252–254
204–206
[300
NH
S
N
S
O
232–233 [64]
226–228 [66]
255–257 [66]
202–205 [67]
[300 [67]
190–193 [67]
–
NH
S
S
Cl
F
O
3g
3h
4a
4b
4c
4d
4e
NH
S
S
O
NH
S
O2N
S
O
N
S
COOH
S
O
N
S
COOH
S
O
182–184
260–262
272–274
N
S
COOH
MeO
S
O
N
S
COOH
r
B
S
O
276–279 [67]
N
S
COOH
N
S
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
Table 2 continued
Compound
Structure
Yields (%)
93
Mp °C (observed)
Mp °C (reported)
O
4f
248–252
250–253 [67]
N
S
COOH
COOH
COOH
Cl
S
O
4g
4h
a
88
87
[300
–
N
S
F
S
O
238–240
240–243 [67]
N
S
O2N
S
Reaction conditions: Aldehyde (1 mmol), rhodanine (1 mmol) or rhodanine acetic acid (1 mmol) and
20 mol% [Et3NH][HSO4] ionic liquid at 80 °C
of an electron-donating/-withdrawing group on the aromatic aldehydes (Table 2),
and hence this protocol is highly effective, promising and general for the synthesis
of 5-arylidene-rhodanine conjugates. All the synthesized compounds 3b–h and 4a–h
1
were well characterized by the advanced spectroscopic techniques, H NMR, 13C
NMR and HRMS analysis.
A plausible mechanism for the synthesis of 5-arylidene-rhodanine conjugates via
Knoevenagel condensation catalyzed by [Et3NH][HSO4] is shown in Fig. 2. As
[Et3NH][HSO4] is a protic ionic liquid, initially aldehyde 1a was protonated to form
intermediate II followed by nucleophilic attack of the active methylene compound
2a producing intermediate III via C–C bond formation. The final compound 3a was
formed by the E1CB elimination (removal of H2O) from intermediate IV and then
regeneration of ionic liquid I takes place.
Biological evaluation
Anti-tubercular activity
The synthesized (Z)-5-benzylidene-2-thioxothiazolidin-4-one 3a–h and (Z)-2-(5-
benzylidene-4-oxo-2-thioxothiazolidin-3-yl)acetic acid conjugates 4a–h were
screened for in vitro antitubercular activity against MTB H37Ra (ATCC 25177)
and M. bovis BCG (ATCC 35743) in liquid medium. In a preliminary screening
(Supporting information, Tables S3 and S4), the anti-mycobacterial activity of these
compounds were assessed at concentrations of 30, 10 and 3 lg/mL, using an
established XTT Reduction Menadione assay (XRMA) anti-tubercular screening
protocol [68–70], with first-line antitubercular drug rifampicin as reference
standard; IC50 values are presented in Table 3. The rhodanine conjugates 3a, 3b,
3c, 3d, 3f, 3g and 3h (IC50 range = 0.9–25.8 lg/mL) were found to be particularly
active against active MTB H37Ra. Compounds 3a, 3b, 3c, 3e, 3g, 3h and 4f (IC50
range = 2.3–15.8 lg/mL) were found to be particularly active against dormant M.
bovis BCG strain.
123

D. D. Subhedar et al.
Fig. 2 Plausible reaction mechanism pathway for the synthesis of (Z)-5-benzylidene-2-
thioxothiazolidin-4-one 3a
The activity data (Table 3) show that rhodanine is important for antitubercular
activity. The activity depends on the substituent present on the phenyl rings. From
compounds 3a–h, the rhodanine core is constant with variation of substituents on
the phenyl ring. Compound 3a shows moderate antitubercular activity with IC50
value 25.8 lg/mL against dormant MTB H37Ra. For compound 3b, (R = methyl-)
shows moderate antitubercular activity against dormant MTB H37Ra with IC50
value 8.5 lg/mL. The best antitubercular activity was displayed by 3c (R = methoxy-)
with IC50 value 12.7 lg/mL against dormant MTB H37Ra compared with 3a and 3b.
The bromo-group in 3d displays a slight decrease in antitubercular activity as
compared to 3c against dormant MTB H37Ra with IC50 value 17.5 lg/mL.
Compound 3e (R = NMe2-) shows less antitubercular activity against dormant
MTB H37Ra with IC50 values greater than 30 lg/mL. Again, compound 3f
(R = chloro-) shows excellent antitubercular activity with IC50 value 0.9 lg/mL
against the dormant MTB H37Ra strain. The introduction of fluoro and nitro groups in
compounds 3g and 3h with IC50 value 9.3 and 2.6 lg/mL, respectively, shows
excellent antitubercular activity against the dormant MTB H37Ra strain. In
conclusion, compounds 3f, 3g and 3h display promising antitubercular activity
against the dormant MTB H37Ra strain.
From the 5-arylidene-rhodanine acetic acid derivatives 4a–h, the antitubercular
activity against the dormant strain with IC50 values greater than 30 lg/mL. Hence,
the activity data suggest that rhodanine acetic acid derivatives 4a–h are less active
as compared to rhodanines derivatives 3a–h.
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
Table 3 Anti-mycobacterial activitiesa
Comp
M. tuberculosis H37Ra
In vitro (dormant)
IC50 (lg/mL)
M. bovis BCG
In vitro (dormant)
IC50 (lg/mL)
MIC90 (lg/mL)
MIC90 (lg/mL)
3a
3b
3c
3d
3e
3f
25.8
8.5
[30
26.5
2.3
[30
13.3
28.7
12.7
17.5
[30
0.9
[30
[30
[30
[30
[30
22.3
12.1
[30
[30
3.7
[30
[30
[30
15.8
[30
3g
3h
4a
4b
4c
4d
4e
4f
9.3
[30
2.6
6.3
25.7
[30
[30
[30
[30
[30
[30
[30
[30
0.05 00.19
[30
[30
[30
[30
[30
[30
[30
[30
0.75 0.21
[30
[30
[30
[30
[30
7.4
[30
[30
[30
[30
[30
[30
4g
4h
RPb
a
[30
[30
0.075 0.028
[30
[30
0.81 0.039
IC50/IC90 in lg/mL. Antitubercular activity of each agent was determined by serial dose dependent
dilutions
b
RP rifampicin. Standard antitubercular drugs and positive controls. Data are expressed as the means of
triplication standard deviation
We have also studied the antitubercular activity against the dormant M. bovis
BCG strain. Compound 3a with IC50 value 2.3 lg/mL against dormant M. bovis
BCG displays excellent antitubercular activity. Compounds 3b and 3c show
promising antitubercular activity against dormant M. bovis BCG with IC50 values
13.3 and 12.1 lg/mL, respectively. Compound 3e (R = NMe2-) is highly potent
against dormant M. bovis BCG with IC50 value 3.7 lg/mL. Compounds 3g and 3h
show excellent antitubercular activity against dormant M. bovis BCG with IC50
values 15.8 and 6.3 lg/mL, respectively.
For rhodanine acetic acid derivatives 4a–h, compound 4f possesses excellent
antitubercular activity with IC50 value 7.4 lg/mL against dormont M. bovis BCG
strain; however, other conjugates do not display any activity. In particular, among all
the synthesized compounds, 3a, 3b, 3c, 3e, 3g and 3h contain rhodanine and 4f with a
rhodanine acetic acid core were highly active against the dormant M. bovis BCG strain.
Antibacterial activity
The selectiveness of the 3a, 3b, 3c, 3e, 3f, 3h and 4f compounds towards
antitubercular activity made us enthusiastic to evaluate these for antibacterial
123

D. D. Subhedar et al.
activity against Gram-negative (E. coli and P. flurescense) and Gram-positive (S.
aureus and B. subtilis) bacteria. Gram-positive and Gram-negative bacteria showed
higher resistance with IC50 and IC90 of [ 30 lg/ml for compounds 3a, 3b, 3c, 3d,
3e, 3f, 3h and 4f. Thus, the observed higher IC90 values against bacteria indicates
the greater specificity of compounds 3a–3h and 4f towards mycobacteria (Table 4).
Cytotoxicity
The synthesized (Z)-5-benzylidene-2-thioxothiazolidin-4-one 3a–h and (Z)-2-(5-
benzylidene-4-oxo-2-thioxothiazolidin-3-yl)acetic acid conjugate 4f were further
screened for their in vitro cytotoxic activity against the three different human cancer
cell lines, MCF-7 (breast cancer cell line), A549 (human lung adenocarcinoma cell
line) and HCT116 (human colorectal carcinoma cell line) obtained from the
European Collection of Cell Cultures (ECCC, Salisbury, UK). The antiproliferative
effect was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-
bromide (MTT cell proliferation) assay [71, 72] with 48 h exposure time of the
tested compounds and paclitaxel was used as positive control. Primary screening of
cytotoxic activity against MCF-7, A549 and HCT116 using MTT cell proliferation
assay (Supporting information Tables S5, S6 and S7). The cytotoxic effect of these
compounds was checked on cancer cell lines using the concentration range up to
100 lg/mL to determine the growth inhibition (GI) GI50 and GI90, and the results
are shown in Table 5.
The selected compounds showed GI50 and GI90 [ 100 lg/mL against all three
cell lines. However, a few compounds, 3a, 3d and 3g, showed some cytotoxicity.
The results indicate that, in MTT cytotoxicity studies, most active compounds are
lantimycobacterial owing to no significant toxicity against MCF-7, A549 and
HCT116 cell lines at the maximum concentration evaluated.
Table 4 Anti-bacterial activity against Gram-negative and Gram-positive organisms in lg/mL
Compound
E. coli
P. flurescense
S. aureus
B. subtilis
MIC
IC50
MIC
IC50
MIC
IC50
MIC
IC50
3a
[30
[30
[30
[30
[30
[30
[30
[30
[30
0.41
0.67
[30
[30
[30
[30
[30
[30
[30
[30
[30
1.46
1.62
[30
[30
[30
[30
[30
[30
[30
[30
[30
2.83
0.012
[30
[30
[30
[30
[30
[30
[30
[30
[30
4.36
0.49
[30
[30
[30
[30
[30
[30
[30
[30
[30
0.12
16
[30
[30
[30
[30
[30
[30
[30
[30
[30
1
[30
[30
[30
[30
[30
[30
[30
[30
[30
5.89
0.25
[30
[30
[30
[30
[30
[30
[30
[30
[30
10.32
1.35
3b
3c
3d
3e
3f
3g
3h
4f
AMP
KAN
[30
AMP ampicillin, KAN kanamycin
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
Table 5 Cytotoxicity of
selected compounds in 3 human
cancer cell linesa
Compound MCF-7
A549
GI50
HCT116
GI50 GI90
GI50
GI90
GI90
3a
8.4
[100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
3b
[100 [100
[100 [100
3c
3d
28.7
[100
3e
[100 [100
[100 [100
[100 [100
[100 [100
[100 [100
3f
a
3g
6.7
[100
GI50/GI90 in lg/mL, after 48 h
b
3h
[100 [100
[100 [100
Standard anticancer drug and
positive control. Data are
4f
Paclitaxelb
0.075 0.0039 0.039 \0.004 5.21
0.026
expressed as the means of
triplication standard deviation
Table 6 Pharmacokinetic parameters
Comp % ABS TPSA n-ROTB MV
(A2)
Rule
MW
miLog P n-ON n-OHNH Lipinski
violation
\500
B5
\10
\5
B1
3a
3b
3c
3d
3e
3f
97.66
97.66
94.47
97.66
96.54
97.66
97.66
81.85
88.54
32.86
32.86
42.10
32.86
36.10
32.86
32.86
78.69
59.30
1
1
1
1
1
1
1
1
1
177.99 221.31 1.82
194.56 235.33 2.27
203.56 251.33 1.88
195.88 300.20 2.63
223.90 264.38 1.93
191.53 255.75 2.50
182.93 239.30 1.99
201.33 266.30 1.78
235.72 313.19 1.87
2
2
3
2
3
2
2
5
4
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
3g
3h
4f
In silico ADME prediction
The success of a drug is determined not only by good efficacy but also by an
acceptable ADME (absorption, distribution, metabolism and excretion) profile. A
computational study of active compounds 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h and 4f was
performed for the prediction of ADME properties and the obtained values are
presented in Table 6.
It can be seen that the compounds exhibited a good % ABS (% absorption)
ranging from 81.85 to 97.66 %. Furthermore, these compounds did not violate
Lipinski’s rule of five (miLog P B 5). All the tested compounds followed the
criteria for orally active drugs and, therefore, these compounds may have a good
potential for eventual development as oral agents.
123

D. D. Subhedar et al.
Conclusions
In conclusion, a novel, environment friendly, highly efficient, cost effective protocol
has been developed for the efficient synthesis of 5-arylidene-rhodanine conjugates
in excellent yields via Knoevenagel condensation using [Et3NH][HSO4] in a short
reaction time. These 5-arylidene-rhodanine conjugates were evaluated antitubercu-
lar activity against the MTB H37Ra and M. bovis BCG strain. However, the
compound 3b, 3c, 3f, 3g and 3h exhibited half maximal concentration (IC50) values
8.5, 12.7, 0.9, 9.3 and 2.6 lg/mL against MTB. Compound 3a, 3b, 3c, 3e, 3h and 4f
shows excellent antitubercular activity with IC50 value 2.3, 13.3, 12.1, 3.7, 6.3 and
7.4 lg/mL, against M. bovis BCG strain. Furthermore, the cytotoxicity and
antibacterial activity shows that compounds 3a, 3b, 3c, 3e, 3f, 3h and 4f are highly
selective towards antitubercular activity against the MTB H37Ra and M. bovis BCG
strain.
Acknowledgments The authors D.D.S and. M.H.S. are very grateful to the Council of Scientific and
Industrial Research (CSIR), New Delhi, for the award of a Senior Research Fellowship. They are also
grateful to the Head, Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad-431 004, India, for providing the laboratory facility.
References
1. World Health Organization, Global Tuberculosis Report. http://www.who.int/tb/publications/global_
report/en/; accessed October 20, 2014. 2012
2. G. Gunther, Clin. Med. 3, 279–285 (2014)
3. D. Donoghue, M. Spigelman, C.L. Greenblatt, G. Lev-Maor, G.K. Bar-Gal, C. Matheson, K. Vernon,
A.G. Nerlich, A.R. Zink, Lancet Infect. Dis. 4, 584–592 (2004)
4. C. Dye, A. Bassili, A.L. Bierrenbach, J.F. Broekmans, V.K. Chadha, P. Glaziou, P.G. Gopi, M.
Hosseini, S. Kim, D. Manissero, I. Onozaki, H.L. Rieder, S. Scheele, F. van Leth, M. van der Werf,
B.G. Williams, Lancet Infect. Dis. 8, 233–243 (2008)
5. A. Zumla, M. Raviglione, R. Hafner, C.F. von Reyn, N. Engl, J. Med. 368, 745–755 (2013)
6. Q. Wang, F. Song, X. Xiao, P. Huang, L. Li, A. Monte, W.M. Abdel-Mageed, J. Wang, H. Guo, W.
He, F. Xie, H. Dai, M. Liu, C. Chen, H. Xu, M. Liu, A.M. Piggott, X. Liu, R.J. Capon, L. Zhang,
Angew. Chem. Int. Ed. 52, 1231–1234 (2013)
7. G.A. Colditz, C.S. Berkey, F. Mosteller, T.F. Brewer, M.E. Wilson, E. Burdick, H.V. Fineberg,
Pediatrics 96, 29–35 (1995)
8. N. Shafii, M. Khoobi, M. Amini, A. Sakhteman, H. Nadri, A. Moradi, S. Emami, E.S. Moghadam, A.
Foroumadi, A. Shafiee, J. Enzyme Inhib. Med. Chem. 30, 389–395 (2015)
9. K. Chauhan, M. Sharma, P. Singh, V. Kumar, P.K. Shukla, M.I. Siddiqi, P.M.S. Chauhan, Med.
Chem. Commun. 3, 1104–1110 (2012)
10. P. Singh, B. Jindal, A. Surolia, D. Panda, Biochemistry 51, 5434–5442 (2012)
11. W. Sinko, Y. Wang, W. Zhu, Y. Zhang, F. Feixas, C.L. Cox, D.A. Mitchell, E. Oldfield, J.A.
McCammon, J. Med. Chem. 57, 5693–5701 (2014)
12. T. Strittmatter, A. Brockmann, M. Pott, A. Hantusch, T. Brunner, A. Marx, ACS Chem. Biol. 9,
282–290 (2014)
13. T. Strittmatter, B. Bareth, T.A. Immel, T. Huhn, T.U. Mayer, A. Marx, ACS Chem. Biol. 6, 314–319
(2011)
14. A. Panico, R. Maccari, V. Cardile, S. Avondo, L. Crascia, R. Ottana, Med. Chem. Commun. 6,
823–830 (2015)
123

[Et3NH][HSO4] catalyzed efficient synthesis of…
15. L. Ma, C. Xie, Y. Ma, J. Liu, M. Xiang, X. Ye, H. Zheng, Z. Chen, Q. Xu, T. Chen, J. Chen, J. Yang,
N. Qiu, G. Wang, X. Liang, A. Peng, S. Yang, Y. Wei, L. Chen, J. Med. Chem. 54, 2060–2068 (2011)
16. M. Mori, F. Moraca, D. Deodato, D.M. Ferraris, P. Selchow, P. Sander, M. Rizzi, M. Botta, Bioorg.
Med. Chem. Lett. 24, 2508–2511 (2014)
17. X. Ge, B. Wakim, D.S. Sem, J. Med. Chem. 51, 4571–4580 (2008)
18. C. Nitsche, V.N. Schreier, M.A. Behnam, A. Kumar, R. Bartenschlager, C.D. Klein, J. Med. Chem.
56, 8389–8403 (2013)
19. T. Tomasic, N. Zidar, R. Sink, A. Kovac, D. Blanot, C.C. Martel, A. Dessen, M.M. Premru, A. Zega,
S. Gobec, D. Kikelj, L.P. Masic, J. Med. Chem. 54, 4600–4610 (2011)
20. S.S. Jadav, B.N. Sinha, R. Hilgenfeld, B. Pastorino, X. de Lamballerie, V. Jayaprakash, Eur. J. Med.
Chem. 89, 172–178 (2015)
21. B. Bulic, M. Pickhardt, I. Khlistunova, J. Biernat, E.M. Mandelkow, E. Mandelkow, H. Waldmann,
Angew. Chem. Int. Ed. 46, 9215–9219 (2007)
22. B. Bulic, M. Pickhardt, B. Schmidt, E.-M. Mandelkow, H. Waldmann, E. Mandelkow, Angew.
Chem. Int. Ed. 48, 1740–1752 (2009)
23. M. Ono, S. Hayashi, K. Matsumura, H. Kimura, Y. Okamoto, M. Ihara, R. Takahashi, H. Mori, H.
Saji, ACS Chem. Neurosci. 2, 269–275 (2011)
24. M.J. Robertson, G. Hadzic, J. Ambrus, D.Y. Pome, E. Hyde, A. Whiting, A. Mariana, L. von Kleist,
N. Chau, V. Haucke, P.J. Robinson, A. McCluskey, ACS Med. Chem. Lett. 3, 352–356 (2012)
25. R. Dayam, T. Sanchez, O. Clement, R. Shoemaker, S. Sei, N. Neamati, J. Med. Chem. 48, 111–120
(2005)
26. S.D. Furdas, S. Shekfeh, S. Kannan, W. Sippl, M. Jung, Med. Chem. Commun. 3, 305–311 (2012)
27. T.N. Reddy, M. Ravinder, P. Bagul, K. Ravikanti, C. Bagul, J.B. Nanubolu, K. Srinivas, S.K.
Banerjee, V.J. Rao, Eur. J. Med. Chem. 71, 53–66 (2014)
28. J.P. Powers, D.E. Piper, Y. Li, V. Mayorga, J. Anzola, J.M. Chen, J.C. Jaen, G. Lee, J. Liu, M.G.
Peterson, G.R. Tonn, Q. Ye, N.P.C. Walker, Z. Wang, J. Med. Chem. 49, 1034–1046 (2006)
29. M. Joshi, C. Vargas, P. Boisguerin, A. Diehl, G. Krause, P. Schmieder, K. Moelling, V. Hagen, M.
Schade, H. Oschkinat, Angew. Chem. Int. Ed. 45, 3790–3795 (2006)
30. P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39, 3772–3789 (2000)
31. M.A.P. Martins, C.P. Frizzo, A.Z. Tier, D.N. Moreira, N. Zanatta, H.G. Bonacorso, Chem. Rev. 114,
1–70 (2014)
32. P. Wasserscheid, T. Welton (eds.), Ionic liquids in synthesis, 1st edn. (Wiley, Weinheim, 2003)
33. C. Yue, D. Fang, L. Liu, T.-F. Yi, J. Mol. Liq. 163, 99–121 (2011)
34. Z.A. Ondrej, W. Steven, B. Polyak, T.A. William, K.A. Kevin, D.C. Huanqin, S.P.A. Daniel, M.B.
Renato, Z.C. Lixin, W.B.B. Grant, D.A. Andrew, Bioorg. Med. Chem. Lett. 22, 2720–2722 (2012)
35. A.I. Khodair, J.-P. Gesson, Carbohydr. Res. 346, 2831–2837 (2011)
36. M. Guo, C.J. Zheng, M.X. Song, Y. Wu, L.P. Sun, Y.J. Li, Y. Liu, H.R. Pio, Bioorg. Med. Chem.
Lett. 23, 4358–4361 (2013)
37. V. Ramesh, B.A. Rao, P. Sharma, B. Swarna, D. Thummuri, K. Srinivas, V.G.M. Naidu, V.J. Rao,
Eur. J. Med. Chem. 83, 569–580 (2014)
38. R. Maccari, A.D. Corso, M. Giglio, R. Moschini, U. Mura, R. Ottana, Bioorg. Chem. Lett. 21,
200–203 (2011)
39. Z.H. Chen, C.J. Zheng, L.P. Sun, H.R. Piao, Eur. J. Med. Chem. 45, 5739–5743 (2010)
40. M. Subbaiah, R. Sekar, E. Palani, A. Sambandam, Tetrahedron Lett. 54, 3132–3136 (2013)
41. T. Meyer, D. Ogermann, A. Pankrath, K. Kleinermanns, T.J. Muller, J. Org. Chem. 77, 3704–3715
(2012)
42. M. Matsui, Y. Asamura, Y. Kubota, K. Funabiki, J. Jin, T. Yoshida, H. Miura, Tetrahedron 66,
7405–7410 (2010)
43. T.T. Talele, P. Arora, S.S. Kulkarni, M.R. Patel, S. Singh, M. Chudayeu, N.K. Basu, Bioorg. Med.
Chem. 18, 4630–4638 (2010)
44. G. Min, S.K. Lee, H.N. Kim, Y.M. Han, R.H. Lee, D. Jeong, D.C. Han, B.M. Kwon, Bioorg. Med.
Chem. Lett. 23, 3769–3774 (2013)
45. V. Opletalova, J. Dolezel, K. Kralova, M. Pesko, J. Kunes, J. Jampilek, Molecules 16, 5207–5227
(2011)
46. K. Gong, Z.W. He, Y. Xu, D. Fang, Z.L. Liu, Monatsh. Chem. 139, 913–915 (2008)
47. J. Weng, C. Wang, H. Li, Y. Wang, Green Chem. 8, 96–99 (2006)
48. Z.N. Siddiqui, K. Khan, ACS Sustainable Chem. Eng. 2, 1187–1194 (2014)
123

D. D. Subhedar et al.
49. A.M. Ghatole, K.R. Lanjewar, M.K. Gaidhane, K.M. Hatzade, Spectrochim. Acta A Mol. Biomol.
Spectrosc. 151, 515–524 (2015)
50. M.H. Shaikh, D.D. Subhedar, F.A.K. Khan, J.N. Sangshetti, B.B. Shingate, Res. Chem. Int. doi:10.
1007/s11164-015-2348-z
51. M.H. Shaikh, D.D. Subhedar, F.A.K. Khan, J.N. Sangshetti, L. Nawale, M. Arkile, D. Sarkar, B.B.
Shingate, J. Hetero. Chem. doi:10.1002/jhet.2598
52. D.D. Subhedar, M.H. Shaikh, F.A.K. Khan, J.N. Sangshetti, V.M. Khedkar, B.B. Shingate, New J.
Chem. doi:10.1039/c6nj00021e
53. M.H. Shaikh, D.D. Subhedar, M. Arkile, V.M. Khedkar, N. Jadhav, D. Sarkar, B.B. Shingate, Bioorg.
Med. Chem. Lett. 26, 561–569 (2016)
54. M.H. Shaikh, D.D. Subhedar, F.A.K. Khan, J.N. Sangshetti, B.B. Shingate, Chin. Chem. Lett. 27,
295–301 (2016)
55. A.H. Kategaonkar, R.U. Pokalwar, S.S. Sonar, V.U. Gawali, B.B. Shingate, M.S. Shingare, Eur.
J. Med. Chem. 45, 1128–1132 (2010)
56. A.H. Kategaonkar, P.V. Shinde, A.H. Kategaonkar, S.K. Pasale, B.B. Shingate, M.S. Shingare, Eur.
J. Med. Chem. 45, 3142–3146 (2010)
57. B.B. Shingate, B.G. Hazra, D.B. Salunke, V.S. Pore, F. Shirazi, M.V. Deshpande, Eur. J. Med. Chem.
46, 3681–3689 (2011)
58. B.B. Shingate, B.G. Hazra, D.B. Salunke, V.S. Pore, F. Shirazi, M.V. Deshpande, Tetrahedron 69,
11155–11163 (2013)
59. M.H. Shaikh, D.D. Subhedar, L. Nawale, D. Sarkar, F.A.K. Khan, J.N. Sangshetti, B.B. Shingate,
Med. Chem. Commun. 6, 1104–1116 (2015)
60. M.A.P. Martins, C.P. Frizzo, D.N. Moreira, L. Buriol, P. Machado, Chem. Rev. 109, 4140–4182
(2009)
61. M. Azizmohammadi, M. Khoobi, A. Ramazani, S. Emami, A. Zarrin, O. Firuzi, R. Miri, A. Shafiee,
Eur. J. Med. Chem. 59, 15–22 (2013)
62. A. Khazaei, H. Veisi, M. Safaei, H. Ahmadian, Sulf. Chem. 35, 270–278 (2014)
63. S.S. Jadav, B.N. Sinha, R. Hilgenfeld, B. Pastorino, X. de Lamballerie, V. Jayaprakash, Eur. J. Med.
Chem. 89, 172–178 (2015)
64. V. Opletalova, J. Dolezel, K. Kralova, M. Pesko, J. Kunes, J. Jampilek, Molecules 16, 5207–5227
(2011)
65. S. Maximiliano, D. Paula, J. Sabina Bioorg. Med. Chem. 15, 484–494 (2007)
66. D.N. Pansare, D.B. Shinde, Tetrahedron Lett. 55, 1107–1110 (2014)
67. J.F. Zhou, Y.Z. Song, F.X. Zhu, Y.L. Zhu, Synth. Commun. 36, 3297–3303 (2006)
68. U. Singh, S. Akhtar, A. Mishra, D. Sarkar, J. Microbiol. Methods 84, 202–207 (2011)
69. A. Khan, S. Sarkar, D. Sarkar, Int. J. Antimicrob. Agents 32, 40–45 (2008)
70. S. Sarkar, D. Sarkar, J. Biomol. Screen. 17, 966–973 (2012)
71. A.A. van de Loosdrecht, R.H. Beelen, G.J. Ossenkoppele, M.G. Broekhoven, M.M. Langenhuijsen, J.
Immunol. Methods 174, 311–320 (1994)
72. C.M. Doyen, Y.M. Moshkin, G.E. Chalkley, K. Bezstarosti, J.A. Demmers, C. Rathke, R. Renkawitz-
Pohl, C.P. Verrijzer, Cell Rep. 4, 59–65 (2003)
73. R. De La Fuente, N.D. Sonawane, D. Arumainayagam, A.S. Verkman, Br. J. Pharmacol. 149,
551–559 (2006)
74. T. Mosmann, J. Immunol. Methods 65, 55–63 (1983)
75. G. Ciapetti, E. Cenni, L. Pratelli, A. Pizzoferrato, Biomaterials 14, 359–364 (1993)
76. J.P. Dzoyem, S.K. Guru, C.A. Pieme, V. Kuete, A. Sharma, I.A. Khan, A.K. Saxena, R.A. Vish-
wakarma, BMC Complement. Altern. Med. 13, 1–6 (2013)
77. C.A. Lipinski, L. Lombardo, B.W. Dominy, P.J. Feeney, Adv. Drug Deliv. Rev. 46, 3–26 (2001)
78. Molinspiration Chemoinformatics Brastislava, Slovak Republic (2014). http://www.molinspiration.
com/cgi-bin/properties
79. Y. Zhao, M.H. Abraham, J. Lee, A. Hersey, N.C. Luscombe, G. Beck, B. Sherborne, I. Cooper,
Pharm. Res. 19, 1446–1456 (2002)
80. Drug-likeness and molecular property prediction. http://www.molsoft.com/mprop/
123