One-pot rapid synthesis of thiazole-substituted pyrazolyl-4-thiazolidinones mediated by diisopropylethylammonium acetate

Article abstract of DOI:10.1007/s11164-015-1940-6

A convenient one-pot, rapid and scalable synthetic protocol has been developed for recently reported anti-inflammatory agents, thiazole-substituted pyrazolyl-4-thiazolidinones. Quantitative multicomponent cyclocondensation of 3-(4-methyl-2-substituted thi

Full text of DOI:10.1007/s11164-015-1940-6

Res Chem Intermed
DOI 10.1007/s11164-015-1940-6
One-pot rapid synthesis of thiazole-substituted
pyrazolyl-4-thiazolidinones mediated
by diisopropylethylammonium acetate
•
Lalit D. Khillare Manisha R. Bhosle
•
•
Amarsinh R. Deshmukh Ramrao A. Mane
Received: 23 November 2014 / Accepted: 27 January 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A convenient one-pot, rapid and scalable synthetic protocol has been
developed for recently reported anti-inflammatory agents, thiazole-substituted pyr-
azolyl-4-thiazolidinones. Quantitative multicomponent cyclocondensation of 3-(4-
methyl-2-substituted thiazol-5-yl)-1-phenyl-1H-pyrazole-4-carbaldehydes (5a, b),
amines and mercaptoacetic acid has been carried out in safer medium, diisopropy-
lethylammonium acetate, at room temperature. The optimisation details of the
developed novel protocol are recorded.
Keywords Diisopropylethylammonium acetate (DIPEAc) ꢀ 4-Thiazolidinone ꢀ
Multicomponent reaction
Introduction
Heterocyclic compounds have attracted a lot of attention because of their varied
biological activities. Specifically, five-membered heterocycles with two heteroat-
oms have received particular attention, having proven utility in medicinal
chemistry. Among them, 4-thiazolidinones represent an important class of five-
membered heterocycles. Some of the thiazolidinones are found to possess
interesting biological activities such as anticancer, antimalarial, tuberculostatic,
antihistaminic, anticonvulsant, antibacterial and antiarrythmic activity [1]. Ralito-
line, etozoline, pioglitazone and thiazolidomycin have this heteryl ring and are
already in the market as medicaments [2]. This diversity in the biological response
profile has attracted much attention from many researchers to explore this skeleton
for its multiple potential activities. Motivated by the above recently reported
L. D. Khillare ꢀ M. R. Bhosle ꢀ A. R. Deshmukh ꢀ R. A. Mane (&)
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad 431 004, MS, India
e-mail: manera2011@gmail.com
123

L. D. Khillare et al.
observations, we reported new anti-inflammatory agents [3] having 4-thiazolydinoyl
moiety.
Thiazolidinones are obtained by either one- or two-step synthesis processes.
However, the most widely used approach to synthesise this ring system is through
one-pot three-component tandem reaction between carbonyl compounds, primary
amines, and mercaptoacetic acid or its derivatives. The reaction proceeds via
initial formation of an imino intermediate, which undergoes attack by a sulphur
nucleophile, i.e. mercaptoacetic acid, followed by intramolecular cyclisation with
expulsion of water to yield the desired product. It is generally believed that the
last step, i.e. removal of the water molecule, is rate determining and seems to be
critical for obtaining 4-thiazolidinones in high yield [4]. Therefore, a number of
strategies have been developed to remove the water molecules formed during the
reaction in order to obtain high yield of the desired product. The media used for
the condensation are volatile organic solvents such as toluene, benzene, 1,4-
dioxane and tetrahydrofuran (THF). There are reports of acceleration of the above
cyclocondensation using catalysts and desiccants such as N,N0-dicyclohexylcar-
bodiimide (DCC) [5], O-(benzotriazolyl)-N,N,N0,N0-tetramethyluronium hexa-
fluorophosphate (HBTU) [6], ferrite [7], ZnCl2 [8], sodium sulphate [9],
[bmim][PF6] [10, 11] and activated fly ash [12]. Use of microwave heating
[13], and solid-phase [14] and polymer-supported [15] systems to run the
cyclocondensation leading to 2,3-disubstituted 4-thiazolidinones has also been
reported.
However, most of the methods described above suffer from certain limitations.
To achieve the cyclocondensation, one must incorporate azeotropic distillation,
microwave irradiation [16], use of stoichiometric amount of hazardous and costly
catalysts, auxiliary reagents such as desiccants, inert and dry atmosphere or
prolonged heating. Thus, a convenient, versatile, rapid and high-yielding
synthetic method remains in demand to fulfil timely supply of a library of
4-thiazolidinones for biological evaluation and enrichment of the medicinal
chemist’s toolbox.
Recently, room-temperature ionic liquids (RTILs) have been found to be useful
as green media for value-added transformations [17]. RTILs act as ‘neoteric
solvents’ for a broad range of chemical and industrial processes. ILs are emerging as
more promising catalysts in various fields such as organic synthesis, materials
science, electrochemistry and separation technology [18–22]. The ability to dissolve
many organic and inorganic substances makes RTILs eco-friendly reaction media/
catalysts. ILs completely consist of weakly coordinating ions, i.e. organic cation
with inorganic/organic anion, possessing desirable properties, and are liquids at or
close to room temperature. An interesting aspect is that ILs not only act as media for
organic substrates but also display catalytic behaviour [17].
Owing to the significance of 4-thiazolidinones and considering the urgent need
for cost-effective and environmentally benign novel synthetic protocols for
therapeutically interesting 4-thiazolidinones, our group has developed and reported
various synthetic methods having one or another type of advantage. While
conducting multicomponent cyclocondensation leading to 4-thiazolidinones and to
accelerate the rate of the cyclocondensation, in our reports we used alternative
123

One-pot rapid synthesis of pyrazolyl-4-thiazolidinones
media such as PEG-400 [23] and N-methylpyridinium tosylate (ionic liquid) [24].
We even reported the catalytic role of heterogeneous catalysts such as silica
chloride [25] and biocatalyst, baker’s yeast [26] in accelerating the cycloconden-
sation. Synthesis of the title compounds has been carried out by traditional/classical
protocols and recently even by our reported alternative synthetic method [3]. It has
been noticed that the cyclocondensations carried out in these reports were under
prolonged heating and finally gave moderate yields. Therefore, we felt that there
was still scope to develop a rapid, benign, cost-effective multicomponent one-pot
method to obtain the title products.
Results and discussion
Considering the significance of RTILs, here in the search for the best experimental
reaction conditions, screening of different room-temperature ionic liquids was
carried out for cyclocondensation of benzaldehyde 1a, aniline 2a and mercapto-
acetic acid 3 (Scheme 1) as a standard model reaction. For evaluation of the effect
of solvents on the one-pot model reaction, initially various solvents such as toluene,
1,4-dioxane, acetonitrile, methanol, ethanol and various freshly prepared room-
temperature ionic liquids [27] were screened at room temperature for the model
reaction (Table 1, entries 1–10). Amongst them, diisopropylethylammonium ace-
tate (DIPEAc) and RTIL were found to be an excellent medium and catalyst,
furnishing the product in excellent yield of 88 % (Table 1, entry 10). In the absence
of solvent, the neat one-pot multicomponent cyclocondensation at room temperature
did not exhibit similar reaction condensations.
To confirm the role of DIPEAc as a catalyst, one-pot cyclocondensation of
benzaldehyde, aniline and mercaptoacetic acid was separately attempted in media,
viz. acetonitrile, ethanol and dichloromethane, in presence of 20, 50 and 100 mol%
of DIPEAc. It was noted that, at R.T. in absence of DIPEAc, condensation could not
occur in these media. However, the condensation was found to take place at
moderate rate. It was also observed that, when equimolar quantity of DIPEAc was
incorporated in these media, the rate of condensation was noticeable (Table 2). This
R'
O
H
NH2
R'
R
O
RTIL
HS
OH r.t. Stirring
15 min.
N
O
S
R
(1 a-f)
(2 a-c)
(3)
(4 a-r)
Scheme 1 Synthesis of 4-thiazolidinones from aromatic carbonyl compounds, aromatic amines and
mercaptoacetic acid in RTIL
123

L. D. Khillare et al.
Yield (%)a
Table 1 Screening of solvents
Entry
Solvent
Time (min)
1
2
3
4
5
6
7
8
9
10
Toluene
15
15
15
15
15
15
15
15
15
15
No condensation
No condensation
No condensation
No condensation
No condensation
No condensation
17
1,4-Dioxane
Acetonitrile
Methanol
Ethanol
Dichloromethane
Piperidine ammonium acetate
Pyrrolidine ammonium acetate
Triethylethylammonium acetate
Diisopropylethylammonium acetate
22
41
88
Reaction conditions: 1a (0.009 mol), 2a (0.009 mol), 3 (0.028 mol), in solvent (5 ml), stirred at room
temperature
a
Isolated yields
Table 2 Effect of catalyst (DIPEAc) concentration in presence of solvent
Entry
Solvent
Time (min)
Yield (%)a in presence of DIPEAc
20 mol%
50 mol%
100 mol%
1
2
3
Acetonitrile
Ethanol
15
15
15
36
28
21
38
34
28
51
43
33
Dichloromethane
Reaction conditions: 1a (0.009 mol), 2a (0.009 mol), 3 (0.028 mol), in solvent (5 ml), stirred at room
temperature
a
Isolated yields
Table 3 Recovery and recycling of DIPEAc
Entry
Run
Yield (%)a
1
2
3
4
I
88
86
80
76
II
III
IV
Reaction conditions: 1a (0.009 mol), 2a (0.009 mol), 3 (0.028 mol), DIPEAc (5 ml) stirred at room
temperature for 15 min
a
Isolated yields
observation is in agreement with the idea that DIPEAc plays a role not only as a
medium but also as a catalyst in this cyclocondensation. Therefore, DIPEAc was
chosen as the medium and catalyst of choice for further optimisation studies.
123

One-pot rapid synthesis of pyrazolyl-4-thiazolidinones
We studied the reusability of room-temperature ionic liquid (DIPEAc). It was
observed that the recovered IL worked with good efficiency up to the second run
(Table 3, entries 1, 2), while in the third and fourth runs (Table 3, entries 3, 4) the
product yield decreased slightly.
Inspired by these observations and to generalise the synthetic procedure, a variety
of electronically divergent aromatic aldehydes and aromatic amines were treated
with mercaptoacetic acid at room temperature in presence of DIPEAc. Aromatic
aldehydes with several functionalities such as Cl, OH, CH3, OCH3 and NO2 were
found to be compatible under the optimised reaction condition. Aldehydes having
no substituent on phenyl ring reacted smoothly, and the products were obtained in
excellent yields ranging from 88 to 93 % (Table 4, entries 1–3). Electron-donating
group such as OCH3, CH3 and OH present in the aldehyde afforded moderate yields
of the products (Table 4, entries 4–12). Presence of Cl group in the aldehydes
(Table 4, entries 13–15) showed good effect on yield. Aldehydes possessing
electron-withdrawing group gave moderate yields (Table 4, entries 16–18). The
products, 4-thiazolidinones, isolated in this study were characterised and their M.P.s
Table 4 Synthesis of 4-thiazolidinones (4a–r)
Entry
Compound
R
R0
Yield (%)a
Melting point (°C)b
1
4a
4b
4c
4d
4e
4f
H
H
88
73
93
80
78
84
74
79
82
76
81
75
75
79
78
76
81
70
105–107
112–115
111–113
110–111
148–150
157–160
115–117
120–122
155–157
182–184
210–211
158–160
130–132
162–165
124–126
105–107
157–159
139–141
2
H
CH3
Cl
3
H
4
OCH3
OCH3
OCH3
CH3
CH3
CH3
OH
OH
OH
Cl
H
5
CH3
Cl
6
7
4g
4h
4i
H
8
CH3
Cl
9
10
11
12
13
14
15
16
17
18
4j
H
4k
4l
CH3
Cl
4m
4n
4o
4p
4q
4r
H
Cl
CH3
Cl
Cl
NO2
NO2
NO2
H
CH3
Cl
Reaction conditions: aryl carbaldehyde (0.009 mol), 2a (0.009 mol), 3 (0.028 mol), DIPEAc (5 ml)
stirred at room temperature for 15 min
a
Isolated yields
b
The melting points of these products prepared in this work are in good agreement with those reported in
literature [24, 25, 28]
123

L. D. Khillare et al.
and other physical data found to be in good agreement with those reported in
literature [24, 26, 28].
Encouraged by this success and in continuation of our endeavour towards
synthesis of bioactive heterocycles, here we applied the above same strategy for
one-pot cyclocondensation of heteryl aldehydes to obtain the thiazolidinones
(Scheme 2). Hence, we attempted one-pot cyclocondensation of 3-(4-methyl-2-
substituted thiazol-5-yl)-1-phenyl-1H-pyrazole-4-carbaldehydes (5a, b), aromatic
amines (2a–e) and mercaptoacetic acid (3) under the above optimised condition to
obtain recently reported anti-inflammatory agents, 2-(3-(4-methyl-2-substituted
thiazol-5-yl)-2-phenyl-1H-pyrazol-4-yl)-3-phenylthiazolidin-4-ones [3].
It was observed that 3-(4-methyl-2-substituted thiazol-5-yl)-1-phenyl-1H-pyra-
zole-4-carbaldehydes reacted cleanly under the above reaction condition
(Scheme 2) and the products were obtained in 73 to 88 % yields without any
difficulty within 30 min at room temperature. Aromatic amines having halide
groups reacted smoothly, and the products were obtained in excellent yields ranging
from 79 to 88 % (Table 5, entries 3–5, 8–10). Slightly moderate yields of the
product were obtained when heterocyclic aldehydes were allowed to react with
aromatic amines having electron-donating group such as CH3 (Table 5, entries 2,
7). Aniline gave moderate yield with the heterocyclic aldehyde (Table 5, entries 1,
6). All the products (6n–w) were characterised by infrared (IR), 1H nuclear
magnetic resonance (NMR), 13C NMR and mass analysis, and the analytical data
were in agreement with our earlier reports [3].
A plausible mechanism for the rapid formation of 4-thiazolidinones using RTIL,
diisopropylethylammonium acetate (DIPEAc) would be as follows:
1. The rate acceleration for the formation of 4-thiazolidinone has been found to be
enhanced due to the dual nature of the ionic liquid, i.e. as a catalyst and
medium. RTIL might be helping to create a high initial concentration of the
reactants in solvation at room temperature.
2. The electrophilic character of carbonyl carbons of the aldehyde would have
been enhanced because of their non-covalent interaction with ammonium cation
of the RTIL, diisopropylethylammonium acetate.
N
R
S
N
N
S
N
NH2
R
S
O
N
DIPEAc
N
HS
N
OH
O
r.t. Stirring
30 min.
O
H
R'
R'
(6 n-w)
(5 a-b)
R- Ph Or CH3
(2a-e)
(3)
Scheme 2 Synthesis of 2-(3-(4-methyl-2-substituted thiazol-5-yl)-2-phenyl-1H-pyrazol-4-yl)-3-
phenylthiazolidin-4-ones
123

One-pot rapid synthesis of pyrazolyl-4-thiazolidinones
Table 5 Synthesis of 2-(3-(4-methyl-2-substituted thiazol-5-yl)-2-phenyl-1H-pyrazol-4-yl)-3-phe-
nylthiazolidin-4-ones
Entry
Compound
R
R0
Yield (%)a
Melting point (°C)b
1
2
6n
6o
6p
6q
6r
6s
Ph
H
73
75
82
85
81
74
77
88
79
80
208–210
169–171
178–180
180–182
165–167
137–139
155–157
163–165
175–177
171–173
Ph
CH3
F
3
Ph
4
Ph
Cl
Br
H
5
Ph
6
CH3
CH3
CH3
CH3
CH3
7
6t
CH3
F
8
6u
6v
6w
9
Cl
Br
10
Reaction conditions: heterocyclic carbaldehyde 5a (0.0028 mol), 2a (0.0028 mol), 3 (0.0086 mol),
DIPEAc (5 ml) stirred at room temperature for 30 min
a
Isolated yields
b
The melting points of these products prepared in this work are in good agreement with those reported in
literature [3]
3. Acetate anion may be responsible for enhancing nucleophilicity of mercapto
group of mercaptoacetic acid, causing its facile addition on the imino
intermediate generated in situ.
These factors might be responsible for accelerating the rate of cyclocondensation
and hence enhancing yields of the desired 4-thiazolidinones.
Experimental
The chemicals used were of laboratory grade. Melting points of all synthesised
1
compounds were determined in open capillary tubes and are uncorrected. H NMR
spectra were recorded on a Bruker DRX-300 and 400 MHz NMR spectrometer and
13C NMR spectra were recorded on a Bruker DRX-75 and 100 MHz NMR in
CDCl3/dimethyl sulphoxide (DMSO)-d6 using tetramethylsilane (TMS) as internal
standard, and chemical shifts are in d (ppm). Direct analysis in real time (DART)
mass spectra were recorded on a Vokudelna ES ? 2000. High-resolution mass
spectra (HRMS) were recorded on an Agilent 6520 quadrupole time-of-flight
(QTOF) electrospray ionisation (ESI) HRMS instrument. The purity of each
compound was checked by thin-layer chromatography (TLC) using silica gel,
60F254 aluminium sheets as adsorbent, with visualisation accomplished by iodine/
ultraviolet light.
123

L. D. Khillare et al.
General procedure for synthesis of 2,3-diaryl-4-thiazolidinones (4a–r)
A mixture of aromatic carbaldehydes (1a–f) (0.009 mol), aromatic amines
(2a–c) (0.009 mol) and mercaptoacetic acid (3) (0.028 mol) was dissolved in
DIPEAc (5 ml), then the solution was stirred at room temperature for 15 min.
Reaction progress was monitored by TLC using ethyl acetate:hexane (3:7) as
solvents. Then, after 15 min, reaction mass was poured on cold water and washed
with NaHCO3, and the obtained crude products were filtered and crystallised from
ethanol. The reaction is depicted in Scheme 1.
General procedure for 2-(3-(4-methyl-2-substituted thiazol-5-yl)-2-phenyl-1H-
pyrazol-4-yl)-3-phenylthiazolidin-4-ones (6n–w)
A mixture of 3-(4-methyl-2-substituted thiazol-5-yl)-1-phenyl-1H-pyrazole-4-car-
baldehydes (5a, b) (0.0028 mol), aromatic amines (2a-e) (0.0028 mol) and
mercaptoacetic acid (3) (0.0086 mol) was dissolved in DIPEAc (5 ml), then the
solution was stirred at room temperature for 30 min. Reaction progress was
monitored by TLC using ethyl acetate:hexane (3:7) as solvents. Then, after 30 min,
reaction mass was poured on cold water and washed with NaHCO3, and the
obtained crude products were filtered and crystallised from ethanol. The reaction is
depicted in Scheme 2.
General procedure for synthesis of diisopropylethylammonium acetate
(DIPEAc)
A mixture of acetic acid (0.02 mol) and N-ethyl-N-isopropylpropan-2-amine
(0.02 mol) was stirred at 0–10 °C for 30 min. The viscous liquid, diisopropylethy-
lammonium acetate, was obtained [26].
Recovery of ionic liquid
An attempt was made to recover the room-temperature ionic liquid (DIPEAc). After
completion of the reaction, the reaction mixture was poured on ice water, and the
product was separated by filtration. The filtrate was subjected to evaporation of
water to get viscous liquid, which on cooling gave the ionic liquid. The recovered
ionic liquid was reused for three more cycles of the same cyclocondensation and
found to act satisfactorily.
Spectral data for representative compounds
2-(4-Chlorophenyl)-3-p-tolylthiazolidin-4-one (4n) 1H NMR (CDCl3, 300 MHz)
d = 2.26 (s, 3H, CH3), 3.83–3.86 (d, methylene 1H), 3.96–4.02 (d, methylene 1H),
5.99 (s, 1H, CH), 7.01–7.29 (m, overlapping 8H, Ar–H). 13C NMR (CDCl3,
75 MHz) d = 21.2, 33.5, 65.1, 125.3, 125.7, 127.3, 129.2, 130.1, 130.3, 134.6,
134.8, 137.5, 142.0, 171.0. DART-MS (ESI?): (m/z, % intensity): 304 (M?, 100)
C16H14ClNOS.
123

One-pot rapid synthesis of pyrazolyl-4-thiazolidinones
2-(3-(4-Methyl-2-phenylthiazol-5-yl)-2-phenyl-1H-pyrazol-4-yl)-3-phenylthiazoli-
1
din-4-one (6n) IR (KBr) Vmax: 3,045, 2,910, 1,680, 1,585, 765 cm-1. H NMR
(DMSO-d6, 400 MHz) d = 2.48 (3H, s, CH3), 3.85–3.89 (1H, d, J = 16 Hz),
3.97–4.01 (1H, d, J = 16 Hz), 6.41 (1H, s), 7.08–7.91 (14H, m, Ar–H), 8.85 (1H, s,
pyrazolyl). 13C NMR (DMSO-d6, 100 MHz) d = 16.2, 32.8, 55.7, 115.4, 115.8,
118.2, 121.1, 122.0, 126.1, 127.0, 128.2, 128.3, 129.2, 129.8, 130.7, 132.8, 133.6,
138.5, 141.9, 152.2, 165.8, 170.1. HRESIMS m/z (pos): 529.0923 [M ? H]? for
C28H21ClN4OS2.
Conclusions
We have developed an exceedingly simple, scalable and novel synthetic protocol for
synthesis of 4-thiazolidinone derivatives. To the best our knowledge, this is the first
report of application of diisopropylethylammonium acetate (DIPEAc) for synthesis
of 4-thiazolidinones at room temperature. The new catalytic and green medium
procedure offers the following distinct advantages: (1) one-pot multicomponent
synthesis of thiazolidinones with broad substrate scope, (2) no requirement for
additional reagents, (3) high yields, (4) ease of product isolation/purification, (5)
ease of preparation/handling of RTIL and (6) catalyst reuse that fulfils the triple
bottom-line philosophy of green chemistry. Therefore, this one-pot multicomponent
procedure clearly represents an appealing methodology for synthesis of 4-thiazo-
lidinones in both academia and pharmaceutical industry, and is even better than our
earlier efforts towards synthesis of 2-(3-(4-methyl-2-substituted thiazol-5-yl)-2-
phenyl-1H-pyrazol-4-yl)-3-phenylthiazolidin-4-ones [3].
Acknowledgments Authors are grateful to Professor D. B. Ingle for invaluable discussion and
guidance. The authors are very grateful to CDRI, Lucknow for providing spectral facilities. Authors are
also grateful to the National Toxicology Centre (NTC), Pune for screening anti-inflammatory activity.
L.D.K. is also grateful to U.G.C., New Delhi, India for financial assistance in the form of a UGC-NET
Senior Research Fellowship.
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