Catalytic N-formylation for synthesis of 6-substituted-2-benzothiazolylimino-5-piperazinyl-4-thiazolidinone antimicrobial agents

Article abstract of DOI:10.1007/s11164-014-1684-8

A new class of piperazine-based 2-benzothiazolylimino-4-thiazolidinones has been efficiently prepared via highly accelerated N-formylation of N-isopropylpiperazine by the use of a mild heterogeneous catalyst, sulfated tungstate. Heterocyclization of N-(benzo[d]thiazol-2-yl)-2-chloroacetamides (3a-j) by use of NH₄SCN in ethanol under reflux efficiently furnished the intermediates 2-benzothiazolyliminothiazolidin-4-ones (4a-j). These were treated with 4-isopropylpiperazine-1-carbaldehyde (2) to prepare the final products 5a-j. The structures of the new derivatives were confirmed by elemental analysis and use of spectroscopic data (FTIR and ¹H NMR). Their pharmacological potential as promising antimicrobial agents was determined in vitro against bacteria and a fungus; the lowest minimum inhibitory concentrations (MIC) observed were in the range 4-8 μg/mL.

Full text of DOI:10.1007/s11164-014-1684-8

Res Chem Intermed
DOI 10.1007/s11164-014-1684-8
Catalytic N-formylation for synthesis of 6-substituted-2-
benzothiazolylimino-5-piperazinyl-4-thiazolidinone
antimicrobial agents
•
Rahul V. Patel Se Won Park
Received: 10 March 2014 / Accepted: 22 April 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract A new class of piperazine-based 2-benzothiazolylimino-4-thiazolidi-
nones has been efficiently prepared via highly accelerated N-formylation of N-iso-
propylpiperazine by the use of a mild heterogeneous catalyst, sulfated tungstate.
Heterocyclization of N-(benzo[d]thiazol-2-yl)-2-chloroacetamides (3a–j) by use of
NH4SCN in ethanol under reflux efficiently furnished the intermediates 2-ben-
zothiazolyliminothiazolidin-4-ones (4a–j). These were treated with 4-iso-
propylpiperazine-1-carbaldehyde (2) to prepare the final products 5a–j. The structures
of the new derivatives were confirmed by elemental analysis and use of spectroscopic
data (FTIR and 1H NMR). Their pharmacological potential as promising antimicrobial
agents was determined in vitro against bacteria and a fungus; the lowest minimum
inhibitory concentrations (MIC) observed were in the range 4–8 lg/mL.
Keywords N-Formylation Á Sulfated tungstate Á Piperazine Á 2-
Benzothiazolylimino-4-thiazolidinones Á Antibacterial activity
Introduction
Rapid growth of bacteria and fungi in immune-compromised individuals quickly
results in disease. The emergence of multidrug-resistant microbial strains as a result
of comprehensive use of antibacterial and antifungal drugs is a serious global
problem [1, 2]. Infections caused by resistant Gram-positive [3, 4] and Gram-
negative [5, 6] bacterial strains often fail to respond to currently used drugs.
R. V. Patel Á S. W. Park (&)
Organic Research Laboratory, Department of Bioresources and Food Science, College of Life and
Environmental Sciences, Konkuk University, Seoul 143 701, South Korea
e-mail: sewpark@konkuk.ac.kr
R. V. Patel
e-mail: rahul.svnit11@gmail.com
123

R. V. Patel, S. W. Park
Infection by these microorganisms is a serious problem for the medical community
and emphasizes the immediate need for new, more effective, and, in particular, non-
traditional antimicrobial agents. Discovery of new antimicrobial agents is an urgent
priority to prevent worsening of the problem.
Numerous reports, including ours [7, 8], have emphasized the antimicrobial
action of compounds containing a piperazine ring. We have also observed that
combination of a piperazine ring with a thiazole ring significant inhibited growth of
a wide range of bacteria and fungi [9]. In particular, the presence of a variety of
6-substituted benzothiazole nuclei has been established as integral to antimicrobial
potency [10, 11]. Hence, during a search for new antimicrobial compounds, we have
investigated a variety of derivatives with specific structures with known biological
efficacy, including piperazines, 6-substituted benzothiazoles, and 4-thiazolidinones,
the later ones of which have not yet been investigated. Much evidence of the
importance of the 4-thiazolidinone ring in medicinal chemistry has emerged in
recent decades [12, 13]. Among the important representatives of this class, 2-aryl/
heteroarylimino-4-thiazolidinones have been observed to have high antimicrobial
potency, and several 2-thiazolylimino-4-thiazolidinones were recently found to have
antibacterial properties [14], with MIC ranging from 0.19 to [100 lg/mL.
2-Heteroarylimino-5-benzylidene-4-thiazolidinones have also been prepared and
tested for in-vitro antimicrobial efficacy; their MIC ranged from 3 to [100 lg/mL
[15]. A similar class of compounds had MIC in the range 0.3 to[100 lg/mL [16].
In an attempt to synthesize 6-substituted-2-benzothiazolylimino-5-piperazinyl-4-
thiazolidinone derivatives for testing as antimicrobial agents, the first stage of
introduction of piperazine groups into 4-thiazolidinones was N-formylation of
piperazine derivatives. Previous research has revealed that sulfated tungstate is a
highly efficient, recyclable, heterogeneous catalyst for facile N-formylation [17].
Materials and methods
Chemistry
Melting points were determined in open capillaries on a Veego VMP-D electronic
apparatus and are uncorrected. IR spectra (4000–400 cm-1) of synthesized compounds,
as KBr pellets, were recorded on a Shimadzu 8400-S FTIR spectrophotometer. Thin-
layer chromatography (TLC) was performed on glass slides (2 9 7.5 cm) coated with
1
silica gel G and spots were visualized under UV irradiation. H NMR spectra were
recorded on a Varian 400 MHz spectrometer with dimethyl sulfoxide (DMSO) as
solvent and TMS (Me4Si) as internal standard. 1H NMR chemical shifts are reported as
parts per million (ppm) downfield from TMS. The splitting patterns are designated as
follows; s, singlet; d, doublet; dd, doublet of doublets; q, quartet, m, multiplet.
Preparation of sulfated tungstate
A solution of chlorosulfonic acid (0.1 mol) in chloroform (75 mL) in a flask fitted
with CaCl2 drying tube was maintained at 0–5 °C in an ice bath, with continuous
123

Catalytic N-formylation for synthesis of antimicrobial agents
stirring, while anhydrous sodium tungstate (0.05 mol) was added in portions. After
completion of the reaction, the mixture was stirred for another 1.5 h. The yellowish-
white solid obtained was filtered, and washed several times with Millipore water
until the filtrate was neutral and free from chloride ions, as confirmed by testing
with AgNO3. The crude product was then dried in an oven at 100 °C for 2 h to
furnish the final catalyst, sulfated tungstate. IR (KBr, cm-1): 3451, 1616, 1222,
1089, 961, 654.
Synthesis of 4-isopropylpiperazine-1-carbaldehyde (2)
Sulfated tungstate (10 wt% ) was added to a stirred solution of 1-isopropylpiper-
azine (1, 0.1 mol) and formic acid (0.12 mol) and the resulting mixture was heated
in an oil bath at 70 °C for 4.5 h. The progress of the reaction was monitored by TLC
with toluene–acetone 8:2 as mobile phase. Also, when conversion was complete the
liquid reaction mixture automatically solidified, which is the main physical change
confirming completion of the reaction. The mixture was left to cool then diluted
with 20 mL ethyl acetate and the insoluble catalyst was recovered by filtration and
washed. The combined filtrate and washings were washed with water (2 9 10 mL),
dried over anhydrous sodium sulfate, and the pure product was obtained by
evaporation of the solvent under reduced pressure. Yield 92 %, m.p. 102–103 °C,
1
1
IR (KBr, cm-1): 1731 (CHO). H NMR (400 MHz, DMSO-d6): d 8.39 (s, H,
CHO), 7.35–7.28 (m, 4H, Ar–H), 4.34 (s, 3H, OCH3), 3.87 (br s, 4H, piperazine
ring), 3.39 (br s, 4H, piperazine ring).
General procedure for synthesis of 2-amino-6-substituted benzothiazoles (2a–j)
The appropriate para-substituted amine derivative (1a–j; 0.1 mol) and an equimo-
lecular amount of potassium thiocyanate were added to 100 mL glacial acetic acid,
with cooling of the reaction mixture in an ice bath. The mixture was left at this
temperature for up to 20 min then bromine (0.1 mol) in glacial acetic acid was
added very slowly, to maintain the temperature of the reaction mixture below 10 °C,
then the mixture was stirred at room temperature for 2–4 h to furnish the
hydrobromide (HBr) salt. The salt was then isolated by filtration, washed with acetic
acid, dried in a vacuum oven, then dissolved in sufficient aqueous ammonia solution
to ensure the pH was 11.0. The solid precipitate thus formed was filtered, washed
with water, and dried in a vacuum oven to yield the intermediates 4a–j. The
progress of the reaction was monitored by TLC with toluene–acetone 8:2 as mobile
phase.
General procedure for synthesis of N-(benzo[d]thiazol-2-yl)-2-chloroacetamides
(3a–j)
Chloroacetyl chloride (0.06 mol) was added dropwise to a mixture of the
appropriate 2-amino-6-Substituted benzothiazole, 2a–j (0.05 mol) and K2CO3
(0.06 mol) in benzene (50 mL) at room temperature. The reaction mixture was
heated under reflux for 6–12 h, then, after cooling to room temperature, it was
123

R. V. Patel, S. W. Park
slowly poured into 100 mL ice water. The precipitate obtained was isolated by
filtration, washed repeatedly with water, then dried under vacuum to furnish 3a–
j. The progress of the reaction was monitored by TLC with toluene–acetone 8:2 as
mobile phase.
General procedure for synthesis of 6-substituted-2-(benzo[d]thiazol-2-
ylimino)thiazolidin-4-ones (4a–j)
A solution of the appropriate N-(benzo[d]thiazol-2-yl)-2-chloroacetamide (3a–j;
5 mmol) and 10 mmol ammonium thiocyanate in ethanol was heated under reflux
for 2–6 h. When completion of the reaction was confirmed by TLC, the mixture was
left to stand overnight. The precipitate obtained was isolated by filtration, washed
with water, then recrystallized, furnishing 4a–j.
For example, 4a [15]: Yield: Yield 59 %, m.p. 191–192 °C, IR (KBr, cm-1):
3160 (N–H), 1728 (C=O), 1567 (N=C), 1659 (C=N, benzothiazole, str.), 647 (CS).
1
1
1H NMR (400 MHz, DMSO-d6): d 12.61 (s, H, NH), 7.81 (d, H, J = 8.4, H-7,
benzothiazole), 7.71 (d, 1H, J = 8.1, H-4, benzothiazole), 7.51 (t, 1H, J = 8.4, H-5,
benzothiazole), 7.35 (t, 1H, J = 8.2, H-6, benzothiazole), 3.57 (s, 2H, CH2,
thiazolidin-4-one).
General procedure for synthesis of 2-benzothiazolylimino-5-piperazinyl-4-
thiazolidinones (5a–j)
A well-stirred solution of 5 mmol 6-substituted-2-(benzo[d]thiazol-2-ylimino)thiaz-
olidin-4-one (4a–j) in 40 mL acetic acid was buffered with 9 mmol sodium acetate.
4-Isopropylpiperazine-1-carbaldehyde (2; 6.5 mmol) was added and the mixture
was heated under reflux for 6–9 h. When reaction was complete, as indicated by
TLC, the mixture was poured into ice-cold water. The precipitate was isolated by
filtration and washed with water, and the resulting crude product was purified by
recrystallization from dioxane to furnish 5a–j [15].
2-(Benzo[d]thiazol-2-ylimino)-5-((4-isopropylpiperazin-1-yl)methylene)thiazolidin-
4-one (5a) Yield 70 %, m.p. 229–231 °C, IR (KBr, cm-1): 3155 (N–H), 1726
(C=O), 1668 (C=N, benzothiazole, str.), 1571 (N=C), 647 (CS). 1H NMR
(400 MHz, DMSO-d6): d 12.75 (s, 1H, NH), 7.90 (d, 1H, J = 8.1, H-7,
benzothiazole), 7.72 (s, 1H, CH), 7.68 (d, 1H, J = 7.6, H-4, benzothiazole),
7.56–7.43 (m, 2H, H-5 and H-6, benzothiazole), 3.80 (br s, 4Hpip), 3.44 (br s, 4Hpip),
2.94–2.98 (m, 1H, N–CH), 1.88 (d, J = 6.6 Hz, 6H, -2CH3). Anal Calcd for
C18H21N5OS2: C, 55.79; H, 5.46; N, 18.07; Found: C, 55.61; H, 5.64; N, 17.93.
2-(6-Fluorobenzo[d]thiazol-2-ylimino)-5-((4-isopropylpiperazin-1-yl)methylene)thiaz-
olidin-4-one (5b) Yield 56 %, m.p. 259–261 °C, IR (KBr, cm-1): 3170 (N–H),
1712 (C=O), 1661 (C=N, benzothiazole, str.), 1565 (N=C), 655 (CS). H NMR
1
(400 MHz, DMSO-d6): d 12.81 (s, 1H, NH), 7.78–7.42, 7.13–6.89 (m, 3H,
benzothiazole), 7.73 (s, 1H, CH), 3.76 (br s, 4Hpip), 3.39 (br s, 4Hpip), 2.94–2.98 (m,
123

Catalytic N-formylation for synthesis of antimicrobial agents
1H, N–CH), 1.91 (d, J = 6.6 Hz, 6H, –2CH3). Anal Calcd for C18H20FN5OS2: C,
53.31; H, 4.97; N, 17.27; Found: C, 53.48; H, 5.11; N, 17.14.
2-(6-Chlorobenzo[d]thiazol-2-ylimino)-5-((4-isopropylpiperazin-1-yl)methylene)thiaz-
olidin-4-one (5c) Yield 62 %, m.p. 248–250 °C, IR (KBr, cm-1): 3160 (N–H),
1
1716 (C=O), 1653 (C=N, benzothiazole, str.), 1580 (N=C), 649 (CS). H NMR
(400 MHz, DMSO-d6): d 12.69 (s, 1H, NH), 7.69–7.39, 7.09–6.93 (m, 3H,
benzothiazole), 7.78 (s, 1H, CH), 3.85 (br s, 4Hpip), 3.48 (br s, 4Hpip), 2.88–2.92 (m,
1H, N–CH), 1.82 (d, J = 6.6 Hz, 6H, –2CH3). Calcd for C18H20ClN5OS2: C, 51.23;
H, 4.78; N, 16.60; Found: C, 51.38; H, 4.91; N, 16.77.
2-(6-Bromobenzo[d]thiazol-2-ylimino)-5-((4-isopropylpiperazin-1-yl)methylene)thiaz-
olidin-4-one (5d) Yield 79 %, m.p. 236–238 °C, IR (KBr, cm-1): 3149 (N–H),
1
1723 (C=O), 1659 (C=N, benzothiazole, str.), 1570 (N=C), 642 (CS). H NMR
(400 MHz, DMSO-d6): d 12.74 (s, 1H, NH), 7.81–7.28 (m, 3H, benzothiazole), 7.69
(s, 1H, CH), 3.79 (br s, 4Hpip), 3.41 (br s, 4Hpip), 2.90–2.95 (m, 1H, N–CH), 1.78 (d,
J = 6.6 Hz, 6H, –2CH3). Anal Calcd for C18H20BrN5OS2: C, 46.35; H, 4.32; N,
15.02; Found: C, 46.48; H, 4.15; N, 15.19.
5-(4-Isopropylpiperazin-1-yl)methylene)-2-(6-nitrobenzo[d]thiazol-2-ylimino)thiaz-
olidin-4-one (5e) Yield 49 %, m.p. 287–289 °C, IR (KBr, cm-1): 3166 (N–H),
1
1709 (C=O), 1671 (C=N, benzothiazole, str.), 1575 (N=C), 639 (CS). H NMR
(400 MHz, DMSO-d6): d 12.80 (s, 1H, NH), 8.09–7.51 (m, 3H, benzothiazole), 7.74
(s, 1H, CH), 3.87 (br s, 4Hpip), 3.42 (br s, 4Hpip), 2.87–2.92 (m, 1H, N–CH), 1.93 (d,
J = 6.6 Hz, 6H, –2CH3). Anal Calcd for C18H20N6O3S2: C, 49.98; H, 4.66; N,
19.43; Found: C, 50.14; H, 4.51; N, 19.28.
5-(4-Isopropylpiperazin-1-yl)methylene)-4-oxothiazolidin-2-ylideneamino)benzo[d]thi-
azole-6-carbonitrile (5f) Yield 44 %, m.p. 279–281 °C, IR (KBr, cm-1): 3158 (N–
1
H), 1699 (C=O), 1656 (C=N, benzothiazole, str.), 1560 (N=C), 650 (CS). H NMR
(400 MHz, DMSO-d6): d 12.70 (s, 1H, NH), 7.66–7.26 (m, 3H, benzothiazole), 7.68
(s, 1H, CH), 3.80 (br s, 4Hpip), 3.45 (br s, 4Hpip), 2.94–2.99 (m, 1H, N–CH), 1.86 (d,
J = 6.6 Hz, 6H, –2CH3). Anal Calcd for C19H20N6OS2: C, 55.32; H, 4.89; N,
20.37; Found: C, 55.42; H, 5.05; N, 20.51.
5-(4-Isopropylpiperazin-1-yl)methylene)-2-(6-methylbenzo[d]thiazol-2-ylimino)thiaz-
olidin-4-one (5g) Yield 59 %, m.p. 271–273 °C, IR (KBr, cm-1): 3171 (N–H),
1
1696 (C=O), 1649 (C=N, benzothiazole, str.), 1564 (N=C), 654 (CS). H NMR
(400 MHz, DMSO-d6): d 12.78 (s, 1H, NH), 7.65–7.09 (m, 3H, benzothiazole), 7.77
(s, 1H, CH), 3.84 (br s, 4Hpip), 3.51 (br s, 4Hpip), 2.89–2.94 (m, 1H, N–CH), 2.19 (s,
3H, CH3), 1.88 (d, J = 6.6 Hz, 6H, –2CH3). Anal Calcd for C19H23N5OS2: C,
56.83; H, 5.77; N, 17.44; Found: C, 56.71; H, 5.90; N, 17.29.
5-(4-Isopropylpiperazin-1-yl)methylene)-2-(6-methoxybenzo[d]thiazol-2-ylimino)thiaz-
olidin-4-one (5h) Yield 66 %, m.p. 252–254 °C, IR (KBr, cm-1): 3159 (N–H),
123

R. V. Patel, S. W. Park
1
1712 (C=O), 1659 (C=N, benzothiazole, str.), 1572 (N=C), 646 (CS). H NMR
(400 MHz, DMSO-d6): d 12.81 (s, 1H, NH), 7.56–7.26, 6.85–6.96 (m, 3H,
benzothiazole), 7.71 (s, H, CH), 3.87 (br s, 4Hpip), 3.76 (s, 3H, OCH3), 3.46 (br s,
1
4Hpip), 2.93–2.97 (m, 1H, N–CH), 1.84 (d, J = 6.6 Hz, 6H, –2CH3). Anal Calcd for
C19H23N5O2S2: C, 54.65; H, 5.55; N, 16.77; Found: C, 54.79; H, 5.38; N, 16.94.
5-(4-Isopropylpiperazin-1-yl)methylene)-2-(6-ethoxybenzo[d]thiazol-2-ylimino)thiaz-
olidin-4-one (5i) Yield 71 %, m.p. 245–247 °C, IR (KBr, cm-1): 3151 (N–H),
1721 (C=O), 1655 (C=N, benzothiazole, str.), 1579 (N=C), 647 (CS). H NMR
1
(400 MHz, DMSO-d6): d 12.75 (s, 1H, NH), 7.41–7.16, 6.81–6.90 (m, 3H,
benzothiazole), 7.75 (s, H, CH), 4.18 (q, J = 6.6 Hz, 2H, OCH2CH3), 3.77 (br s,
1
1
4Hpip), 3.41 (br s, 4Hpip), 2.91–2.95 (m, H, N–CH), 2.21 (t, J = 6.8 Hz, 3H,
CH2CH3), 1.78 (d, J = 6.6 Hz, 6H, –2CH3). Anal Calcd for C20H25N5O2S2: C,
55.66; H, 5.84; N, 16.23; Found: C, 55.52; H, 6.01; N, 16.11.
N-5-(4-Isopropylpiperazin-1-yl)methylene)-4-oxothiazolidin-2-ylideneamino)benzo[d]
thiazol-6-yl)acetamide (5j) Yield 50 %, m.p. 239–241 °C, IR (KBr, cm-1): 3168
(N–H), 1729 (C=O), 1669 (C=N, benzothiazole, str.), 1567 (N=C), 642 (CS). H
1
NMR (400 MHz, DMSO-d6): d 12.79 (s, 1H, NH), 9.42 (s, 1H, –NH), 7.47–7.25 (m,
3H, benzothiazole), 7.68 (s, 1H, CH), 3.83 (br s, 4Hpip), 3.46 (br s, 4Hpip), 2.93–2.98
1
(m, H, N–CH), 2.19 (s, 3H, CH3), 1.91 (d, J = 6.6 Hz, 6H, –2CH3). Calcd for
C20H24N6O2S2: C, 54.03; H, 5.44; N, 18.90; Found: C, 53.92; H, 5.59; N, 19.07.
Microbiology
Derivatives 5a–j were tested for antimicrobial activity against Gram-positive
bacteria (Staphylococcus aureus, Bacillus subtilis), Gram-negative bacteria (E. coli,
P. aeruginosa), and the fungus (Candida albicans). Ciprofloxacin and fluconazole
were used as control drugs for antibacterial and antifungal activity, respectively.
Details of the biological assays have been reported elsewhere [18]. Results were
obtained as minimum inhibitory concentrations (MIC) by use of the agar streak
dilution method [19]. A stock solution of the compound (100 lg/mL) was prepared
in DMSO, and serial dilutions of the test compounds at lg/mL concentrations were
incorporated in molten sterile agar (nutrient agar and Sabouraud dextrose agar for
evaluation of antibacterial activity and antifungal activity, respectively). The
medium containing the test compound was poured into a Petri dish to a depth of
4–5 mm and left to solidify under aseptic conditions. A suspension of the respective
microorganism, approximately 105 CFU/mL, was prepared and applied to the
plates, which were then incubated at 37 1 °C for 24 h (bacteria) or 48 h (fungus).
The lowest concentration of the substance that prevented development of visible
growth was recorded as the MIC value. Zones of inhibition were determined by use
of the paper disk diffusion technique [20].
123

Catalytic N-formylation for synthesis of antimicrobial agents
Results and discussion
Chemistry
The reactions outlined in Scheme 1 were used for synthesis of the intermediates and
title compounds (5a–j). Sulfated tungstate as mild, heterogeneous, solid acid
catalyst was obtained as a yellowish–white solid by reaction of anhydrous sodium
tungstate with chlorosulfonic acid in chloroform. Its formation was confirmed by
comparison of its FTIR spectrum with literature data. The catalyst (10 wt%)
efficiently promoted rapid N-formylation of N-isopropylpiperazine with formic acid,
yielding 4-isopropylpiperazine-1-carbaldehyde (2) [17]. Its FTIR and 1H NMR
spectra contained peaks at 1731 cm-1 and 8.39 ppm, respectively, indicative of
CHO functionality. We have previously reported synthesis of 2-amino-6-substituted
benzothiazoles 2a–j from para-substituted amines in the presence of KSCN and
bromine in glacial acetic acid, and their 2-chloroacetamide congeners 3a–j obtained
by use of chloroacetyl chloride [10]. The structures of the benzothiazole-based
1
intermediates were confirmed by comparison of FTIR and H NMR spectral data
with literature results [21]. Heterocyclization of 3a–j in the presence of NH4SCN in
ethanol under reflux furnished the 6-substituted-2-(benzo[d]thiazol-2-ylimino)thiaz-
olidin-4-one intermediates 4a–j in good yields and purity. The structures of 4a–
1
j were, again, confirmed by study of FTIR and H NMR spectra. The spectra of
derivative 4a were in perfect agreement with the proposed structure. Peaks at 3160,
1728, and 1567 cm-1 in the FTIR spectrum confirmed cyclization to form an
1
iminothiazolidinone. In addition, the H NMR spectrum of 4a contained an –NH
proton signal at 12.61 ppm and a methylene proton signal of the cyclized ring at
3.57 ppm. Similar results confirmed successful synthesis of 4a–j. The imino
intermediates were then treated with N-(benzo[d]thiazol-2-yl)-2-chloroacetamides
3a–j in the presence of anhydrous sodium acetate in glacial acetic acid to furnish
2-benzothiazolylimino-5-piperazinyl-4-thiazolidinones 5a–j. Analytical data for
5a–j confirmed the Z configuration—NH lactam proton peaks occurred at
approximately 12.70 ppm rather than below 10 ppm (where signals from imine
protons are observed). Moreover, because of the deshielding effect of the adjacent
carbonyl functionality, the methine protons of exocyclic C=C bonds resonated at
high chemical shifts, approximately 7.70 ppm, further confirming the Z isomer-
ism [16, 22]. Furthermore, all other aspects of the 1H NMR and FTIR spectra were
indicative of the presence of benzothiazole and piperazine rings, in accordance with
our previous research results [7, 23]. The purity of the synthesized compounds was
monitored by TLC and by elemental analysis.
Pharmacology
Bioassay results from investigation of the activity of 5a–j against two Gram-positive
bacteria (S. aureus and B. subtilis), two Gram-negative bacteria (E. coli and P.
aeruginosa), and the fungus C. albicans are summarized in Table 1. In general, the
results for antibacterial action were encouraging with lowest MIC of 4–8 lg/mL for
some of the derivatives. None of the derivatives had antifungal activity, however (MIC
123

R. V. Patel, S. W. Park
O
N
N
Cl
i
ii
R
NH2
NH2
NH
3a-j
S
S
R
R
1a-j
2a-j
Sodium tungstate
iv
iii
H
N
O
H3C
H3C
N
S
H3C
H3C
O
Sulfated tungstate
v
N
CH N
N C
CH N
NH
S
H
1
2
4a-j
vi
H
N
O
N
N
S
S
R
N
N
5a-j
Reagents & Conditions: i. KSCN, Br2, AcOH, R.T.; ii. ClCH2COCl, CHCl3, K2CO3,
reflux; iii. NH4SCN, EtOH, reflux; iv. ClSO3H, CHCl3, 0-5oC; v. Sulfated tungstate
(10 wt %), HCOOH, 70 oC; vi. anhydrous NaOAc in AcOH, 120 oC.
Scheme 1 Synthesis of piperazine-based benzothiazolylimino-4-thiazolidinones 5a–j
[32 lg/mL). Gram-positive strains were more sensitive than Gram-negative strains
(MIC 4 lg/mL and 4–8 lg/mL, respectively). Compounds with benzothiazole rings
containing electron-withdrawing (EWD) fluorine (b) and nitro (e) and electron-
releasing (ER) acetamide (j) were most potent. In general, however, EWD substituents
resulted in better antibacterial activity than ER groups. The order of activity among
EWD groups was F [ NO2 [ Br [ Cl [ CN. Overall, SARs (Structure Activity
Relationships) suggested that combined benzothiazole and thiazolidinone rings
positively enhanced antibacterial action, and the presence of the piperazine ring was
essential to reduce MIC compared with MIC reported in the literature.
The MIC of the 6-substituted-2-benzothiazolylimino-5-piperazinyl-4-thiazolidi-
nones were determined by measurement of their growth inhibitory effects, as zone
of inhibition obtained for a concentration of 32 lg/disk. Compound 5b was the most
potent of all the compounds (MIC 4 lg/mL against S. aureus, B. subtilis, and
P. aeruginosa and 8 lg/mL against E. coli). These MIC suggest that the presence of
the highly electronegative and EWD fluorine atom may be a crucial aspect of
potency. In addition, 5e, with an EWD nitro substituent was promisingly active
(MIC 8 lg/mL against both Gram-positive bacteria and Gram-negative E. coli).
Derivative 5j with the ER acetamide group had an MIC of 4 lg/mL against both
Gram-positive strains. Compound 5c with the EWD chlorine atom and compound
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R. V. Patel, S. W. Park
5g with the ER methoxy functional group on the benzothiazole ring had MIC of
8 lg/mL against P. aeruginosa and compound 5d with the bromine substituent had
a similar MIC against B. subtilis. Growth inhibitory effects of these compounds, as
zone of inhibition, ranged from 17 to 22 mm compared with 15–24 mm for the
control drug ciprofloxacin. Overall, the activity of the 6-substituted-2-benzothia-
zolylimino-5-piperazinyl-4-thiazolidinones was comparable with that of ciproflox-
acin. Some were almost equipotent with ciprofloxacin whereas the others had MIC
of 16–32 lg/mL against all the bacterial strains.
Conclusion
The purpose of this research study was rational development of a new class of
compounds with promising antibacterial action. The novelty of the work includes
attachment of piperazine at the C-1 position of the 4-thiazolidinone ring and use of
more than one thiazole ring to enhance the overall potency of the compounds. As
expected, the 6-substituted-2-benzothiazolylimino-5-piperazinyl-4-thiazolidinones
were potent inhibitors of the growth of Gram-positive and Gram-negative bacteria
with MIC of 4–8 lg/mL and inhibitory zones of 17–22 mm compared with
3.12–6.25 lg/mL and 15–24 mm, respectively, for the control drug ciprofloxacin.
Growth of fungal species was not inhibited. These preliminary research results
could aid discovery, by drug-discovery scientists, of other compounds with activity
against several biological targets.
Acknowledgments This article was supported by the SMART Research Professor Program of Konkuk
University, Seoul, South Korea.
Conflict of interest The authors report no conflicts of interest.
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