Synthesis of novel 2-[4-(4-substitutedbenzamido/phenylacetamido) phenyl]benzothiazoles as antimicrobial agents

Article abstract of DOI:10.1007/s00044-011-9918-4

A new series of 2-[4-(4-substitutedbenzamido/ phenylacetamido)phenyl] benzothiazole derivatives (6a-k) were synthesized and evaluated for antibacterial and antifungal activities against Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Escherichia coli with their drug-resistant isolates and a yeast Candida albicans. Microbiological results indicated that the compounds possessed a broad spectrum of activity against the tested microorganisms at minimum inhibitory concentration (MIC) values between 100 and 6.25 lg/ml. Compounds 6d and 6k exhibited significant antibacterial activity showing 6.25 lg/ml MIC values against drugresistant S. aureus and P. aeruginosa isolates, respectively. Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s00044-011-9918-4

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2012) 21:3818–3825
DOI 10.1007/s00044-011-9918-4
ORIGINAL RESEARCH
Synthesis of novel 2-[4-(4-substitutedbenzamido/phenylacetamido)
phenyl]benzothiazoles as antimicrobial agents
•
•
•
Kayhan Bolelli Ismail Yalcin Tugba Ertan-Bolelli
¨
•
•
•
Selda Ozgen Fatma Kaynak-Onurdag Ilkay Yildiz
Esin Aki
Received: 21 June 2011 / Accepted: 17 November 2011 / Published online: 7 December 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A new series of 2-[4-(4-substitutedbenzamido/
phenylacetamido)phenyl] benzothiazole derivatives (6a–k)
were synthesized and evaluated for antibacterial and anti-
fungal activities against Staphylococcus aureus, Bacillus
subtilis, Klebsiella pneumoniae, Pseudomonas aeruginosa,
and Escherichia coli with their drug-resistant isolates and a
yeast Candida albicans. Microbiological results indicated
that the compounds possessed a broad spectrum of activity
against the tested microorganisms at minimum inhibitory
concentration (MIC) values between 100 and 6.25 lg/ml.
Compounds 6d and 6k exhibited significant antibacterial
activity showing 6.25 lg/ml MIC values against drug-
resistant S. aureus and P. aeruginosa isolates, respectively.
available, methicillin and vancomycin. These drugs are
reserved to treat only the most intractable infections to
slow development of resistance to them (Fridkin and
Gaynes, 1999). So, there is still need for the new classes of
antimicrobial agents.
The compounds which possess benzothiazole nucleus in
their structure are involved in research aimed at evaluating
new chemotherapeutically active agents, such as antimi-
crobial (Trapani et al., 1994; Yalc¸in et al., 1992; Yildiz-
Oren et al., 2004; Ku¨c¸u¨kbay and Durmaz, 1997), a topical
carbonic anhydrase inhibitor (Kalina et al., 1988), a cyclo-
oxygenase inhibitor (Paramashivappa et al., 2003), antitu-
bercular (Koc et al., 2002; Katz, 1953), anti-nematode
(Surin, 1995), a dual inhibitor of thromboxane A2 synthetase
and 5-lipoxygenase (Komatsu and Minami, 1995), a selec-
tive and reversible inhibitor of monoamine oxidase type A
(MAO-A) (Kagaya et al., 1996), antiallergic (Ager et al.,
1988), multi-drug resistance cancer cell activities with
inhibiting activity on eukaryotic topoisomerase II enzyme in
cell-free system (Pinar et al., 2004; Temiz-Arpaci et al.,
2005; Tekiner-Gulbas et al., 2006), and antitumor agents
(Shi et al., 1996; Hutchinson et al., 2002; Chua et al., 1999).
Currently, a new series of benzothiazoles have been
synthesized as antitumor agents and showed potent inhib-
itory activity against human breast cancer cell lines in
vitro and in vivo (Shi et al., 1996). Among them, lysyl-
amide of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiaz-
ole (Structure 1) had been selected for phase 1 clinical
evaluation (Hutchinson et al., 2002).
Keywords Benzothiazole Á Antibacterial activity Á
Antifungal activity Á Multi-drug resistance
Introduction
Disease-causing microbes that have become resistant to
drug therapy are an increasing public health problem.
Tuberculosis, gonorrhea, malaria, and childhood ear
infections are just a few of the diseases that have become
hard to treat with antibiotic drugs. The hospital-acquired
infections are resistant to the most powerful antibiotics
K. Bolelli Á I. Yalcin Á T. Ertan-Bolelli Á I. Yildiz (&) Á E. Aki
Department of Pharmaceutical Chemistry, Faculty of Pharmacy,
In the last years, we reported the synthesis of several
2-substitutedbenzothiazole derivatives as the antimicrobial
agents (Yalc¸in et al., 1992; Yildiz-Oren et al., 2004) as
seen in Structure 2. According to these studies, the com-
pounds were found to have inhibitory effect with minimum
inhibitory concentration (MIC) value of 3.12–50 lg/ml
˘
Ankara University, Tandogan, 06100 Ankara, Turkey
e-mail: iyildiz@pharmacy.ankara.edu.tr
¨
S. Ozgen Á F. Kaynak-Onurdag
Department of Pharmaceutical Microbiology,
Faculty of Pharmacy, Gazi University, Etiler, 06330 Ankara,
Turkey
123

Med Chem Res (2012) 21:3818–3825
3819
CH3
by thin layer chromatography (TLC). Silicagel HF254
chromatoplates (0.3 mm) were used for TLC and the sol-
vent systems were n-hexane:ethyl acetate (2:1) for 2-(4-
aminophenyl)benzothiazole (3) and chloroform:methanol
(20:1) for compounds 6a–k. Melting points were taken on
a Buchi SMP 20 capillary apparatus and are uncorrected.
IR spectra were recorded by FT/IR-420 in KBr disks.
1H-NMR spectra were obtained with a Varian Mercury 400
High Performance Digital FT-NMR-400 MHz spectrome-
ter in d6-DMSO, tetramethylsilane (TMS) was used as an
internal standard. Elemental analyses were carried out with
CHNS-932 (LECO) apparatus. The results (C, H, N) were
within 0.4% of the calculated values. Mass analysis was
obtained by Waters 2695 Alliance ZQ Micromass LC–MS
working with ESI apparatus. All of the synthesized com-
pounds are original except 6f (Dilworth et al., 2007).
S
NH2
N
F
Structure 1
against some of Gram-positive, Gram-negative bacteria,
and Candida albicans as yeast. Among the tested com-
pounds, 2-(phenoxymethyl)benzothiazole was found as the
most active derivative at a MIC value of 3.12 lg/ml
against the tested S. aureus (Yildiz-Oren et al., 2004).
Moreover, the same compound was found very potent as an
eukaryotic topoisomerase II inhibitor exhibiting a better
inhibitor activity than reference drug etoposide (Pinar
et al., 2004; Temiz-Arpaci et al., 2005; Tekiner-Gulbas
et al., 2006).
General procedure for the synthesis of 2-(4-
aminophenyl)benzothiazole (3)
The goal of outset of this research is to develop new
effective antimicrobial agents possessing benzothiazole
nuclei in their structure. Herein, we have described the
synthesis a series of 2-[4-(4-substitutedbenzamido/pheny-
lacetamido)phenyl]-benzothiazole derivatives (Structure 3)
as a new class of synthetic antimicrobial agents along with
their in vitro antimicrobial activity tested against some
Gram-negative, Gram-positive bacteria, and the drug
resistance isolates as well as the yeast C. albicans. The
tested compounds are synthesized as possessing phenyl
acetamide or benzamide moiety holding different substit-
uents on position R showing various physicochemical
properties, such as electron donating, electron withdraw-
ing, and steric effects to be able to discuss the effect of the
substituent on the activity.
2-(4-Aminophenyl)benzothiazole (3) was prepared by
heating 1 mmol o-aminothiophenol (1) with 1 mmol
p-aminobenzoic acid (2) in 2.4 g polyphosphoric acid and
stirring for 4 h (Scheme 1). After then, the residue was
poured into ice–water mixture and neutralized with excess
of 10% NaOH solution. The precipitate was boiled with
activated charcoal in ethanol and then, filtered and re-
crystallized in ethanol (M.p.: 155–157°C) (Hein et al.,
1957; Stevens et al., 1995).
General procedure for the synthesis of compounds
(6a–k)
The
final
products,
2-[4-(4-substitutedbenzamido/
phenylacetamido)phenyl]benzothiazole derivatives (6a–k)
were synthesized by heating thionyl chloride (0.3 ml) and
appropriate carboxylic acid (4) (1 mmol) in benzene
(0.5 ml) at 80°C for 3 h, and then excess thionyl chloride
was removed in vacuo (Scheme 2). The residue was dis-
solved in ether (1 ml) and the solution was added during
1 h to a stirred, ice-cold mixture of 2-(4-aminophenyl)
benzothiazole (1 mmol), sodium bicarbonate (2 mmol),
diethylether (1 ml), and water (1 ml) (Yalcin et al., 1997).
The mixture was stirred overnight at room temperature and
then filtered. After then, the precipitate was washed with
water, 2N HCl, and water. Ethanol was used for recrys-
tallization and crystals are dried in vacuo.
Experimental procedures
Chemistry
The chemicals were purchased from the commercial
venders and were used without purification. The reactions
were monitored and the purity of the products was checked
S
CH2
R
2-[4-(4-Fluorobenzamido)phenyl]benzothiazole (6a)
N
Yield: 27% mp 259–261°C. IR (cm-1): 3351, 3039, 1653
(Amide I), 1602–1589, 1534 (Amide II),1502, 1483, 1434,
1232, 970–600, 728–620. 1H NMR (400 MHz, CDCl3):
R = cyclohexylmethyl, phenylmethyl, phenoxy,
p-chlorophenoxy, phenylmercaptoxy
Structure 2
123

3820
Med Chem Res (2012) 21:3818–3825
O
S
N
NH C X
R
X= −, -CH2-
R= -F, -Cl, -Br, -CH3, -C2H5, -C(CH3)3, -NO2, -OCH3
Structure 3
SH
S
HO
PPA
NH2
+
NH2
O
N
NH2
3
2
1
Scheme 1 The synthesis of 2-(4-aminophenyl)benzothiazole (3)
R
R
Benzene
HCl
+
SO2
+
+
SOCl2
COCl
COOH
X
X
5a-5k
4
R
+
S
N
NH2
COCl
0oC
water/ether/NaHCO3
X
5a-5k
3
O
C
S
N
NH
X
R
6a-6k
X= −, CH2
R= -F, -Cl, -Br, -CH3, -C2H5, -C(CH3)3, -NO2, -OCH3
Scheme 2 The synthesis of 2-[4-(4-substitutedbenzamido/phenylacetamido)phenyl]-benzothiazole derivatives (6a–k)
2-[4-(4-Chlorobenzamido)phenyl]benzothiazole (6b)
7.258 (t, 2H), 7.305 (t, 1H), 7.399 (t, 1H), 7.853–8.001 (m,
8H), 10,45 (s, 1H). ESI (?) m/e: 348,88 (M?, 68%), 349,16
(M? ? H, 100%). Anal. Found: C, 68.89; H, 3.775; N,
8.171; S, 9.166. Calcd. for C20H13FN2OS: C, 68.95; H,
3.761; N, 8.041; S, 9.204.
Yield: 35% mp 278–280°C. IR (cm-1): 3358, 3054–2918,
1653 (Amide I), 1591, 1532 (Amide II), 1511, 1481, 1434,
1092, 969–606, 728–620. 1H NMR (400 MHz, CDCl3):
123

Med Chem Res (2012) 21:3818–3825
3821
7.458 (t, 1H), 7.549 (t, 1H), 7.648 (dd, 2H), 8.002–8.058
(m, 5H), 8.122 (d, 2H), 8.153 (s, 1H), 10.628 (s, 1H. ESI
(?) m/e: 364.88 (M?, 61%), 365.09 (M? ? H, 100%),
367,06 (M? ? H ? 2, 29%). Anal. Found: C, 66.17; H,
3.592; N, 7.818; S, 8.809. Calcd. for C20H13ClN2OS: C,
65.84; H, 3.591; N, 7.678; S, 8.789.
C20H13N3O3S–0.2 HOH: C, 63.38; H, 3.564; N, 11.087; S,
8.461.
2-[4-(4-Fluorophenylacetamido)phenyl]benzothiazole
(6g)
Yield: 56% mp 218–220°C. IR (cm-1): 3269, 3028, 2919,
2850, 1664 (Amide I), 1608, 1532 (Amide II),1511, 1484,
1434, 1218, 971–610, 727–625. 1H NMR (400 MHz,
CDCl3): 3.695 (s, 2H), 7.159 (t, 2H), 7.376 (t, 2H), 7.425
(t, 1H), 7.518 (t, 1H), 7.798 (d, 2H), 8.010 (d, 1H), 8.043
(d, 2H), 8.112 (d, 1H), 10.529 (s, 1H). ESI (?) m/e: 363.17
(M? ? H, 100%). Anal. Found: C, 68.99; H, 4.125; N,
7.731; S, 8.688. Calcd. for C21H15FN2OS–0.1 HOH: C,
69.25; H, 4.206; N, 7.691; S, 8.804.
2-[4-(4-Bromobenzamido)phenyl]benzothiazole (6c)
Yield: 32% mp 283–285°C. IR (cm-1): 3364, 2923–2852,
1657 (Amide I), 1587, 1530 (Amide II), 1517, 1478, 1434,
1072, 966–607, 731–620. 1H NMR (400 MHz, CDCl3):
7.457 (t, 1H), 7.548 (t, 1H), 7.787 (dd, 2H), 7.955 (dd, 2H),
8.002–8.057 (m, 3H), 8.108–8.154 (m, 3H), 10.642 (s, 1H).
ESI (?) m/e: 409.06 (M?, 91%), 411.06 (M? ? 2, 91%).
Anal. Found: C, 57.42; H, 3.239; N, 6.769; S, 7.820. Calcd.
for C20H13BrN2OS–0.4 HOH: C, 57.67; H, 3.340; N,
6.726; S, 7.700.
2-[4-(4-Chlorophenylacetamido)phenyl]benzothiazole
(6h)
Yield: 58% mp 229–231°C. IR (cm-1): 3305, 3049, 2923,
2850, 1659 (Amide I), 1601, 1544 (Amide II), 1532, 1484,
1433, 1091, 973–585, 721–620. 1H NMR (400 MHz,
CDCl3): 3.708 (s, 2H), 7.369 (m, 4H), 7.420 (t, 1H), 7.514
(t, 1H), 7.798 (d, 2H), 8.008 (d, 1H), 8.044 (d, 2H), 8.104
(d, 1H), 10.549 (s, 1H). ESI (?) m/e: 379.01 (M? ? H,
100%), 381.15 (M? ? H ? 2, 44%). Anal. Found: C,
66.14; H, 3.982; N, 7.460; S, 8.206. Calcd. for
C21H15ClN2OS–0.1 HOH: C, 66.25; H, 4.025; N, 7.359; S,
8.423.
2-[4-(4-Ethylbenzamido)phenyl]benzothiazole (6d)
Yield: 43% mp 238–240°C. IR (cm-1): 3347, 3052, 2957,
2872, 2924, 2851, 1656 (Amide I), 1591, 1529 (Amide II),
1514, 1479, 1434, 967–622, 727–622. 1H NMR (400 MHz,
CDCl3): 1.229 (t, 3H), 2.706 (q, 2H), 7.402 (d, 2H), 7.457
(t, 1H), 7.549 (t, 1H), 7.937 (d, 2H), 8.018–8.156 (m, 6H),
10.510 (s, 1H). ESI (?) m/e: 359.16 (M? ? H, 100%).
Anal. Found: C, 72.88; H, 4.995; N, 7.840; S, 8.596. Calcd.
for C22H18N2OS–0.2 HOH: C, 72.98; H, 5.122; N, 7.737;
S, 8.857.
2-[4-(4-Bromophenylacetamido)phenyl]benzothiazole (6i)
Yield: 55% mp 224–226°C. IR (cm-1): 3320, 3047, 2924,
2851, 1658 (Amide I), 1601, 1544 (Amide II),1532, 1483,
1433, 1071, 964–608, 721–620. 1H NMR (400 MHz,
CDCl3): 3.689 (s, 2H), 7.303 (d, 2H), 7.428 (t, 1H),
7.501–7.538 (m, 3H), 7.798 (dd, 2H), 8.011 (d, 1H), 8.044
(dd, 2H), 8.115 (d, 1H), 10.536 (s, 1H). ESI (?) m/e:
423.07 (M?, 80%), 425.02 (M? ? 2, 100%). Anal. Found:
C, 59.65; H, 3.618; N, 6.653; S, 7.516. Calcd. for
C21H15BrN2OS: C, 59.58; H, 3.572; N, 6.618; S, 7.575.
2-[4-(4-tert-Butylbenzamido)phenyl]benzothiazole (6e)
Yield: 37% mp 228–230°C. IR (cm-1): 3385, 3060, 2954,
1656 (Amide I), 1604–1585, 1529 (Amide II),1500, 1482,
1434, 967–580, 728–622. 1H NMR (400 MHz, CDCl3):
1.334 (s, 9H), 7.452 (t, 1H), 7.544–7.587 (m, 3H), 7.929 (d,
2H), 8.009–8.054 (m, 3H), 8.098–8.150 (m, 3H), 10.510
(s, 1H). ESI (?) m/e: 387.17 (M? ? H, 100%). Anal.
Found: C, 74.36; H, 5.642; N, 7.239; S, 8.074. Calcd. for
C24H22N2OS: C, 74.58; H, 5.737; N, 7.248; S, 8.296.
2-[4-(4-Methylphenylacetamido)phenyl]benzothiazole (6j)
2-[4-(4-Nitrobenzamido)phenyl]benzothiazole (6f)
(Dilworth et al., 2007)
Yield: 64% mp 227–229°C. IR (cm-1): 3252, 3052, 3180,
3108, 2924, 2853, 1659 (Amide I), 1598, 1541 (Amide II),
1515, 1480, 1433, 962–592, 720–619. 1H NMR (400 MHz,
CDCl3): 2.282 (s, 3H), 3.646 (s, 2H), 7.145 (d, 2H), 7.242
(d, 2H), 7.439 (t, 1H), 7.531 (t, 1H), 7.810 (d, 2H), 8.023
(d, 1H), 8.053 (d, 2H), 8.125 (d, 1H), 10.497 (s, 1H). ESI
(?) m/e: 359.16 (M? ? H, 100%). Anal. Found: C, 73.57;
H, 4.972; N, 7.832; S, 8.809. Calcd. for C22H18N2OS: C,
73.71; H, 5.061; N, 7.815; S, 8.946.
Yield: 25% mp 296–298°C. IR (cm-1): 3350, 2919–2851,
1653 (Amide I), 1599–1588, 1526 (Amide II), 1489, 1477,
1434, 1347, 968–603, 713–620. 1H NMR (400 MHz,
CDCl3): 7.460 (t, 1H), 7.551 (t, 1H), 8.013–8.061 (m, 3H),
8.130–8.151 (m, 3H), 8.228 (d, 2H), 8.400 (d, 2H), 10.873
(s, 1H). ESI (?) m/e: 376.12 (M? ? H, 100%). Anal.
Found: C, 63.51; H, 3.477; N, 10.970; S, 8.161. Calcd. for
123

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Med Chem Res (2012) 21:3818–3825
2-[4-(4-Methoxyphenylacetamido)phenyl]benzothiazole
(6k)
(SDA) plates at 35°C for 24–48 h. The microorganisms
were passaged at least twice to ensure purity and viability.
The solution of the newly synthesized compounds (6a–k)
and standard drugs were prepared at 400, 200, 100, 50, 25,
12.5, 6.25, 3.125, 1.562, 0.78, 0.39, 0.19, 0.095, 0.047, and
0.024 lg/ml concentrations, in the wells of microplates by
diluting in Mueller–Hinton broth (MHB).
Yield: 28% mp 198–199°C. IR (cm-1): 3327, 3032, 2963,
2927, 2910, 2832, 1670 (Amide I), 1608–1584, 1527
(Amide II), 1513, 1485, 1437, 1244, 1032, 970–593,
733–620. 1H NMR (400 MHz, CDCl3): 3.606 (s, 2H),
3.720 (s, 3H), 6.891 (d, 2H), 7.259 (d, 2H), 7.425 (t, 1H),
7.518 (t, 1H), 7.794 (d, 2H), 8.009 (d, 1H), 8.037 (d, 2H),
8.112 (d, 1H), 10.459 (s, 1H). ESI (?) m/e: 375.11
(M? ? H, 100%). Anal. Found: C, 70.29; H, 4.891; N,
7.390; S, 8.384. Calcd. for C22H18N2O2S–0.1 HOH: C,
70.23; H, 4.876; N, 7.445; S, 8.522.
Bacterial susceptibility testing was performed according
to the guidelines of CLSI M100-S16 (2006b). The bacterial
suspensions used for inoculation were prepared at 105 CFU/
ml by diluting fresh cultures at MacFarland 0.5 density
(107 CFU/ml). Suspensions of the bacteria at 105 CFU/ml
concentration were inoculated to the twofold diluted solu-
tion of the compounds. There were 104 CFU/ml bacteria in
the wells after inoculations. MHB was used for diluting the
bacterial suspension and for twofold dilution of the com-
pound. 80% DMSO–20% EtOH, methanol, DMSO, PBS,
pure microorganisms, and pure media were used as control
wells. A 10 ll bacteria inoculum was added to each well of
the microdilution trays. The trays were incubated at 37°C
and MIC endpoints were read after 24 h of incubation.
Candida albicans was subcultured in SDA plates, and
incubated at 35°C for 24–48 h before antifungal suscepti-
bility testing. Susceptibility testing was performed in
RPMI-1640 medium with L-glutamine buffered pH 7 with
3-[N-morpholino]-propansulfonic acid (MOPS) and culture
suspensions were prepared through the guideline of CLSI
M27-A3 (2006c). Yeast suspensions were prepared
according to McFarland 0.5 density and a working sus-
pension was made by a 1:100 dilution followed by a 1:20
dilution of the stock suspension (2.5 9 103 CFU/ml). A
10 ll yeast inoculum was added to each well of the mic-
rodilution trays. The trays were incubated at 35°C and MIC
end points were read after 48 h of incubation.
Microbiology
Microorganisms
Klebsiella pneumoniae isolate, which has an extended
spectrum beta lactamase enzyme (ESBL), Pseudomonas
aeruginosa isolate (gentamicin-resistant), Escherichia coli
isolate, which has an extended spectrum beta lactamase
enzyme (ESBL), Bacillus subtilis isolate (resistant to cef-
triaxon), Staphylococcus aureus isolate [methicillin-resis-
tant (MRSA)], K. pneumoniae RSKK 574 (Refik Saydam
National Public Health Agency Culture Collection),
P. aeruginosa ATCC 25853 (American Type Culture
Collection), E. coli ATCC 25922, B. subtilis ATCC 6633,
S. aureus ATCC 25923, and C. albicans ATCC 10231.
Methods
Standard strains of K. pneumoniae RSKK 574, P. aeru-
ginosa ATCC 25853, E. coli ATCC 25922, B. subtilis
ATCC 6633, S. aureus ATCC 25923, C. albicans ATCC
10231, and clinical isolates of these microorganisms
resistant to various antimicrobial agents were included in
the study. Resistance was determined by Kirby Bauer Disk
Diffusion method according to the guidelines of Clinical
and Laboratory Standards Institute (CLSI) (2006a) in the
clinical isolates.
All organisms were tested in triplicate in each run of the
experiments. The lowest concentration of the compound
that completely inhibits macroscopic growth was deter-
mined and MICs are reported in Table 1.
Results and discussion
Standard powders of ampicillin trihydrate, gentamycin
sulfate, rifampicin, ofloxacin, fluconazole, and amphoteri-
cin B were obtained from the manufacturers. Stock solu-
tions were dissolved in dimethylsulfoxide (ofloxacin), pH 8
phosphate buffer saline (PBS) (ampicillin trihydrate)
methanol (rifampicin), and distilled water (gentamicin
sulfate, fluconazole, and amphotericin B). Newly synthe-
sized compounds (6a–k) were dissolved in 80% DMSO–
20% EtOH.
Chemistry
The synthetic pathways for preparation of the target
compounds are shown in Schemes 1 and 2. The final
compounds were easily obtained in two steps. In the first,
2-(4-aminophenyl)benzothiazole as a starting material was
performed by condensing of appropriate aminophenols and
suitable acids in polyphosphoric acid (Chua et al., 1999;
Hein et al., 1957; Stevens et al., 1995) (Scheme 1). In the
second reaction, an amidification was done. For this reac-
tion, 2-(4-aminophenyl)benzothiazole was treated with
Bacterial isolates were subcultured in Mueller–Hinton
Agar (MHA) plates and incubated over night at 37°C and
C. albicans was subcultured in Sabouraud dextrose agar
123

Med Chem Res (2012) 21:3818–3825
3823
Table 1 The antibacterial and antifungal activity (MIC in lg/ml) of the newly synthesized compounds (6a–k) with the control drugs
O
S
NH
C
X
R
N
Compounds
X
R
K.p.
K.p.a
P.a.
P.a.b E.c.
E.c.c
B.s.
B.s.d
S.a.
S.a.e
C.a.
6a
–
F
25
50
50
25
50
25
50
50
50
50
50
50
50
50
50
–
25
25
25
25
25
25
25
25
12.5
25
25
–
25
25
25
25
25
25
25
25
25
25
50
50
50
25
25
25
25
50
25
50
50
50
50
50
50
50
50
50
50
50
25
50
50
50
50
50
50
50
50
50
50
12.5
50
50
50
50
50
50
50
50
100
50
50
50
50
25
25
6b
–
Cl
12.5
25
25
6c
–
Br
100
50
25
6d
–
C2H5
6.25
12.5
12.5
50
12.5
12.5
12.5
25
6e
–
C(CH3)3 25
100
50
6f
–
NO2
F
50
50
50
50
25
12.5
–
6g
CH2
25
6h
CH2 Cl
50
50
25
6i
CH2 Br
50
25
12.5
25
6j
CH2 CH3
CH2 OCH3
50
25
6k
6.25 12.5
50
12.5
–
12.5
–
Ampicillin trihydrate
Gentamycin sulfate
Rifampicin
Ofloxacin
–
3.125 400
0.78
0.78
0.78
0.39
6.25
–
1.562 50
0.78
–
12.5
0.39
0.19
0.19
–
0.19
3.125
3.125
–
0.78
0.0225
0.19
–
6.25
1.562
1.562
–
–
–
–
–
50
–
–
–
0.19
–
3.125
–
6.25
–
0.19
–
6.25
–
–
Fluconazol
Amphotericin B
0.78
0.78
–
–
–
–
–
–
–
–
–
–
K.p. Klebsiella pneumoniae RSKK 574, P.a. Pseudomonas aeruginosa ATCC 25853, E.c. Escherichia coli ATCC 25922, B.s. Bacillus subtilis
ATCC 6633, S.a. Staphylococcus aureus ATCC 25923, C.a. Candida albicans ATCC 10231
a
b
K. pneumoniae isolate, P. aeruginosa isolate, E. coli isolate, B. subtilis isolate, S. aureus isolate
c
d
e
In vitro antibacterial and antifungal activity
suitable carboxylic chloride obtained by treating suitable
carboxylic acid with thionyl chloride to get compounds 6a–k
as given in Scheme 2. The structures were supported by
spectral data. The IR, 1H-NMR, mass spectra, and ele-
mental analysis results are in agreement with the proposed
structures.
All the newly synthesized 2-[4-(4-substituted-benzamido/
phenylacetamido)phenyl]-benzothiazole derivatives (6a–k)
were in vitro tested for antibacterial activity against
K. pneumoniae RSKK 574, P. aeruginosa ATCC 25853,
E. coli ATCC 25922, K. pneumoniae isolate, which has an
extended spectrum beta lactamase enzyme (ESBL), E. coli
isolate (ESBL), P. aeruginosa isolate (gentamicin-resis-
tant) as Gram-negative bacteria, B. subtilis ATCC 6633,
S. aureus ATCC 25923, B. subtilis isolate (resistant to cef-
triaxon), S. aureus isolate [meticilline-resistant (MRSA)] as
Gram-positive bacteria, and the antifungal activity was
evaluated against C. albicans ATCC 10231. The standard
agents, ampicillintrihydrate, gentamycin sulfate, rifampicin,
and ofloxacin for antibacterial activity and fluconazole and
amphotericin B for antifungal activity were also screened
under identical conditions for quality control and compari-
son. The MIC values were determined by microdilution
method according to the guidelines of Clinical and Labora-
tory Standards Institute (CLSI) (2006b, c).
According to the spectroscopic data of the final com-
pounds the IR showed characteristic C=O stretching bands
in the 1,653–1,670 cm-1 (amide I) and 1,530–1,500 cm-1
(amide II) regions, respectively. Besides, C=N stretching
bands were observed in the 1,489–1,532 cm-1 region. In
the 1H-NMR spectra of the compounds 6a–k, the signal of
NH proton was observed at 10.459–10.873 ppm as a sin-
glet band; benzylic CH2 protons were observed at
3.606–3.708 ppm as a singlet band for compounds 6g–k.
Aromatic methyl proton appeared at 2.282 ppm (for com-
pound 6j) as a singlet band, as well. Besides, all the aro-
matic protons were observed at the expected regions. On
the other hand, mass spectra of the compounds showed
M? ? H peaks, since the electrospray ionization method
was employed, in accordance with their formulas.
123

3824
Med Chem Res (2012) 21:3818–3825
All the observed in vitro antimicrobial activity results of
C. albicans than the compared standard drugs, fluconazol
and amphotericin B.
the tested compounds are given in Table 1. The synthe-
sized compounds were found showing an antibacterial
activity at MIC values between 6.25 and 50 lg/ml for
Gram-negative bacteria. Among the tested compounds,
2-[4-(4-methoxyphenylacetamido)phenyl]benzothiazole (6k)
was found as the most potent derivative at a MIC value of
6.25 lg/ml against the screened drug-resistant enteric
Gram-negative rod P. aeruginosa isolate providing three-
fold higher potency than the compared standard drugs,
gentamycin sulfate and ofloxacin. It revealed that the
compound having phenyl acetamide moiety by holding a
methoxy group on position R (6k) was performed twofold
better activity against drug-resistant P. aeruginosa isolate
than the other tested compounds. Besides, the derivatives
6a–j exhibited onefold better activity than the reference
drugs. However, all the compounds displayed lower anti-
bacterial activity against P. aeruginosa ATCC 25853 with
MIC values between 12.5 and 25 lg/ml than the compared
control drugs, gentamycin sulfate and ofloxacin. Moreover,
the compound 6k also indicated a better inhibitory effect
than the other tested compounds against K. pneumoniae
and E. coli.
According to the antimicrobial results given in Table 1,
it reveals that the binding a phenylacetamide moiety at
position 2 holding a methoxy group on position R in the
benzothiazole nuclei is important for increasing the
potency against Gram-negative bacteria especially against
the drug-resistant P. aeruginosa isolate. On the other hand,
the compound having a benzamide moiety at position 2
was found more effective for increasing potency against
the drug-resistant Gram-positive S. aureus isolate.
These observations provide some predictions to design
further antimicrobial active compounds before their syn-
thesis following with QSAR and molecular modeling
studies.
Acknowledgments This study was supported by TUBITAK
107S455 (SBAG-COST BM0701-19). The Central Laboratory of the
Faculty of Pharmacy of Ankara University provided support for
acquisition of the NMR, mass spectrometer, and elemental analyzer
used in this study.
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