New conjugated benzothiazole-N-oxides: Synthesis and biological activity

Article abstract of DOI:10.3390/molecules14125382

Eleven new 2-styrylbenzothiazole-N-oxides have been prepared by aldol - type condensation reactions between 2-methylbenzothiazole-N-oxide and para-substituted benzaldehydes. Compounds with cyclic amino substituents showed typical push-pull molecule proper

Full text of DOI:10.3390/molecules14125382

Molecules 2009, 14, 5382-5388; doi: 10.3390/molecules14125382
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
New Conjugated Benzothiazole-N-oxides: Synthesis and
Biological Activity
Peter Gajdoš 1, Peter Magdolen 1,*, Pavol Zahradník 2 and Pavlína Foltínová 2
1
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská
dolina CH-2, 842 15 Bratislava, Slovakia
2
Institute of Subcellular Biology, Faculty of Natural Sciences, Comenius University, Mlynská dolina
CH-2, 842 15 Bratislava, Slovakia
* Author to whom correspondence should be addressed; E-Mail: magdolen@fns.uniba.sk.
Received: 28 October 2009; in revised form: 12 December 2009 / Accepted: 18 December 2009 /
Published: 23 December 2009
Abstract: Eleven new 2-styrylbenzothiazole-N-oxides have been prepared by aldol – type
condensation reactions between 2-methylbenzothiazole–N-oxide and para-substituted
benzaldehydes. Compounds with cyclic amino substituents showed typical push-pull
molecule properties. Four compounds were tested against various bacterial strains as well
as the protozoan Euglena gracilis as model microorganisms. Unlike previously prepared
analogous benzothiazolium salts, only weak activity was recorded.
Keywords: benzothiazole; N-oxides; push-pull systems; antibacterial; Euglena gracilis
Introduction
Various benzothiazoles are known as industrial chemicals, dyes and drugs [1–3]. The biological
activity of compounds with benzothiazole skeletons includes anticancer, antibacterial, antifungal and
anthelmintic properties [4,5]. Some of the active antimicrobial substances have a dipolar push-pull
structures with benzothiazole as the acceptor part of the molecule and an amino-substituted benzene
ring as a donor part, linked together with a conjugated bridge. A previous QSAR study of such a type
of compounds revealed that 2-styrylbenzothiazoles with p-dialkylamino-substituted benzene rings
presented interesting antimicrobial activity [6]. The next generation of derivatives showed enhanced
activity when a conjugated bridge was enlarged to two or three double bonds, the dialkylamino

Molecules 2009, 14
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substituent was changed to N-cyclic amino and the nitrogen in benzothiazole part of molecule was
quaternized to the corresponding N-alkylbenzothiazolium salt, preferentially with allyl, propargyl or
methyl halogenides [7]. The substitution at the benzene ring of benzothiazole did not add any
substantial bonus. As the quaternization of the benzothiazole nitrogen by an alkyl group improved the
biological activity 2-3 fold [8,9], we shifted our interest to the benzothiazolium-N-oxide derivatives
assuming a similar activity effect might result. The N-oxide structure could also influence the electron
density in the benzothiazole ring what could extend the synthetic route to derivatives substituted on the
heterocycle phenyl ring. Since the benzothiazole push-pull dipolar structures also show important
nonlinear optical properties, photonics and optoelectronics could be other fields of possible application
[9–11]. These were the main reasons to synthesize the target compounds with the general structure
presented in Figure 1.
Figure 1. Structure of target benzothiazole-N-oxides.
7
6
S
N
a
2
3´
4´
5´
2´
5
R
b
4
O
6´
R = -H
-CN
-N(CH3)2 1c
1a
1b
N
N
N
N CH3
1i
1f
N
N
N
N
-NO2
1d
N COOC2H5
1j
1g
1h
O
1k
S
1e
N
N
N
N
In this paper the synthesis and characterization of eleven new 2-styrylbenzothiazole-N-oxides is
reported. Diverse substituents R are bonded to the 4´-position of the styryl part of the molecule:
electron-acceptors –NO2 and –CN as well as electron-donors represented by the dimethylamino group
and various N-cyclic amines. Four chosen compounds were also tested against various model
microorganisms.
Results and Discussion
As the quaternization of benzothiazole heterocyclic nitrogen causes increased acidity of the
2-methyl hydrogens, the most appropriate way to prepare the target styryl compounds consists of the
aldol-type condensation of 2-methylbenzothiazole-N-oxide and a substituted benzaldehyde
(Scheme 1). Methanol was chosen as a suitable solvent due to its good solvatation of the reactants. The
use of dimethylsulfoxide, which was beneficial in the case of the reaction between simple
2-methylbenzothiazole and the respective aldehydes did not show any advantage in this case as the
products remain dissolved in the reaction mixture. 2-Methylbenzothiazole-N-oxide was prepared from
2-methylbenzothiazole by the known oxidation procedure with hydrogen peroxide–maleic anhydride
mixture [12]. The required p-substituted benzaldehydes were synthesized via nucleophilic aromatic
substitution from 4-fluorobenzaldehyde [13].

Molecules 2009, 14
5384
Scheme 1. Synthesis of benzothiazole-N-oxides.
S
N
S
KOH
O
Me
+
R
N
O
R
MeOH
O
1
The prepared derivatives were identified by 1H- and 13C-NMR, UV-vis spectroscopy and elemental
analysis. The physicochemical properties of prepared compounds and their biological activity were
compared with the same data of analogous condensation products of 2-methyl-3-alkylbenzothiazolium
salts with the corresponding aldehydes.
UV–VIS absorption spectra of the prepared compounds were measured and they consist of the
long-wavelength band located between 340–420 nm and other band at shorter wavelength sometimes
separable to two bands. The long wavelength band is usually the most intensive one and represents the
S0JS1 electronic transition. The position of λmax of this band strongly depends on the character of a
substituent in the para position on the styrene moiety. The electron-donating cyclic amine substituents
cause a higher push-pull effect, expressed with the batochromic shift of λmax to 400–420 nm. These
compounds appear as orange – red solids, contrary to yellow colour of the compounds with electron-
accepting or heteroaromatic substituents at the same position.
In all, of the eleven new benzothiazole derivatives prepared, four of them with the divers types of
aminosubstituents were tested for their antimicrobial and antifungal activity. The activities of prepared
compounds have been measured and compared with the corresponding neutral conjugated
benzothiazoles and N-alkyl benzothiazolium salts. Unfortunately the new derivatives are less active
than the analogous benzothiazolium salts. They are comparable to the corresponding neutral
conjugated benzothiazole derivatives. The results are listed in Table 1. The activities against the
unicellular flagelate Euglena gracilis were also low and did not exceed a value of 3 expressed as
log(1000/ED50). The activities of the corresponding N-alkyl benzothiazolium salts were found to be
more than 6 [8].
Table 1. The quantitative antibacterial and antifungal activities of new benzothiazole-N-oxides.
MIC values (μg/mL) for individual organisms
Compound
S. aureus
50
B. subtilis
E. coli
P. aeruginosa C. albicans M. gypseum
1c
1e
1f
500
500
500
500
>500
>500
>500
>500
>500
250
250
250
250
500/250
> 500/250
> 500/500
> 500/500
250
250
250
>500
>500
>500
1h
Conclusions
In summary, an aldol type condensation reaction between 2-methylbenzothiazole–N-oxide and
divers types of p-substituted benzaldehydes has been used to prepare eleven new
2-styrylbenzothiazole-N-oxides. The UV-vis spectra show typical push-pull character in derivatives
with amino substituents. Four compounds were tested against various bacterial strains as well as the

Molecules 2009, 14
5385
protozoan Euglena gracilis. However only weak activity was recorded compared with previously
prepared analogous benzothiazolium salts.
Experimental
General
NMR spectra were recorded on a Varian Gemini 2000 spectrometer, at 300 MHz (1H) and 75 MHz
(13C) using tetramethylsilane as an internal standard. UV-vis spectra were measured on Hewlett-
Packard Diode Array 8245 in methanol (UV grade) and ε values are expressed in mol-1·m2. Elemental
analysis was determined on a Carlo Erba Science 1106 instruments and obtained values were in
acceptable error range (0.4%). Solvents were dried and distilled before use. Condensation products
were purified by crystallization.
2-Methylbenzothiazole-N-oxide
To a solution of 2-methylbenzothiazole (7.5 g, 0.05 mol) and maleic anhydride (6.10 g, 0.062 mol)
in CH2Cl2 (25 mL) 30% aqueous H2O2 (6.35 g, 0.18 mol) was added dropwise. The mixture was
stirred and heated to reflux for 1 h, then cooled in the ice-water bath. The resulting precipitated maleic
acid was filtered off and washed with CH2Cl2. The combined filtrates were dried and concentrated
under reduced pressure. Chromatography of the residue on silica gel using MeOH-CH2Cl2 (1:15) gave
1
2-methylbenzothiazole-N-oxide (2.9 g, 36%) as a white solid, m.p. 46–48 °C. H-NMR (CDCl3)
δ 8.12–8.15 ( 1H, m, 4-H), 7.96–8.00 (1H, m, 7-H), 7.58–7.70 (2H, m, 5,6-H), 2.65 (3H, s, CH3).
General procedure for the preparation of condensation products
2-Methylbenzothiazole-N-oxide (0.16 g, 0.001 mol) and the appropriate substituted benzaldehyde
(0.001 mol) were dissolved in methanol (2 mL). Potassium hydroxide (0.05 mL of 50% aqueous
solution) was added as a base catalyst and the mixture was heated to reflux for 3 h. The precipitated
crude product was crystallized from ethanol to give pure compound.
2-[(E)-2-Phenylvinyl]-1,3-benzothiazole-3-oxide (1a): Yield 74%; M.p 241–244 °C; 1H-NMR
(DMSO-d6) δ 8.15–8.18 (1H, m, H-7), 7.99–8.02 (1H, m, H-4), 7.89 (1H, d, J = 16.5, H-β), 7.76 (2H,
d, J = 8.1, H-2,6), 7.73 (1H, d, J = 16.5, H-α), 7.64–7.67 (2H, m, H-5,6), 7.37–7.48 (3H, m, H-3,4,5);
13C- NMR: (DMSO-d6) δ 142.86, 141.38, 135.91, 135.51, 129.57, 128.99, 128.52, 127.55, 127.38,
125.86, 123.86, 116.81, 114.48; UV-vis: λmax /nm (log ε): 344 (3.19).
1
2-[(E)-2-[4-Cyanophenyl]vinyl]-1,3-benzothiazole-3-oxide (1b): Yield 66%. M.p. 224- 228 °C. H
NMR (DMSO) δ 8.18–8.21 (1H, m, H-7), 8.01–8.04 (1H, m, H-4), 7.96 (2H, d, J = 8.4, H-3`,5`),
7.86–7.91 (2H, m, H-α,β), 7.89 (2H, d, J = 8.4, H-2`,6`), 7.66–7.69 (2H, m, H-5,6). 13C NMR:
(DMSO) δ 142.74, 141.88, 140.09, 135.96, 133.41, 132.59, 128.72, 127.99, 127.36, 126.13, 118.55,
117.66, 116.81, 110.98. UV-vis: λmax /nm (log ε): 347 (3.95).

Molecules 2009, 14
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2-[(E)-2-[4-(Dimethylamino)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1c): Yield 31%. M.p.
1
255–259 °C. H NMR (CDCl3) δ 8.11–8.14 (1H, m, H-7), 7.66–7.72 (1H, m, H-4), 7.62 (1H, d,
J = 16.4, H-β), 7.52 (2H, d, J = 8.9, H-2`,6`), 7.42–7.55 (2H, m, H-5,6), 7.38 (1H, d, J = 16.4, H-α),
13
6.70 (2H, d, J = 8.9, H-3`,5`), 3.04 (6H, s, N(CH3)2). C NMR (CDCl3) δ 150.95, 145.69, 141.73,
137.32, 132.98, 128.92, 127.74, 127.19, 126.55, 124.37, 116.89, 111.54, 109.25, 39.74.
2-[(E)-2-[4-Nitrophenyl]vinyl]-1,3-benzothiazole-3-oxide (1d): Yield 54%; M.p. 230–234 °C;
1H- NMR: (DMSO-d6) δ 8.10–8.14 (1H, m, H-7), 7.94–7.97 (1H, m, H-4), 7.71 (1H, d, J = 16.5, H-β),
7.61–7.63 (4H, m, H-5,6, H-3`,5`), 7.51 (1H, d, J = 16.4, H-α), 6.99 (2H, d, J = 8.8, H-2`,6`);
13C-NMR: (DMSO-d6) δ 151.79, 146.68, 143.44, 141.65, 139.68, 132.17, 127.70, 126.94 (2C), 126.73,
126.63, 123.27 (2C), 116.57, 109.31; UV-vis: λmax /nm (log ε): 276 (2.85).
2-[(E)-2-[4-(1H-Benzo[d]imidazole-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1e): Yield 40%;
1
M.p 186–188 °C; H-NMR: (DMSO-d6) δ 8.64 (1H, s, H-2benzim), 8.17–8.20 (1H, m, H-7), 8.00–8.05
(1H, m, H-4), 8.03 (2H, d, J = 8.8, H-2`,6`), 7.83 (1H, d, J = 16.7, H-β), 7.79 (2H, d, J = 8.8, H-3`,5`),
7.78 (1H, d, J = 16.7 H-α), 7.73–7.80 (2H, m, H-4,7benzim), 7.66–7.73 (2H, m, H-5,6), 7.29–7.42 (2H,
m, H-5,6benzim); 13C-NMR (DMSO-d6) δ 145.48, 143.53, 143.23, 142.98, 142.82, 142.04,
133.75,132.48, 132.88, 129.13 (2C), 128.58, 127.42, 125.91, 123.73, 123.59, 122.61, 119.95, 116.78,
115.17, 110.82; UV-vis: λmax /nm (log ε) : 276 (3.48), 350 (2.77).
2-[(E)-2-[4-(1H-Benzo[d]triazole-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1f): Yield 35%; M.p
1
219–223 °C; H-NMR: (DMSO-d6) 8.13–8.18 (1H, m, H-7), 8.01–8.05 (1H, m, H-4), 7.92 (1H, d,
J = 16.3, H-β), 7.89 (2H, d, J = 8.5, H-2`,6`), 7.88 (1H, d, J = 16.3 H-α), 7.75 (2H, d, J = 8.5, H-
3`,5`), 7.62–7.70 (6H, m, H-5,6, Hbenzotri); 13C-NMR (DMSO-d6) δ 145.67, 144.90, 141.71, 135.33,
134.16, 132.77, 131.76, 129.09, 128.60, 127.67, 127.34 (2C), 126.80, 124.62, 123.63, 122.49 (2C),
119.60, 116.60, 115.51, 110.87; UV-vis: λmax /nm (log ε): 304 (3.48), 356 (2.82).
2-[(E)-2-[4-(Morpholine-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1g): Yield 49%; M.p
240–244 °C; 1H-NMR (DMSO-d6) δ 8.10–8.13 (1H, m, H-7), 7.94–7.97 (1H, m, H-4), 7.72 (1H, d, J =
16.5, H-β), 7.54–7.69 (4H, m, H-5,6,2`,6`), 7.52 (1H, d, J = 16.2, H-α), 7.00 (2H, d, J = 8.7, H-3`,5`),
13
3.74 (4H, m, OCH2), 3.28 (4H, m, NCH2); C-NMR (DMSO-d6) δ 151.79, 143.37, 142.95, 136.24,
135.97, 128.87, 127.98, 126.64, 125.78, 123.60, 116.47, 114.43 (2C), 110.64, 65.69, 46.37; UV-vis:
λmax /nm (log ε): 316 (2.77), 420 (3.41).
2-[(E)-2-[4-(Imidazole-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1h): Yield 50%; M.p.
1
196–198 °C; H-NMR (DMSO-d6) δ 8.38 (1H, s br, H-2im), 8.16–8.19 (1H, m, H-7), 8.00–8.04 (1H,
m, H-4), 7.93 (1H, d, J = 16.5, H-β), 7.92 (2H, d, J = 8.7, H-3`,5`), 7.85 (1H, t, H-4im) 7.83 (1H, d,
J = 16.5 H-α), 7.65–7.68 (2H, m, H-5,6), 7.63 (2H, d, J = 8.7 H-2`,6`), 7.13 (1H, t, H-5im); 13C-NMR:
(DMSO-d6) δ 142.73, 142.03, 137.08, 135.17, 134.24, 133.89, 131.35, 129.76, 128.64 (2C), 126.97,
125.62, 123.40, 120.27, 117.43, 116.50, 114.67; UV-vis: λmax /nm (log ε) : 344 (3.54).

Molecules 2009, 14
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2-[(E)-2-[4-(4-Methylpiperazine-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1i): Yield 63%; M.p.
222–225 °C; 1H-NMR (DMSO-d6) δ 8.10–8.13 (1H, m, H-7), 7.96 (1H, m, H-4), 7.71 (1H, d, J = 16.4,
H-β), 7.59–7.66 (2H, m, H-5,6), 7.60 (2H, d, J = 8.8 H-2`,6`), 7.50 (1H, d, J = 16.4 H-α), 6.99 (2H, d,
J = 8.8 H-3`,5`), 3.25–3.29 (4H, m, NCH2), 2.42–2.46 (4H, m, NCH2), 2.22 (3H, s, NCH3); 13C-NMR
(DMSO-d6) δ 151.41, 142.99, 142.78, 136.53, 135.98, 128.56, 127.61, 126.79, 125.49, 125.10, 116.20,
114.27, 110.18, 54.03, 46.79, 45.29; UV-vis: λmax /nm (log ε) : 316 (3.00), 420 (3.57).
2-[(E)-2-[4-(4-Etoxycarbonylpiperazine-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1j): Yield
1
50%; M.p. 216–219 °C; H-NMR: (DMSO-d6) δ 8.11–8.14 (1H, m, H-7), 7.95–7.98 (1H, m, H-4),
7.72 (1H, d, J = 16.4, H-β), 7.62–7,66 (2H, m, H-5,6), 7.63 (2H, d, J = 8.8 H-2`,6`), 7.52 (1H, d,
J = 16.4 H-α), 7.00 (2H, d, J = 8.8 H-3`,5`), 4.07 (2H, q, J = 7.0, COOCH2), 3.48–3.53 (4H, m,
NCH2), 3.25–3.30 (4H, m, NCH2), 1.20 (3H, t, J = 7.0, CH3); 13C-NMR: (DMSO-d6) δ 154.64, 151.46,
143.72, 142.92, 136.35, 133.76, 129.05 (2C), 128.30, 128.11, 125.69, 125.01, 116.53, 114.99 (2C),
110.60, 61.02, 45.74, 42.62, 14.61; UV-vis: λmax /nm (log ε) : 276 (3.82), 400 (3,04).
2-[(E)-2-[4-(Thiomorpholine-1-yl)phenyl]vinyl]-1,3-benzothiazole-3-oxide (1k): Yield 64%; M.p.
1
229–231 °C; H-NMR: (DMSO-d6) δ 8.10–8.13 (1H, m, H-7), 7.94–7.97 (1H, m, H-4), 7.70 (1H, d,
J = 16.5, H-β), 7.60–7.64 (2H, m, H-5,6), 7.60 (2H, d, J = 9.0, H-2`,6`), 7.49 (1H, d, J = 16.5, H-α),
6.97 (2H, d, J = 9.0, H-3`,5`), 3.70–3.73 (4H, m, NCH2), 2.61–2.66 (4H, m, SCH2); 13C-NMR:
(DMSO-d6) δ 150.43, 143.70, 142.28, 136.34, 131.62, 129.17, 127.93, 127.16. 125.27, 124.71, 116.38,
114.81, 110.10, 58.79, 46.95; UV-vis: λmax /nm (log ε): 424 (3.46).
Biological testing
The compounds 1c-1h have been tested for their antimicrobial activities against Gram positive
(Staphylococcus aureus and Bacillus subtilis) and Gram-negative bacteria (Escherichia coli and
Pseudomonas aeruginosa) as well as a yeast (Candida albicans) and a mould (Microsporum gypseum).
The standard plate diffusion method using Mueller-Hinton and Saboraud agar, or standard dilution
method in Saboraud medium [14] were used. The tested compounds were dissolved in DMSO.
Minimum inhibitory concentrations (MIC in g/mL) are given in Table 1.
Acknowledgements
This work has been supported by the Slovak Grant Agencies VEGA (No. 1/4470/07) and APVV
(No. 0259-07).
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Sample Availability: Samples of the compounds are available from the authors.
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