Screening of antimicrobial activity of diarylamines in the 2,3,5-trimethylbenzo[b]thiophene series: A structure-activity evaluation study

Article abstract of DOI:10.1016/j.bmcl.2004.09.038

In vitro antimicrobial data against Gram positive, Gram negative bacteria, and Candida albicans are presented. Gram positive (Bacillus cereus, B. subtilis), Gram negative (Pseudomonas aeruginosa, Escherichia coli) bacteria, and Candida albicans as a repre

Full text of DOI:10.1016/j.bmcl.2004.09.038

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5831–5833
Screening of antimicrobial activity of diarylamines in
the 2,3,5-trimethylbenzo[b]thiophene series:
a structure–activity evaluation study
a
a
Isabel C. F. R. Ferreira,a, Ricardo C. Calhelha, Letıcia M. Estevinho and
*
´
Maria-Joa˜o R. P. Queirozb
a
´
´
´
Escola Superior Agraria, Instituto Politecnico de Braganc¸a, Campus de Sta. Apolonia, Apartado 1172, 5301-854 Braganc¸a, Portugal
b
´
Departamento de Quımica, Campus de Gualtar, Universidade do Minho, 4710-057 Braga, Portugal
Received 25 May 2004; revised 24 August 2004; accepted 17 September 2004
Available online 5 October 2004
Abstract—Gram positive (Bacillus cereus, B. subtilis), Gram negative (Pseudomonas aeruginosa, Escherichia coli) bacteria, and Can-
dida albicans as a representative of fungi were used for screening the in vitro antimicrobial activity of diarylamines in the 2,3,5-tri-
methylbenzo[b]thiophene series bearing different substituents, synthesized by us using the palladium-catalyzed C–N coupling
methodology. The minimal inhibitory concentration (MIC) and structure–activity relationships (SARs) were evaluated.
Ó 2004 Elsevier Ltd. All rights reserved.
The enhance prevalence of infectious diseases is becom-
ing a world wide problem. Additionally, the resistance
problem demands that a renewed effort should be made
to seek antimicrobial agents effective against pathogenic
microorganisms resistant to current treatment.
Benzo[b]thiophenes are important heterocycles as bio-
logical active molecules.1–5 Recently we have been inter-
ested in the palladium-catalyzed aryl amination of
benzo[b]thiophenes either in the benzene or in the thio-
phene ring to obtain the corresponding diarylamines
and in some cases the tetracyclic aromatic compounds
resulting from intramolecular cyclizations.6,7 We have
been able to establish that under the same C–N coupling
conditions [Pd(OAc)2 (3mol%), Cs2CO3 (1.4equiv), and
BINAP (4mol%) in toluene] it was possible to obtain
either primary amines8 or diarylamines7,8 in the benzene
ring of the benzo[b]thiophene moiety.
high yields (50%-quantitative yield) using either the bro-
mo compound 2 or the amino derivative 3 as coupling
components.8 The latter was prepared from compound
2 using also a C–N palladium-catalyzed cross-coupling
with benzophenone imine, followed by acidic hydrolysis
in a 60% overall yield (Scheme 1).8
A screening of antibacterial activities using two Gram
negative (Escherichia coli and Pseudomonas aeruginosa)
and two Gram positive bacteria (Bacillus subtilis and
B. cereus) and antifungal using Candida albicans was
performed for the diarylamines 4–8 and for their
benzo[b]thiophene precursors 1–3, to determine the
importance of the diarylamine skeleton in this series.
The minimal inhibitory concentration (MIC in lg/mL)
was determined (Table 1) using an adaptation of agar
streak dilution method based on radial diffusion.9,10 Sus-
pensions of the microorganism were prepared to contain
approximately 108 cfu/mL and the plates were inocu-
lated. A stock solution of the synthesized compound
(1000lg/mL) in DMSO was prepared and graded dilu-
tions of the tested compounds were incorporated in a
cavity (depth 3mm, diameter 4mm) made in the center
of the petridish (nutrient agar for antibacterial activity
and sabouraud dextrose agar medium for antifungal
activity). The plates were incubated at 37°C (bacteria)
or 30°C (fungi) for 24h in duplicate. In the same condi-
tions different concentrated solutions of Ampicillin,
Chloramphenicol (antibacterial), and Cyclohexamide
Herein, we describe the structure–activity relationship of
a novel series of diarylamines as antimicrobial agents.
Differently substituted diarylamines derivatives of
2,3,5-trimethylbenzo[b]thiophene 1 were obtained by
C–N palladium-catalyzed cross-couplings in good to
Keywords: Diarylamines; Antimicrobial activity; SAR; Benzothio-
phenes.
*
Corresponding author. Tel.: +351 273 303219; fax: +351 273
325405; e-mail: iferreira@ipb.pt
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.09.038

5832
I. C. F. R. Ferreira et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5831–5833
S
1
Br2, ether
2
1
2
R
R
i
+
1
S
R
NH
2
N
H
S
R
Br
2
4, R1 = R2 = H, 17 h, 50%
5, R1 = H, R2 = OMe, 21h, 60%
6, R1 = R2 = OMe, 17 h, 70%
1. HN=C(Ph)2
i
2. ii
R4
3
R
3
4
i
R
R
+
N
S
H N
2
Br
H
S
3, 22 h, 60%
7, R3= F, R4= H, 20 h, 90%
8, R3 = H, R4 = F, 21 h, quantitative yield
Scheme 1. Synthesis of diarylamines and their precursors in the 2,3,5-trimethylbenzo[b]thiophene series.8 Reagents and conditions: (i) Pd(OAc)2
(3mol%), Cs2CO3 (1.4equiv), BINAP (4mol%), dry toluene, 100°C, Ar; (ii) HCl 2M/THF.
Table 1. Antimicrobial activity of the synthesized compounds
Table 1 (continued)
Compounds Microorganism
MIC (lg/mL)
(zone of inhibition)
(mm)
Compounds
Microorganism
MIC (lg/mL)
(zone of inhibition)
(mm)
1
2
3
B. cereus (CECT 148)
B. subtilis (CECT 498)
E. coli (CECT 101)
Not active
Not active
Not active
E. coli
Not active
Not active
3.13 (13)
12.5 (10)
6.25 (15)
6.25 (12)
P. aeruginosa
B. cereus
B. subtilis
E. coli
Ampicillin
P. aeruginosa (CECT 108) Not active
C. albicans (CECT 1394)
Not active
P. aeruginosa
B. cereus
1000 (12)
500 (18)
B. subtilis
E. coli
Chloramphenicol
Cyclohexamide
B. cereus
3.13 (8)
12.5 (5)
Not active
Not active
Not active
C. albicans
P. aeruginosa
C. albicans
CECT—Spanish type culture collection of Valencia University.
B. cereus
500 (10)
500 (17)
B. subtilis
E. coli
(antifungal) in DMSO were used as standards. The MIC
was considered to be the lowest concentration of the
tested compound, which inhibits growth of bacteria or
fungi on the plate. The diameters of the inhibition zones
corresponding to the MICs are presented in Table 1.
Positive control using only inoculation and negative
control using only DMSO in the cavity were carried
out. From analysis of Table 1 we can conclude that
Not active
Not active
1000 (13)
P. aeruginosa
C. albicans
4
5
B. cereus
B. subtilis
E. coli
200 (18)
200 (8)
200 (4)
200 (16)
P. aeruginosa
B. cereus
100 (4)
50 (19)
1000 (4)
1000 (8)
12.5 (6)
2,3,5-trimethylbenzo[b]thiophene
1 is not effective
B. subtilis
E. coli
against none of the microorganisms tested. Insertion
of a bromine atom in the 6-position (compound 2)
shows activity only for Gram positive bacteria but with
high MICs. The substitution of the halogen atom by
an amino group (compound 3) lowers the MIC for
B. cereus and turns it active against C. albicans but still
with high MICs. Diarylamine 4, without substituents in
the phenyl ring, shows activity either against Gram
negative or Gram positive bacteria (MIC 200lg/mL).
The introduction of a OMe group in the para position
(compound 5) lowers the MIC to 100lg/mL for
B. cereus and to 50lg/mL for B. subtilis but increases
it to 1000lg/mL for Gram negative bacteria. The intro-
duction of an additional OMe group (compound
6) makes the compound inactive against E. coli.
P. aeruginosa
C. albicans
6
B. cereus
B. subtilis
E. coli
200 (24)
200 (10)
Not active
1000 (10)
25 (8)
P. aeruginosa
C. albicans
7
8
B. cereus
B. subtilis
E. coli
200 (12)
200 (10)
Not active
Not active
P. aeruginosa
B. cereus
B. subtilis
200 (9)
50 (8)

I. C. F. R. Ferreira et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5831–5833
5833
Compounds with a F atom in the phenyl ring (7 and 8)
show selectivity against Gram positive bacteria. The
presence of the F atom in the ortho position (8) instead
of in the para position (7) lowers the MIC for B. subtilis
from 200 to 50lg/mL. Although the MICs were higher
than the values obtained for ampicillin it is the first time
that this type of compounds was submitted to antibacte-
rial studies. Diarylamines 5 and 6 were also submitted to
antifungi preliminary studies using Candida albicans and
the MIC obtained for compound 5 is comparable to the
obtained for cyclohexamide (antifungal).
CIMO-ESA Braganc¸a, Calouste Gulbenkian Founda-
tion for the financial support through the Research
Incitement programme.
References and notes
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J. Org. Chem. 2003, 68, 5992–5999.
In conclusion, a new diarylamine series as antimicrobial
agents based on the 2,3,5-trimethylbenzo[b]thiophene
moiety has been developed. The diarylamine skeleton
and the different substituents (H, 1 OMe, 2 OMe, or F)
proved to be important for activity, changing selectivi-
ties and MICs, when compared with the functionalized
(Br and NH2) benzo[b]thiophene precursors.
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Acknowledgements
10. Rameshkumar, N.; Ashokkumar, M.; Subramanian, E.
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Foundation for Science and Technology (Portugal) for
financial support through CQ-Univ. Minho and