Synthesis and antimicrobial activity of novel tetrabromo-bis(substituted benzyl)cycloalkanones

Article abstract of DOI:10.2298/JSC110408005M

A series of novel tetrabromo-bis(substituted benzyl)cycloalkanones have been synthesized through a rapid, simple, and efficient methodology in an excellent isolated yield and characterized via IR, NMR (1H-, 13C-NMR, DEPT 135, DEPT 90) and mass spectrometry. All compounds were assayed for their in vitro antimicrobial activities against eight bacteria and five fungi. They showed stronger antibacterial than antifungal activities. Compounds 4c, 4d and 4i, containing a methoxy or chloro substituent on the para or meta position of the phenyl ring, showed comparable minimum inhibitory concentration (MIC) values to those of the standard antibiotics streptomycin and tetracycline. Among all the tested compounds, 4i exhibited good to moderate antifungal activity against all the tested fungal strains used.

Full text of DOI:10.2298/JSC110408005M

J. Serb. Chem. Soc. 77 (6) 717–723 (2012)
UDC 547.583.1+547.21+5-42.913:615.28–188
JSCS–4302
Original scientific paper
Synthesis and antimicrobial activity of novel
tetrabromo-bis(substituted benzyl)cycloalkanones
1
*
2,3**
A. F. M. MOTIUR RAHMAN , MOHAMMAD SAYED ALAM
and ADNAN A. KADI1
1Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University,
Riyadh 11451, Saudi Arabia, 2Division of Bioscience, Dongguk University,
Geyonju 780–714, Republic of Korea and 3Department of Chemistry,
Jagannath Univeristy, Dhaka 1100, Bangladesh
(Received 8 April 2011, revised 6 January 2012)
Abstract: A series of novel tetrabromo-bis(substituted benzyl)cycloalkanones
have been synthesized through a rapid, simple, and efficient methodology in an
excellent isolated yield and characterized via IR, NMR (1H-, 13C-NMR, DEPT
135, DEPT 90) and mass spectrometry. All compounds were assayed for their
in vitro antimicrobial activities against eight bacteria and five fungi. They showed
stronger antibacterial than antifungal activities. Compounds 4c, 4d and 4i, con-
taining a methoxy or chloro substituent on the para or meta position of the phe-
nyl ring, showed comparable minimum inhibitory concentration (MIC) values
to those of the standard antibiotics streptomycin and tetracycline. Among all
the tested compounds, 4i exhibited good to moderate antifungal activity against
all the tested fungal strains used.
Keywords: antibacterial; antifungal; tetrabromo-bis-(substituted benzyl)cyclo-
alkanones.
INTRODUCTION
In the past few decades, the incidence of serious antimicrobial infection has
1
increased dramatically due to the problem of multi-drug resistant microorga-
nisms that has reached alarming levels around the world. For the treatment of
microbial infections, the synthesis of new anti-infectious compounds has become
a critical need. The organohalogen compounds have received much attention due
2–4
to their important use in the field of medicinal and agro-chemistry.
A survey
of the literature also revealed that different kinds of bis(substituted benzyli-
dene)cycloalkanones, including organohalogen compounds, have been used for
the treatment of the central nervous system, cardiovascular and obesity diseases,
*,** Corresponding authors. E-mail: *afmrahman@ksu.edu.sa; **alam_sms@yahoo.com
doi: 10.2298/JSC110408005M
717
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RAHMAN, ALAM and KADI
5
5
6,7
and also showed antibacterial, antifungal, anti-angiogenic, quinine reductase
8
9,10
11
inducer, cytotoxic,
and cholesterol-lowering activity.
In 1980, Hoeve and Wynberg prepared tetrabromo-di-m-anisalcyclopen-
12
tanone from 2,5-dibenzylidenecycloalkanones using bromine in chloroform in a
13
47 % yield. Later, in 1988, Desiraju and Kishan used the same starting material
and obtained dibromo derivatives in which the bromine addition occurred on the
methylene carbon of 2,5-dibenzylidenecycloalkanones. In the present study, we
have synthesized and performed antimicrobial evaluation of a series of tetra-
bromo-bis(substituted benzyl)cycloalkanone analogues (4a–j). The structures of
1
13
the new compounds were established by IR, NMR ( H-, C-, DEPT 135 and
DEPT 90) and mass spectrometry.
EXPERIMENTAL
Chemistry
General. Melting points were recorded on a Fisher–Jones melting point apparatus and
are uncorrected. IR spectra were obtained on an FTIR-300E (Jasco Corporation, Tokyo, Ja-
pan) in KBr disc. 1H-NMR (250 MHz) and 13C-NMR (62.5 MHz) spectra were recorded on a
Bruker 250 spectrometer (Bruker, USA) in CDCl3 with TMS as an internal standard. ESI-
-MS were measured on an LCQ advantage-trap mass spectrometer (Thermo Finnigan, San
Jose, CA, USA).
Synthesis of bis(substituted benzyl)cycloalkanones 3a–j
The bis(substituted-benzyl)cycloalkanones were obtained by the cross-aldol condensa-
tion of substituted aldehydes with cycloalkanones in the presence of sodium acetate–acetic
acid, Scheme 1.14
Synthesis of tetrabromo-bis(substituted benzyl)cycloalkanones 4a–j
The tetrabromo-bis(substituted benzyl)cycloalkanones (4a–j) were prepared by adding a
Br2 solution (2 mmol) in dioxane (10 mL) dropwise over 10 min to a stirred solution of the
respective bis(substituted benzyl)cycloalkanone (3a–j, 1 mmol) in dioxane (20 mL). The stirring
was continued for 3–4 h. The completion of the reaction was monitored by thin layer chroma-
tography (TLC). The solvent was evaporated and the dry residue subjected to column chroma-
tography (n-hexane/dichloromethane, 1:1), which afforded the respective compound 4a–j as a
white solid.
Data of synthesized compounds are given in Supplementary material.
Biology
Antibacterial screening. The in vitro antibacterial activity of the synthesized compounds
4a–j were evaluated against eight bacterial strains, i.e., Staphylococcus aureus (KCTC 1916),
Bacillus subtilis (ATCC 6633), Listeria monocytogenes (ATCC 19166), Salmonella enteritidis
(KCCM 12021), Pseudomonas aeruginosa (KCTC 2004), Enterobacter aerogenes (KCTC
2190), Salmonella typhimurium (KCTC 2515) and Escherichia coli (ATCC 8739). All the
bacterial strains were obtained from the Korea Food and Drug Administration (KFDA),
Daegu, South Korea. Cultures of each bacterial strain were maintained on Luria broth (LB)
agar medium at 4 °C.
The minimum inhibitory concentrations (MICs) of compounds 4a–j were tested by the
two-fold serial dilution method.15 The test compounds 4a–j were incorporated into Luria–
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SYNTHESIS AND ANTIMICROBIAL ACTIVITY OF ORGANOHALOGENS
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–Broth medium to obtain a concentration of 1,000 μg ml-1 and serially diluted to a concen-
tration in the range from 500 to 7.8 μg ml-1. A standardized suspension (10 μl) of each of the
tested organism (108 CFU ml-1) was transferred to each tube and incubated at 37 °C for 24 h.
Control tubes containing only bacterial suspensions were also incubated at 37 °C for 24 h. The
lowest concentrations of the test samples that did not show any growth of the tested organism
after macroscopic evaluation were determined as the MICs.
Compound
n
R1
R2
3a, 4a
3b, 4b
3c, 4c
3d, 4d
3e, 4e
3f, 4f
3g, 4g
3h, 4h
3i, 4i
1
1
1
1
1
2
2
2
2
2
H
H
Cl
H
H
H
H
H
H
H
H
CH3
H
OCH3
F
H
CH3
F
OCH3
Br
3j, 4j
Scheme 1. Reagents and conditions: a) AcONa, AcOH, 3–8 h, 120 °C;
b) Br2–dioxane, 3–4 h, room temperature.
Antifungal screening. The in vitro antifungal activities of the synthesized compounds 4a–j
were evaluated against various fungal species, i.e., Botrytis cinerea (KACC 40573), Rhizoc-
tonia solani (KACC 40111), Fusarium oxysporum (KACC 41083), Sclerotonia sclerotiorum
(KACC 41065) and Phytophthora capsici (KACC 40157). The fungal cultures were obtained
from the Korean Agricultural Culture Collection (KACC). Cultures of each fungal species
were maintained on potato dextrose agar (PDA) slants and stored at 4 °C.
Potato dextrose agar (PDA) medium was used as the basal medium for the antifungal
activity assay, which was performed by disc diffusion method.16 Sterile Whatman No. 1 paper
discs (6 mm diameter) were pierced in agar plates, equidistant and near the border, where 10
μl of compounds 4a–j (200 μg disc-1) were applied. A disc of fungal inoculum 6 mm in dia-
meter was removed from the pre-grown cultures of all the tested fungal strains and placed
upside down in the center of the Petri dishes. The plates were incubated at 28 °C for 5–7 days,
the time in which the growth of the control would have reached the edges of the plates. Discs
with CHCl3 and nystatin were used as negative and positive controls, respectively. The in-
hibitory activity was measured (in mm) as the diameter of the observed inhibition zones.
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RAHMAN, ALAM and KADI
The minimum inhibitory concentration (MIC) was determined by the two-fold dilution
method,15 against B. cinerea, R. solani, F. oxysporum, S. sclerotiorum and P. capsici. Samples
were dissolved in CHCl3 accordingly with their respective known weight. These solutions
were serially diluted with CHCl3 and were added to PDA to final concentrations of 500, 250
125 and 62.5, 31.2, 15.6 and 7.8 μg ml-1, respectively. A 10 μl spore suspension of each test
strain was inoculated in test tubes in PDA medium and incubated for 2–7 days at 28 °C. The
control tubes containing PDA medium were inoculated only with fungal suspension. The mi-
nimum concentration at which no visible growth was observed was defined as the MIC, which
was expressed in μg ml-1.
RESULTS AND DISCUSSION
Chemistry
The synthetic route to the tetrabromo-bis(substituted benzyl)cycloalkanones
(4a–j) are outlined in Scheme 1. The compounds 4a–j were obtained from the
intermediate compounds 3a–j, respectively, in excellent yields (70–93 %). The
structures of the newly synthesized compounds were established based on their
1
13
IR, H-NMR, C-NMR, DEPT 135, DEPT 90 and mass spectral data. IR spectra
–1
of compounds 4a–j showed an absorption peak in the 1660–1681 cm region,
indicating the presence of a carbonyl (>C=O) group. In the H-NMR spectra, the
1
characteristic benzylic protons of compounds 4a–j appeared as a singlet at 5.50–
–6.03 ppm. The aliphatic protons of the cyclopentanone ring of compounds 4a–e
appeared as two AB-quartets at around 3.28–3.32 and 2.37–2.42 ppm equivalent
to two protons, respectively. On the other hand, the six protons of the cyclohexa-
none ring of compounds 4f–j were observed as triplets of a doublet at 2.99–3.07
ppm and two multiplets at around 2.14–2.46 and 1.90–2.03 ppm equivalent to
two, three and one proton(s), respectively. The aromatic protons of the phenyl
groups of compounds 4a–j appeared in usual way. The methyl protons of com-
pounds 4b and 4g were observed as a singlet at 2.35–2.36 ppm, whereas the me-
thoxy protons of compounds 4d and 4i appeared as a singlet at 3.81–3.82 ppm. In
13
the C-NMR spectra of compounds 4a–j, the characteristics carbonyl carbons
and benzylic carbons appeared at 197.77–198.57 ppm and 51.40–55.30 ppm,
respectively. In addition, the α-carbons were observed in the range of 63.95–
–67.98 ppm. In summary, all the newly synthesized compounds exhibited satis-
factory spectral data consistent with the proposed structures. In addition, electron
spray ionization mass spectra (ESI-MS) of 4a–j showed a metastable ion peak as
79
+
[M( Br)+H] (100 %) in the positive mode along with other isotopic peaks.
Antibacterial activity
The in vitro antibacterial activities of the novel compounds 4a–j were as-
sayed against three Gram-positive bacteria viz. S. aureus, B. subtilis and L. mo-
nocytogenes and five Gram-negative bacteria, viz. S. enteritidis, P. aeruginosa,
E. aerogenes, S. typhimurium and E. coli. The minimum inhibitory concentration
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SYNTHESIS AND ANTIMICROBIAL ACTIVITY OF ORGANOHALOGENS
721
–1
15
(MIC / μg ml ) was determined by the serial dilution method for all the com-
pounds and compared with the controls (streptomycin and tetracycline). The MIC
values of the tested compounds are presented in Table I, from which it can be
seen that all the tetrabromo-bis-benzylcycloalkanone analogues (4a–j) showed
antibacterial activity against both the Gram-positive and Gram-negative bacteria.
Among the tested compounds, compound 4i exhibited the highest activity against
both Gram-positive and Gram-negative strains, followed by compounds 4c and
4d. The MIC values of compounds 4i, 4c and 4d are almost comparable to those
of the standard drugs used in the present study against all bacterial strains.
TABLE I. Antibacterial activity (MIC / μg ml-1) of compounds 4a–j
S. au- B. sub- L. mono- S. enteri- P. aeru- E. aero- S. typhi-
Compound
E. coli
reus
tilis cytogenes tidis
ginosa
genes murium
4a
4b
4c
4d
4e
4f
4g
4h
4i
125
62.5
31.2
31.2
31.2
62.5
62.5
31.2
31.2
62.5
125
62.5
15.6
15.6
31.2
125
62.5
62.5
15.6
62.5
15.6
15.6
125
31.2
31.2
32.2
31.2
62.5
62.5
62.5
31.2
31.2
31.2
31.2
125
62.5
15.6
15.6
62.5
125
62.5
62.5
15.6
31.2
31.2
15.6
125
125
125
62.5
15.6
15.6
31.2
62.5
125
62.5
15.6
31.2
15.6
15.6
62.5
62.5
31.2
31.2
31.2
125
62.5
31.2
15.6
31.2
31.2
15.6
62.5
125
15.6
31.2
31.2
125
32.2
31.2
15.6
31.2
15.6
15.6
31.2
31.2
62.5
62.5
62.5
62.5
31.2
62.5
31.2
31.2
4j
Streptomycin 31.2
Tetracycline 15.6
Structure–activity relationships (SAR) may be explained briefly as follows:
the ring size of cycloalkanones has no strong effect on the antibacterial activity,
whereas the unsubstituted compounds (4a and 4f) showed lower activity than the
substituted compounds (4b–d and 4g–j). Introduction of hydrophilic substituents
(halogen, OMe) at the R or R position resulted in better activity than the hydro-
2
1
phobic substituent (CH ). Among the compounds containing halogen substitution
3
at the phenyl ring, the chlorine analogue (4c) showed better antibacterial activity
than the fluorine or bromine analogues (4e, 4h and 4j).
Antifungal activity
The newly synthesized novel 4a–j were evaluated for their in vitro antifungal
activities against five fungal strains, i.e., B. cinerea, R. solani, F. oxysporum, S.
sclerotiorum and P. capsici by disc diffusion methods. The inhibition zones of the
–1
synthesized compounds were measured at a dose of 200 μg disc and nystatin, a
–1
positive control, were evaluated at a dose of 30 μg disc . As presented in Table
II, among the all tetrabromo-bis-benzylcycloalkanone derivatives (4a–j) tested, 5
compounds inhibited the growth of some fungal strains. In detail, compound 4i
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RAHMAN, ALAM and KADI
showed good to moderate activity against all the tested fungal strains, while
compounds 4d and 4j exhibited moderate activity against R. solani and F. oxy-
sporum. Compounds 4f and 4h demonstrated moderate activity against one bac-
terial strain only. Compounds 4a–c, 4e and 4g showed no activity against all the
tested fungal strains. Therefore, the minimum inhibitory concentrations (MICs) of
compound only 4i against B. cinerea, R. solani, F. oxysporum, S. sclerotiorum,
and P. capsici were determined and found MIC values of 125, 31.2, 31.2, 125
–1
and 250 µg ml , respectively, were found (Table III).
TABLE II. Antifungal profile of compounds 4a–j in terms of the inhibition zone in mm (data
are means ± SD for at least three experiments. Concentrations of compounds 4a–j and netil-
mycin were 200 and 30 μg disc-1, respectively)
Compound
B. cinerea R. solani
F. oxysporum
S. sclerotiorum
P. capsici
4d
4f
4h
4i
–
–
–
08±1.1
09±1.1
–
19±1.0
11±1.1
25±1.2
10±1.1
–
07±1.1
19±1.1
11±1.2
25±1.1
–
–
–
–
–
–
07±1.2
–
23±1.1
08±1.1
–
27±1.1
07±1.0
–
33±1.0
4j
Nystatin
TABLE III. Antifungal activity (MIC / μg ml-1) of compound 4i
Compound
B. cinerea R. solani
F. oxysporum
S. sclerotiorum
P. capsici
4i
125
15.6
31.2
15.6
31.2
15.6
125
31.2
250
30.2
Nystatin
CONCLUSIONS
In this paper, an efficient synthesis of novel tetrabromo-bis(substituted
benzyl)cycloalkanone derivatives and their antimicrobial activities against eight
bacterial and five fungal strains were presented. All the tetrabromo-bis(substi-
tuted benzyl)cycloalkanone analogues showed potent antibacterial activity com-
pared to their antifungal activity. Compounds 4i, 4c and 4d, having a methoxy- or
chloro-substituent on the para or meta position of the phenyl ring exhibited
strong activity against all the tested bacterial strains. Hydrophilic substitution on
the phenyl ring resulted in a better antibacterial activity than that of hydrophobic
substitution. Among all the tested compounds, 4i exhibited good antifungal ac-
tivity against R. solani and F. oxysporum. The potential antimicrobial activity of
compounds 4i, 4c and 4d would be helpful in synthesis of a huge number of
tetrabromo-bis(substituted benzyl)cycloalkanone analogues for extensive anti-
microbial studies, which could be used to develop more suitable drug candidates.
SUPPLEMENTARY MATERIAL
Аnalytical and spectral data of synthesized compounds are available electronically from
http://www.shd.org.rs/JSCS/, or from the corresponding author on request.
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SYNTHESIS AND ANTIMICROBIAL ACTIVITY OF ORGANOHALOGENS
723
И З В О Д
СИНТЕЗА И АНТИМИКРОБНА АКТИВНОСТ НОВИХ
ТЕТРАБРОМ-БИС(СУПСТИТУИСАНИХ БЕНЗИЛ)ЦИКЛОАЛКАНОНА
A. F. M. MOTIUR RAHMAN1, MOHAMMAD SAYED ALAM,2,3 и ADNAN A. KADI1
1Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, Riyadh 11451, Saudi
Arabia, 2Division of Bioscience, Dongguk University, Geyonju 780–714, Republic of Korea и
3Department of Chemistry, Jagannath Univeristy, Dhaka 1100, Bangladesh
Синтетисана је серија нових тетрабром-бис(супституисаних бензил)циклоалканона,
брзим и ефикасним поступком. Производи су добијени у високом приносу и окарактерисани
су спектроскопским методама (ИЦ, NMR (1H-, 13C-NMR, DEPT 135, DEPT 90)) и масеном
спектрометријом. Испитана је in vitro антимикробна активност синтетисаних једињења
према осам врста бактерија и 5 врста гљива. Тестирана једињења показују добру антибак-
теријску и антифунгалну активност. Једињења 4c, 4d and 4i која садрже метокси- и хлор-
-супституенте у para и meta положају на фенил групи имају вредности MIC које су блиске
вредностима које дају стандардни антибиотици стрептомицин и тетрациклин. Од свих испи-
тиваних једињења, дериват 4i показује добру до умерену активност према све четири врсте
гљива.
(Примљено 8. априла 2011, ревидирано 6. јануара 2012)
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