Synthesis and evaluation of novel 1,3,4-oxadiazole derivatives of marine bromopyrrole alkaloids as antimicrobial agent

Article abstract of DOI:10.1016/j.bmcl.2012.08.061

In an attempt to identify new potential lead as antimicrobial agent, twenty hybrids of marine bromopyrrole alkaloids with 1,3,4-oxadiazole were designed based on molecular hybridization technique and synthesized. Synthesized molecules were evaluated for t

Full text of DOI:10.1016/j.bmcl.2012.08.061

Bioorganic & Medicinal Chemistry Letters 22 (2012) 6429–6432
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and evaluation of novel 1,3,4-oxadiazole derivatives of marine
bromopyrrole alkaloids as antimicrobial agent
⇑
Rajesh A. Rane , Shweta D. Gutte, Niteshkumar U. Sahu
S.P.P. School of Pharmacy and Technology Management, NMIMS University, Vile Parle, Mumbai 400056, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 April 2012
Revised 18 July 2012
Accepted 16 August 2012
Available online 23 August 2012
In an attempt to identify new potential lead as antimicrobial agent, twenty hybrids of marine bromopyr-
role alkaloids with 1,3,4-oxadiazole were designed based on molecular hybridization technique and syn-
thesized. Synthesized molecules were evaluated for their antibacterial, antifungal and antitubercular
activities. Hybrids 5d, 5i, 5j and 5k exhibited equivalent antibacterial activity (MIC of 1.56
pared with standard drug ciprofloxacin against Staphylococcus aureus andEscherichia coli. Equal antifungal
activity (MIC of 1.56 g/mL) was shown by of hybrids 5j, 5k and 7d compared with standard Amphoter-
icin-B. The inhibition of Mycobacterium tuberculosis at concentrations as low as 1.6 and 1.5 g/mL by
lg/mL) com-
l
Keywords:
l
Bromopyrrole alkaloids
1,3,4-Oxadiazole derivatives
Antitubercular agent
compounds 5f and 7d respectively indicates that these compounds can act as leads for development of
newer anti-TB compounds.
Ó 2012 Elsevier Ltd. All rights reserved.
1,3,4-Oxadiazole, a privileged structure, endows its derivatives
with broad and potent biological functions.1 These includes anti-
inflammatory,2 hypoglycemic,3 antianxiety,4 antidepressant activi-
ties.5 Recently derivatives of 1,3,4-oxadiazole have been reported
for their antiproliferative,6 antifungal,7,8 antibacterial9 and antitu-
bercular activities.10 Molecular modeling and pharmacokinetic
studies have demonstrated that the introduction of 1,3,4-oxadiaz-
ole ring to the inhibitors can change their polarity, flexibility as
well as metabolic stability, and 1,3,4-oxadiazole scaffold can also
act as acceptors of hydrogen bonds formation, which make it pos-
sible to be used as a isosteric substituent for amide or ester
groups.11 We therefore are interested in exploring the biological
activity of such molecules through structural modifications.
Heterocycles containing pyrrole ring system are found to exhi-
bit wide spectrum of biological activities. Our interest in haloge-
nated pyrrole derivatives led to synthesis and antimicrobial
evaluation of few analogues of pyolueterins.12 In continuation to
this we found that bromopyrrole alkaloids; a family of marine alka-
loids represents a fascinating example of the large variety of sec-
ondary metabolites formed by marine sponges. Most of these
alkaloids are defined by the signature 4,5-dibromopyrrole ring fall
under the oroidin class of alkaloids, defined by the signature
bromopyrrole carboxamide.13 Many of these compounds are re-
ported to have intriguing biochemical activities, such as blocking
reported to be antitubercular, antibacterial, antifungal and inhibi-
tors of enoyl-ACP reductase13–15 (Fig. 1a).
In view of the above-mentioned findings, the purpose of the
present work was to design and synthesize novel series of
bromopyrrole alkaloids derivatives by molecular hybridization of
4,5-dibromopyrrole scaffold with pharmacologically privileged
scaffold 1,3,4-oxadiazole. Further we want to evaluate effect of
replacement of amide and ester linkage present in most of these
alkaloids with its bioisoter, 1,3,4-oxadiazole ring. Another objec-
tive of the study was to evaluated effect of various substituent at-
tached to 1,3,4-oxadiazole ring like SH group, S-alkyl/aryl,
substituted aromatic or heteroaromatic ring on antimicrobial and
antitubercular activity (Fig. 1b).
In the present work, proposed hybrids were synthesized utiliz-
ing the reaction sequence as shown in Scheme 1. Trichloroacetyla-
tion of 1H-pyrrole using equimolar quantity of trichloroacetyl
chloride gave an excellent yield of 2-trichloroacetylpyrrole 2.16
Bromination of 2 using 2 equiv of bromine in chloroform gave
4,5-dibromo-2-trichloroacetyl-1H-pyrrole 3 in excellent yield.17
4,5-Dibromo-2-trichloroacetyl-1H-pyrrole 3 on stirring with excess
of hydrazine hydrate at room temperature gave its acid hydrazide
derivative 4 in good yield. Compound 4 on condensation with
equimolar quantities different aromatic and heteroaromatic acids
in presence of excess of phosphorus oxychloride at reflux condition
gave 2-(4,5-dibromo-1H-pyrrol-2-yl)-5-aryl-1,3,4-oxadiazoles 5.
a-adrenoceptors, antihistamine, antagonist of serotonergic recep-
tors, activating actomyosin ATPase, inhibiting kinase activity,
5-(4,5-Dibromo-1H-pyrrol-2-yl)-1,3,4-oxadiazole-2-thiol
6
was
antineoplastic activity.12–14 Moreover these natural products are
synthesized by reaction of 4 with carbon disulfide and alcoholic
potassium hydroxide at reflux condition. Compound 6 was S-
alkylated by reaction with different aryl/alkyl halides in alcoholic
sodium hydraxide to give S-alkylated derivatives 7 (Scheme 1).
⇑
Corresponding author. Tel.: +91 9224294690; fax: +91 22 26114512.
E-mail address: rajeshrane_uict@yahoo.co.in (R.A. Rane).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.08.061

6430
R. A. Rane et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6429–6432
Br
Br
HN
N
N
NH
O
HO
OH
O
Br
N
H
N
H
O
O
O
O
O
Br
Br
Br
Br
Br
H
N
X
O
N
H
O
Br
N
H
N
H
O
O
H
Br
Br
H
N
N
NH2
N
N
O
H
X = O or NH
Figure 1a. Reported bromopyrrole alkaloids.
Br
inhibitory concentrations (MICs) were determined by broth dilu-
tion technique.18,19 The nutrient broth, which contained logarith-
mic serially two fold diluted amount of test compound and
controls were inoculated with approximately 5 Â 105 CFU/mL of ac-
tively dividing bacteria cells. The cultures were incubated for 24 h
at 37 °C and the growth was monitored visually and spectrophoto-
metrically. Ciprofloxacin was used as a standard drug. The lowest
concentration (highest dilution) required to arrest the growth of
bacteria was regarded as minimum inhibitory concentration
(MIC). The minimum inhibitory concentrations are given in Table 1.
The newly prepared compounds were screened for their anti-
fungal activity against Candida albicans in DMSO by agar dilution
method20,21 The nutrient broth, which contained logarithmic seri-
ally twofold diluted amount of test compound and controls inocu-
lated with approximately 1.6 Â 104–6 Â 104 CFU/mL, was used.
The cultures were incubated for 48 h at 35 °C and the growth
was monitored. The lowest concentration (highest dilution) re-
quired to arrest the growth of fungus was regarded as minimum
inhibitory concentration (MIC). The fungal activity of each com-
pound was compared with Amphotericin-B as standard drug. The
fungal zone of inhibition and minimum inhibitory concentration
values are given in Table 1.
N
N
x
A
Br
B
O
Ar/Het
N
R
H
O
Br
Br
R
O
A
B
N
N
N
H
X = O- or NH-
R = SH or S-alkyl or aryl or heteroaromatic ring or NH-aryl
Figure 1b. Design of novel 2-(4,5-dibromopyrrol-2-yl)-5-substituted-1,3,4-oxadi-
azoles using molecular hybridization approach.
The synthesized compounds were screened against Mycobacte-
rium tuberculosis H37Rv in order to determine the minimum inhibi-
tory concentration (MIC) with Resazurin microtiter assay (REMA).22
Homogenous mycobacterial (H37Rv) culture suspension was seeded
The spectral data (IR, 1H NMR and MS) of all synthesized com-
pounds were in agreement with the proposed structures. The MS of
all the compounds exhibited the [M+Å] as molecular ion peak, con-
firming the molecular weight. Bromination of pyrrole ring was
confirmed by the presence of [M+2] peak in mass spectra. 1H
NMR spectra of these hybrids showed singlet peak of pyrrole-NH
proton in the range of 13.5–13.4 ppm. Thiol group of 6 showed
NMR peak at 3.8 ppm. The proton of aryl substitutions attached
to oxadiazole ring resonated in between 8.2 and 6.1 ppm. 5-(4,5-
in microtitre plates at density of 105 cells per well in 100
lL of the
Middlebrook 7H9 broth (Difco laboratories, Detroit, MI, USA) and
the test compounds were serially diluted directly on the plate. The
control received equivalent amount of DMSO. The plates were incu-
bated at 37 °C for 7 days. Freshly prepared resazurin dye (0.02%)
was added and plates were again incubated for 48 h. Isoniazid
was used as the reference drug. MIC is the lowest concentration at
which complete inhibition was observed and was determined by vi-
sual inspection (colour change from blue to pink) (Table 1).
The investigation of antimicrobial screening revealed that syn-
thesized hybrids showed some promising antibacterial, antituber-
cular and antifungal activity. Antibacterial activities of these
hybrids exhibited broader spectrum of activities against both
gram-positive and gram-negative bacteria. Hybrids 5c, 5d, 5i, 5j
Dibromo-1-methyl-1H-pyrrol-2-yl)-1,3,4-oxadiazole-2-thiol
6
showed peculiar peaks at 2751 cmÀ1 of –SH group. The alkylated
product, 5-(4,5-dibromo-1-methyl-1H-pyrrol-2-yl)-1,3,4-oxadiaz-
ole-2-methylthiol did not have the peak of –SH group at 2751 cm
À1. Also the product showed peak at 711.9 cmÀ1 of S-alkyl group.
The newly prepared compounds were screened for their antibac-
terial activity against Escherichia coli (ATCC-25922), Staphylococcus
aureus (ATCC-25923), Pseudomonas aeruginosa (ATCC-27853), and
Klebsiella pneumoniae (recultured) bacterial strains. Minimum
and 5k showed equivalent antibacterial activity (MIC of 1.56
lg/
mL) compared with standard ciprofloxacin against E. coli. Presence

R. A. Rane et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6429–6432
6431
Br
CCl3COCl
Br2/Chloroform
anhydrous ether
K2CO3
N
H
1
COCCl
3
N
H
Br
COCCl3
N
H
2
3
NH2NH2H2O
RT, 1 h
Br
Br
Br
Ethanol,reflux 12 hr
CS2/KOH
O
Br
R-COOH,
POCl3,reflux
N
H
R
O
Br
Br
CONHNH2
N
H
N
H
SH
4
N
N
6
N
5
N
5a - R = 2-hydroxyphenyl-
5b - R = phenyl acetic-
Alkyl/Aryl halide,NaOH
Ethanol,stir for 10 hr
5c - R = 4-amino phenyl-
5d - R = 4-chloro phenyl-
5e - R = 4-methoxy phenyl-
5f - R = 2,4- dichlorophenyl-
5g - R = 2-phenylethenyl
5h - R = 4-hydroxyphenyl-
5i - R = 5-flouro-2-chlorophenyl-
Br
O
Br
N
H
SR
5j - R = 4-nitrophenyl-
7
N
N
5k - R = 4,5-dibromo-1H-pyrrol-2-yl-
5l - R = phenyl-
5m - R = 2-methoxy-4-vinylphenol-
5n - R = pyridin-4-yl-
7a - R = methyl-
7b - R = ethyl-
7c - R = phenyl-
5o - R = 4H-chromen-3-yl-vinyl-
7d - R = acetophenone-
Scheme 1. Synthesis of 2-(4,5-dibromopyrrol-2-yl)-5-substituted-1,3,4-oxadiazoles.
Table 1
Antimicrobial test results: (MIC
l
g/mL)
S. No.
E. colia
S. aureusa
P. aeruginosaa
K. pneumoniaea
C. abilcansb
Antitubercular activityc
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
6
7a
7b
7c
3.125
3.125
1.56
1.56
3.125
12.5
12.5
12.5
1.56
1.56
1.56
1.56
3.125
3.125
1.56
3.125
6.25
3.125
12.5
1.56
1.56
1.56
6.25
1.56
1.56
1.56
12.5
12.5
12.5
12.5
12.5
1.56
12.5
12.5
6.25
6.25
6.25
6.25
6.25
6.25
6.25
6.25
6.25
6.25
12.5
12.5
12.5
12.5
25
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1.56
6.25
12.5
12.5
12.5
12.5
12.5
12.5
12.5
12.5
1.56
1.56
12.5
12.5
12.5
12.5
12.5
12.5
12.5
6.25
1.56
1.56
36.50
145.80
16.50
9.50
56.50
1.60
78.50
25.00
6.50
9.00
3.50
98.70
112.50
3.50
12.5
12.5
12.5
3.125
2.00
12.5
12.5
12.5
12.5
3.125
1.56
54.50
89.50
118.00
23.50
1.50
25
25
12.5
3.125
7d
StandardX
0.40
X = (Standard) ciprofloxacin for antibacterial study, Amphotericin-B for antifungal study and Isoniazid for antitubercular study.
a
MIC values are determined by broth dilution method (twofold dilution).
MIC values are determined by agar dilution method (twofold dilution).
MIC values are determined by Resazurin microtiter assay (REMA).
b
c
of electron donating groups like amino, chloro, flouro and with-
drawing groups like nitro on aromatic ring seem to be important
for activity against E. coli. For gram-positive S. aureus, hybrids 5a,
5d, 5i, 5j, 5k, 5m, 5n and 5o showed equivalent activity at MIC of
1.56 lg/mL compared with standard ciprofloxacin. However hybrid
6, 7a, 7b, 7c, 7d containing thiol group or S-alkyl/aryl groups are
found to be less active especially against S. aureus in comparison
with other derivatives. All compounds were found to be less active

6432
R. A. Rane et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6429–6432
against K. pneumonia while hybrids 5c, 5d, 5e, 5f, 5g, 5h, 5i, 5j, 5k, 5l
were found to be moderately active against P. aeruginosa (MIC of
6.25 lg/mL). Among the tested compounds best antifungal activity
was showed by three hybrids 5j, 5k, 7d (MIC of 1.56 lg/mL) equiv-
alent to standard Amphotericin-B.
References and notes
1. Omar, F. A.; Mehfouz, N. M.; Rahman, M. A. Eur. J. Med. Chem. 1996, 31, 819.
2. Palaska, E.; Sahin, G.; Kelicen, P.; Durlu, N. T.; Altinok, G. Farmaco 2002, 57, 101.
3. Ramalingum, T.; Sattur, P. B. Eur. J. Med. Chem. 1990, 25, 541.
4. Harfenist, M.; Heuser, D. J.; Joyner, C. T.; Batchelor, J. F.; White, H. L. J. Med.
Chem. 1996, 39, 1857.
5. Ergun, Y.; Orhan, F. O.; Ozer, U. G.; Gisi, G. Eur. J. Pharmacol. 2010, 630, 74.
6. Jin, L.; Chen, J.; Song, B.; Chen, Z.; Yang, S.; Li, Q.; Hu, D.; Xu, R. Bioorg. Med.
Chem. Lett. 2006, 16, 5036.
7. Liu, F.; Luo, X. Q.; Song, B. A.; Bhadury, P. S.; Yang, S.; Jin, L. H.; Xue, W.; Hu, D. Y.
Bioorg. Med. Chem. Lett. 2008, 16, 3632.
8. Chen, C. J.; Song, B. A.; Yang, S.; Xu, G. F.; Bhadury, P. S.; Jin, L. H.; Hu, D. Y.; Li, Q.
Z.; Liu, F.; Xue, W.; Lu, P.; Chen, Z. Bioorg. Med. Chem. Lett. 2007, 15, 3981.
9. Xu, W. M.; Han, F. F.; He, M.; Hu, D. Y.; He, J.; Yang, S.; Song, B. A. J. Agric. Food
Chem. 2012, 60, 1036.
Promising antitubercular activity was displayed by hybrids 5f,
5k, 5n, 5o and 7d at MIC of 1.60, 3.50, 3.50, 2.00 and 1.50 g/mL
l
respectively against M. tuberoculosis. Highest activity showed by
7d indicated that addition of S-aroyl substitution at 5-position of
1,3,4-oxadiazole ring leads increased antitubercular activity com-
pared to free SH, or S-alkyl or S-aryl substitutions. Replacement
of heterocyclic rings like 4,5-dibromopyrrole, pyridine-4-yl and
4H-chromen-3yl-vinyl instead of aryl ring at 5-position of 1,2,4-
oxadiazole resulted in to enhancement of antitubercular activity
10. Ahsan, M. J.; Samy, J. G.; Khalilullah, H.; Nomani, M. S.; Saraswat, P.; Gaur, R.;
Singh, A. Bioorg. Med. Chem. Lett. 2011, 21, 7246.
(5k, 5n, 5o MIC of 3.50, 3.50 and 2.00
l
g/mL).
11. Guimaraes, C. R. W.; Boger, D. L.; Jorgensen, W. L. J. Am. Chem. Soc. 2005, 127,
17377.
In summary, 20 new compounds based on the molecular
hybridization of bromopyrrole alkaloids with 1,3,4-oxadiazole
were synthesized and evaluated for their antimicrobial activity.
The synthesized hybrids showed promising activity against S. aur-
eus, E. coli and Mtb. Equal antifungal activity show by of hybrids 5j,
5k and 7d with standard Amphotericin-B indicated their potential
for further lead development. The inhibition of Mtb at concentra-
tions as low as 1.6 and 1.5 lg/mL by compounds 5f and 7d respec-
tively indicates that these compounds can act as leads for
development of newer anti-TB compounds.
12. Rane, R. A.; Telvekar, V. N. Bioorg. Med. Chem. Lett. 2010, 20, 5681.
13. Aiello, A.; Fattorusso, E.; Menna, M; Taglialatela-Scafati, O. Modern Alkaloids
Structure, Isolation, Synthesis and Biology; Wiley-VCH, 2008, p 271.
14. Tasdemir, D.; Topaloglu, B.; Perozzo, R.; Brun, R.; O’Neill, R.; Carballeira, N. M.;
Zhang, X.; Tonge, P. J.; Linden, A.; Ruedi, P. Bioorg. Med. Chem. 2007, 15, 6834.
15. Xiong, H.; Pang, H.; Fan, J.; Ge, F.; Yang, X.; Liu, Q.; Liao, X.; Xu, S. Br. J.
Pharmacol. 2010, 159, 909.
16. Bailey, D. M.; Johnson, R. E.; Salvador, J. U. J. Med. Chem. 1973, 16, 1298.
17. Linington, R. G.; Williams, D. E.; Tahir, A.; Soest, R.; Andersen, R. J. Org. Lett.
2003, 2735.
18. Schillaci, D.; Petruso, S.; Sciortino, V. Int. J. Antimicrob. Agents 2005, 25, 338.
19. NCCLS Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria, 5th ed.;
NCCLS: Villanova, PA, 2000.
20. Khan, Z. K. In vitro and vivo screening techniques for bioactivity screening and
evaluation, Proceedings of the International Workshop UNIDO-CDRI, 1997, 210.
21. Antifungal Agents: Past, Present and Future prospects; Varma, R. S., Ed.; National
Academy of Chemistry & Biology: Lucknow, India, 1998.
22. Palomino, J. C.; Martin, A.; Camacho, M.; Guerra, H.; Swings, J.; Portaels, F.
Antimicrob. Agents Chemother. 2002, 46, 2720.
Acknowledgments
R.A.R. thanks Professors R.S. Gaud and Anil Thaker for providing
facility to carry out these experiments.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.08.061.