Synthesis and antimicrobial activity of novel fluorine containing 4-(substituted-2-hydroxybenzoyl)-1H-pyrazoles and pyrazolyl benzo[d]oxazoles

Article abstract of DOI:10.1016/j.bmcl.2010.07.019

A series of fluorine containing 4-(substituted-2-hydroxybenzoyl) pyrazoles and pyrazolyl benzo[d]oxazoles were synthesized and evaluated for their antibacterial activity against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Bacillus subtilis and antifungal activity against Candida albicans. The antibacterial activities were expressed as the minimum inhibitory concentration (MIC₅₀) in μg/ml. The compounds 1-(3,4-difluorophenyl)-4-(5-fluoro-2-hydroxybenzoyl)-1H-pyrazole (4b), oxime derivatives such as 1-(3,4-difluorophenyl)-1H-pyrazol-4-yl)(2-hydroxy-4- methylphenyl)methanone oxime (5b) and (5-chloro-2-hydroxyphenyl)(1-(3,4- difluorophenyl)-1H-pyrazol-4-yl)methanone oxime (5e) exhibited promising activities against tested bacterial strains. Except compound 1-(3,4-difluorophenyl)-4-(2-hydroxybenzoyl)-1H-pyrazole (4d), none of the other compounds showed promising antifungal activity.

Full text of DOI:10.1016/j.bmcl.2010.07.019

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5572–5576
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and antimicrobial activity of novel fluorine containing
4-(substituted-2-hydroxybenzoyl)-1H-pyrazoles and pyrazolyl benzo[d]oxazoles
c
Amol V. Gadakh a, Chetan Pandit b, Sahebrao S. Rindhe a, , Bhausaheb K. Karale
*
a P.G. Department of Chemistry, New Arts, Science and Commerce College, University of Pune, Ahmednagar 414 003, India
b Aurigene Discovery Technologies Ltd, 39-40 KIADB Industrial Area, Electronic City, Phase-2, Hosur Road, Bangalore 560 100, India
c Department of Chemistry, Radhabai Kale Mahila Mahavidyalaya, University of Pune, Ahmednagar 414 003, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of fluorine containing 4-(substituted-2-hydroxybenzoyl) pyrazoles and pyrazolyl benzo[d]oxaz-
oles were synthesized and evaluated for their antibacterial activity against Staphylococcus aureus, Esche-
richia coli, Pseudomonas aeruginosa and Bacillus subtilis and antifungal activity against Candida albicans.
Received 5 March 2010
Revised 24 June 2010
Accepted 7 July 2010
Available online 17 August 2010
The antibacterial activities were expressed as the minimum inhibitory concentration (MIC50) in
lg/ml.
The compounds 1-(3,4-difluorophenyl)-4-(5-fluoro-2-hydroxybenzoyl)-1H-pyrazole (4b), oxime deriva-
tives such as 1-(3,4-difluorophenyl)-1H-pyrazol-4-yl)(2-hydroxy-4-methylphenyl)methanone oxime
(5b) and (5-chloro-2-hydroxyphenyl)(1-(3,4-difluorophenyl)-1H-pyrazol-4-yl)methanone oxime (5e)
exhibited promising activities against tested bacterial strains. Except compound 1-(3,4-difluoro-
phenyl)-4-(2-hydroxybenzoyl)-1H-pyrazole (4d), none of the other compounds showed promising anti-
fungal activity.
Keywords:
Synthesis
Hydroxybenzoyl pyrazoles and
benzoxazoles
Antimicrobial activities
Antifungal
Ó 2010 Published by Elsevier Ltd.
Antibacterial
The clinical applications of antibacterial chemotherapy began in
1935 with the discovery of prontosil and over the next three dec-
ades virtually all the major classes of antibiotics and synthetic anti-
bacterial agents in current use were introduced. The widespread
introduction of these antimicrobials into clinical practice have
made possible the treatment of life threatening diseases and have
enabled advances in surgical techniques by permitting the prophy-
laxis and treatment of surgery related infections.
However, the emergence of antibacterial drug resistance posed
by bacteria is recognized as a serious problem worldwide.1–4 It has
been postulated that the development of resistance to known anti-
biotics could be overcome by identifying new drug targets via
genomics, improving the existing nature of antibiotics and identi-
fying new antibacterial agents with novel mode of action and
structure.5–7
Over the last decade, a variety of benzoxazole derivatives were
synthesized and studied for their antibacterial activity.8–13 A calci-
mycin (I) (Fig. 1), well known antibiotic contains a 2-substituted
benzoxazole ring in its structure and is reported to be active
against Bacillus cereus, Bacillus megaterium and Micrococcus lutes.14
A careful literature survey has also revealed many hydrazone
derivatives to be reported as anticancer and antibacterial agents
(II and III) (Fig. 1).15,16 Moreover, 4-(2-hydroxybenzoyl) pyrazoles
are one of the precursors of benzoxazole derivatives and also
important intermediates for the synthesis of biologically active
benzofuran and coumarins systems.17–19
Although various synthetic methods are reported in literature
for the synthesis of 4-(2-hydroxybenzoyl) pyrazoles,20–22 but anti-
microbial activities of these classes of compounds has received
little attention.23 Pyrazoles and substituted pyrazoles are also
reported to be potent inhibitors of many bacteria.24,25 Synthesis
of compounds containing benzoxazole anchored with pyrazole ring
system is known in the literature,26 but there are no reports avail-
able describing antibacterial activity with both heterocycles in a
single moiety. Therefore an attempt made here to synthesize series
of 4-(2-hydroxybenzoyl) pyrazoles containing 3,4-difluorophenyl
ring system in the present study. Further, it was extended to also
cover corresponding fluorine containing pyrazolyl benzo[d]oxaz-
oles and investigate their antimicrobial activities.
The synthetic route for the preparation of 4-(2-hydroxybenzoyl)
pyrazoles and pyrazolyl benzo[d]oxazole derivatives are shown in
Scheme 1. Substituted formyl chromones27,28 1a–h were treated
with 3,4-difluorophenyl hydrazine 2 in ethanol and catalytic
amount of acetic acid at 40 °C to yield hydrazones 3a–h. The
hydrazones 3a–h on further treatment with KOH at 50 °C yielded
respective 1-(3,4-difluorophenyl)-4-(substituted-2-hydroxyben-
zoyl)-1H-pyrazoles 4a–h.29 The compounds 4a–h on treatment
with hydroxylamine hydrochloride in ethanol and sodium acetate
yielded oxime 5a–e, which underwent Beckmann rearrangement
using POCl3 to yield pyrazolyl benzo[d]oxazoles 6a–f. All the
* Corresponding author. Mobile: +91 9422221414.
E-mail address: rindhe_ss@yahoo.com (S.S. Rindhe).
0960-894X/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2010.07.019

A. V. Gadakh et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5572–5576
5573
R
NH
O
N
N
N
O
N
O
O
O
N
N
O
N
H
NH CH3
NH CH3
HO
O
a, R = Cl,
b, R = NO2
II
III
Calcimycin (I)
Figure 1.
R1
R2
R3
R1
O
NHNH2
R2
R3
O
O
N
a
b
NH
F
+
H
O
F
O
F
F
3a-h
2
1a-h
F
F
F
F
F
F
R1
R1
R1
R2
R3
OH
R2
R3
OH
O
R2
R3
N
N
c
d
N
N
O
N
N
N
N
OH
5a-e
4a-h
6a-f
Scheme 1. Reagents and conditions: (a) EtOH, 40 °C, Cat. AcOH, ꢀ99%; (b) KOH, 50 °C, 3 h and HCl, 65–75%; (c) NH2OHÁHCl, NaOAc, EtOH, reflux, 80–90%; (d) POCl3, 60–70%.
Table 1
Antibacterial and antifungal activities of newly synthesized compounds (4a–h), (5a–e) and (6a–f) indicated by MIC50 values (
lg/ml)
Compound
Structures
MIC50 values ( g/mL)
l
E. coli
P. aeruginosa
S. aureus
B. subtilis
C. albicans
F
F
Cl
4a
500
500
1000
500
250
OH
O
N
N
Cl
F
F
4b
62.5
50
100
62.5
500
OH
N
N
F
O
F
F
CH3
4c
500
250
500
500
1000
OH
O
N
N
H3C
(continued on next page)

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A. V. Gadakh et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5572–5576
Table 1 (continued)
Compound
Structures
MIC50 values (lg/mL)
E. coli
P. aeruginosa
S. aureus
B. subtilis
C. albicans
F
F
4d
4e
4f
250
555
500
12.5
100
OH
N
N
O
F
F
125
500
500
250
62.5
500
250
500
200
125
500
1000
100
100
500
500
500
H3C
OH
O
N
N
F
F
>1000
1000
500
OH
N
N
Br
O
F
F
4g
OH
N
N
H3C
O
F
F
4h
OH
O
N
N
Cl
F
F
OH
5a
5b
5c
500
500
500
250
1000
N
N
F
N
OH
F
F
H3C
OH
N
25
50
250
500
500
N
N
OH
F
F
OH
N
500
500
>1000
>1000
1000
N
N
OH

A. V. Gadakh et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5572–5576
5575
Table 1 (continued)
Compound
Structures
MIC50 values (lg/mL)
E. coli
P. aeruginosa
S. aureus
B. subtilis
C. albicans
F
F
F
OH
N
5d
200
100
500
1000
500
N
N
Br
OH
F
OH
5e
12.5
125
100
62.5
1000
N
N
Cl
Br
N
OH
F
F
6a
6b
6c
6d
6e
250
125
500
200
250
250
100
62.5
500
50
250
125
50
250
200
200
25
500
1000
500
500
250
O
N
N
N
F
F
O
N
N
N
F
F
O
N
N
N
H3C
H3C
F
F
200
500
O
N
N
N
F
F
200
O
N
N
N
F
F
F
6f
100
125
125
200
500
O
N
N
N
Cl
Gentamycin
Ampicillin
Chloramphenicol
Ciprofloxacin
Norfloxacin
Nystatin
0.05
100
50
25
10
1
0.25
250
50
50
10
1
250
50
50
100
100
50
25
10
100
500
Greseofulvin
compounds were isolated in pure form with a yield of 60–70%.
The compounds were analyzed by 1H NMR, LC–MS spectral
methods.
1H NMR spectrum of (3,5-dichloro-2-hydroxyphenyl)(1-(3,4-
difluorophenyl)-1H-pyrazol-4-yl)methanone (4a) showed broad
singlet at d 9.9 corresponds to phenolic –OH group and singlets

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A. V. Gadakh et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5572–5576
at d 8.3 and d 9.2 showed characteristic peak for pyrazole ring.
Molecular mass was confirmed by LC–MS at m/z 368.8 (M+1,
100%). The 1H NMR spectrum of (5-fluoro-2-hydroxyphenyl)
(1-(3,4-difluorophenyl)-1H-pyrazol-4-yl)methanone oxime (5a)
showed one more deshielded singlet at d 11.8 which corresponds
to hydroxyl group of oxime in addition to phenolic –OH proton
at d 9.8. The compound exhibited a mass peak at m/z 334 (M+1,
100%) which further confirms the structure. For the compound
5-bromo-2-(1-(3,4-difluorophenyl)-1H-pyrazol-4-yl)benzo[d]oxa-
zole (6a), we observed characteristic singlets at d 8.4 and d 9.4 due
to pyrazole ring. Molecular mass has been confirmed by LC–MS
with a peak at m/z 375.9 (M+1, 100%).
Acknowledgments
The authors thank Mr. C.S.N. Murthy, C.E.O. of Aurigene Discov-
ery technologies Ltd, Bangalore, India for his constant motivation
and support for this work.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.07.019.
References and notes
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fungal strain, Candida albicans (MTCC 227), using broth dilution
method and serial dilution techniques. The results were deter-
mined using minimum inhibitory concentration (MIC) values in
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(3,4-difluorophenyl)-4-(2-hydroxybenzoyl)-1H-pyrazole
(4d)
exhibited excellent activity against C. albicans in comparison to
Nystatin. None of the other compounds were promising in terms
of antifungal activity. A detailed structure activity relationship
could not be evolved from the present studies.
In conclusion, we have designed and synthesized a series of
fluorine containing 4-(substituted-2-hydroxybenzoyl) pyrazoles
and pyrazolyl benzoxazoles, which exhibited promising antibacte-
rial activity. We believe the insights gained in this study would be
useful for the development of potential drug candidates derived
from 4-(2-hydroxybenzoyl) pyrazoles to pyrazolyl benzo[d]oxaz-
oles in the development of novel anti-infective agents.