Microwave-Assisted Rapid and Efficient Synthesis of New Series of Chromene-Based 1,2,4-Oxadiazole Derivatives and Evaluation of Antibacterial Activity with Molecular Docking Investigation

Article abstract of DOI:10.1002/jhet.3430

A new series of novel chromene-based oxadiazole derivatives were synthesized from a variety of chromene-based amidoximes with readily available carboxylic acids under conventional oil bath heating as well as under microwave irradiation. The use of commercially available EDCI and HOBt as coupling reagents in DMF combined with microwave heating resulted in high yields and purities of the product 1,2,4-oxadiazoles in an expeditious manner. This methodology is successfully applied to synthesize 18 numbers of new 2H-chromene-substituted 1,2,4-oxadiazole derivatives in good to high yields. The structure of the product was ascertained by X-ray crystallographic analysis. All the synthesized compounds were evaluated for their in vitro antibacterial activity against two different pathogenic bacterial strains, that is, Escherichia coli (MTCC614) and Klebsiella pneumoniae (MTCC4031). The obtained results from in vitro antimicrobial assays indicated that 6g and 6h exhibited good antibacterial activity nearer to the standard drug, gentamicin. The molecular docking studies showed that compounds 6g and 6h show hydrogen bonding interaction with the bacterial target DNA gyrase of E.?coli.

Full text of DOI:10.1002/jhet.3430

Month 2018
Microwave-Assisted Rapid and Efficient Synthesis of New Series of
Chromene-Based 1,2,4-Oxadiazole Derivatives and Evaluation of Anti-
bacterial Activity with Molecular Docking Investigation
a
a
Nilofar Baral, Seetaram Mohapatra,
Sabita Nayak,a
Satyendra Kumar Pandey, P. Sudhir Kumar, and Chita Ranjan Sahoo
Bishnu Prasad Raiguru,a Nilima Priyadarsini Mishra,a Pravati Panda,a
*
b
c
c
*
aDepartment of Chemistry, Ravenshaw University, Cuttack, Odisha 753 003, India
bDepartment of Chemistry, Banaras Hindu University, Utter Pradesh 221005, India
cDepartment of Pharmaceutical Chemistry, School of Pharmaceutical Sciences, Siksha ‘O’ Anusandhan University,
Bhubaneswar, Odisha 751003, India
*E-mail: seetaram.mohapatra@gmail.com; sairampaidesetty@gmail.com
Received July 4, 2018
DOI 10.1002/jhet.3430
Published online 00 Month 2018 in Wiley Online Library (wileyonlinelibrary.com).
A new series of novel chromene-based oxadiazole derivatives were synthesized from a variety of
chromene-based amidoximes with readily available carboxylic acids under conventional oil bath heating
as well as under microwave irradiation. The use of commercially available EDCI and HOBt as coupling
reagents in DMF combined with microwave heating resulted in high yields and purities of the product
1,2,4-oxadiazoles in an expeditious manner. This methodology is successfully applied to synthesize 18 num-
bers of new 2H-chromene-substituted 1,2,4-oxadiazole derivatives in good to high yields. The structure of
the product was ascertained by X-ray crystallographic analysis. All the synthesized compounds were evalu-
ated for their in vitro antibacterial activity against two different pathogenic bacterial strains, that is,
Escherichia coli (MTCC614) and Klebsiella pneumoniae (MTCC4031). The obtained results from in vitro
antimicrobial assays indicated that 6g and 6h exhibited good antibacterial activity nearer to the standard
drug, gentamicin. The molecular docking studies showed that compounds 6g and 6h show hydrogen bond-
ing interaction with the bacterial target DNA gyrase of E. coli.
J. Heterocyclic Chem., 00, 00 (2018).
INTRODUCTION
Uncomplicated infections can be treated with short courses
of antibiotics, while complicated UTIs require longer and
more intensive courses of antibiotics. Effective therapy is
based on antibiotics, but bacterial resistance is an ongoing
issue for the management of UTI. In an era of increasing
bacterial resistance to classical antibacterial agents, the
design and synthesis of new antibacterial agents become an
imperative need to support for the battle against pathogenic
bacteria [10]. The development of novel structure leads
remains a key challenge for medicinal chemists to design
new, effective, and broad spectrum antimicrobial drug
targets via genomic improving or overcoming the
phenomenon of multiple drug resistance strains of bacteria
in existing antibiotics and most importantly by identifying
new antibacterial agents with novel structures and mode of
action with enhanced activity profile and high potency
without or with at least reduced systemic adverse effects. A
wide variety of approaches are being used in the search for
In recent years, the occurrence of bacterial infections has
reached alarming levels around the world. The widespread
use of antibacterial agents considered as the leading
weapons used for the treatment of infectious diseases
resulted in resistance to drug therapy against bacterial
infections, which led to serious health hazards [1,2].
Among several bacterial infections, the urinary tract
infections (UTIs) are common infectious diseases in clinical
practice. An estimated 150 million people worldwide are
diagnosed with a UTI in each year [3], and 40–50% of
women experiences at least one UTI during their lifetime
[4,5]. Most cases of UTI are caused by Gram-negative
bacilli, with Escherichia coli accounting for over 90% of
uncomplicated UTIs [6]. Other pathogens included such
as Klebsiella species, Proteus species, Pseudomonas
aeruginosa,
and
Enterococcus
species
[7–9].
© 2018 Wiley Periodicals, Inc.

N. Baral, S. Mohapatra, B. P. Raiguru, N. P. Mishra, P. Panda, S. Nayak, S. K. Pandey,
P. S. Kumar, and C. R. Sahoo
Vol 000
Figure 1. Examples of biologically active 1,2,4-oxadiazole derivatives. [Color figure can be viewed at wileyonlinelibrary.com]
novel antibacterial agents [11]. For a long period of time,
natural products are still one of the major sources of new
antibacterial agents [12]. Following in this vein, it was
apparent that heterocycles containing nitrogen(s) and an
oxygen atom particularly oxadiazoles have received
considerable attention in the field of medicinal chemistry
due to their interesting and diverse clinical applications
such as antibacterial [13], antifungal [14], anthelmintic
[15], antitubercular [16], anticancer [17], anti-HIV [18],
antioxidant [19], anti-inflammatory [20], and anticonvulsant
[21] activities (Fig. 1). Due to their huge biomedical
importance, several synthetic methods for preparing the
oxadiazole ring systems have been reported [22]. Literature
survey reveals the synthesis of 1,3,4-oxadiazole derivatives
as antibacterial agents [2,13–22]. However, there are very
few reports on the antibacterial activity of 1,2,4-oxadiazole
derivatives, which encouraged us to move further
[11,23,24]. Various methods reported for the preparation of
1,2,4-oxadiazoles such as cyclization of O-acylamidoximes
obtained from acylation of amidoximes with acyl halides,
esters, or anhydrides [25]. Katritzky and co-workers have
used N-acyl benzotriazoles for the synthesis of 1,2,4-
oxadiazoles [26]. A strong recent trend toward the use of a
carboxylic acid that is activated in situ and then reacted
with an amidoxime has emerged. Several acid activators
have been reported for this purpose including BOP-Cl,
scope for the development of better alternative protocols for
promoting this reaction.
On the other hand, chromenes are key structural units of
a variety of biologically important compounds, many of
which are pharmaceutically significant. In recent times,
there are several drug moieties that are in use, bearing
chromene entity in the treatment of various diseases
like antiallergic, antitumor, antiviral, antioxidant, anti-
inflammatory, antibacterial, and anticancer [28–35]. A
key feature is that the lipophilic nature of the benzopyran
derivatives helps to cross the bacterial cell membrane
easily [36], which encouraged the chemist to develop
novel methodologies for the synthesis of various 2H-
chromene derivatives.
Nowadays, the synthesis of hybrid compound drawn
much interest because hybrid compounds are more potent
than their individual counterpart [37–41]. Hybrid
compounds that possess important skeletons present in
drugs/clinical agents have become an important
research area for medicinal chemists. This can be an
effective
approach
to
possibly
avoid
the
TBTU,
1,10-carbonyldiimidazole,
CDMT,
and
propylphosphonicanhydride (T3P) [27]. Microwave-
assisted synthesis of 1,2,4-oxadiazole derivatives was
reported by Santagada et al. [27] in 2004 and Wang et al.
[27] in 2005 by using polymer supported reagents as well
as using suitable coupling reagents. Despite the wide
generality and the high efficiency of the aforementioned
methodologies with their own merits, some limitations still
remain, that is, most of the reaction conditions such as high
temperature, long duration of reaction time, and
chromatographic separation of the products do not satisfy
each and every aspect of ideal and green synthesis.
Therefore, in view of environmental concerns, there is still
Figure 2. Designed new chromene-based oxadiazole derivative. [Color
figure can be viewed at wileyonlinelibrary.com]
Journal of Heterocyclic Chemistry
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Synthesis of Chromene-Fused Oxadiazole Derivatives
physicochemical/pharmacokinetic/toxicity problems that
appear in the later stages of development. Literature
report reveals that chromene-based enones [34] and
method for the synthesis of chromene-fused oxadiazole
derivatives to provide privileged scaffolds for the
generation of target compounds for drug discovery. In our
preliminary studies, in this protocol, chromene-fused
oxadiazole derivatives were synthesized starting from
chromene-3-carbonitrile 3(a–i). Initially, the starting
chromene nitriles were prepared by following a green and
efficient one-pot two component approach using
salicylaldehyde 1(a–i) and acrylonitrile 2 in the presence
of DABCO under solvent-free conditions in good to high
yield following Morita–Bayllis–Hillman reaction [43]. All
quinoline-based
oxadiazoles
[11]
show
potent
antibacterial activity. All these observations and our
interest in chromene and oxadiazole derivatives prompted
us to explore a series of new hybrid compounds (Fig. 2).
The activities of chromene, as well as oxadiazoles, lead
us to embark both the pharmacophores in a single
framework and to synthesize
a set of chromene-
substituted oxadiazole derivatives and to study its
antibacterial activity. To the best of our knowledge, these
are the newly synthesized chromene-fused 1,2,4-
oxadiazole derivatives and have not been reported before.
Herein, we have designed the molecules based on
structure–activity relationship studies and explored their
antibacterial activity against two different pathogenic
bacteria, that is, E. coli (MTCC614) and Klebsiella
pneumoniae (MTCC4031). Among all the tested
compounds, 6g and 6h exhibited good antibacterial
activity nearer to the standard drug, gentamicin. Bacterial
DNA gyrase is playing a vital role for catalyzing negative
super coiling of DNA and involves with binding and
cleaving the protein of the DNA. The literature reported
that novobiocin has inhibited DNA gyrase, which is a
chromene derivative. Thus, the selected antibacterial
DNA gyrase has been performed for molecular docking
[42]. The molecular docking studies showed that the
compounds 6g and 6h have hydrogen bonding interaction
with the bacterial target DNA gyrase of E. coli.
the
synthesized
chromene-3-carbonitriles
were
successfully characterized by 1H-NMR, 13C-NMR, IR,
and mass spectroscopy. In the subsequent step, compound
3(a–i) was treated with hydroxylamine hydrochloride
using Et3N in ethanol for 2 h to obtain N0-hydroxy-2H-
chromene-3-carboximidamide 4(a–i) in 87–93% yield
(Scheme 1). All the synthesized molecules were
successfully characterized by 1H-NMR and 13C-NMR.
We mainly focused on developing a facile procedure to
generate
2H-chromene-substituted
1,2,4-oxadiazole
library with a variety of substitutions. It requires
investigating the scope of the reaction by screening with
various functionalities. In the process of evaluating best
methodology, for the synthesis of 1,2,4-oxadiazole
derivative, we have tried with various reagents such as
EDCI, EDCI/HOBt, and DCC in different solvents such
as DMF, CH3CN, toluene, and THF (Scheme 2). As
shown in Table 1, EDCI/HOBt in DMF under heating at
80°C provided higher yield of 75% (Table 1, entry 5).
The yields were not improved further even on raising the
temperature and elongating the time period. Therefore,
EDCI/HOBt in DMF under reflux condition was chosen
as optimal conditions for all further reactions.
RESULTS AND DISCUSSION
In continuation of our enormous research interest in the
development of efficient and environmentally friendly
procedures for the synthesis of biologically important
heterocycles, we devised a convenient and eco-friendly
In order to reduce the reaction time as well as to enhance
the yield of chromene-substituted oxadiazole derivatives,
the reaction was subsequently investigated under
microwave irradiation. This technique is a valuable
Scheme 1. Synthesis of chromene-3-carbontrile 3(a–i) and N0-hydroxy-2H-chromene-3-carboximidamide 4(a–i).
Scheme 2. Methodology screening.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

N. Baral, S. Mohapatra, B. P. Raiguru, N. P. Mishra, P. Panda, S. Nayak, S. K. Pandey,
P. S. Kumar, and C. R. Sahoo
Vol 000
Table 1
Reaction system screening results.
Entry
Coupling reagent
Solvent
Temp. (°C)
80
Time (h)
12
Yield (%)a
50
MW (W)
Temp. (°C)
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
9
DCC
DCC
DCC
DMF
DMF
DMF
DMF
DMF
DMF
DMF
CH3CN
Toluene
THF
30
30
30
30
30
30
30
30
30
30
50
60
70
80
50
70
80
80
80
80
10
10
10
10
10
10
10
10
10
10
30
40
50
60
70
85
92
n.r
52
48
DCC
EDCI/HOBt
EDCI/HOBt
EDCI/HOBt
EDCI/HOBt
EDCI/HOBt
EDCI/HOBt
80
6
75
80
80
80
12
12
12
n.r
30
35
10
Bold signifies for best result.
n.r = No Reaction.
aIsolated yield.
alternative to conventional heating, introduces energetic
radiations into the reaction, and thereby, enhances the
rate and yields of the reaction and simultaneously
decreases the time. Similar to conventional heating, the
coupling reagents, as well as the solvents, were again
screened under microwave irradiation. It has been
observed that among all the reagents and solvents used,
EDCI/HOBt in DMF at 80°C for 10 min provided 92%
yield (Table 1, entry 7) of chromene-substituted
oxadiazole derivative without any unwanted side products
by simple recrystallization. To our delight in microwave
irradiation excellent yield of the product, the formation
was observed, and the reaction time period reduced from
hours to minutes, which encouraged us to move further.
Having these preliminary observations in hand, we
wished to broaden the scope of this protocol for the
synthesis of a new class of 2H-chromene-substituted
1,2,4-oxadiazoles 6(a–r) (Scheme 3). The reaction of 2H-
chromene-3-carboximidamide 4(a–i) with a variety of
carboxylic acid derivatives 5(a–h) in the presence
of EDCI/HOBt in anhydrous DMF was carried out. All
of them underwent the reaction smoothly affording
the corresponding chromene-substituted oxadiazole
derivatives. The results obtained for both conventional
and microwave irradiation methods are summarized in
Table 2, which clearly shows that microwave irradiation
led to an enhancement in the rate as well as yield of
the products (80–92%) over the conventional method
(65–78%).
As shown in Table 2, a variety of aromatic carboxylic
acid derivatives bearing functional groups such as halo,
methoxy, and alkyl participated effectively with
chromene-3-carboximidamide and resulted in chromene-
substituted 1,2,4-oxadiazoles with good to excellent yields
(Table 2). To our delight, a wide range of chromene-3-
carboximidamide containing an electron-donating as well
as electron-withdrawing groups was well tolerated.
However, from Table 2, it was noticed that the product
yield is affected by the substituents and their positions at
the aromatic ring of the carboxylic acid. Without
substitution in the carboxylic acid functionalities,
compounds (6a, 6b, 6d, 6f, 6g, 6h, 6i, and 6k) provided
an excellent yield (82–92%). Presence of electron-
donating groups (─OMe) at 2,4,5-position of the
aromatic ring of carboxylic acid (6c, 6e, 6j, and 6l) leads
to decrease in the yield (80–82%). However, when the
electron-donating group is changed from ─OMe to
─CH3 in the carboxylic acid, good yield was observed
(6q, 90%). Excitingly halogenated carboxylic acid
derivatives such as ─Cl and ─Br were tolerated in
entries 6m, 6n, and 6p and provided better yield. Also,
presence of single ─OMe group at the carboxylic acid
ring (6o and 6r) decreases the yield (80–85%). The
1
structures of the products 6(a–r) were confirmed by H-
Scheme 3. Synthesis of chromene-fused oxadiazole derivatives 6(a–r).
Journal of Heterocyclic Chemistry
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Synthesis of Chromene-Fused Oxadiazole Derivatives
Table 2
Substrate scope for the synthesis of chromene-fused 1,2,4-oxadiazole derivatives 6(a–r).
Conventional method: A (DMF, 80°C)
Microwave method: B (DMF, 80°C, 30 W)
Entry
Product
Time (h)
6
Yield (%)a
71
Time (min)
10
Yield (%)a
92
6a
6b
6c
6d
6
6
4
68
65
72
10
15
15
88
81
88
6e
6f
6
2
65
75
72
20
10
10
80
86
89
6g
2.5
6h
5
67
15
82
6i
6j
3
6
78
65
12
20
92
82
(Continues)
Journal of Heterocyclic Chemistry
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N. Baral, S. Mohapatra, B. P. Raiguru, N. P. Mishra, P. Panda, S. Nayak, S. K. Pandey,
P. S. Kumar, and C. R. Sahoo
Vol 000
Table 2
(Continued)
Conventional method: A (DMF, 80°C)
Microwave method: B (DMF, 80°C, 30 W)
Entry
Product
Time (h)
5
Yield (%)a
65
Time (min)
15
Yield (%)a
6k
82
6l
6
65
20
80
6m
6n
6o
6p
6q
6
5
4
2
3
75
71
66
72
70
20
15
12
10
10
83
82
80
90
90
6r
5
67
15
85
aIsolated yield.
NMR, 13C-NMR, and HRMS. Also, the structure of the
compound was unambiguously confirmed by single-
crystal X-ray analysis of compound 6m (Figs 3 and 4) [44].
pathogenic bacteria E. coli (MTCC614) and
K. pneumoniae (MTCC4031). The minimum inhibitory
concentration values were determined by comparison
with standard drug gentamicin (Table 3). The
antibacterial activity studies indicated that all the
synthesized compounds showed moderate to excellent
activity against the tested gram-negative bacterial
Antibacterial activities. After successful synthesis of
chromene-fused 1,2,4-oxadiazole derivatives 6(a–r), we
further studied the antibacterial activity of the newly
synthesized molecules against two gram-negative
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Month 2018
Synthesis of Chromene-Fused Oxadiazole Derivatives
pathogens. From the antibacterial activity data, it was
observed that compounds 6g and 6h are the most active
among all the tested compound, while compounds 6a,
6b, 6d, and 6p showed less antibacterial activity against
E. coli. Similarly, compounds 6a, 6d, and 6r exhibit less
antibacterial activity against K. pneumoniae in
comparison with gentamicin. Rest of the compounds of
the series showed moderate to good antibacterial activity
against both the bacteria.
Figure 3. Single crystal analysis data of compound 6m. [Color figure can
be viewed at wileyonlinelibrary.com]
Structure–activity relationship study.
The structure–
activity relationship indicates that the potency of
antibacterial activity exhibited by compounds is
preferably due to the presence of halogen substituents at
the structures because the introduction of halogen atoms
into the chromene scaffold can be beneficial for
antimicrobial activity so far as this improves the lipid
solubility of the active ingredients. Most potent activity
was observed when chromene moiety was substituted
with electron-withdrawing halogen atom chlorine at C-6
position and chlorine/bromine at C-8 positions (6g–h).
Also, chromene-fused 1,2,4-oxadiazole derivatives with
electron-donating group methoxy at C-2, C-4, and C-5
positions of aromatic ring of carboxylic acid and
methoxy, ethoxy, ─Cl, and ─H at C-8 position of
chromene ring resulted in better antibacterial activity (6c,
6e, 6l, and 6j). Introduction of halogen atoms (6f, 6i, 6k,
6m, and 6n) and methoxy group at C-2 position of
carboxylic ring (6o) also led to an increase in the
microbial activity. Inhibition was decreased when there is
Figure 4. View of the dimeric unit formed from the C─H⋯Cl hydrogen
bonding (cyan-colored dotted line) of 5-(2-chlorophenyl)-3-(2H-chromen-
3-yl)-1,2,4-oxadiazole 6m. [Color figure can be viewed at
wileyonlinelibrary.com]
Table 3
Antibacterial activity as size of zone of inhibition and MIC of synthesized compounds against two pathogenic bacteria.
Tested
compound
code
Diameter of inhibition zone in three different concentration (mm in μg/disc)
Escherichia coli (MTCC614) Klebsiella pneumoniae (MTCC4031)
MIC (μg/mL)
K. pneumoniae
Serial
no.
E. coli
1
2
3
4
5
6
7
8
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
6q
6r
—
9
8
10
8
10
10
10
9
11
10
11
12
14
12
20
16
11
11
14
11
12
12
13
9
7
7
10
7
11
11
12
11
8
10
11
10
9
9
8
12
8
8
8
13
8
80
80
60
80
60
60
40
40
60
40
60
60
60
60
60
80
60
60
60
80
60
60
80
60
60
40
40
60
60
60
60
60
60
60
60
60
80
60
11
10
14
12
10
10
12
10
11
11
9
12
12
17
13
11
11
13
12
12
12
9
12
12
14
14
10
12
13
12
10
12
11
8
13
13
19
17
13
13
16
13
11
14
12
10
15
11
17
9
10
11
12
13
14
15
16
17
18
Gena
10
8
8
9
7
7
10
8
8
10
9
12
13
18
10
10
15
15
16
14
Bold signifies for best result.
MIC, minimum inhibitory concentration.
aGentamicin used as reference antibiotic as positive control.
Journal of Heterocyclic Chemistry
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N. Baral, S. Mohapatra, B. P. Raiguru, N. P. Mishra, P. Panda, S. Nayak, S. K. Pandey,
P. S. Kumar, and C. R. Sahoo
Vol 000
Table 4
docking study. The docking study was performed by
Discovery Studio 4.0 program. Molecular docking was
achieved by employing two targets E. coli (PDB ID:
1KZN) and K. pneumoniae (PDB ID: 4G89). The
standard ligands were (6g–h, gentamicin). The docking
energy gained by two effective compounds 6g and 6h
against two bacterial targets was depicted in Table 4. It is
stimulating to note that both compounds 6(g–h) exhibited
binding energy in the range between ꢀ6.76 and
ꢀ8.57 kcal/mol. The 2D diagram illustrated that
compounds 6(g–h) and the standard drugs have
interrelated to the active binding sites through various
bindings. Through molecular docking study from
Figures 5, 6, and 7, it was noticed that the most potent
compounds 6g and 6h selectively bind to the DNA
gyrase of the target bacteria E. coli with respect to
confirmation of hydrogen bonding, alkyl, π–π interaction,
π–donor hydrogen bonding, and π–σ interactions. A
greater number of interactions found in compounds 6g
and 6h that exactly fit into the DNA gyrase in the
model resulted in greater interaction, which explains their
potent antibacterial activity as compared with standard
drug, gentamicin.
Docking energy of two effective compounds against two bacterial targets.
Docking scores (kcal/mol)
Chemical
PDB ID: 1KZN
PDB ID: 4G89
6g
6h
ꢀ8.57
ꢀ8.85
ꢀ6.71
ꢀ6.76
ꢀ6.93
ꢀ6.56
Gentamicin
PDB, protein data band.
no substitution in the aromatic ring (6a). Also, from the
result, it was observed that methoxy substitution at C-4
position of carboxylic acid (6r) and introduction of
bromo group at C-6 position and methoxy at C-8 position
of (6d) of chromene ring decrease the activity.
Molecular modeling.
Molecular docking study was
performed parallel to the synthesis and in vitro evaluation
of chromene-based 1,2,4-oxadiazole to gain a more
exhaustive perception of the interactions and binding
ability of the molecule to the active sites of bacterial
DNA gyrase. In order to find out the molecular binding
interaction of compounds with bacterial DNA strain,
compounds 6g and 6h were subjected to molecular
Figure 5. Interactions of chemical 6g, with the bacterial target DNA gyrase of Escherichia coli visualization during molecular docking. [Color figure can
be viewed at wileyonlinelibrary.com]
Figure 6. Interactions of chemical 6h, with the bacterial target DNA gyrase of Escherichia coli visualization during molecular docking study. [Color
figure can be viewed at wileyonlinelibrary.com]
Journal of Heterocyclic Chemistry
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Month 2018
Synthesis of Chromene-Fused Oxadiazole Derivatives
Figure 7. Interactions of antibiotic gentamicin with the bacterial target DNA gyrase of Escherichia coli visualization during molecular docking study.
[Color figure can be viewed at wileyonlinelibrary.com]
recorded on 400 MHz (100 MHz for 13C-NMR) JEOL
NMR spectrometer with CDCl3 as solvent and
tetramethylsilane as an internal standard. Chemical shifts
were reported in parts per million (ppm, δ scale)
downfield from tetramethylsilane at 0.00 ppm and
1
referenced to the CDCl3 at 7.26 ppm (for H-NMR) or
77.00 ppm (for 13C-NMR). HRMS analysis was
conducted on a Brucker micro-TOF-Q-MS analyzer. All
reagents and solvents used in this study were
commercially available (from Sigma-Aldrich) and were
used without further purification.
CONCLUSIONS
Synthesis of chromene-3-carbonitrile 3(a–i).
A neat
reaction was carried out by taking substituted
salicylaldehydes 1(a–i) (1.0 mmol), DABCO (0.2 mmol),
and acrylonitrile 2 (1.0 mmol) in one pot and stirred at
40°C for 4–5 h. The progress of the reaction was
monitored by TLC. After completion, reaction mixture
was diluted with water and extracted with EtOAc. The
organic layer was separated, dried over anhydrous
Na2SO4, and concentrated under reduced pressure. The
crude product was purified by general silica gel (100–200
mesh) column chromatography using ethyl acetate/hexane
to furnish the pure compound 3(a–i) in 82–93% yield.
All compounds were characterized through 1H-NMR,
13C-NMR, IR, and mass data.
In summary, a series of new chromene-fused 1,2,4-
oxadiazole derivatives were synthesized and evaluated for
their in vitro antibacterial activity against two pathogenic
bacteria, which are causing UTI: E. coli (MTCC614) and
K. pneumoniae (MTCC4031). Among all the synthesized
molecules, in particular, compounds 6g and 6h were
identified as the most potent antibacterial candidates
against E. coli. Through molecular docking study, it was
noticed that the most potent compounds 6g and 6h
selectively bind to the DNA gyrase of the target bacteria.
Together, given their potent antibacterial activities, these
newly synthesized hybrid molecules can be potentially be
developed into useful antimicrobial agents that can
prompt future researcher to synthesize a series of
oxadiazole derivatives containing wide varieties of
substituents with the aim of obtaining some novel
heterocyclic candidates for the potential treatment of
bacterial infection.
2H-chromene-3-carbonitrile (3a).
White solid. (93%).
m.p.: 53–55°C; IR (KBr): ν 3436, 2215, 1606, 1484,
1
1459, 1211, 1150, 1037, 757 cmꢀ1; H-NMR (400 MHz,
CDCl3): δ 7.28–7.23 (m, 1H, Ar─H), 7.13 (s, 1H, CH),
7.12–7.09 (dd, J = 4.0 Hz, 8.0 Hz, 1H, Ar─H), 6.98–
6.94 (m, 1H, Ar─H), 6.87 (d, J = 8.0 Hz, 1H, Ar─H),
4.79 (s, 2H, CH2); 13C-NMR (100 MHz, CDCl3): δ
154.4, 138.9, 132.8, 128.6, 122.5, 120.1, 116.7, 116.6,
103.4, 64.4. ESI-HRMS (m/z): Anal. Calcd for C10H7NO
[M + H]+ 157.05276; found: 157.05265.
EXPERIMENTAL
Melting points were recorded on SMP10 digital melting
point apparatus using open capillary tubes and uncorrected.
IR spectra were recorded on a Nicolet FT-IR500
spectrophotometer using KBr. 1H-NMR spectra were
8-Ethoxy-2H-chromene-3-carbonitrile (3b).
Off white
solid. (92%). m.p.: 92–95°C; IR (KBr): ν 2931, 2209,
1626, 1578, 1475, 1398, 1335, 1282, 1212, 1113, 1087,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

N. Baral, S. Mohapatra, B. P. Raiguru, N. P. Mishra, P. Panda, S. Nayak, S. K. Pandey,
P. S. Kumar, and C. R. Sahoo
Vol 000
1015, 910, 894, 775, 728 cmꢀ1
;
1H-NMR (400 MHz,
122.8, 121.9, 115.5, 105.8, 65.1. ESI-HRMS (m/z): Anal.
CDCl3): δ 7.50 (s, 1H, CH), 7.25–7.22 (m, 2H, Ar─H),
7.17–7.14 (m, 1H, Ar─H), 5.01 (s, 2H, CH2), 4.12 (q,
J = 8.0 Hz, 2H, CH2), 1.79 (t, J = 8.0 Hz, 3H, CH3);
13C-NMR (100 MHz, CDCl3): δ 147.5, 143.7, 139.1,
122.2, 120.9, 120.4, 116.7, 116.5, 103.4, 64.7, 64.6,
14.9. ESI-HRMS (m/z): Anal. Calcd for C12H11NO2
[M + H]+ 201.07898; found: 201.07811.
Calcd for C10H5Cl2NO [M + H]+ 224.97482; found:
224.97470.
8-Bromo-6-chloro-2H-chromene-3-carbonitrile (3h).
Yellow solid. (88%). m.p.: 143–145°C; IR (KBr, νmax
/
cmꢀ1): 2925, 2854, 2209, 1732, 1465, 1252, 1222, 1193,
1021, 916, 864, 817, 729, 694; 1H-NMR (400 MHz,
CDCl3): δ 7.46 (s, 1H, CH), 7.06 (d, J = 8.0 Hz, 2H,
Ar─H), 4.91 (s, 2H, CH2); 13C-NMR (100 MHz, CDCl3):
δ 149.7, 136.9, 135.2, 127.6, 126.9, 121.6, 115.4, 111.1,
105.7, 65.1. ESI-HRMS (m/z): Anal. Calcd for
C10H5BrClNO [M + H]+ 268.92430; found: 268.92420.
8-Methoxy-2H-chromene-3-carbonitrile (3c).
solid. (90%). m.p.: 101–103°C; IR (KBr): ν 3429, 2924,
Off white
2205, 1606, 1577, 1483, 1464, 1275, 1223, 1174, 1098,
1
1021, 974, 775, 729 cmꢀ1; H-NMR (400 MHz, CDCl3):
6,8-Dibromo-2H-chromene-3-carbonitrile (3i).
Yellow
δ 7.13 (s, 1H, CH), 6.90–6.87 (m, 2H, Ar─H), 6.70–6.68
(m, 1H, Ar─H), 4.83 (s, 2H, CH2), 3.84 (s, 3H, OCH3);
13C-NMR (100 MHz, CDCl3): δ 148.1, 143.3, 139.0,
122.3, 120.8, 120.4, 116.5, 115.3, 103.5, 64.6, 56.2. ESI-
HRMS (m/z): Anal. Calcd for C11H9NO2 [M + H]+
187.06333; found: 187.06333.
6-Bromo-8-methoxy-2H-chromene-3-carbonitrile (3d).
Yellow solid. (87%). m.p.: 97–101°C; IR (KBr): ν 3433,
solid. (83%). m.p.: 161–163°C; IR (KBr, νmax/cmꢀ1):
2924, 2853, 2210, 1732, 1630, 1465, 1252, 1224, 1192,
1
1017, 914, 726, 661; H-NMR (400 MHz,CDCl3): δ 7.62
(s, 1H, CH), 7.19 (s, 1H, Ar─H), 7.09 (s, 1H, Ar─H),
4.94 (s, 2H, CH2); 13C-NMR (100 MHz,CDCl3): δ 150.1,
137.9, 136.8, 129.8, 122.2, 115.4, 114.4, 111.5, 105.6,
65.1. ESI-HRMS (m/z): Anal. Calcd for C10H5Br2NO
[M + H]+ 312.87379; found: 312.8770.
3082, 2924, 2853, 2211, 1624, 1469, 1283, 1215, 1114,
1
851, 735 cmꢀ1; H-NMR (400 MHz, CDCl3): δ 7.10 (s,
Synthesis
of
(Z)-N0-hydroxy-2H-chromene-3-
1H, CH), 7.02 (s, 1H, Ar─H), 6.90 (d, J = 4.0 Hz, 1H,
Ar─H), 4.88 (s, 2H, CH2), 3.88 (s, 3H, OCH3); 13C-
NMR (100 MHz, CDCl3): δ 148.9, 142.2, 137.7, 122.5,
121.8, 118.2, 116.0, 114.2, 104.8, 64.7, 56.5. ESI-HRMS
(m/z): Anal. Calcd for C11H8BrNO2 [M + H]+ 264.97384;
carboximidamide 4(a–i).
In
a
reaction bottle,
hydroxylamine hydrochloride (NH2OH.HCl) (1.5 eqv.),
Et3N (1.5 eqv.), and ethanol were taken and stirred at
room temperature for half an hour. Then chromene-3-
carbonitrile (1 eqv.) was added to it, and the reaction
mixture was stirred at room temperature. The progress of
the reaction was monitored by TLC. The reaction was
found completed after 2 h. After completion, reaction
mixture was diluted with water and extracted with EtOAc.
The organic layer was separated, dried over anhydrous
Na2SO4, and concentrated under reduced pressure to get
the pure product 4(a–i) in 87–93% yield. All compounds
were characterized through 1H-NMR and 13C-NMR.
(Z)-N0-hydroxy-2H-chromene-3-carboximidamide (4a).
found: 264.97370.
6-Chloro-2H-chromene-3-carbonitrile (3e). Faint yellow
solid. (82%). m.p.: 132–134°C; IR (KBr, νmax/cmꢀ1):
3444, 3066, 2923, 2213, 1629, 1480, 1239, 1213, 1113,
1
1021, 914, 815, 675, 628; H-NMR (400 MHz, CDCl3):
δ 7.19 (d, J = 8.0 Hz, 1H, CH), 7.09–7.06 (m, 2H,
Ar─H), 6.80 (d, J = 8.0 Hz, 1H, Ar─H), 4.80 (s, 2H,
CH2); 13C-NMR (100 MHz, CDCl3): δ 152.8, 137.7,
132.4, 127.8, 127.4, 121.2, 118.1, 116.0, 104.9, 64.5.
ESI-HRMS (m/z): Anal. Calcd for C10H6ClNO [M + H]+
191.01379; found: 191.01365.
White solid. (93%). m.p.: 200–202°C; 1H-NMR
(400 MHz, CDCl3): δ 7.18–7.14 (td, J = 7.7 Hz, 1.7 Hz,
1H, Ar─H), 7.08–7.06 (dd, J = 7.6 Hz, 1.6 Hz, 1H,
Ar─H), 6.92–6.88 (m, 1H, Ar─H), 6.84 (d, J = 8.0 Hz,
1H, Ar─H), 6.69 (s, 1H, CH), 4.97 (s, 2H, CH2), 4.64 (s,
2H, NH2); 13C-NMR (100 MHz, CDCl3): δ 154.5, 145.0,
130.2, 127.5, 124.5, 122.0, 121.7, 121.5, 116.0, 64.3.
6-Bromo-2H-chromene-3-carbonitrile (3f). Yellow solid.
(82%). m.p.: 105–107°C; IR (KBr): ν 3064, 2919, 2857,
2211, 1672, 1476, 1413, 1212, 1112, 1021, 917, 815,
1
757, 662 cmꢀ1; H-NMR (400 MHz, CDCl3): δ 7.37 (d,
J = 8.0 Hz, 1H, CH), 7.25–7.23 (m, 1H, Ar─H), 7.13–
7.09 (m, 1H, Ar─H), 6.78 (d, J = 8.0 Hz, 1H, Ar─H),
4.80 (s, 2H, CH2); 13C-NMR (100 MHz, CDCl3): δ
153.2, 137.5, 135.3, 130.7, 121.7, 118.5, 116.0, 114.6,
104.8, 64.5. ESI-HRMS (m/z): Anal. Calcd for
C10H6BrNO [M + H]+ 234.96328; found: 234.96344.
(Z)-8-ethoxy-N0-hydroxy-2H-chromene-3-carboximidamide
1
(4b).
White solid. (91%). m.p.: 112–115°C; H-NMR
(400 MHz, CDCl3): δ 6.86–6.78 (m, 2H, Ar─H, CH),
6.73–6.65 (m, 2H, Ar─H), 5.03 (s, 2H, CH2), 4.68 (s,
2H, NH2), 4.10 (q, J = 7.0 Hz, 2H, CH2), 1.45 (t,
J = 7.2 Hz, 3H, CH3); 13C-NMR (100 MHz, CDCl3): δ
149.7, 147.0, 143.6, 124.4, 122.3, 122.1, 121.2, 119.7,
114.5, 64.6, 64.4, 14.8.
6,8-Dichloro-2H-chromene-3-carbonitrile (3g).
Yellow
solid. (86%). m.p.: 94–96°C; IR (KBr, νmax/cmꢀ1): 3446,
2924, 2854, 2227, 1634, 1467, 1216, 762; 1H-NMR
(400 MHz, CDCl3): δ 7.32 (s, 1H, CH), 7.10 (s, 1H,
Ar─H), 7.0 (s, 1H, Ar─H), 4.92 (s, 2H, CH2); 13C-NMR
(100 MHz, CDCl3): δ 148.7, 137.0, 132.5, 127.3, 126.4,
(Z)-N0-hydroxy-8-methoxy-2H-chromene-3-
carboximidamide (4c).
White solid. (90%). m.p.: 150–
152°C; 1H-NMR (400 MHz, CDCl3): δ 6.87–6.78 (m,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Synthesis of Chromene-Fused Oxadiazole Derivatives
2H, Ar─H, CH), 6.71–6.63 (m, 2H, Ar─H), 5.01 (s, 2H,
CH2), 4.68 (s, 2H, NH2), 3.86 (s, 3H, OCH3); 13C-NMR
(100 MHz, CDCl3): δ 149.5, 147.8, 143.3, 124.5, 122.2,
and was diluted with water and extracted with EtOAc.
The organic layer was separated, dried over anhydrous
Na2SO4, and concentrated under reduced pressure. The
pure product was obtained by recrystallization from
isopropyl alcohol to provide compound 6(a–r) in
65–78% yield.
121.9, 121.2, 119.7, 112.9, 64.5, 56.0.
(Z)-6-bromo-N0-hydroxy-8-methoxy-2H-chromene-3-
carboximidamide (4d). Yellow solid. (87%). m.p.: 158–
160°C; 1H-NMR (400 MHz, CDCl3): δ 6.88–6.81 (m,
2H, Ar─H), 6.57 (s, 1H, CH), 4.98 (s, 2H, CH2), 4.65 (s,
2H, NH2), 3.85 (s, 3H, OCH3); 13C-NMR (100 MHz,
CDCl3): δ 149.2, 148.6, 142.5, 125.7, 123.4, 122.0,
120.6, 115.9, 113.1, 64.5, 56.2.
Method B.
EDCI/HOBt (1.5 mmol) and benzoic acid
(1 mmol) was added to a solution of hydroxy-chromene-
carboximidamide (1 mmol) in DMF (2 mL), and the
mixture was irradiated under microwave irradiation (30W
power) at 80°C for 10–20 min. The progress of the
reaction was monitored by TLC. On completion of the
reaction, the reaction mixture was cooled to room
temperature and was diluted with water and extracted
with EtOAc. The organic layer was separated, dried over
anhydrous Na2SO4, and concentrated under reduced
pressure. The pure product was obtained by
recrystallization from isopropyl alcohol to provide
(Z)-6-chloro-N0-hydroxy-2H-chromene-3-carboximidamide
1
(4e).
White solid. (89%). m.p.: 215–217°C; H-NMR
(400 MHz, DMSO-d6): δ 9.99 (s, 1H, OH), 7.15–7.12
(m, 1H, Ar─H), 7.10–7.09 (m, 1H, Ar─H), 6.93 (s, 1H,
Ar─H), 6.79 (d, J = 8.0 Hz, 1H, CH), 5.62 (s, 2H, NH2),
4.85 (s, 2H, CH2); 13C-NMR (100 MHz, DMSO-d6): δ
152.9, 149.1, 129.3, 127.2, 126.8, 125.6, 124.3, 119.9,
117.6, 64.5.
(Z)-6-bromo-N0-hydroxy-2H-chromene-3-carboximidamide
compound 6(a–r) in 80–92% yield.
3-(2H-chromen-3-yl)-5-phenyl-1,2,4-oxadiazole (6a).
1
(4f). Yellow solid. (91%). m.p.: 210–212°C; H-NMR
Light yellow solid. (92%). m.p.: 118–120°C; 1H-NMR
(400 MHz, CDCl3): δ 8.20 (d, J = 8.0 Hz, 2H, Ar─H),
7.65–7.55 (m, 4H, Ar─H), 7.27–7.20 (m, 2H, Ar─H),
6.97 (t, J = 8.0 Hz, 1H, Ar─H), 6.91 (d, J = 8.0 Hz, 1H,
Ar─H), 5.27 (s, 2H, CH2); 13C-NMR (100 MHz,
CDCl3): δ 175.2, 166.2, 154.6, 132.9, 131.0, 129.1,
128.2, 124.0, 121.8, 121.4, 119.3, 116.0, 64.5. ESI-
HRMS (m/z): Anal. Calcd for C17H12N2O2 [M + H]+
277.0899; found: 277.0998.
(400 MHz, DMSO-d6): δ 10.00 (s, 1H, OH), 7.27–7.22
(m, 2H, Ar─H), 6.93 (s, 1H, Ar─H), 6.74 (d, J = 8.0 Hz,
1H, CH), 5.62 (s, 2H, NH2), 4.86 (s, 2H, CH2); 13C-
NMR (100 MHz, DMSO-d6): δ 153.4, 149.1, 132.2,
129.7, 127.1, 124.8, 119.8, 118.1, 113.2, 64.5.
(Z)-6,8-dichloro-N0-hydroxy-2H-chromene-3-
carboximidamide (4g).
White solid. (87%). m.p.: 202–
204°C; 1H-NMR (400 MHz, CDCl3): δ 7.18–7.06 (m,
1H, Ar─H), 6.92–6.83 (m, 1H, Ar─H), 6.69 (s, 1H,
CH), 4.97 (s, 2H, CH2), 4.65 (s, 2H, NH2); 13C-NMR
(100 MHz, CDCl3): δ 154.4, 149.9, 130.2, 127.4, 124.4,
121.9, 121.6, 121.5, 115.9, 64.2.
3-(8-Ethoxy-2H-chromen-3-yl)-5-phenyl-1,2,4-oxadiazole
1
(6b).
White solid. (88%). m.p.: 180–182°C; H-NMR
(400 MHz, CDCl3): δ 8.19 (d, J = 8.0 Hz, 2H, Ar─H),
7.64–7.60 (m, 1H, Ar─H, CH), 7.58–7.54 (m, 3H,
Ar─H, CH), 6.90–6.83 (m, 3H, Ar─H), 5.33 (s, 2H,
CH2), 4.14 (q, J = 8.0 Hz, 2H, CH2), 1.49 (t, J = 8.0 Hz,
3H, CH3); 13C-NMR (100 MHz, CDCl3): δ 174.8, 165.9,
146.8, 143.5, 132.5, 128.8, 127.8, 123.6, 121.8, 121.1,
120.1, 119.0, 114.8, 64.4, 64.2, 14.5. ESI-HRMS (m/z):
Anal. Calcd for C19H16N2O3 [M + H]+ 321.1161; found:
(Z)-8-bromo-6-chloro-N0-hydroxy-2H-chromene-3-
carboximidamide (4h).
White solid. (87%). m.p.: 194–
1
197°C; H-NMR (400 MHz, DMSO-d6): δ 10.11 (s, 1H,
OH), 7.47 (s, 1H, Ar─H), 7.12 (s, 1H, Ar─H), 6.93 (s,
1H, CH), 5.66 (s, 2H, NH2), 4.97 (s, 2H, CH2); 13C-
NMR (100 MHz, DMSO-d6): δ 149.8, 148.8, 131.6,
127.9, 126.4, 126.0, 125.2, 119.4, 110.0, 65.4.
(Z)-6,8-dibromo-N0-hydroxy-2H-chromene-3-
321.1248.
carboximidamide (4i).
White solid. (87%). m.p.: 194–
3-(8-Methoxy-2H-chromen-3-yl)-5-(2,4,5-trimethoxyphenyl)-
196°C; 1H-NMR (400 MHz, CDCl3):
δ
7.49 (d,
1,2,4-oxadiazole (6c).
Yellow solid. (81%). m.p.: 203–
1
J = 2.4 Hz, 1H, Ar─H), 7.12 (d, J = 2.4 Hz, 1H, Ar─H),
6.57 (s, 1H, CH), 5.08 (s, 2H, CH2), 4.62 (s, 2H, NH2);
13C-NMR (100 MHz, CDCl3): δ 149.1, 135.2, 134.2,
128.9, 126.4, 124.0, 120.0, 113.4, 110.7, 65.1.
205°C; H-NMR (400 MHz, CDCl3): δ 7.58(s, 1H, CH),
7.55 (s, 1H, Ar─H), 6.91–6.87 (m, 2H, Ar─H), 6.83 (dd,
J = 4.0 Hz, 8.0 Hz, 1H, Ar─H), 6.60 (s, 1H, Ar─H), 5.31
(s, 2H, CH2), 3.98 (s, 6H, OCH3), 3.94 (s, 3H, OCH3),
3.89 (s, 3H, OCH3); 13C-NMR (100 MHz, CDCl3): δ
174.4, 165.4, 154.5, 153.9, 147.8 143.4, 143.2, 127.7,
122.1, 121.4, 120.3, 119.7, 113.4, 112.9, 104.2, 64.9,
56.9, 56.5, 56.1, 56.0. ESI-HRMS (m/z): Anal. Calcd for
Synthesis of chromene-based oxadiazole 6(a–r).
Procedure. Method A.
EDCI/HOBt (1.5 mmol) and
benzoic acid (1 mmol) were added to a solution of
hydroxy-chromene-carboximidamide (1 mmol) in DMF
(2 mL), and the mixture was refluxed for 2–6 h in a
sealed tube at 80°C. The progress of the reaction was
monitored by TLC. On completion of the reaction,
the reaction mixture was cooled to room temperature
C21H20N2O6 [M + H]+ 397.13214; found: 397.13979.
3-(6-Bromo-8-methoxy-2H-chromen-3-yl)-5-phenyl-1,2,4-
oxadiazole (6d).
Off white solid. (88%). m.p.: 200–
202°C; 1H-NMR (400 MHz, CDCl3):
δ
8.12 (d,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

N. Baral, S. Mohapatra, B. P. Raiguru, N. P. Mishra, P. Panda, S. Nayak, S. K. Pandey,
P. S. Kumar, and C. R. Sahoo
Vol 000
J = 8.0 Hz, 2H, Ar─H), 7.63–7.46 (m, 5H, Ar─H), 6.97 (s,
1H, Ar─H), 5.31 (s, 2H, CH2), 3.89 (s, 3H, OCH3); 13C-
NMR (100 MHz, CDCl3): δ 175.4, 165.9, 148.6, 133.8,
133.0, 130.2, 129.3, 129.1, 128.5, 128.2, 126.8, 122.6,
116.6, 113.3, 64.8, 56.3. ESI-HRMS (m/z): Anal. Calcd
64.6. ESI-HRMS (m/z): Anal. Calcd for C17H11ClN2O2
[M]+ 310.0509; found: 310.0673.
3-(2H-chromen-3-yl)-5-(2,4,5-trimethoxyphenyl)-1,2,4-
oxadiazole (6j).
Dirty yellow solid. (82%). m.p.: 200–
1
202°C; H-NMR (400 MHz, CDCl3): δ 7.61 (s, 1H), 7.58
(s, 1H), 7.21–719 (m, 2H), 6.95 (td, J = 7.4, 1.1 Hz, 1H),
6.90 (d, J = 8.0 Hz, 1H), 6.62 (s, 1H), 5.27 (s, 2H), 4.00
(s, 6H, OCH3), 3.97 (s, 3H, OCH3).13C-NMR (100 MHz,
CDCl3): δ 174.7, 165.8, 154.9, 154.8, 154.3, 143.5,
131.1, 128.4, 128.0, 122.0, 121.8, 119.9, 116.3, 113.2,
104.5, 64.9, 57.2, 56.8, 56.4. ESI-HRMS (m/z): Anal.
Calcd for C20H18N2O5 [M + H]+ 367.1216; found:
for C18H13BrN2O3 [M + H]+ 385.0110; found: 385.0119.
3-(6-Chloro-2H-chromen-3-yl)-5-(2,4,5-trimethoxyphenyl)-
1,2,4-oxadiazole (6e).
Yellow solid. (80%). m.p.: 158–
1
160°C; H-NMR (400 MHz, CDCl3): δ 7.56 (s, 1H, CH),
7.46–7.44 (m, 1H, Ar─H), 7.36–7.33 (m, 1H, Ar─H),
6.91 (d, J = 8.0 Hz, 1H, Ar─H), 6.79 (d, J = 8.0 Hz,
1H, Ar─H), 6.60 (s, 1H, Ar─H), 5.22 (s, 2H, CH2), 3.97
(s, 6H, OCH3), 3.93 (s, 3H, OCH3); 13C-NMR
(100 MHz, CDCl3): δ 174.5, 165.2, 154.6, 154.2, 153.0,
143.2, 130.3, 127.5, 126.8, 126.5, 125.7, 123.4, 117.9,
117.3, 113.0, 103.9, 64.7, 56.7, 56.5, 56.2. ESI-HRMS
367.1329.
3-(6,8-Dibromo-2H-chromen-3-yl)-5-phenyl-1,2,4-
oxadiazole (6k). Brown solid. (82%). m.p.: 220–222°C;
1H-NMR (400 MHz, CDCl3): δ 8.19 (d, J = 8.0 Hz, 2H),
7.65–7.62 (m, 1H, Ar─H), 7.59–7.57 (m, 2H, Ar─H),
7.52–7.48 (m, 2H, Ar─H), 7.17 (d, J = 4.0 Hz, 1H,
Ar─H), 5.39 (s, 2H, CH2); 13C-NMR (100 MHz,
CDCl3): δ 175.0, 165.8, 149.2, 136.0, 133.1, 129.8,
129.7, 129.5, 129.2, 128.6, 128.2, 126.2, 123.8, 121.3,
113.4, 108.4, 65.4. ESI-HRMS (m/z): Anal. Calcd for
(m/z): Anal. Calcd for C20H17ClN2O5 [M
+
H]+
401.0826; found: 401.0922.
3-(6-Bromo-2H-chromen-3-yl)-5-phenyl-1,2,4-oxadiazole
1
(6f). Light brown solid. (86%). m.p.: 190–193°C; H-
NMR (400 MHz, CDCl3): δ 8.18 (d, J = 8.0 Hz, 2H,
Ar─H), 7.67–7.53 (m, 3H, Ar─H), 7.48 (s, 1H, Ar─H),
7.41–7.30 (m, 2H, Ar─H), 6.78–6.76 (m,1H, Ar─H),
5.26 (s, 2H, CH2); 13C-NMR (100 MHz, CDCl3): δ
175.3, 165.9, 153.6, 133.4, 132.9, 130.4, 129.1, 128.2,
126.8, 123.9, 123.1, 120.6, 117.8, 113.7, 64.6. ESI-
HRMS (m/z): Anal. Calcd for C17H11BrN2O2 [M + Na]+
C17H10Br2N2O2 [M + H]+ 434.9089; found: 434.1509.
3-(8-Ethoxy-2H-chromen-3-yl)-5-(2,4,5-trimethoxyphenyl)-
1,2,4-oxadiazole (6l).
Brown solid. (80%). m.p.: 183–
185°C; 1H-NMR (400 MHz, CDCl3): δ 7.61 (s, 1H,
Ar─H), 7.57 (s, 1H, Ar─H), 6.89–6.82 (m, 3H, Ar─H),
6.62 (s, 1H, Ar─H), 5.33 (s, 2H, CH2), 4.14 (q,
J = 8.0 Hz, 2H, CH2), 4.0 (s, 6H, OCH3), 3.96 (s, 3H,
OCH3), 1.48 (t, J = 8.0 Hz, 3H, CH3); 13C-NMR
(100 MHz, CDCl3): δ 174.4, 165.5, 154.5, 153.9, 147.1,
143.8, 143.2, 127.8, 125.2, 122.3, 121.3, 120.3, 119.6,
115.0, 112.9, 104.3, 64.8, 64.6, 56.9, 56.5, 56.1, 14.8.
ESI-HRMS (m/z): Anal. Calcd for C22H22N2O6 [M + K]+
377.0004; found: 377.2188.
3-(6,8-Dichloro-2H-chromen-3-yl)-5-phenyl-1,2,4-
oxadiazole (6g). Yellow solid. (89%). m.p.: 112–114°C;
1H-NMR (400 MHz, CDCl3): δ 8.18 (d, J = 8.0 Hz, 2H),
7.62 (d, J = 8.0 Hz, 1H), 7.56 (m, 2H), 7.48 (s, 1H),
7.28 (s, 1H), 7.085 (d, J = 2.0 Hz, 1H), 5.37 (s, 2H).
13C-NMR (100 MHz, CDCl3): δ 175.5, 165.7, 148.9,
133.1, 130.6, 129.2, 128.2, 126.4, 126.3, 126.1, 123.8,
123.4, 121.9, 121.4, 65.3. ESI-HRMS (m/z): Anal.
Calcd for C17H10Cl2N2O2 [M + H]+ 345.0119; found:
449.1478; found: 449.9642.
5-(2-Chlorophenyl)-3-(2H-chromen-3-yl)-1,2,4-oxadiazole
1
(6m). Light yellow solid. (83%). m.p.: 109–110°C; H-
NMR (400 MHz, CDCl3): δ 8.11 (dd, J1 = 7.8,
J2 = 1.4 Hz, 1H, Ar─H), 7.39–7.35 (m, 4H, Ar─H),
7.26–7.20 (m, 2H, Ar─H), 6.98–6.93 (m, 1H, Ar─H),
6.91 (d, J = 8.0 Hz, 1H, Ar─H), 5.29 (s, 2H, CH2); 13C-
NMR (100 MHz, CDCl3): δ 174.1, 166.3, 154.9, 135.0,
133.9, 133.6, 132.8, 132.2, 131.8, 131.3, 128.8, 128.7,
127.4, 127.0, 122.1, 116.4, 64.7. ESI-HRMS (m/z): Anal.
345.0167.
3-(8-Bromo-6-chloro-2H-chromen-3-yl)-5-phenyl-1,2,4-
oxadiazole (6h). White solid. (82%). m.p.: 185–187°C;
1H-NMR (400 MHz, CDCl3): δ 8.17 (d, J = 8.0 Hz, 2H),
7.65–7.61 (m, 1H, Ar─H), 7.57–7.54 (m, 2H, Ar─H),
7.46 (s, 1H, Ar─H), 7.44 (d, J = 2.0 Hz, 1H), 7.11 (d,
J = 2.4 Hz, 1H), 5.37 (s, 2H); 13C-NMR (100 MHz,
CDCl3): δ 175.5, 165.6, 150.0, 139.0, 133.4, 133.1,
130.7, 129.2, 128.2, 126.8, 126.3, 123.3, 121.4, 110.5,
65.4. ESI-HRMS (m/z): Anal. Calcd for C17H10BrClN2O2
Calcd for C17H11ClN2O2 [M ꢀ H]+ 309.05091; found:
309.04273.
5-(4-Chlorophenyl)-3-(2H-chromen-3-yl)-1,2,4-oxadiazole
1
[M + H]+ 388.9614; found: 388.9583.
(6n). Yellow solid. (82%). m.p.: 114–116°C; H-NMR
(400 MHz, CDCl3): δ 8.13 (d, J = 8.0 Hz, 2H), 7.56–
7.53 (m, 3H), 7.23–7.19 (m, 2H), 6.96 (td, J = 7.4,
1.1 Hz, 1H), 6.90 (d, J = 8.0 Hz, 1H), 5.25 (s, 2H) .13C-
NMR (100 MHz, CDCl3): δ 174.4, 166.4, 154.7, 139.5,
131.2, 129.7, 129.6, 128.5, 128.4, 122.5, 121.9, 121.4,
119.2, 116.2, 64.5. ESI-HRMS (m/z): Anal. Calcd for
C17H11ClN2O2 [M ꢀ H]+ 309.05091; found: 309.04283.
3-(6-Chloro-2H-chromen-3-yl)-5-phenyl-1,2,4-oxadiazole
(6i).
White solid. (92%). m.p.: 104–106°C; 1H-NMR
(400 MHz, DMSO-d6): δ 8.12 (d, J = 8.0 Hz, 2H), 7.69–
7.50 (m, 5H), 7.26 (dd, J = 8.8, 2.4 Hz, 1H), 6.90 (d,
J = 8.8 Hz, 1H), 5.18 (s, 2H). 13C-NMR (100 MHz,
DMSO-d6): δ 175.5, 166.1, 153.3, 134.1, 131.1, 130.2,
128.5, 128.4, 127.2, 126.0, 123.6, 123.2, 120.8, 118.0,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Synthesis of Chromene-Fused Oxadiazole Derivatives
3-(2H-chromen-3-yl)-5-(2-methoxyphenyl)-1,2,4-oxadiazole
(6o). Brown solid. (80%). m.p.: 124–126°C; H-NMR
were in the tabular form [45]. The reference drug
gentamicin were aliquot in 100 micro leter at
concentration 10 microgram per mL for both, gram -ve
bacteria. The zone of inhibition rate 20 mm and/or more
than consider as highest active and consequently, less
than 20 mm as measured moderate active. Gentamicin
10 μg/mL with an average size of the zone of inhibition
of 20 mm and no zone of inhibition by DMSO 10%
solution was taken [46].
Determination of minimum inhibitory concentration.
The six different concentrations (μg/mL) of an individual
synthesized test sample and along with standard antibiotic
gentamicin were prepared in DMSO by serial dilution
technique; the values of concentration of tested samples
were 80, 60, and 40 μg/mL, respectively. The calculated
value of test sample was added to 96-well plates along
aliquots 20 μL inoculated around 107 CFU/mL. Five
microliter of an aliquot with 0.5% 2,3,5-
triphenyltetrazolinium chloride was added in each well of
96-well plates. The bacterial growth was observed by
pink coloration. After incubation of the well plates in
37°C for 24 h, the minimum inhibitory concentration
value of tested samples was determined by observing the
absence of bacterial growth in well plate.
1
(400 MHz, CDCl3): δ 8.10 (dd, J = 7.8, 1.8 Hz, 1H),
7.58–7.55 (m, 2H), 7.23–7.17 (m, 2H), 7.12–7.06 (m,
2H), 6.96 (td, J = 7.6, 0.9 Hz, 1H), 6.90 (d, J = 8.0 Hz,
1H), 5.27 (s, 2H), 4.0 (s, 3H, OCH3). 13C-NMR
(100 MHz, CDCl3): δ 174.6, 165.7, 158.5, 154.6, 134.2,
131.6, 130.8, 128.2, 127.9, 121.7, 121.4, 120.8, 119.5,
116.0, 113.6, 112.1, 64.6, 56.0. ESI-HRMS (m/z): Anal.
Calcd for C18H14N2O3 [M + H]+ 307.10044; found:
307.10751.
5-(4-Bromophenyl)-3-(2H-chromen-3-yl)-1,2,4-oxadiazole
1
(6p). Yellow solid. (90%). m.p.: 198–200°C; H-NMR
(400 MHz, CDCl3): δ 8.05 (d, J = 8.0 Hz, 2H), 7.70 (d,
J = 8.0 Hz, 2H), 7.56 (s, 1H), 7.27–7.19 (m, 2H), 6.98–
6.96 (m, 1H), 6.90 (d, J = 8.0 Hz, 1H), 5.25 (s, 2H). 13C-
NMR (100 MHz, CDCl3): δ 174.0, 166.0, 154.3, 132.2,
130.7, 129.2, 128.1, 127.9, 127.6, 122.5, 121.5, 120.9,
118.8, 115.7, 64.1. ESI-HRMS (m/z): Anal. Calcd for
C17H11BrN2O2 [M + H]+ 355.0004; found: 354.9960.
3-(2H-chromen-3-yl)-5-(m-tolyl)-1,2,4-oxadiazole (6q).
Brown solid. (90%). m.p.: 162–164°C; 1H-NMR
(400 MHz, CDCl3): δ 7.99–7.96 (m, 2H), 7.86–7.83 (m,
1H), 7.56 (s, 1H), 7.19 (td, J = 7.9, 1.6 Hz, 2H, Ar─H),
6.94 (td, J = 7.4, 1.1 Hz, 1H), 6.91–6.85 (m, 2H), 5.25
(s, 2H), 2.45 (s, 3H, OCH3). 13C-NMR (100 MHz,
CDCl3): δ 175.7, 166.5, 154.9, 139.4, 134.0, 131.2,129.3,
129.0, 128.5, 128.4, 126.8, 125.6, 122.1, 116.3, 64.8,
21.6. ESI-HRMS (m/z): Anal. Calcd for C18H14N2O2
[M + H]+ 291.1055; found: 291.1142.
Acknowledgments. S. R. M. and S. N. are thankful to CSIR,
New Delhi (02(0218)/14/EMR-II) and DRDO, New Delhi
(ERIP/ER/1203083/M/01) for providing research grant and also
thankful to DST-FIST, New Delhi, for providing NMR facility.
3-(2H-chromen-3-yl)-5-(4-methoxyphenyl)-1,2,4-oxadiazole
1
(6r).
White solid. (85%). m.p.: 128–130°C; H-NMR
REFERENCES AND NOTES
(400 MHz, CDCl3): δ 8.13 (d, J = 12 Hz, 2H, Ar─H),
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2H), 6.96 (td, J = 7.6, 0.9 Hz, 1H), 6.90 (d, J = 8.0 Hz,
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114.2, 64.2, 55.2. ESI-HRMS (m/z): Anal. Calcd for
C18H14N2O3 [M + H]+ 307.1004; found: 307.1093.
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SUPPORTING INFORMATION
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Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet