An efficient synthesis of some new 3-bipyridinyl substituted coumarins as potent antimicrobial agents

Article abstract of DOI:10.1016/j.cclet.2013.01.041

As a part of the ongoing studies in developing new antimicrobials, a series of structurally novel 3-bipyridinyl substituted coumarin derivatives 4a-f and 5a-f were synthesized by a single-step reaction protocol under Krohnke's reaction conditions. ^1<

Full text of DOI:10.1016/j.cclet.2013.01.041

Chinese Chemical Letters 24 (2013) 227–229
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
An efficient synthesis of some new 3-bipyridinyl substituted coumarins as potent
antimicrobial agents
*
Hemali B. Lad, Rakesh R. Giri, D.I. Brahmbhatt
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India
A R T I C L E I N F O
A B S T R A C T
Article history:
As a part of the ongoing studies in developing new antimicrobials, a series of structurally novel 3-
bipyridinyl substituted coumarin derivatives 4a–f and 5a–f were synthesized by a single-step reaction
protocol under Krohnke’s reaction conditions. 1H NMR, 13C NMR, IR and mass spectral techniques were
employed for the structural elucidation of the synthesized compounds. An evaluation of antimicrobial
activity showed that almost all compounds exhibited better results than the referenced drugs. Among
the synthesized derivatives 4f, 5a and 5d were found to be the most potent analogs. Thus they could be
promising lead for novel drugs.
Received 29 September 2012
Received in revised form 10 January 2013
Accepted 14 January 2013
Available online 7 March 2013
Keywords:
Antimicrobial activity
Bipyridinyl coumarins
Coumarins
ß 2013 D.I. Brahmbhatt. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Krohnke’s reaction
1. Introduction
we report the synthesis and antimicrobial activity of some new 3-
bipyridinyl substituted coumarins 4a–f and 5a–f by reacting various
In recent years, the mounting threat of bacterial resistance has
heightened the urgency to discover and develop effective agents
with novel mechanisms of action and enhanced activity. Thus
considerable research efforts have been directed to the discovery of
potent local antimicrobial agents with reduced or negligible
systemic adverse effects. The development of potent and effective
antimicrobial agents is most important to overcome the emerging
multi-drug resistance strains of bacteria and fungi. Recent observa-
tions suggested that coumarins and bipyridines have a wide range of
applications in the field of pharmaceuticals. Coumarins have been
reported to possess a significant range of biological properties such
as antitumor [1], anti-inflammatory [2] and antibacterial [3]
activities. 3-Pyridyl coumarins have been reported to function as
CNS depressant [4], antimicrobial agents [5], fish toxin [6] and
bactericidalagents[6]. Asurveyof the literaturerevealedthatcertain
bipyridinyl moieties had been reported to possess antimicrobial [7],
cardiotonic [8], and DNA interacting activity and show some
cytotoxicity [9]. In view of the aforementioned properties and as a
continuation of our ongoing research aiming at the synthesis of new
biologically active coumarin derivatives [10], we planned the
synthesis and evaluation of biological activity of coumarin deriva-
tivesbearingbipyridinemoietyatthe3-position. Inthepresentletter
3-coumarinoyl methyl pyridinium salts 1a–b with appropriate 3-
(pyridin-3-yl)-1-arylprop-2-ene-1-ones 2a–c and 3-(pyridin-4-yl)-
1-arylprop-2-ene-1-ones 3a–c, respectively, under the Krohnke’s
reaction conditions [11] with the belief that structural hybridization
of more than one bioactive moiety into a single scaffold may lead to
novel heterocycles having fascinating antimicrobial activity.
2. Experimental
Starting precursors, 3-coumarinoyl methyl pyridinium salts
1a–b, were prepared using the reported procedures [12]. The
procedure for the preparation of 2a–c and 3a–c is given in
Supplementary data.
General procedure for the synthesis of 3-(60-aryl-300,40-bipyridin-
20-yl)coumarins 4a–f and 3-(60-aryl-400,40-bipyridin-20-yl)coumar-
ins 5a–f:
A solution of appropriate 3-coumarinoyl methyl
pyridinium salt 1a–b (0.003 mol) in glacial acetic acid (50 mL)
was added in a round bottom flask. To this material was added
ammonium acetate (0.03 mol) and appropriate 3-(pyridin-3-yl)-
1-arylprop-2-ene-1-one 2a–c or 3-(pyridin-4-yl)-1-arylprop-2-
ene-1-one 3a–c (0.003 mol) with stirring. The stirring was further
continued for 45 min. Then the reaction mixture was refluxed for
8 h and was allowed to cool to room temperature. The reaction
mixture was poured into ice-cold water and extracted with
chloroform (3ꢀ 30 mL). The chloroform extracts were then
washed with water (2ꢀ 20 mL) and then dried over anhydrous
* Corresponding author.
E-mail address: drdib317@gmail.com (D.I. Brahmbhatt).
1001-8417/$ – see front matter ß 2013 D.I. Brahmbhatt. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.01.041

228
H.B. Lad et al. / Chinese Chemical Letters 24 (2013) 227–229
O
Het
O
10% aqueous NaOH
2a-c : Het=3-pyridyl
3a-c : Het =4-pyridyl
CHO
+
Het
R1
ethanol
R1
H3C
2a-c, 3a-c
R
O
1
5''
2''
8
O
O
2
6''
N1''
7
6
4''
3'
3
2'
4'
3''
2a-c
NH4OAc/AcOH
4a : R=H, R1=CH3
4b : R=H, R1=OCH3
4c : R=H, R1=Cl
N
R1
4
5
N
5'
1'
6'
4d : R=OCH3, R1=CH3
4a-f
4e : R=OCH
3, R =OCH3
R
4f : R=OCH3, R1=Cl
O
O
O
1
R1
3'
R
1
6''
8
PyBr
O
O
2
3
1''
2''
7
6
N
5''
4'
O
5a : R=H, R1 =CH3
5b : R=H, R1=OCH3
5c : R=H, R1=Cl
1a-b
2'
4''
4
3''
5
N
N
5'
1'
3a-c
NH OAc/AcOH
R1
6'
5d : R=OCH3, R1=CH3
5e : R=OCH3, R1=OCH3
5f : R=OCH3, R1=Cl
4
5a-f
R1
Scheme 1. Synthetic pathway for compounds 4a–f and 5a–f.
Na2SO4. The removal of chloroform under reduced pressure gave
crude products, which were subjected to column chromatography
using ethyl acetate–petroleum ether (1:4) as eluent to obtain the
pure products 4a–f and 5a–f (Scheme 1). The physical, analytical
and spectral data for compounds 4a and 5a are given below:
3-(60-p-Tolyl-300,40-bipyridin-20-yl)coumarin 4a: Yield: 75%; mp:
carbonyl functionality in 2a–c and 3a–c to afford the 1,5-
dicarbonyl intermediates, which in the presence of ammonia
cyclized to give the target compounds.
The structures of all the newly synthesized compounds were
confirmed by elemental analysis, FT-IR, 1H NMR and 13C NMR
spectral analysis. The tertiary and quaternary carbons were
discriminated by APT spectra. The structures of compounds 4a
and 5a were also supported by mass spectral analysis.
184 8C; IR (KBr, cmꢁ1):
str.), 1721 (C55O str.), 1596 and 1454 (Ar. C55C and C55N str.), 802
n 3033 (Ar. C–H str.), 2918 (aliphatic C–H
(C–H bending vib.); 1H NMR (400 MHz, CDCl3):
d
2.47 (s, 3H, CH3),
The IR spectrum of compound 5a exhibited a strong band at
7.35–8.12 (m, 11H, Ar–H), 8.68–9.04 (m, 4H, C4–H, C020ꢁH, C30 ꢁH
1731 cmꢁ1 for
d
-lactone C55O stretching. C55C and C55N aromatic
and C060ꢁH); 13C NMR (100 MHz, CDCl3):
d
21.35 (CH3), 116.43 (CH),
stretching bands were observed at 1610 cmꢁ1 and 1455 cmꢁ1
,
117.86 (CH), 119.59 (C), 120.21 (CH), 123.84 (CH), 124.63 (CH),
125.12 (C), 126.99 (CH), 129.02 (CH), 129.62 (CH), 132.28 (CH),
134.50 (C), 134.85 (CH), 136.26 (C), 139.59 (C), 142.99 (CH), 146.80
(C), 148.26 (CH), 150.02 (CH), 151.62 (C), 154.02 (C), 157.67 (C) and
160.42 (CO of coumarin); Anal. Calcd. for C26H18N2O2: C, 79.98; H,
4.65; N, 7.17. Found: C, 79.78; H, 4.67; N, 7.14.
respectively. The aromatic C–H bending vibrations for p-disubsti-
tuted benzene ring appeared at 816 cmꢁ1. The aliphatic and
aromatic C–H stretching vibrations were observed at 2922 cmꢁ1
and 3024 cmꢁ1, respectively. The 1H NMR spectrumof compound5a
showed a singlet at
d
2.47for themethylprotons. The C4–Happeared
as a singlet at
d
9.06. The chemical shift of C4–H was due to the para
3-(60-p-Tolyl-400,40-bipyridin-20-yl)coumarin 5a: Yield: 70%; mp:
237–239 8C; IR (KBr, cmꢁ1):
effect of the bipyridine nitrogen ðN01Þ as well as the deshielding
n
3024 (Ar. C–H str.), 2922 (aliphatic C–
effects of the coumarin carbonyl. The C020ꢁH and C600ꢁH appeared as
H str.), 1731 (C55O str.), 1610 and 1455 (Ar. C55C and C55N str.), 816
an ortho coupled doublet centered at
d
8.80 (J = 6.0 Hz). The C020ꢁH
(C–H bending vib.); 1H NMR (400 MHz, CDCl3):
d
2.47 (s, 3H, CH3),
and C060ꢁH appeared in the downfield region due to their attachment
7.36–8.09 (m, 11H, Ar–H), 8.71 (concealed doublet, 1H, C03ꢁH), 8.80
to carbons that are directly attached to the N010 in the bipyridine. The
(d, 2H, J = 6.0 Hz, C200ꢁH and C600ꢁH), 9.06 (s, 1H, C4–H); 13C NMR
C03ꢁH appeared as concealed meta coupled doublet centered at
d
(100 MHz, CDCl3):
d
21.36 (CH3), 116.45 (CH), 117.73 (CH), 119.55
8.71. The separation of C03ꢁH from other aromatic protons is due to
the para effect of the coumarin carbonyl group. The other remaining
(C), 119.98 (CH), 121.77 (CH), 124.66 (CH), 124.97 (C), 126.98 (CH),
129.05 (CH), 129.64 (CH), 132.35 (CH), 136.14 (C), 139.70 (C), 143.06
(CH), 146.23 (C), 147.18 (C), 150.56 (CH), 151.76 (C), 154.03 (C),
157.79 (C), 160.40 (CO of coumarin); Anal. Calcd. for C26H18N2O2: C,
79.98; H, 4.65; N, 7.17. Found: C, 79.70; H, 4.59; N, 7.18.
eleven aromatic protons appeared as a multiplet signal
In the 13C NMR spectrum of compound 5a, the signal appeared at
21.36wasassignedtothemethylcarbon.Themostdeshieldedsignal
that appeared at 160.40 can be assigned to the carbonyl carbon of
the -lactone ring in the coumarin. The signals for aromatic carbons
appeared between 116.45 and
157.79 in the 13C NMR spectrum.
d 7.36–8.09.
d
d
d
3. Results and discussion
d
d
The mass spectrum of compound 5a showed M+ peak at m/z 390
(100%) along with other fragment peaks, which supported the
structure of compound 5a.
3-Bipyridinyl substituted coumarins 4a–f and 5a–f were
synthesized according to the sequences shown in Scheme 1. The
reaction proceeded via a Michael addition by the nucleophilic
All the compounds 4a–f and 5a–f were screened for their
antimicrobial activity using Ampicillin, Norfloxacin, Nystatin and
addition of active methylene group in 1a–b to the a,b-unsaturated

H.B. Lad et al. / Chinese Chemical Letters 24 (2013) 227–229
229
Table 1
In vitro antimicrobial activity of compounds 4a–f and 5a–f.
Compound
Gram-positive bacteria
Gram-negative bacteria
Fungi
Sa. MTCC 96
Bs. MTCC 441
Ec. MTCC 443
St. MTCC 98
Ca. MTCC 227
An. MTCC 282
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
5f
A
100
250
200
100
500
500
250
500
100
100
250
200
250
10
100
500
125
125
200
500
100
250
100
62.5
100
100
250
100
–
250
125
250
500
250
62.5
100
500
100
500
250
200
100
10
250
125
100
500
500
200
62.5
500
250
500
200
250
100
10
500
>1000
1000
>1000
1000
500
1000
>1000
>1000
>1000
>1000
1000
500
500
1000
>1000
200
>1000
250
>1000
1000
1000
–
250
500
–
B
–
–
C
–
–
–
100
100
D
–
–
–
–
500
100
Sa., Staphylococcus aureus; Bs., Bacillus subtilis; Ec., Escherichia coli; St., Salmonella typhi; Ca., Candida albicans; An., Aspergillus niger; A, Ampicillin; B, Norfloxacin; C, Nystatin; D,
Greseofulvin.
Greseofulvin as standard drugs by the broth microdilution method
as recommended by NCCLS [13]. The assessment of antimicrobial
screening data (Table 1) revealed that almost all the compounds
4a–f and 5a–f showed good antibacterial activity against gram
positive bacteria and moderate activity against gram negative
bacteria. Among compounds 4a–f, having a (300, 40) linkage in the
Acknowledgments
The authors are thankful to the Head, Department of Chemistry,
Sardar Patel University for providing research amenities. Financial
assistance to HJP and RRG from the UGC, New Delhi, India, is highly
acknowledged.
bipyridine moiety, compounds 4a (MIC = 100 and 100
4d (MIC = 100 and 125 g/mL) exerted an improved inhibitory
activity against gram positive bacteria compared to Ampicillin
(MIC = 250 and 250 g/mL). It is perceived by examining the
mg/mL) and
Appendix A. Supplementary data
m
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2013.01.041.
m
antimicrobial data that introducing methoxy group in coumarin
ring does not affect the activity to a great extent. However,
replacing the methyl group in phenyl ring by a methoxy or a
chlorine group caused a drastic reduction in activity. The observed
pattern may be attributed to the enhanced lipophilicity due to the
methyl group. Compounds 5a–f, bearing (400, 40) linkage in
bipyridine moiety, exhibited excellent activity against Bacillus
subtilis compared to the standard drugs. It is interesting to note
that altering the substitution in the coumarin ring as well as in the
phenyl ring does not influence the activity to a large extent.
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mg/mL) showed excellent
activity against gram positive bacteria B. subtilis compared to both
standard drugs Ampicillin and Norfloxacin. Compounds 4f
(MIC = 62.5
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4. Conclusion
An efficient and straightforward protocol for the synthesis of a
series of 3-bipyridinyl substituted coumarin derivatives was
described and the synthesized compounds were evaluated for
antimicrobial activity. The results indicated that all the deriva-
tives exhibited appreciable antibacterial activities. In particular,
compounds 4f, 5a and 5d exhibited the more potent inhibitory
activity against bacterial and fungal strains compared to other
derivatives and emerged as potential lead compounds for further
investigations.
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