Bioorganic & Medicinal Chemistry Letters 23 (2013) 2714–2717

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Bioorganic & Medicinal Chemistry Letters

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Synthesis of pyrazole encompassing 2-pyridone derivativesas antibacterial agents

0960-894X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.bmcl.2013.02.077

⇑ Corresponding author. Tel./fax: +91 278 2439852.E-mail address: dnisheeth@rediffmail.com (N.C. Desai).

Nisheeth C. Desai ⇑, Kiran M. Rajpara, Vivek V. JoshiDivision of Medicinal Chemistry, Department of Chemistry, (DST-FIST Sponsored Department) Mahatma Gandhi Campus, Maharaja Krishnakumarsinhji Bhavnagar University,Bhavnagar 364 002, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 January 2013Revised 6 February 2013Accepted 15 February 2013Available online 26 February 2013

Keywords:Pyrazole2-PyridoneAntibacterial activityCytotoxicity

A series of novel compounds 6-amino-1-((1,3-diphenyl-1H-pyrazole-4-yl)methyleneamino)-4-(aryl)-2-oxo-1,2-dihydropyridine-3,5-dicarbonitriles (4a–t) were synthesized and characterized by IR, 1H NMR,13C NMR and mass spectral data. These compounds were screened for their in vitro antibacterial activityagainst Staphylococcus aureus, Streptococcus pyogenes (Gram positive), Escherichia coli, Pseudomonas aeru-ginosa (Gram negative) by serial broth dilution and cytotoxic activity (NIH 3T3 & HeLa) by MTT assay. Theresults indicated that compounds 4g, 4i, 4m, 4o, 4r and 4t exhibit potent antibacterial activity againstbacterial strains at non-cytotoxic concentrations.

� 2013 Elsevier Ltd. All rights reserved.

Since resistance of pathogenic bacteria towards available anti- activities.22 Recently, we have reported pyrazole containing com-

biotics is rapidly becoming a major worldwide problem, the designof new compounds to deal with resistant bacteria has become oneof the most important areas of antibacterial research today.1 De-spite the development of several new antibacterial agents, theirclinical value is limited to treating an increasing array of life-threatening systemic infections because of their relatively high riskof toxicity, emergence of drug resistant strains, pharmacokineticdifferences, and/or insufficiencies in their activity.

Pyridone and their derivatives play an essential role in severalbiological processes and have considerable chemical and pharma-cological importance.2–4 2-Pyridones represent a unique class ofpharmacophores, which are observed in various therapeuticagents5 and antibiotics.6 In recent years, 2-pyridones have capti-vated much importance as these compounds have been found toexhibit several biological activities, such as HIV-1 non-nucleosidereverse transcriptase inhibitors (NNRTIs),7 antifungals,8 sedatives9

and cardiotonic agents.10–12 Moreover, 2-pyridones are a class ofrecently discovered potent antibacterial agents that are of particu-lar interest due to their in vitro and in vivo antibacterial potenciesagainst the bacterial type II DNA topoisomerases, which includestwo highly homologous enzymes DNA gyrase and topoisomeraseIV.13 Furthermore, pyrazoles occupy a distinct place in heterocyclicchemistry and represent a key motif in medicinal chemistry due totheir capability to exhibit an array of bioactivities such as antimi-crobial,14–16 anticancer,17 anti-inflammatory,18 antidepressant,19

anticonvulsant,20 antipyretic21 and selective enzyme inhibitory

pounds as antibacterial agents (NCD1–17) (Fig. 1).23 Recognizingthese facts and in continuation of our previous work24,25 to lookfor new antibacterial agents with possible novel mechanisms of ac-tion, it was thought worthwhile to synthesize some new congenersof 2-pyridones by incorporating 2-pyridones and pyrazoles in asingle molecular framework.

The quest for synthesis of 6-amino-1-((1,3-diphenyl-1H-pyra-zole-4-yl)methyleneamino)-4-(aryl)-2-oxo-1,2-dihydropyridine-3,5-dicarbonitriles (4a–t) was accomplished in two steps as out-lined in Scheme 1. In the first step, 1,3-diphenylpyrazole-4-carbox-aldehyde (1), prepared according to previously publishedmethods26 was condensed with cyanoacetic acid hydrazide using1,4-dioxane as a solvent to yield the intermediate 2-cyano-N-((1,3-diphenyl-1H-pyrazol-4-yl)methylene)acetohydrazide (2).27

In the second step, the intermediate 2 was further reacted with(2-arylidene)malononitriles (3a–t) in the presence of catalyticamount of piperidine utilizing ethanol (95%) as a solvent to yieldthe desired products 6-amino-1-((1,3-diphenyl-1H-pyrazole-4-yl)methyleneamino)-4-(aryl)-2-oxo-1,2-dihydropyridine-3,5-dicarbonitriles (4a–t).28 The yields of the products were obtainedin the range of 54–65%. Designed series of molecules 4a–t werecharacterized by IR, 1H NMR, 13C NMR, and mass spectrometrytechniques before evaluating for in vitro antibacterial and cytotox-icity essays.

IR spectrum of compound 4o showed stretching band of car-bonyl group at 1684 cm�1 and absorption bands of NH2 group at3439 and 3448 cm�1. Strong intense absorption bands wereobserved at 2210 and 2214 cm�1 due to stretching vibration of–CBN groups. In 1H NMR spectra of 4o, characteristic signal of

NN

NN

S

N

O

F

N

N

N

O

NNN

H2N

NCD1-17

Present work

R

R

R = Differentsubstituents

Figure 1. Structural similarity between newly synthesized compounds and previ-ously synthesized compounds.

N. C. Desai et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2714–2717 2715

primary amine showed a broad singlet at d = 5.93 ppm which isexchangable with D2O. The aromatic and CH@N protons appearedat d = 7.29–8.50 ppm. The 13C NMR spectrum of compound 4o dis-played characteristic signal of carbonyl carbon at d = 171.2 ppm.Carbons of –CBN group showed a chemical shift atd = 115.6 ppm. Carbon of nitrophenyl ring showed a chemical shiftat d = 148.2 ppm. The mass spectrum revealed a molecular ionpeak at m/z = 526. In mass spectra, molecular ion peak is in agree-ment with proposed molecular weight and elemental analysis.

A plausible mechanism for the formation of compounds (4a–t)is suggested in Scheme 2. Firstly, the Michael addition of Schiffbase cyanoacetic hydrazide a to Knovenagel product b yieldedthe intermediate c, which further underwent intramolecular nucle-ophilic attack on cyano carbon followed by annulation to giveintermediate e. The compound e was converted to final compoundby intramolecular electron transfer to nitrogen atom.

All the newly synthesized compounds (4a–t) were initiallyscreened for their in vitro antibacterial activity against Gram posi-tive bacteria (Staphylococcus aureus (MTCC-96), Streptococcus pyog-enes (MTCC-442)) and Gram negative bacteria (Escherichia coli(MTCC-443), Pseudomonas aeruginosa (MTCC-1688)) using conven-tional broth-dilution method.29 The MIC (minimum inhibitory con-centration) values were determined by using ciprofloxacin asreference drug. The results of antibacterial studies of newly syn-thesized compounds revealed that these compounds exhibited sig-nificant antibacterial activity. From antibacterial activity data(Table 1), it was observed that compounds 4f (2-Cl), 4g (2-F), 4i(4-F), 4m (2-NO2), 4o (4-NO2), 4p (2-Br), 4r (4-Br) and 4t (4-CF3)were most active compounds. Compound 4m (2-NO2) exhibitedinhibition at MIC = 25 lg/mL and compounds 4i (4-F) and 4o (4-

NN

O

H

(1)

CNCH2CONHNH2

1,4-DioxaneReflux, 3h

N

N

R = Different subs

Scheme 1. Synthesis of ti

NO2) displayed strong inhibition action at MIC = 12.5 lg/mLagainst E. coli. Compounds 4i (4-F) and 4o (4-NO2) showed appre-ciable activity at MIC = 25 lg/mL against P. aeruginosa. Compounds4g (2-F), 4m (2-NO2) and 4t (4-CF3) exhibited significant potentialat MIC = 25 lg/mL while compounds 4i (4-F), 4p (2-Br) and 4r(4-Br) exerted highest inhibition at MIC = 12.5 lg/mL against S.aureus. Compound 4g (2-F) indicated good inhibition at MIC =25 lg/mL and compounds 4f (4-Cl) and 4o (4-NO2) showedremarkable activity at MIC = 12.5 lg/mL against S. pyogenes.

As shown in our results, some analogues of this series werefound to have more potency than the standard drug ciprofloxacinwhile some of them have comparable potency. The antimicrobialactivity was considerably affected by substitution pattern on thephenyl ring and the most active compounds contained an electronwithdrawing substituent at ortho and para positions of the phenylring. The role of electron withdrawing group in improving antimi-crobial activity is very well supported by previous studies.30,31

Compounds 4g, 4i, 4m, 4o, 4r and 4t bearing F, Cl, NO2, Br, andCF3 groups were most potent. Thus, compounds bearing substitu-ents at 2- or 4-position of the terminal benzene ring of 2-pyridonepart were found to have higher potency than compounds bearinggroups at 3-position or at both. Moreover all active compoundswere effective against Gram negative bacterial strain S. aureus. Restof the compounds bearing these substituents at 3-position showedmoderate activity with respect to standard drug against the teststrains.

All the newly synthesized compounds 4a–t were tested forcytotoxic activity on Mouse embryonic fibroblasts cell line (NIH3T3) and human cervical cancer cell line (HeLa) using MTT colori-metric assay.32 The IC50 values obtained for these compounds areshown in Table 2. Pyrazole analogues 4g, 4i, 4m, 4r and 4t showedno toxicity at concentration of 200 lg/mL (IC50 >200 lg/mL), whileother compounds showed moderate toxicity against NIH 3T3 cells.Moreover, none of the derivatives showed cytotoxicity againstHeLa cells (IC50 >200 lg/mL).

In conclusion, our attempts at exploring pyrazole based 2-pyri-done derivatives have unexpectedly led to the identification of anovel chemotype with substantial antibacterial activity againstvarious bacterial strains. Among the newly synthesized com-pounds 4a–t, analogues 4g (2-F), 4i (4-F), 4m (2-NO2), 4o (4-NO2), 4r (4-Br) and 4t (4-CF3) showed the highest inhibitionagainst nearly all of the tested bacteria, superior to the referencedrugs, and displayed antibacterial activity at non-cytotoxic con-centrations. Our preliminary results suggest that the mechanismof action may be distinct from that of known antibacterial agents.

NNH

ON

(2)

N

N

R+

PiperidineEthanol

Reflux, 3h

(3a-t)

Yield 73%

N

NN

N

H2N

ON

N

R

tituents (4a-t)

tle compounds 4a–t.

NO

HN

NR1

O

N

NH

N

N

+O

HN

NR1

N

C

NN

R = Different substituentsR1 = 1,3-Diphenyl-1H-pyrazol-4-yl

+

(4a-t)

N

N

..

O

HN

NR1 ..

N

NN

N

NH

O

N

N

N

R1

H

N

NH2

O

N

N

NR1

R

R

RR

R

R

R

(3a-t)

a

bc

d

e

Scheme 2. Plausible mechanistic pathway for the synthesis of compounds 4a–t.

Table 1Results of antibacterial screening of compounds 4a–t

Entry –R Minimum inhibitory concentration (MIC) in lg/mL ± SD

E. coliMTCC 443

P. aeruginosaMTCC 1688

S. aureusMTCC 96

S. pyogenesMTCC 442

4a –H 250 ± 3.05 500 ± 1.00 500 ± 4.04c 500 ± 3.46c

4b –2-OH 500 ± 2.64c 500 ± 1.00b 500 ± 3.21 250 ± 3.60b

4c –4-OH 500 ± 3.21 500 ± 2.88 500 ± 2.05b 500 ± 2.054d –2-C1 100 ± 1.60b 100 ± 2.04b 100 ± 3.05 100 ± 3.21c

4e –3-CI 50 ± 3.61c 250 ± 4.04 500 ± 1.16c 100 ± 3.214f –4-C1 500 ± 3.60 500 ± 1.60b 100 ± 1.05a 12.5 ± 4.16�4g –2-F 250 ± 1.20b 50 ± 3.60c 25 ± 2.04c 25 ± 1.92c

4h –3-F 100 ± 1.78c 50 ± 2.21b 50 ± 4.16 50 ± 3.78b

4i –4-F 12.5 ± 3.78c 25 ± 3.78 12.5 ± 2.04b 100 ± 3.464j –4-CH3 500 ± 2.44c 250 ± 1.21c 500 ± 4.16 250 ± 1.784k –4-OCH3 500 ± 4.40 500 ± 3.26 500 ± 3.78b 250 ± 4.61c

41 –3,4,5-(OCH3)3 500 ± 5.19c 250 ± 1.04c 500 ± 4.60c 250 ± 3.78c

4m –2-NO2 25 ± 1.60a 100 ± 3.78c 25 ± 2.64 250 ± 2.64b

4n –3-NO2 100 ± 1.52c 100 ± 2.04b 250 ± 4.04c 50 ± 2.05a

4o –4-NO2 12.5 ± 3.60 25 ± 1.16c 50 ± 4.50a 12.5 ± 3.60c

4p –2-Br 100 ± 2.52 250 ± 2.98a 12.5 ± 1.86b 250 ± 2.84c

4q –3-Br 100 ± 1.24a 250 ± 2.16c 250 ± 2.48c 500 ± 2.48a

4r –4-Br 500 ± 2.14c 100 ± 1.28 12.5 ± 2.28 100 ± 3.64c

4s –2-CF3 100 ± 1.38 250 ± 1.84c 100 ± 1.34a 250 ± 1.524t –4-CF3 250 ± 3.46c 25 ± 1.24c 25 ± 1.68c 250 ± 2.10c

Ciprofloxacin 25 ± 2.05 25 ± 1.0 50 ± 1.52 50 ± 2.06

±SD = standard deviation.a P <0.001 extremely significant.b P <0.01 moderately significant.c P <0.05 significant. All values are presented as mean of 6 experiments (n = 6). All significant differences are considered from control value 0.00.

2716 N. C. Desai et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2714–2717

Structure–activity relationship studies have led to identification ofpositions of pharmacophores on the scaffold for additionalstructural modifications. These modifications would allow theintroduction of probes designed to examine cognate bindingpartners and molecular targets, at the same time not significantlycompromising antibacterial potency.

Acknowledgment

We would like to express our sincere gratitude to the Depart-ment of Chemistry, (DST-FIST Sponsored), Mahatma GandhiCampus, Maharaja Krishnakumarsinhji Bhavnagar University,Bhavnagar for providing research and library facilities.

Table 2Cytotoxic activity of compounds 4a–t against NIH 3T3 and HeLa cell line

Entry Cytotoxicity (IC50)(lg/mL)a

NIH 3T3b HeLac

4a 180 ± 2.4 >2004b 162 ± 4.8 >2004c 110 ± 4.4 >2004d 84 ± 2.4 >2004e 52 ± 1.2 >2004f 64 ± 3 >2004g >200 >2004h 154 ± 3.2 >2004i >200 >2004j 98 ± 2.6 >2004k 50 ± 1.4 >2004l 64 ± 2.4 >2004m >200 >2004n 124 ± 1.8 >2004o 84 ± 3.6 >2004p 174 ± 2.8 >2004q 42 ± 3.2 >2004r >200 >2004s 56 ± 2.6 >2004t >200 >200

The values represent mean ± SD.a IC50 is the concentration required to inhibit 50% of cell growth.b NIH 3T3: Mouse embryonic fibroblasts cell line.c HeLa: Human cervical cancer cell line.

N. C. Desai et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2714–2717 2717

Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.bmcl.2013.02.077.

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yl)methylene)acetohydrazide (2): A mixture containing 1,3-diphenylpyrazole-4-carbaldehyde (1) (0.01 mol) and 2-cyanoacetohydrazide (0.01 mol) in 1,4-dioxane (30 mL) was refluxed for 1 h. After cooling, the crystals formed werefiltered, washed with water and recrystallized from ethanol (95%) to givecompound (2).

28. General procedure for the synthesis of 6-amino-1-((1,3-diphenyl-1H-pyrazole-4-yl)methyleneamino)-4-(4-methoxyphenyl)-2-oxo-1,2-dihydropyridine-3,5-dicarbonitriles (4a–t): A mixture containing 2-cyano-N-((1,3-diphenyl-1H-pyrazol-4-yl)methylene)acetohydrazide (2) (0.01 mol), 2-(arylidene)malononitriles (3a–t) (0.01 mol) and catalytic amount ofpiperidine in ethanol (95%) (30 mL) was refluxed for 3 h. After cooling, thecrystals formed were filtered, washed with water and recrystallized fromaqueous DMF to give compounds 4a–t.

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