Quinoxaline-PABA bipartite hybrid derivatization approach: Design and search for antimicrobial agents

Article abstract of DOI:10.1016/j.molstruc.2019.02.074

In an attempt to find a new class of antimicrobial agents, in the present study we report the synthesis of bipartite hybrid styryl derivatives of quinoxaline containing para-aminobenzoic acid (PABA) and their biological evaluation as antimicrobial agents. The series of new substituted styryl based derivatives 5(a–k) were evaluated for antimicrobial potential against various bacteria including Staphylococcus aureus, Vibrio cholerae, Escherichia coli, Bacillus subtilis, Escherichia coli, Mycobacterium smegmatis, Pseudomonas aeruginosa and fungi; C. albicans, with ampicillin and amphotericin B as standards. Similarly these compounds were also screened for anti-cancer activity using MCF-7 cell line. Among the synthesized compounds, 5(c) was observed to be the most active compound against various strains with MIC in a range of 7.9–31 μM of the series and compound 5i came out with significant anti-cancer activity with IC₅₀ value of 7 μM.

Full text of DOI:10.1016/j.molstruc.2019.02.074

Accepted Manuscript
Quinoxaline-PABA bipartite hybrid derivatization approach: Design and search for
antimicrobial agents
Harun M. Patel, Varun Bhardwaj, Poonam Sharma, Malleshappa N. Noolvi, Sandeep
Lohan, Sumit Bansal, Amit Sharma
PII:
S0022-2860(19)30204-2
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.074
MOLSTR 26231
Reference:
To appear in: Journal of Molecular Structure
Received Date: 4 December 2018
Revised Date: 18 February 2019
Accepted Date: 19 February 2019
Please cite this article as: H.M. Patel, V. Bhardwaj, P. Sharma, M.N. Noolvi, S. Lohan, S. Bansal, A.
Sharma, Quinoxaline-PABA bipartite hybrid derivatization approach: Design and search for antimicrobial
agents, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.02.074.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Quinoxaline-PABA Bipartite Hybrid Derivatization Approach: Design and Search for Antimicrobial Agents
Harun M. Patel, Varun Bhardwaj, Poonam Sharma, Malleshappa N. Noolvi, Sandeep Lohan, Sumit Bansal, Amit Sharma

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Quinoxaline-PABA Bipartite Hybrid
Derivatization Approach: Design and Search
for Antimicrobial Agents
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Harun M. Patel a, Varun Bhardwaj *, b, Poonam Sharma a, Malleshappa N. Noolvi c, Sandeep
Lohan a, Sumit Bansal a, Amit Sharma d
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aDepartment of Pharmaceutical Chemistry, R.C. Patel Institute of Pharmaceutical Education and Research,
Shirpur (Dhule) 425405, Maharashtra, India.
bDepartment of Biotechnology, Bioinformatics and Pharmacy, Jaypee University of Information Technology,
Waknaghat, Solan, 173234 (Himachal Pradesh) India.
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cDepartment of Pharmaceutical Chemistry, Shree Dhanvantary Pharmacy College, Kim (Surat) 394110,
Gujarat, India.
dSchool of Public Health, Postgraduate Institute of Medical Education & Research (PGIMER), Chandigarh,
160012, India.
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*Corresponding Author:
Tel: +91-94189-05865, Fax: +91-1792-245362
Email: varunmilton@gmail.com
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Abstract
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In an attempt to find a new class of antimicrobial agents, in the present study we report the
synthesis of bipartite hybrid styryl derivatives of quinoxaline containing para-aminobenzoic
acid (PABA) and their biological evaluation as antimicrobial agents.The series of new
substituted styryl based derivatives 5(a-k) were evaluated for antimicrobial potential against
various bacteria including Staphylococcus aureus, Vibrio cholerae, Escherichia coli,
Bacillus subtilis, Escherichia coli, Mycobacterium smegmatis, Pseudomonas aeruginosa and
fungi; C. albicans, with ampicillin and amphotericin B as standards. Similarly these
compounds were also screened for anti-cancer activity using MCF-7 cell line. Among the
synthesized compounds, 5(c) was observed to be the most active compound against various
strains with MIC in a range of 7.9-31 µM of the series and compound 5i came out with
significant anti-cancer activity with IC50 value of 7 µM.
Keywords: Synthesis, Quinoxaline, PABA, Anti-microbial activity, Anti-cancer activity
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1. Introduction
Morbidity and mortality because of bacterial infection stay imperative health issues
worldwide, mostly in developing nations and regions, for example, the Indian sub-continent,
some portions of South America and tropical parts of Africa [1,2]. Patients usually
demonstrate wide variety of symptoms for instance, stomach-ache, cramps, bloating or
tenderness [3]. Amoxicillin, norfloxacin and ciprofloxacin are the most well-known drugs
utilized for such infection [4] yet are related with severe side effects. Heterocyclic
compounds provide chemical scaffolds on top of which, pharmacophores can organize to
yield potential and selective drugs [5,6]. The magnitude of heterocyclic compounds was
perceived long in synthetic organic/medicinal chemistry. It is notable that the many of
heterocyclic compounds having sulfur and nitrogen showed a wide assortment of biological
activities that have been investigated and explored for growing pharmaceutically vital
molecules [7]. Among the different types of nitrogen bearing heterocyclic compounds,
quinoxaline derivatives are well known as antimicrobial agents. Quinoxaline ring with certain
fused moiety drastically boosts the variety of certain biological activity [8]. Different
quinoxaline derivatives have been recorded as antimycobacterium [9], antibacterial [10],
anticonvulsant [11], anti-inflammatory and analgesic [12], antimalarial [13], antiviral [14]
and anticancer activity [15].
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There are recognized compounds having pyrazine identified with quinoxaline ring structure,
the most perceived are cyridina, aspergillic acid, luciferin, echinomycin, methoxy Pyrazine,
triostin A and cholenterazine etc. Quinoxaline derivatives, including carbadox, mequindox
and quinocetone, are a class of synthetic heterocycles that are known as potent antibacterial
agents against many Gram-positive and Gram-negative bacteria, especially anaerobes.
Carbadox, mequindox are commonly used as food additives to prevent bacterial infectious
disease and improve animal growth in livestock and poultry as shown in Fig 1. [16].
Conversely, para-amino benzoic acid is additionally known to have antiviral, antioxidant and
ophthalmology properties [2]. It is well known that PABA is synthesized by bacteria to be
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used in folic acid synthesis. PABA was one of the main biologically active constituent to be
utilized in sunscreen as Patented in 1943 [17]. The initial in vivo experiment on mice
demonstrated that PABA lessened UV damage and in addition ensures against skin cancer in
rodents (Flindt-Hansen et al., 1990) [18]. Hence in the current study, we synthesized and
evaluated quinoxaline-PABA bipartite hybrid derivatives 5(a-k) for antibacterial and
antifungal activity. Akberova revealed that when combined with some antibiotics for instance
ciprofloxacin and polymixin B, PABA applies a synergistic antibacterial impact against
different bacterial strain for instance P. mirabilis and S. aureus [19]. Multiple efforts were
made to contemplate the impacts of various functional groups on the homocycle /the
heterocycle for improvement in the bioactivity [20-25]. The present work pointed towards
developing new molecules with better potential for anti-microbial and anti-cancer activity.
Our approach is aimed at designing a variety of ligand by adding alternative binding moiety
with varied chemical properties on quinoxaline, for example, PABA at C-2 position with
substituted styryl derivatives at C-4 position. The new bipartite compounds were designed, in
light of the fusion between quinoxaline and PABA as appeared in Fig 2. The designing of
compounds depends on molecular hybridization approach where amalgamation of
pharmacophoric scaffold of dissimilar bioactive compounds were done to construct a novel
hybrid compound together with better affinity and efficacy as described by Viegas-Junior et
al [26]. In the perspective of biological importance of these two moieties, the new
quinoxaline-PABA bipartite hybrid derivatives 5(a-k) were synthesized and screened for
anti-bacterial, anti-fungal and anti-cancer activity.
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2. Materials and Methods
2.1. Chemistry
The title compounds 5(a-k) (Quinoxaline-PABA Bipartite Hybrid) were synthesized as
depicted in Scheme-1. Condensation of o-phenylenediamine 1 with pyruvic acid in an
aqueous medium alone yielded 3-methyl quinoxalione-2(1H)-one 2. The active methyl
functional group is reacted with various aldehydes using acetic acid as solvent and drops of
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sulfuric acid as catalyst to afford 3(a-k). Chlorination of synthesized compounds 3(a-k) with
phosphorous oxychloride (POCl3) in the presence of catalytic extent of dimethyl formamide
(DMF) under reflux condition yielded the corresponding chloro compounds 4(a-k). This 4(a-
k) on reaction with para-aminobenzoic acid gave the corresponding substituted final
compounds 5(a-k).
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2.2. Experimental
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All the solvents and chemicals have been provided by Spectrochem and Sigma-Aldrich. The
reactions have been monitored with the aid of pre-coated silica gel TLC aluminium sheets.
Melting points were established using an Analab Scientific Melting point apparatus. FTIR
spectra were documented utilizing FTIR-8400S Shimadzu spectrometer. 1HNMR
(DMSO/CDCl3) spectra of the compounds have been determined by Bruker Avance-II
spectrometer at 400 MHz (Punjab University-Chandigarh). Chemical shifts have been
assessed relative to the internal standard TMS and are reported in δ ppm. Mass spectra of the
compounds were determined at Oxygen Heath Care Pvt.Ltd.at Ahmadabad, Gujarat.
2.2.1 3-methylquinoxalin-2(1H)-one (2)
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It is prepared as per the reported procedure by Noolvi et al., 2011 [27].
2.2.2 Synthesis of 3-(4-substitutedstyryl) quinoxalines-2(1H)-ones 3(a-k)
It is prepared as per the reported procedure by Noolvi et al., 2011 [27].
2.2.3 Synthesis of 2-chloro-3-substituted styryl quinoxalines 4(a-k)
It is prepared as per the reported procedure by Noolvi et al., 2011 [27].
2.2.4 General procedure for the synthesis of 4-{3-[2-(substituted styryl)]-quinoxalin-2-
ylamino}-benzoic acid 5(a-k)
A mixture of p-amino benzoic acid (10mM), triethylamine (20 mM) and 2-chloro-3
substituted styryl quinoxalines 4 (a-k) (10 mM) in methanol was refluxed for 11 hrs. It was
then cooled to RT the separated solid was filtered off, washed and recrystallized with ethanol.
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2.2.4.1 4-((3-(3-nitro styryl)quinoxalin-2-yl)amino)benzoic acid (5a)
°
Yield 43 %; mp 220-223 C; IR (KBr) ѵmax 3404 (OH str. of COOH ), 3252 (NH str.), 3023
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(CH str.), 1712 (C=O), 1348 and 1549 (NO2) cm-1; H NMR (DMSO-d6) δ ppm: 7.26-9.00
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(m, 12H, Ar-H), 5.92 (d, 1H, olefinic CH), 5.27 (d, 1H, olefinic CH), 10.63 (s, 1H, NH),
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11.53 (s, 1H, COOH); C NMR (DMSO-d6) δ ppm:168.70, 157.45, 148.60, 141.10, 134.83,
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134.20, 133.92, 130.28, 129.65, 129.63, 128.82, 128.64, 127.99, 127.74, 127.66, 127.45,
127.34, 126.56, 126.56, 125.08, 122.12, 119.26 ; HRMS (EI) m/z calcd for C23H16N4O4:
412.1172; found: 412.1178.
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2.2.4.2 4-{3-[2-(3-bromo styryl)]-quinoxalin-2-ylamino}-benzoic acid (5b)
°
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Yield 52 %; mp 226-228 C; IR (KBr) ѵmax 3412 (OH str. of COOH), 3261 (NH str.), 3083
(CH str.), 1708 (C=O), 652 (C-Br) cm-1; 1H NMR (DMSO-d6) δ ppm: 7.18-8.25 (m, 12H, Ar-
H), 5.98 (d, 1H, olefinic CH), 5.34 (d, 1H, olefinic CH), 10.34 (s, 1H, NH), 11.84 (s, 1H,
COOH); 13C NMR (DMSO-d6) δ ppm: 172.28, 161.85, 158.19, 148.81, 144.29, 143.86,
141.49, 139.88, 139.33, 135.74, 134.28, 132.17, 130.77, 130.52, 129.35, 128.72, 125.85,
125.83, 124.29, 121.83, 115.26; HRMS (EI) m/z calcd for C23H16BrN3O2: 445.0426; found:
445.0419 [M+] and 447.0419 [M+2].
2.2.4.3 4-{3-[2-(4-cyano styryl)]-quinoxalin-2-ylamino}-benzoic acid (5c)
°
Yield 41 %; mp 246-248 C; IR (KBr) ѵmax 3396 (OH str. of COOH), 3231 (NH str.), 3062
(CH str.), 2243 [CN (nitrile)], 1716 (C=O) cm-1; H NMR (DMSO-d6) δ ppm: 7.32-8.24 (m,
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12H, Ar-H), 5.99 (d, 1H, olefinic CH), 5.42 (d, 1H, olefinic CH), 10.98 (s, 1H, NH), 12.13
(s, 1H, COOH); 13C NMR (DMSO-d6) δ ppm: 170.65, 162.84, 155.38, 145.41, 142.80,
141.47, 141.19, 138.82, 135.72, 134.60, 132.55, 130.87, 128.46, 128.01, 125.56, 125.95,
124.37, 122.84, 115.48, 115.04; HRMS (EI) m/z calcd for C24H16N4O2: 392.1273; found:
392.1279.
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2.2.4.4 4-{3-[2-(4-methyl styryl)]-quinoxalin-2-ylamino}-benzoic acid (5d)
°
Yield 40 %; mp 200-202 C; IR (KBr) ѵmax 3402 (OH str. of COOH), 3272 (NH str.), 3042
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(CH str.), 1705 (C=O) cm-1; H NMR (DMSO-d6) δ ppm: 2.54 (s, 3H, CH3), 7.24-8.34 (m,
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12H, Ar-H), 5.88 (d, 1H, olefinic CH), 5.25 (d, 1H, olefinic CH), 10.78 (s, 1H, NH), 12.02 (s,
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1H, COOH); C NMR (DMSO-d6) δ ppm: 172.57, 161.96, 152.46, 144.95, 142.08, 141.92,
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140.58, 137.49, 135.89, 134.64, 134.07, 130.73, 129.39, 128.75, 125.58, 125.64, 125.26,
121.48, 117.49, 21.83; HRMS (EI) m/z calcd for C24H19N3O2: 381.1477; found: 381.1471.
2.2.4.5 4-{3-[2-(4-hydroxy styryl)]-quinoxalin-2-ylamino}-benzoic acid (5e)
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°
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Yield 48 %; mp 194-196 C; IR (KBr) ѵmax 3532 (OH str. phenolic), 3382 (OH str. of
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COOH), 3234 (NH str.), 3063 (CH str.), 1721 (C=O) cm-1; 1H NMR (DMSO-d6) δ ppm: 5.45
(s, 1H, OH), 5.01 (s, 1H, NH), 7.21-8.24 (m, 12H, Ar-H), 6.11 (d, 1H, olefinic CH), 5.70 (d,
1H, olefinic CH), 11.92 (s, 1H, COOH); 13C NMR (DMSO-d6) δ ppm: 171.64, 158.29,
151.48, 145.27, 144.66, 140.16, 138.78, 138.25, 135.50, 135.71, 134.55, 132.56, 129.18,
128.18, 127.27, 125.08, 124.19, 118.34, 115.26; HRMS (EI) m/z calcd for C23H17N3O3:
383.1270; found: 383.1277.
2.2.4.6 4-{3-[2-(4-dimethylamino styryl)]-quinoxalin-2-ylamino}-benzoic acid (5f)
°
Yield 45 %; mp 234-236 C; IR (KBr) ѵmax 3392 (OH str. of COOH), 3221 (NH str.), 3020
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(CH str.), 1723 (C=O) cm-1; H NMR (DMSO-d6) δ ppm: 2.83 (s, 6H, CH3), 5.80-8.46 (m,
12H, Ar-H), 6.01 (d, 1H, olefinic CH), 5.68 (d, 1H, olefinic CH), 10.69 (s, 1H, NH), 11.86
(s, 1H, COOH); 13C NMR (DMSO-d6) δ ppm: 172.42, 158.76, 152.90, 147.69, 142.36,
140.95, 139.84, 137.82, 135.43, 135.41, 134.79, 130.46, 128.35, 125.19, 125.25, 125.44,
122.77, 115.37, 115.41, 43.57; HRMS (EI) m/z calcd for C25H22N4O2: 410.1743; found:
410.1748.
2.2.4.7 4-(3-styryl-quinoxalin-2-ylamino)-benzoic acid (5g)
°
Yield 40 %; mp 196-198 C; IR (KBr) ѵmax 3394 (OH str. of COOH), 3234 (NH str.), 3024
(CH str.), 1711 (C=O) cm-1; 1H NMR (DMSO-d6) δ ppm: 5.92-8.37 (m, 13H, Ar-H), 6.12 (d,
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1H, olefinic CH), 5.78 (d, 1H, olefinic CH), 10.68 (s, 1H, NH), 11.97 (s, 1H, COOH); C
NMR (DMSO-d6) δ ppm: 173.12, 155.29, 154.25, 148.17, 145.36, 144.14, 142.66, 140.72,
138.24, 135.88, 135.33, 134.86, 132.42, 130.14, 128.25, 125.20, 124.44, 121.29, 115.42;
HRMS (EI) m/z calcd for C23H17N3O2: 367.1321; found: 367.1328.
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2.2.4.8 4-{3-[2-(2,5-dimethyoxy styryl)]-quinoxalin-2-ylamino}-benzoic acid (5h)
°
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Yield 49 %; mp 188-190 C; IR (KBr) ѵmax 3392 (OH str. of COOH), 3221 (NH str.), 3020
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(CH str.), 1723 (C=O) cm-1; 1H NMR (DMSO-d6) δ ppm: 3.81 (s, 6H, OCH3), 7.02-8.14 (m,
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11H, Ar-H), 6.56 (d, 1H, olefinic CH), 5.92 (d, 1H, olefinic CH), 10.45 (s, 1H, NH), 12.07 (s,
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1H, COOH); C NMR (DMSO-d6) δ ppm: 172.26, 160.45, 155.33, 152.15, 150.39, 148.68,
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142.29, 142.24, 140.42, 138.58, 135.42, 132.20, 129.83, 128.47, 128.26, 124.26, 123.23,
115.62, 115.14, 114.63, 110.28, 55.61, 55.01; HRMS (EI) m/z calcd for C25H21N3O4:
427.1532; found: 427.1538.
2.2.4.9 4-{3-[2-(2-hydroxy-phenyl)- styryl]-quinoxalin-2-ylamino}-benzoic acid (5i)
°
Yield 38 %; mp 182-184 C; IR (KBr) ѵmax 3456 (OH str. phenolic), 3391 (OH str. of
COOH), 3221 (NH str.), 3063 (CH str.), 1708 (C=O) cm-1; 1H NMR (DMSO-d6) δ ppm: 4.94
(s, 1H, NH), 5.16 (s, 1H, OH), 5.95-8.40 (m, 12H, Ar-H), 5.70 (d, 1H, olefinic, CH), 5.50 (d,
1H, olefinic CH), 11.82 (s, 1H, COOH); 13C NMR (DMSO-d6) δ ppm: 170.41, 153.22,
151.30, 148.13, 142.86, 140.53, 138.37, 135.76, 135.20, 134.87, 134.34, 132.04, 129.38,
128.35, 127.57, 125.16, 124.84, 122.34, 117.21, 115.21; HRMS (EI) m/z calcd for
C23H17N3O3: 383.1270; found: 383.1277.
2.2.4.10 4-{3-[2-(4-fluoro styryl)]-quinoxalin-2-ylamino}-benzoic acid (5j)
°
Yield 44 %; mp 178-180 C; IR (KBr) ѵmax 3379 (OH str. of COOH), 3211 (NH str.), 3023
(CH str.), 1712 (C=O), 1212 (C-F) cm-1; 1H NMR (DMSO-d6) δ ppm: 7.14-8.30 (m, 12H, Ar-
H), 5.90 (d, 1H, olefinic CH), 5.40 (d, 1H, olefinic CH), 10.67 (s, 1H, NH), 12.05 (s, 1H,
COOH); 13C NMR (DMSO-d6) δ ppm: 172.42, 154.32, 151.17, 145.42, 144.40, 142.16,
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139.12, 138.47, 135.26, 134.29, 134.14, 131.42, 129.38, 127.28, 127.31, 125.95, 124.45,
117.26, 115.18; HRMS (EI) m/z calcd for C23H16FN3O2: 385.1227; found: 385.1222.
2.2.4.11 4-{3-[2-(4-chloro styryl)]-quinoxalin-2-ylamino}-benzoic acid (5k)
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°
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Yield 61 %; mp 184-186 C; IR (KBr) ѵmax 3389 (OH str. of COOH), 3242 (NH str.), 3042
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(CH str.), 1714 (C=O), 724 (C-Cl) cm-1; H NMR (DMSO-d6) δ ppm: 7.26-8.40 (m, 12H,
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Ar-H), 5.88 (d, 1H, olefinic CH), 5.38 (d, 1H olefinic CH), 10.98 (s, 1H, NH), 12.12 (s, 1H,
COOH); 13C NMR (DMSO-d6) δ ppm: 171.38, 155.55, 152.14, 148.75, 145.24, 142.34,
138.98, 138.33, 135.35, 135.82, 134.41, 132.23, 129.44, 128.86, 127.24, 125.40, 122.92,
115.40, 115.63; HRMS (EI) m/z calcd for C23H16ClN3O2: 401.0931; found: 401.0938 [M+]
and 402.0927 [M+1]
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2.2.5 Antimicrobial activity
All the synthesized derivatives 5(a-k) were assessed for antibacterial activity towards the S.
aureus (ATCC: 25923), B. subtilis (ATCC-6633), V. cholerae (ATCC-39315), E. coli
(ATCC:25922), M. smegmatis (ATCC-700084), and Pseudomonas aeruginosa
(ATCC:27853); and for antifungal activity against one pathogenic fungi C. albicans (ATCC:
24433) utilizing amphotericin B and ampicillin and as standard. The antimicrobial testing
have been carried out by the disc diffusion and the agar dilution methods of NCCLS utilizing
nutrient agar and potato dextrose agar for bacteria and fungi correspondingly [28,29].
2.2.6 Anti-cancer Activity
A tetrazolium salt 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT),was
used as a colorimetric substrate for measuring cytotoxicity (MTT assay). Briefly, MCF-7
cells were cultured in 96 well plates and 5, 10, 50, 100 and 200 µg/mL quinoxalines were
o
added. The plates were incubated for 24 h at 37 C in 5% CO2 humidified in cubator to get
her with untreated control sample. After incubation period, 20µL MTT dye was added and
the plates were measured with a ELx808-IUBio-Tek apparatus at 540 nm. Control cell
viability was regarded as100%. Stock solutions of the test compounds were dissolved in
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Dimethyl sulfoxide (DMSO) and further concentrations were prepared in cell culture media.
All experiments were repeated three times. For each of the compound doses, four
independent wells were used. In this study, Doxorubicin was used as standard drug [30].
3. Results and Discussion
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3.1 Spectral Characterization
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In the present work, derivatives are prepared according to the indicated reactions in Scheme
1. Spectral data was found in full concurrenc with proposed structure within the scheme. The
synthesis of compound 2, 3(a-k) and 4(a-k) was carried out as per the reported procedure
given by Noolvi et al [27]. The synthesis of title compound 5(a-k) was confirmed by the IR
band at ~ 3400 cm-1 for OH stretch (-COOH) and 2900 cm-1 for olefinic -CH-. Singlet ~ 11-
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12 δ ppm in H NMR and peak ~ 170 δ ppm in C NMR confirms COOH group of para
amino benzoic acid of final compounds 5(a-k). Further molecular ion peaks in HRMS
corresponding to molecular weight of synthesized compounds confirm the synthesis of final
compounds.
3.2 Anti-Microbial Activity
We designed and synthesized novel compounds containing quinoxaline-PABA bipartite
system 5(a-k), so as to explore the relationships between antimicrobial activity and structure.
The MIC values of the synthesized compounds were established against the Gram-positive S.
aureus, V. cholera, B. subtilis, the Gram-negative E. coli, M. smegmatis, P. aeruginosa and
the fungi C. albicans. Their antimicrobial activity was compared to ampicillin and
amphoterecin B as standard drugs. All the MIC results are shown in Table 1. The results of
antimicrobial activity revealed that compounds were capable of inhibiting the growth of the
selected microorganisms in vitro having MIC values between 7.9 to 262 µM. Newly
synthesized derivatives 5(a-k) showed satisfactory to good potential against various bacteria
and a fungi, as tabulated in Table 1. Amongst the synthesized compounds 5c, 5f, 5i, 5k were
the most promising candidates against all strains. Compound 5c showed significant inhibition
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against S. aureus (MIC 13.3 µM), B. subtilis (MIC 7.9 µM) and E. coli (MIC 7.9 µM),
moreover 5(c) responded against all strains, V. cholerae, M. smegmatis, P. aerugnuosa and
fungi; C. albicans in a range of MIC 13.3-31 µM. Compound 5f was found significantly
active against V. cholera (MIC 12 µM), M. smegmatis (MIC 7 µM) and E. coli with MIC 7
µM. Compound 5a, 5c and 5j were found to be the most active candidate against fungi; C.
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albicans with MIC 12 µM, 13.3 µM and 13 µM. Some other compounds in the series were
found averagely active and few of them were found inactive or less active.
8
A concise analysis of the structure–activity reveals that the antimicrobial and anti-cancer
activity is significantly influenced by different substituents at the quinoxaline ring. Para
substitution is more active than ortho (5i) and meta (5a, 5b, 5h) on styryl at 3rd position of
quinoxaline. The order of antimicrobial activity at styryl group on 3rd position of quinoxaline
is para substitution > ortho substitution > meta substitution. Electron donating group favours
the antimicrobial activity on para position of styryl (at 3rd position of quinoxaline) as
compared to the electron withdrawing group (5f and 5k). Among the electron withdrawing
group, low magnitude electron withdrawing group have significant effect on antimicrobial
activity compared to the high magnitude [Cyano substitution (5c), chloro substitution (5k)
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and fluoro substitution (5j)]
.
3.3 Anti-Cancer activity
Anti-Cancer activity of all the synthesized compounds was carried out by MTT assay using
the MCF-7 cell line. Among the synthesized derivatives, compounds 5i and 5h have come out
with significant anti-cancer activity with IC50 value of 7 µM and 10 µM. It is followed by
compound 5j and 5k with value of 15 µM and 32 µM. In case of anti-cancer activity against
the MCF-7 cell line, ortho-hydroxy group on styryl at 3rd position of quinoxaline has the
significant impact followed by methoxy substitution (Compound 5i and 5h). Electron
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withdrawing group at para position on styryl at 3rd position of quinoxaline also favours the
anticancer activity (5j and 5k).
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4. Conclusion
4
Conclusively, this research work involves synthesis of new quinoxaline-PABA bipartite
derivatives 5(a-k) to be examined for their antimicrobial activity. Literature survey revealing
the pharmacological importance of quinoxaline and PABA prompted us to synthesize new
series of bipartite hybrid compounds 5(a-k). It is inferred from the biological studies that
among all synthesized compounds, the styryl derivatives 5c, 5f, 5i and 5k having cyano, -
N(CH3)2-, 2-hydroxy and 4-chloro substitution respectively, were found to be the most potent
inhibitors of microorganism. The feasible enhancements in the activity of these compounds
could be accomplished further by a slight variation in the ring substituent. Our findings can
inspire researchers looking for potent anti-bacterial agents for further investigation in this
field.
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Conflict of Interest
Author declares no conflict of interest.
Acknowledgements
The authors would like to thank “Science and Engineering Research Board (SERB) of
Department of Science and Technology (DST) Govt. of India” (Grant No. YSS/2015/002017)
for funding the project.
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Legends
Fig 1. Quinoxaline derivatives with anti-bacterial and anti-cancer potential
Fig 2. Design concept of the new synthesized compounds using molecular hybridization
approach.
Scheme 1. Systematic route of synthesizing target compounds 5 (a-k).
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Table 1. Antimicrobial and Anticancer activity of the synthesized compounds 5(a-k)
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Fig 1. Quinoxaline derivatives with anti-bacterial and anti-cancer potential
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Fig 2. Design concept of the new synthesized compounds using molecular hybridization approach.
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Table 1. Antimicrobial and Anticancer activity of the synthesized compounds 5(a-k)
Anti-microbial Activity MIC (µM)
Anti-cancer Activity IC50 (µM)
MCF-7
Compounds
S. aureus
V. cholerae
B. subtilis
E. coli
M. smegmatis
P. aeruginosa
C. albicans
121
> 485
> 485
242
30
> 485
12
156
5a
> 448
112
112
> 448
> 448
56
112
> 449
5b
13.3
31
7.9
7.9
31
13.3
13.3
52
5c
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O
OH
N
N
NH
32
> 524
65.6
262
65
262
262
> 524
5d
65
261
261
65
32
522
32
178
5e
30
12
30
7
7
12
60
> 487
5f
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30
68
> 544
544
272
136
136
134
5g
117
29
> 468
> 468
58.5
> 468
234
10
5h
13
65
13
261
32
10
32
7
5i
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32
> 519
> 519
32
259
32
13
15
5j
62
> 498
31
249
62
62
242
32
5k
1
4
4
0.5
4
0.5
-
-
Ampicillin
-
-
-
-
-
-
-
-
-
-
-
-
3
-
-
Amphotericin B
Doxorubicin
0.41
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Scheme 1. Systematic route for synthesizing target compounds 5(a-k)
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Highlights
► Quinoxaline-PABA bipartite hybrid derivatives were synthesized and characterized
► Compound 5c showed significant inhibition against B. subtilis and E. coli
► Electron donating group favours the antimicrobial activity
► Compound 5i came out with significant anti-cancer activity against MCF-7 cell line
► o-hydroxy group on styryl ring has the significant impact on anti-cancer activity