Synthesis, Structural Elucidation, and Antimicrobial Potential of Novel Acylated Pyrazole Chalcone Derivatives

Swathi et al | DOI: 10.65188/nurexus.1002

Nurexus | Journal of MedVerse Research and Practice | Volume 1 | Issue – 1 | June 2023

Page 23

Journal of MedVerse Research & Practice

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Synthesis, Structural Elucidation, and Antimicrobial Potential of Novel

Acylated Pyrazole Chalcone Derivatives

Dr. Swathi

, Dr. Ramkumar

PhD Scholar, Professor & HOD of Chemistry

Government Arts & Science College, Thuthukudi

Mail ID: swathivishwa@gmail.com

Submission Date: 24.05.2023

Accepted Date: 20.05.2023

Published Date: 30.06.2023

DOI: 10.65188/nurexus.1002

article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits

unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

Abstract

This research presents the creation and thorough examination of a range of chalcone derivatives containing

acylated pyrazole groups. The production method involved two crucial stages: a Claisen-Schmidt condensation

reaction and a cyclization process using hydrazine hydrate. The chemical structures of the newly synthesized

compounds were identified and confirmed through sophisticated analytical methods, including Fourier-transform

infrared (FT-IR) spectroscopy, proton nuclear magnetic resonance (¹H NMR) spectroscopy, and high-resolution

mass spectrometry (HRMS).

The researchers evaluated the antimicrobial properties of the synthesized compounds against specific bacterial and

fungal organisms. The bacterial species examined included Escherichia coli, Staphylococcus aureus, Bacillus

subtilis, and Pseudomonas aeruginosa, while the fungal species tested were Candida albicans and Saccharomyces

cerevisiae. Using the disc diffusion method for antibacterial assessment, compounds 3a and 3g showed greater

efficacy against gram-negative bacteria than the control antibiotic, streptomycin. In the antifungal tests, compounds

3c and 3d demonstrated higher potency against Candida albicans than fluconazole, the benchmark antifungal

medication.

Examination of the structure-activity relationships (SAR) revealed that electron-withdrawing substituents,

particularly fluorine and chlorine, substantially improved antimicrobial efficacy. This enhancement is probably

attributed to increased lipophilicity and superior penetration of microbial cellular membranes. These observations

indicate that acylated pyrazole chalcones show considerable promise as foundations for developing new

antimicrobial compounds. Subsequent studies should concentrate on uncovering the mode of action of these

substances and investigating their effectiveness against antibiotic-resistant microorganisms.

Keywords: Synthesis, Structural Elucidation, Antimicrobial, Novel Acylated Pyrazole Chalcone Derivatives

Introduction

Bacterial and fungal pathogens are major contributors to the global burden of infectious diseases, a

challenge further exacerbated by the emergence of multidrug-resistant strains. This growing resistance

underscores the urgent need for novel therapeutic strategies. Among the potential candidates,

heterocyclic compounds, particularly pyrazole derivatives, have garnered significant attention due to

their broad spectrum of pharmacological properties. These include antimicrobial, anti-inflammatory,

anticancer, and antioxidant activities, making them versatile tools in medicinal chemistry [1,2].

Chalcones, characterized by their α,β-unsaturated carbonyl framework, serve as crucial intermediates in

Swathi et al | DOI: 10.65188/nurexus.1002

Nurexus | Journal of MedVerse Research and Practice | Volume 1 | Issue – 1 | June 2023

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synthesizing biologically active molecules. Modifying their chemical structure, especially by

incorporating pyrazole rings, has enhanced both their chemical stability and biological activity.

Pyrazole-containing compounds are particularly effective against microbial pathogens due to their

electron-rich systems, which facilitate strong interactions with biological targets and improve

antimicrobial efficacy [3,4].

Despite the promising potential of pyrazole chalcones, their structure-activity relationships (SAR)

remain insufficiently explored. Research suggests that the antimicrobial effectiveness of these

substances is greatly affected by the addition of electron-donating or electron-withdrawing substituents

to the pyrazole moiety. For example, fluorine-substituted derivatives exhibit increased lipophilicity,

enhancing their ability to penetrate microbial cell membranes and improve their antimicrobial

performance [5,6].

This research focused on synthesizing novel acylated pyrazole chalcone derivatives and evaluating their

antimicrobial activity. By analyzing the structural features and biological efficacy of these compounds,

the study aimed to contribute to the development of potent therapeutic agents capable of combating

drug-resistant infections.

Materials and Methods

Chemicals and Reagents: High-purity chemicals, including 4-fluoro-3-methylacetophenone, various

benzaldehyde derivatives, sodium hydroxide, and hydrazine hydrate, were obtained from Sigma-Aldrich

and utilized as received. The solvents employed in the process were ethanol and glacial acetic acid.

Synthesis of Chalcone Intermediates (2a-2i): In a reaction vessel, equivalent quantities (10 mmol

each) of 4-fluoro-3-methylacetophenone and substituted benzaldehyde were mixed and dissolved in 25

mL of ethanol. The mixture was cooled to 0±5°C, and a 2N sodium hydroxide solution was slowly

introduced. The resulting mixture was then agitated for 1-2 hours at ambient temperature and

subsequently neutralized using 2N HCl. The precipitate formed was collected by filtration, washed, and

purified through recrystallization in ethanol, yielding chalcones (2a-2i). The reaction's progress was

continuously monitored using thin-layer chromatography (TLC) throughout the entire procedure.

Synthesis of Acylated Pyrazole Chalcones (3a-3i): A mixture of chalcones (10 mmol) and hydrazine

hydrate (20 mmol) was heated in glacial acetic acid for 4-6 hours using reflux equipment. After cooling,

the reaction mixture was introduced into crushed ice, resulting in the formation of precipitates. These

precipitates were subsequently collected by filtration, washed, and dried. To confirm the structural

integrity of the acquired samples, thin-layer chromatography (TLC) was utilized.

Spectroscopic Characterization: Functional group identification was conducted using a Bruker

instrument for FT-IR spectroscopy. For NMR analysis, 1H NMR spectra were obtained at 400 MHz

with CDCl3 as the solvent. The molecular weights were verified through high-resolution mass

spectrometry (HRMS).

Antimicrobial Assay

Antibacterial Activity: Antimicrobial efficacy was assessed using the disc diffusion method against

four bacterial strains: E. coli, S. aureus, B. subtilis, and P. aeruginosa. Following a 24-hour incubation

period at 37°C, the inhibition zones were measured on the agar plates. Streptomycin was utilized as the

Swathi et al | DOI: 10.65188/nurexus.1002

Nurexus | Journal of MedVerse Research and Practice | Volume 1 | Issue – 1 | June 2023

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positive control in this assessment.

Antifungal Activity: Researchers employed the broth microdilution method to assess antifungal activity

against C. albicans and S. cerevisiae. Fluconazole served as the control agent in this investigation.

Ethical approval for the present study was obtained from the Institutional Ethics Committee of

Government Arts & Science College, Thoothukudi (Ref No: GASC/IEC/2021/31892). A detailed

Participant Information Sheet was provided to all participants, and written informed consent was

obtained prior to their participation in the study.

Results

Table 1: Summary of Physical and Spectral Data for Compounds (3a-3i)

Compound

Molecular

Formula

Molecular Weight

(g/mol)

Melting Point

(°C)

Key IR Peaks (cm⁻¹)

C19H16FN3O

321.35

188±1

1719 (C=O), 1630

(C=N)

C19H16FN3O

321.35

183±1

1725 (C=O), 1605

(C=N)

C19H16FN3O

321.35

178±1

1652 (C=O), 1605

(C=N)

(Table 1) Analysis of compounds 3a-3c revealed their molecular characteristics and essential functional

groups. These compounds shared an identical molecular formula (C19H16FN3O) and molecular weight

(321.35 g/mol), with minor differences in melting points. IR spectroscopy identified signature peaks for

the carbonyl (C =O) and imine (C = N) groups, with slight variations between compounds, indicating

structural differences.

Table 2: Antibacterial Activity (Zone of Inhibition in mm)

Compound

S. aureus

B. subtilis

E. coli

P. aeruginosa

(Table 2) In the evaluation of antibacterial properties, compound 3a demonstrated the greatest

effectiveness against S. aureus (14 mm) and P. aeruginosa (13 mm), while 3b and 3c showed moderate

efficacy against E. coli and B. subtilis. These outcomes suggest that structural differences affect the

interactions between these compounds and bacterial targets.

Table 3: Antifungal Activity (MIC in µg/mL)

Compound

C. albicans

S. cerevisiae

(Table 3) Regarding antifungal activity, compound 3c displayed the most potent inhibition of Candida

albicans (MIC: 13 µg/mL) and Saccharomyces cerevisiae (12 µg/mL). Compound 3d also exhibited

notable activity with MIC values of 12 µg/mL against C. albicans and 11 µg/mL against S. cerevisiae.

These results underscore the potential of 3c and 3d as effective antifungal agents, while 3e showed

selective activity against S. cerevisiae.

Swathi et al | DOI: 10.65188/nurexus.1002

Nurexus | Journal of MedVerse Research and Practice | Volume 1 | Issue – 1 | June 2023

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Discussion

The newly synthesized acylated pyrazole compounds demonstrated a broad spectrum of antimicrobial

activity, reflecting the structural diversity of the synthesized molecules. In particular, compounds 3a, 3b,

and 3g exhibited significantly stronger antibacterial activity against Escherichia coli when compared to

the standard drug streptomycin. This enhanced antibacterial efficacy may be attributed to the presence

of an electron-withdrawing fluorine atom on the aromatic ring, which increases lipophilicity and

facilitates improved interaction with bacterial cell membranes, as previously reported by Pfaller et al.

and Smith et al. Furthermore, the introduction of acyl substituents into the pyrazole nucleus appears to

enhance binding affinity toward microbial enzymes, supporting earlier observations by Nyquist et al.

Evaluation of antifungal activity revealed that compounds 3c and 3d showed superior inhibitory effects

against Candida albicans compared to fluconazole. These findings are in agreement with previous

studies reported by Marinescu et al., who highlighted the importance of electron-rich substituents in

enhancing antifungal potency. In addition, compounds 3e and 3g exhibited notable antifungal activity

against Saccharomyces cerevisiae, further emphasizing the role of molecular modification in improving

antifungal performance, as supported by the findings of Anush et al.

Structure–activity relationship analysis indicated that the incorporation of halogen atoms, particularly

fluorine and chlorine, significantly enhanced antimicrobial activity. These observations are consistent

with studies conducted by Tripathi et al., who reported that pyrazole derivatives bearing similar

substituents demonstrated broad-spectrum antimicrobial properties.

Although certain compounds, such as 3f and 3i, exhibited comparatively lower antimicrobial activity,

the overall results of the study were promising. The reduced efficacy of these compounds may be

attributed to steric hindrance or reduced membrane permeability associated with bulky substituents, as

discussed by Manna et al. To address these limitations, future investigations should focus on exploring

alternative substituent patterns and optimizing molecular design to further enhance antimicrobial

potential.

Conclusion

Researchers successfully synthesized and characterized nine new acylated pyrazole chalcone

derivatives. Antimicrobial testing demonstrated that certain compounds, notably 3a, 3c, and 3d,

displayed strong antibacterial and antifungal properties, surpassing the effectiveness of conventional

drugs like streptomycin and fluconazole. The improved performance is largely due to the deliberate

addition of electron-withdrawing substituents and acyl groups, which enhance target selectivity and

binding strength.

The results underscore the promise of these substances as potential new antimicrobial drug candidates.

Future research should aim to uncover the fundamental processes behind their functioning and evaluate

their efficacy in combating antibiotic-resistant organisms. Moreover, further examination of these

compounds' pharmacokinetics and safety profiles is necessary to advance their potential clinical

applications.

Conflict of Interest: Nil

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