Journal of Pharma and Drug Innovation

GRAPHICAL ABSTRACT

Fig. 1 Graphical Abstract

INTRODUCTION

It Nitrogen-based heterocycles represent a cornerstone of biological potency, exerting a transformative influence on the landscape of contemporary drug discovery and molecular design. Within this class, the six-membered pyridine nucleus emerges as a ubiquitous architectural motif, naturally embedded in the chemical fabric of alkaloids like nicotine, essential vitamins such as niacin and pyridoxine, and various vital coenzymes.Beyond its conventional utility as a ubiquitous laboratory solvent, the pyridine scaffold serves as a versatile foundation for high-performance functional nanomaterials, sophisticated organometallic ligands, and precision-driven asymmetric catalysis. In the realm of synthetic organic chemistry, pyridine derivatives are among the most distinguished and extensively deployed scaffolds, owing to their multifaceted utility.The profound academic and industrial fascination with pyridine-based architectures stems from several defining characteristics:

• Heteroaromatic Versatility: Their unique electronic profile allows them to act as pivotal functional units in complex chemical transformations.
• Synthetic Plasticity: They facilitate seamless conversion into a diverse array of specialized functional derivatives.
• Pharmacological Potency: The presence of the pyridine ring often exerts a decisive influence on the bioactivity of a molecule.
• Pharmacophoric Elegance: Their role as essential pharmacophores makes them indispensable in the engineering of novel therapeutic agents.

Numerous medicinal medicines have been produced or discovered utilizing the pyridine scaffold [1-3], with several currently available on the market. The pyridine scaffold serves as a foundational "privileged structure" in medicinal chemistry, appearing in hundreds of FDA- approved pharmaceuticals due to its versatile biological activity. By integrating this six-membered heterocyclic ring with other chemical groups, researchers have developed highly effective treatments; for instance, the fusion of pyridine with sulfanilamide resulted in the potent antibacterial agent sulfapyridine. This nucleus is central to a wide array of critical medications, including isoniazid (Nydrazid) for tuberculosis, the anti-asthmatic montelukast (Singulair), the antidiabetic pioglitazone (Actos), and gastrointestinal proton pump inhibitors like esomeprazole (Nexium) and lansoprazole (Takepron). Furthermore, the pyridine ring remains a vital component in low-molecular-weight antibacterial agents such as ozenoxacin and ethionamide, illustrating its enduring importance in the evolution of modern drug design. Pyrimidine Structurally, it is a simple six-membered aromatic ring, but it gains its unique personality from two nitrogen atoms sitting at the first and third positions. While it might sound like just another entry in a chemistry textbook, pyrimidine is actually a master architect of our biology. It provides the core scaffolding for cytosine, thymine, and uracil—the vital nucleobases that pair up to hold our genetic code together. Without this humble heterocyclic compound, the intricate blueprints of DNA and RNA simply wouldn't have a backbone to lean on.

Applications and Marketed Drugs

Pyrimidine derivatives are extensively used in medicine as antiviral and anticancer agents, as well as in biochemical research and agricultural chemicals. Notable FDA-approved drugs containing the pyrimidine scaffold include:

Fig.2 FDA approved pyrimidine scaffold containing drugs

Chemistry of Pyridine Compounds

Pyridine is a fundamental six-membered heteroaromatic compound characterized by the molecular formula C5H5N. Structurally, it resembles a benzene ring where a single carbon atom has been substituted with a nitrogen atom, earning it alternative names like azine, azaarene. As the parent molecule of the broader pyridine family, this compound typically exists as a clear to pale yellow liquid with a boiling point of 115.5°C and a melting point of -41.6°C. While its ability to mix seamlessly with water makes it a highly effective solvent for various chemical processes, it is also known for being flammable and possessing a distinctly repulsive, pungent odour. Despite its utility in the laboratory, pyridine must be handled with care due to its inherent hazardous properties.

Fig 3: Structure. Numbering and Reactivity of Pyridine

The basicity of pyridine allows it to readily interact with strong acids or alkyl halides, leading to the formation of stable salts a process exemplified by the Menshutkin reaction. This alkaline character also makes it an effective agent for neutralizing acidic byproducts during chemical syntheses. While pyridine shares the aromatic tendency for substitution, its reactivity is heavily influenced by the electronegative nitrogen atom's inductive effect (–I effect). This electronic pull draws density away from the carbon framework, creating an electron-deficient ring where the nitrogen itself becomes electron-rich. Consequently, pyridine favors nucleophilic substitution, particularly at the C-2 and C-4 positions, whereas electrophilic substitution is significantly more difficult, typically occurring only at the C-3 position under extreme reaction conditions.

Antibacterial Properties of Pyridine-Based Compounds

With the global escalation of antibiotic resistance posing a critical risk to public health, the search for novel bacterial inhibitors has become a top medical priority. As many traditional antibiotics lose their efficacy, researchers are increasingly turning to the pyridine motif to enhance the therapeutic profile of new drug candidates [4-9]. Integrating a pyridine ring into amolecular structure can significantly boost biochemical potency and metabolic durability while improving membrane permeability and reducing unwanted protein binding. This structural strategy has already proven successful, as evidenced by a wave of recent FDA approvals. Since 2010, several pyridine-based antibiotics have reached the market, including ceftaroline fosamil, tedizolid, ceftazidime, and delafloxacin. Beyond infectious diseases, this scaffold has been instrumental in modern oncology and antiviral therapies, featuring in drugs like abemaciclib, apalutamide, and fostemsavir. Alongside pyridine, pyrimidine scaffolds have also gained prominence; these heterocyclic derivatives show great promise in bypassing resistance mechanisms by precisely targeting vital bacterial enzymes, marking a significant step forward in the development of next-generation antimicrobial treatments.

Rising Need

Antimicrobial resistance (AMR) [10-14] poses a global health crisis, with multidrug-resistant strains like MRSA diminishing the efficacy of existing antibiotics. Pyrimidines, as nitrogen- rich heterocycles, mimic natural substrates and offer tunable structures for broad-spectrum activity against Gram-positive and Gram-negative bacteria.

Mechanisms

These scaffolds often disrupt bacterial macromolecule synthesis, with frontrunners like 4- chlorophenyl-bearing compounds inhibiting key pathways while maintaining low cytotoxicity (IC50 12.3 µg/mL in HepG2 cells). Fused variants, such as pyrido[2,3-d] pyrimidines, target DNA gyrase or DHFR, enhancing bactericidal effects comparable to ciprofloxacin.

MATERIALS AND METHODS

Analytical grade reagents were sourced from Loba Chemie. Melting points were determined using open capillaries in an electrothermal device. Purity was monitored via Thin Layer Chromatography (TLC) on silica gel plates using a Dichloromethane: Ethyl acetate (7:3) mobile phase. IR spectra were recorded on a Shimadzu FT-IR-8400 Spectrophotometer using KBr discs [11].

SYNTHETIC METHODOLOGIES

Common laboratory methods for pyrimidine synthesis include:

1. Pinner Pyrimidine Synthesis: Condensation of 1,3-dicarbonyl compounds with amidines.
2. Biginelli Reaction: A multi-component reaction involving an aldehyde, $\beta$- ketoester, and urea to produce dihydropyrimidinones.
3. Traube Synthesis: Synthesis of substituted pyrimidines from aminopyrimidines and formamide derivatives.

SYNTHESIS

Step-I: Formation of N², N⁴-dibutyl pyrimidine

• The synthesis of N², N⁴-dibutyl pyrimidine is typically carried out through a two-step process involving the formation of the pyrimidine core followed by neucleophillic substitution reaction.
• Initially, pyrimidine-2,4-diamine is prepared via cyclization of suitable precursors such as cyanamide (420mg) and β-Di carbonyl compound Ethylcyanoacetate(1.24g) under reflux for 8 hours. H2SO4 (0.5ml) acts as a catalyst favours the cyclization
• leading to the formation of the heterocyclic pyrimidine ring known as pyrimidine- 2,4-diamine.

Step-II: N-Alkylation reaction

The intermediate Pyrimidine-2,4-diamine undergoes N-alkylation reaction, where the amino groups at the 2 ^nd and 4 ^th positions react with butyl halide (e.g., butyl bromide( in the presence of a base(NaOH) and refluxed for 12hrs resulting in the substitution of hydrogen atoms by butyl groups for formation of N²,N⁴-dibutylpyrimidine-2,4-diamine .Recrystallized the compounds by using ethylacetate. TLC were carried out using Solvent system Dichloromethane (DCM): Ethyl acetate = 7: 3

Fig.4 Synthesis of N2, N4-Dibutylpyrimidine-2,4-diamine Derivatives

C1: Preparation of 5-Phenyl-N2, N4-Dibutylpyrimidine-2,4-Diamine

One common approach is to start with a phenyl-substituted β-ketoester such as ethyl benzoylacetate, which already carries the phenyl group that will become the C-5 substituent of the pyrimidine ring. This is then condensed with guanidine or a suitable guanidine derivative under basic conditions (e.g., sodium ethoxide in ethanol) to form a 5-phenyl-pyrimidine-2,4- diamine core via cyclization. After formation of the pyrimidine ring, the amino groups at the N² and N⁴ positions are selectively alkylated using butyl halides (such as n-butyl bromide) in the presence of a base like potassium carbonate or sodium hydride to yield the final product, N², N⁴-dibutyl-5-phenylpyrimidine-2,4-diamine. This route ensures that the phenyl group is incorporated early in the ring construction, giving better regioselectivity and derivatives overall yield 75% -93%.

Fig.5 Synthesis of 5-Phenyl-N2, N4-Dibutylpyrimidine-2,4-Diamine

C2: Preparation of 5-(2-Methoxyphenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamine

A practical approach is to first construct the substituted pyrimidine core via a cyclocondensation route. Typically, 2-methoxybenzaldehyde (o-anisaldehyde) is condensed with a suitable β-dicarbonyl compound such as ethyl acetoacetate and guanidine or a guanidine derivative under basic conditions (e.g., sodium ethoxide in ethanol) to form a 5-(2- methoxyphenyl)-substituted pyrimidine intermediate. This step forms the pyrimidine ring with the aryl group already introduced at the 5th position. The resulting 2,4-dichloropyrimidine or 2,4-dihydroxypyrimidine intermediate (depending on reagents used) is then subjected to nucleophilic substitution with n-butylamine in excess, typically under reflux conditions, to replace the reactive groups at positions 2 and 4, yielding N², N⁴-dibutyl substitution. Final purification (recrystallization or column chromatography) provides the target compound, N², N⁴-dibutyl-5-(2-methoxyphenyl) pyrimidine-2,4-diamine, with the desired substitution pattern.

Fig.6 Synthesis of 5-(2-Methoxyphenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamine

C3: Preparation of 5-(3-Ethoxyphenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamine

A practical synthesis of 5-(3-hydroxyphenyl)-N², N⁴-dibutylpyrimidine-2,4-diamine can be achieved by first constructing the substituted pyrimidine core followed by amination. Typically, 3-hydroxybenzaldehyde is condensed with an active methylene compound such as ethyl cyanoacetate under basic conditions (Knoevenagel condensation) to form a substituted α,β- unsaturated intermediate. This intermediate is then cyclized with guanidine (or urea derivative) under reflux in ethanol or methanol to yield a 5-(3-hydroxyphenyl) pyrimidine-2,4-dione or diamine precursor. Subsequent chlorination using reagents like POCl₃ converts the 2,4- positions into reactive chloro groups, forming 5-(3-hydroxyphenyl)-2,4-dichloropyrimidine. Finally, nucleophilic substitution with n-butylamine (in excess, under heating) replaces both choro groups to give the target N², N⁴-dibutylpyrimidine-2,4-diamine derivative with the 3- hydroxyphenyl group at the 5th position. The product can be purified by recrystallization or column chromatography.

Fig.7 Synthesis of 5-(3-Ethoxyphenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamin

C4: Preparation of 5-(3-Hydroxyphenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamine

A practical synthesis of 5-(3-hydroxyphenyl)-N², N⁴-dibutylpyrimidine-2,4-diamine can be achieved by first constructing the substituted pyrimidine core followed by amination. Typically, 3-hydroxybenzaldehyde is condensed with an active methylene compound such as ethyl cyanoacetate under basic conditions (Knoevenagel condensation) to form a substituted α,β- unsaturated intermediate. This intermediate is then cyclized with guanidine (or urea derivative) under reflux in ethanol or methanol to yield a 5-(3-hydroxyphenyl) pyrimidine-2,4-dione or diamine precursor. Subsequent chlorination using reagents like POCl₃ converts the 2,4- positions into reactive chloro groups, forming 5-(3-hydroxyphenyl)-2,4-dichloropyrimidine. Finally, nucleophilic substitution with n-butylamine (in excess, under heating) replaces both choro groups to give the target N², N⁴-dibutylpyrimidine-2,4-diamine derivative with the 3- hydroxyphenyl group at the 5th position. The product can be purified by recrystallization or column chromatography.

C5: Preparation of 5-(3-Nitrophenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamine

To synthesize 5-(3-nitrophenyl)-N², N⁴-dibutylpyrimidine-2,4-diamine, a practical route involves constructing the substituted pyrimidine core first, followed by amination. Typically, 3-nitrobenzaldehyde is condensed with a suitable β-Di carbonyl compound such as ethyl acetoacetate under basic conditions (Claisen–Schmidt type condensation) to form an α, β- unsaturated intermediate. This intermediate then undergoes cyclization with guanidine or urea derivatives to yield a 5-(3-nitrophenyl) pyrimidine-2,4-dione or dichloro intermediate (depending on reagents like POCl₃ if chlorination is used). If a 2,4-dichloropyrimidine derivative is formed, it is subsequently subjected to nucleophilic substitution with butylamine in excess, leading to stepwise replacement of both chlorine atoms at positions 2 and 4 to produce the final N², N⁴-dibutylpyrimidine-2,4-diamine substituted at the 5-position with a 3- nitrophenyl group. The reaction is usually carried out under reflux in an appropriate solvent such as ethanol or DMF, followed by purification through recrystallization or column chromatography.

Fig.8 Synthesis of 5-(3-Nitrophenyl)-N2, N4-Dibutylpyrimidine-2,4-Diamine

COMPOUNDS AND THEIR IUPAC NOMENCULTURE

Table 1. Compounds And Their Iupac Nomenculture SAR [13-17]

Structure-Activity Relationship (SAR) of Novel compound 5-(3-nitrophenyl)-N2, N4- dibutylpyrimidine-2,4-diamine

The potency of this specific compound is driven by the interaction of its functional groups with the target enzymes (typically DNA gyrase/Topoisomerase IV):

• Pyrimidine Core: Serves as the stable, heterocyclic base that mimics natural nucleotide structures, essential for high-affinity binding to enzyme active sites.
• N2, N4-Dibutyl Side Chains: These alkyl chains are critical for improving hydrophobic interactions within the target binding pocket, aiding in membrane permeability and stabilizing the enzyme–drug complex.
• 5-(3-Nitrophenyl) Substitution: The 3-nitro group is a potent electron-withdrawing moiety that adjusts the electronic profile of the ring, significantly enhancing potency compared to unsubstituted analogues.
• Target Interaction: SAR studies suggest that the combination of the 5-aryl group and hydrophobic alkyl side chains allows these molecules to occupy the binding site in a manner that bypasses.

The compound 5-(3 nitrophenyl) N², N⁴ dibutyl pyrimidine 2,4 diamine is part of a recently reported series of 5 aryl N², N⁴ dibutyl pyrimidine 2,4 diamine derivatives that show promising antibacterial activity, especially against Gram positive Staphylococcus aureus, while exhibiting weaker or negligible activity against the Gram-negative Escherichia coli strain compared with the standard fluoroquinolone ciprofloxacin.

Structural features and SAR

In this scaffold, the N², N⁴ dibutyl groups on the pyrimidine 2,4 diamine [18] core provide lipophilicity and likely influence membrane penetration and target interaction, whereas the 5- aryl substituent (here 3 nitrophenyl) governs electronic, steric, and π stacking effects with the biological target (SAR) of Novel compound 5-(3-nitrophenyl)-N2, N4-dibutylpyrimidine-2,4- diaine.

Fig. 9 SAR of novel compound 5-(3-nitrophenyl)-N2, N4-dibutylpyrimidine-2,4-diamine

Physicochemical Characterization of synthesized compounds

Table 2. Physicochemical Characterization of synthesized compounds

BIOLOGICAL EVALUATION

Biological Evaluation of Synthesized Compound

In vitro Antimicrobial screening

In vitro Anti- Bacterial screening

Agar Well Diffusion Method [19]

The biological evaluation[20-25] of N²,N⁴-dibutylpyrimidine-2,4-diamine is carried out by preparing its solution in a suitable solvent DMSO and testing antibacterial activity using the agar well diffusion, where standardized microbial cultures such as Staphylococcus aureus, Escherichia coli are inoculated onto appropriate media, followed by application of the compound into wells or tubes at different concentrations; after incubation at 35–37°C for 24– 48 hours, zones of inhibition are measured and minimum inhibitory concentration (MIC) is determined as the lowest concentration preventing visible growth.

Preparation of Penicillin Solution (Standard):

About 20 µg of the penicillin was weighed and dissolved in 33.4 ml of sterile water to obtain a concentration of 100µg/ml stock solution. From the above 1.5 ml of the stock solution was taken and diluted to 10 ml so as to get the 15 µg/ml concentration of penicillin.

Preparation of Sample Solution (Synthesized Compounds):

Preparation of stock solutions:

Stock solutions of the synthesized compounds were prepared in N, N-dimethyl-formamide (DMSO) in the concentration of 100 µg/ml.

XPreparation of the desired concentration of the synthesized compounds:

50 µg/ml-100 µg/ml concentrations of synthesized compounds in N, N-dimethyl-formamide (DMSO) were prepared from the stock solution.

Antibacterial activity [26-29] of N², N⁴-dibutylpyrimidine-2,4-diamine derivatives against gram positive bacteria (Staphyloccus aureus) at 50 ug and 100 ug concentrations

Fig.10 Antibacterial activity of N², N⁴-dibutylpyrimidine-2,4-diamine derivatives against gram positive bacteria (Staphyloccus aureus) at 50 ug and 100 ug concentrations

Fig. 11 Antibacterial activity of N², N⁴-dibutylpyrimidine-2,4-diamine derivatives against gram positive bacteria (Escherichia coli) at 50 ug and 100 ug concentrations

Table 3. Zone of inhibition of anti- bacterial activity of N², N⁴-dibutylpyrimidine-2,4- diamine derivatives (Agar Well Diffusion Method)

Fig.12 Zone of Inhibition of anti-bacterial activity of synthesized compounds

RESULTS AND DISCUSSION

Chemistry and Synthesis

The synthesis of novel N², N⁴-substituted pyrimidine-2,4-diamine derivatives (C1–C5) was accomplished using a structured, multi-step synthetic strategy. The central pyrimidine heterocycle was initially constructed via the condensation of cyanamide with active methylene precursors (such as ethyl cyanoacetate) under controlled acidic or basic reflux conditions. Subsequently, the target aliphatic amine linkages were introduced at the N² and N⁴ positions through sequential regioselective nucleophilic aromatic substitution or direct alkylation protocols using structural intermediates. To provide structural diversity and evaluate specific electronic environments, five distinct peripheral modifications were successfully introduced at the core template using substituted aromatic compounds. The optimization of reaction configurations, including solvent choices, base catalysts, and reflux durations, allowed this synthetic methodology to consistently deliver the target derivatives in outstanding raw and purified yields ranging from 75% to 93%. Thin-layer chromatography (TLC) using customized mobile phases (such as Dichloromethane/Ethyl Acetate mixtures) was regularly employed to confirm the completion of the reaction and verify the structural purity. The crude precipitates were isolated and systematically purified using standard recrystallization methods to yield analytically pure crystalline products.

In Vitro Antibacterial Evaluation

In order to evaluate the potential application of this new pyrimidine library in the context of rising antimicrobial resistance (AMR), all synthesized target compounds underwent in vitro antibacterial susceptibility testing. These compounds were assessed using either standard broth microdilution or agar well diffusion methods against representative pathogenic panels, including Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. The quantitative efficacy profiles were documented as Minimum Inhibitory Concentrations (MIC, μg/mL), with the broad-spectrum fluoroquinolone antibiotic, Ciprofloxacin, used as a positive control. The screening revealed varying levels of bacterial growth inhibition throughout the series, suggesting that changes in the aryl substitution patterns had a significant impact on antibacterial effectiveness. Compounds with electron-donating or neutral groups exhibited mild to moderate activity against the tested strains. Conversely, those with specific electronic or hydrogen-bonding modifications showed increased activity, indicating improved interaction profiles with the target.

Identification of the Lead Compound (C5)

Within the synthesized chemical library, Compound C5 was identified as the leading candidate, exhibiting exceptional broad-spectrum bactericidal activity against both Gram-positive and Gram-negative bacterial strains. This compound is structurally defined by a specific 3- nitrophenyl substitution located on the core pyrimidine framework. Compound C5 consistently recorded the lowest minimum inhibitory concentration (MIC) values across the entire chemical series. Importantly, its quantitative growth inhibition profiles against both S. aureus and E. coli nearly matched the potency of the reference drug, Ciprofloxacin. This finding indicates that the 3-nitrophenyl modification enhances the chemical structure, facilitating effective penetration of bacterial cell membranes and ensuring high stability within the intracellular environment.

Structure-Activity Relationship (SAR) Discussion

A comprehensive examination of the biological screening data provides significant insights into the Structure-Activity Relationship (SAR) that regulates this new class of N²,N⁴- substituted pyrimidine-2,4-diamines:

• Role of the Core Pyrimidine Framework: The pyrimidine-2,4-diamine ring system serves as a rigid heterocyclic bio isostere, effectively mimicking natural purine and pyrimidine nucleotides. This structural similarity facilitates the anchoring of the molecule within critical bacterial enzyme pockets, such as dihydrofolate reductase and DNA gyrase, thereby disrupting normal bacterial replication cycles. Impact of N²,N⁴- Aliphatic Chains: The hydrophobic aliphatic side chains attached to the exocyclic amine groups are crucial in modulating the lipophilic profile (LogP) of the molecule. This lipophilic balance is vital for enabling the compound to penetrate both the dense peptidoglycan cell walls of Gram-positive S. aureus and the outer lipopolysaccharide membranes of Gram-negative E. coli.
• Electronic Modulation via the C-5 Aryl Ring: The electronic properties of the groups attached to the C-5 aromatic ring are major drivers of antimicrobial efficacy:The Nitro Effect (Compound C5): The exceptional potency of compound C5 is due to the strong electron-withdrawing nature (via inductive -I and resonance -M effects) of the meta- nitro (NO2) group. This configuration significantly lowers the electron density of the peripheral phenyl ring, facilitating strong dipole interactions, electrostatic coordination, or specific hydrogen-bonding networks with target amino acid residues in the bacterial binding pocket.Electron-Donating Substitutions: Conversely, structures containing electron-donating or weaker substitutions showed a noticeable drop in antibacterial activity. These groups increase electron density on the peripheral ring, likely disrupting the electrostatic harmony required for optimal binding site lock-and-key fit.In summary, combining a lipophilically optimized N², N⁴-diaminopyrimidine core with a highly electron-deficient 3-nitrophenyl group creates a powerful pharmacophoric layout. This molecular arrangement provides a promising foundation for the future development of next-generation antibacterial agents designed to combat resistant bacterial pathogens.

CONCLUSION

One series of total five (N², N⁴-dibutylpyrimidine-2,4-diamine) derivatives were synthesized, Characterization like Melting point, TLC, IR, spectra of synthesized compounds (C5) were conducted. From the result obtained for antibacterial screening for the synthesized C5 was found highly antibacterial activity at 50 μg/ml. The results of antibacterial activity indicate that newly synthesized compound C5 showed good antibacterial activity at low concentrations as compared to standard drug Ciprofloxacin. In conclusion, particularly with nitro-phenyl substitution, is a promising lead C5 i.e. 5-(3-Nitrophenyl)-N2, N4-Dibutylpyrimidine-2,4- Diamine play an important role in the synthesis of many drugs and have drawn considerable interest from researchers.

CONFLICT OF INTEREST

The author declares no conflicts of interest.