Facile One-Pot Synthesis and Anti-Microbial Activity of Novel 1,4-Dihydropyridine Derivatives in Aqueous Micellar Solution under Microwave Irradiation

The current study describes a novel and eco-conscious method to synthesize 1,4-dihydropyridine derivatives utilizing an aqueous micellar solution containing aluminum dodecyl sulfate, Al(DS)3, using readily available starting material. The final products were synthesized with excellent yields within remarkably quick reaction durations, promoting remarkable atom economy and minimizing environmental impacts. The present protocol has several advantages over other methodologies in terms of high yield (up to 97%) with excellent purity. Further, the synthesized 1,4-DHPs exhibit favorable to excellent resistance against examined bacterial and fungal species. Intriguingly, polar groups on the phenyl ring (5b, 5c, 5i and 5j) make the 1,4-DHPs equally potent against the microbes as compared to the standard drugs.

Persisting on our work, in the present work, we describe the synthesis of novel 1,4-DHP scaffolds using a one-pot method using an aqueous micellar solution as a greener solvent.The microwave-assisted reaction method stands out as superior due to its rapid reaction times, cost-effectiveness, and straightforward operational procedures compared to other methods.To begin our investigation into a sustainable and effective approach for synthesizing 1,4-DHPs, we commenced by employing a catalytic quantity of Al(DS) 3 with a combination of substituted benzaldehyde, diethyl acetylene dicarboxylate, and either ammonium acetate or aniline, along with malononitrile under microwave irradiations at ambient temperature.The SEM images depicted in Figure 1a-f, magnified from 500 to 7500 times, provide valuable insights into the morphology of the Al(DS) 3 surfactant.This compound was produced as dense aggregates (Figure 1a) which are irregular in shape.At higher magnifications, it becomes evident that randomly organized microplates arranged to compose the micro-objects are in an edge-to-face style (Figure 1a-d).Based on the elemental investigation conducted on microscopic sectors of Al(DS) 3 using EDS (as shown in Figure S1), the atomic percentages of Al and S were determined to be 22.40% and 77.60%, respectively, which indicates a 1:3 stoichiometry.

XRD Characterization
The XRD pa erns in the range of 2θ = 5-25° for the Al(DS)3 sample are shown in Figure 2. The prepared sample of Al(DS)3 gave rise to XRD peaks at 2θ = 6.596, 7.184, 10.965, 13.166, 17.587, 20.387, 20.674, and 21.875, corresponding to (110), ( 110), ( 111), ( 111), ( 200), ( 210), ( 211), (211), and (211) diffraction peaks, respectively.According to the above data, the XRD pa erns of Al(DS)3 show a prominent diffraction peak at (110) with preferential orientation around 2θ = 6.596° with 100% relative intensity, which shows a reasonable degree of crystallinity of 65.3%.Various solvents, including water, ethanol, ethanolic solution of p-TSA, glycerol, aqueous solution of SDS, and aqueous solution Al(DS) 3 , were employed for the condensation of benzaldehyde 1, dialkyl acetylene dicarboxylate 2, ammonium acetate 3, and malononitrile 4.Among these, water as a solvent proved to be the most effective, as indicated in Table 1.The maximum yield was achieved through an Al(DS) 3 + water catalyst system.Table 2 illustrates the optimal temperature for a reaction carried out in an aqueous micellar solution and it was also observed that increasing the temperature has a significant increase in yield with time.Conversely, further temperature increases, result in a decrease in product yield as the products start decomposing at higher temperatures.Similarly, various other aldehydes 3b-g will undergo reactions with diethyl acetylene dicarboxylate, malononitrile, and either ammonium acetate or aniline using the same procedure, with the formation monitored by TLC and melting point analysis.The reactions proceed smoothly even in the company of various electron-donating (-Me and -OMe) as well as electron-withdrawing substituents (-NO 2 ) on the aldehyde, facilitating efficient reactions.The effectiveness of this method is notable, yielding high percentages (92-97%) for the final products.The structural elucidation of the novel scaffold is accomplished by using various spectroscopic methods such as FTIR, 1 H NMR, 13 C NMR, Mass, and elemental analysis.In the series, the IR spectra of compound 5a reveal three significant absorptions at 3360.7, 2250.6, and 1740.5 cm −1 corresponding to the N-H, C≡N, and C=O groups stretching.In 1 H NMR spectra (500 MHz, CDCl 3 ) of compound 5a, three singulets at δ 11.94, 6.59, and 4.54 were observed for NH, NH 2 , and CH, respectively, confirming the formation of 1,4-DHP.The multiplet for 5 protons was at δ 7.13-7.27for the phenyl ring attached at the C-4 position.One quadruplet and one triplet at δ 4.17-4.13and 0.90-0.91 were observed for CH 2 and CH 3 protons of ester linkage.

Anti-Microbial Activity
The novel synthesized compounds 5a-n underwent testing against two Gram-positive bacteria (Streptococcus pyogenes MTCC 442, and Bacillus subtilis MTCC 441), three Gramnegative bacteria (Klebsiella pneumonia MTCC 3384, Escherichia coli MTCC 443, and Staphylococcus aureus MTCC 96), as well as three fungal strains (Aspergillus niger MTCC 281, Aspergillus janus MTCC 2751, and Aspergillus sclerotiorum MTCC 1008).Fungal strains were cultivated in malt extract medium prior to inoculation for a duration of 72 h at 28 • C, whereas bacterial samples were grown in nutrient broth for 24 h at 37 • C. Each synthesized chemical underwent triplicate testing after being dissolved in DMSO at concentrations of 2, 4, 8, 16, 32, 64, and 128 g/mL using a serial dilution procedure.

Chemicals
All chemicals used in this study were obtained from Sigma-Aldrich (St. Louis, MO, USA) which were employed without additional distillation, while the solvents were ordered from Loba Chemie (Mumbai, India).

Analytical Instruments
The digital melting point apparatus was employed to measure the melting point of all the resulting products via the open capillary method.IR spectra of the targeted compound were taken using ATR mode on Perkin Elmer (Waltham, MA, USA) Spectrum II.NMR such as 1 H and 13 C are collected on a Bruker (Billerica, MA, USA) Avance NEO 500 MHz NMR spectrometer using DMSO as solvent.Chemical shifts (δ) are accounted for in ppm relative to that of TMS as an internal standard.The mass spectroscopy was recorded on LC-MS Spectrometer Model Q-ToF Micromass Thermo Scientific (Waltham, MA, USA) (FLASH 2000) CHN Elemental Analyser is used for fundamental analysis.The thin layer chromatographic (TLC) technique was used to observe the reaction time as well as to check the purity of the compound, and then the visualization of TLC was performed with the help of a UV chamber.XRD patterns of the dried (lyophilized) samples were captured at room temperature using a Bruker D8 advance.The compounds were exposed to monochromatic Cu-Kα radiation (λ = 1.5418Å, 50 kV, 40 mA) across the 2θ range between <1 and >150 • , with steps of 0.02 • .SEM micrographs were obtained utilizing a JSM IT500 scanning electron microscope.Elemental analysis on microscopic sections of the Al(DS) 3 sample was conducted via EDS.SEM images were acquired under high vacuum mode, ranging from 30 nm (30 kV) to 15.0 nm (1.0 kV).

Synthesis of Aluminum Dodecyl Sulfate
The preparation of aluminum dodecyl sulfate was conducted following the procedure outlined in the literature (Scheme 1) [28].Anhydrous AlCl 3 (0.1 mol) and sodium dodecyl sulfate (SDS) (0.3 mol) were dissolved using the minimum amount of water required.The solutions were gradually diluted with continuous stirring at room temperature, resulting in the precipitation of a colorless solid.This solid was filtered to isolate the solid Al(DS) 3 .
Molecules 2024, 29, x FOR PEER REVIEW 6 of 13 temperature, resulting in the precipitation of a colorless solid.This solid was filtered to isolate the solid Al(DS)3.
Scheme 1. Preparation of Al(DS)3 in water.

Synthesis of 1,4-DHPs Derivatives (5a-n)
Using Al(DS) 3 catalyst (5 mole%), a mixture composed of substituted benzaldehyde (5 mmol), diethyl acetylene dicarboxylate (5 mmol), and either ammonium acetate or aniline (5 mmol), along with malononitrile (5 mmol), was subjected to MW irradiation at 80 watts for 5 min in H 2 O (Scheme 2).The reaction's progress was monitored using TLC (Merck, Darmstadt, Germany) (EtOAc: Toluene; 8:2).After determining that the reaction had concluded, the mixture was cooled to ambient temperature, filtered, rinsed with water, and subsequently subjected to extraction using ethyl acetate.Subsequently, the resulting solid was recrystallized using ethyl alcohol to yield colorless crystals with an efficiency of 93-97%.80 wa s for 5 min in H2O (Scheme 2).The reaction's progress was monitored using TLC (Merck, Darmstadt, Germany) (EtOAc: Toluene; 8:2).After determining that the reaction had concluded, the mixture was cooled to ambient temperature, filtered, rinsed with water, and subsequently subjected to extraction using ethyl acetate.Subsequently, the resulting solid was recrystallized using ethyl alcohol to yield colorless crystals with an efficiency of 93-97%.By the optimized reaction conditions, a diversity of 1,4-DHP was prepared using various substituted aldehydes in aqueous micellar solution under MW irradiations at 80 wa s for 5 min (Table 3).By the optimized reaction conditions, a diversity of 1,4-DHP was prepared using various substituted aldehydes in aqueous micellar solution under MW irradiations at 80 watts for 5 min (Table 3).Compound 80 wa s for 5 min in H2O (Scheme 2).The reaction's progress was monitored using TLC (Merck, Darmstadt, Germany) (EtOAc: Toluene; 8:2).After determining that the reaction had concluded, the mixture was cooled to ambient temperature, filtered, rinsed with water, and subsequently subjected to extraction using ethyl acetate.Subsequently, the resulting solid was recrystallized using ethyl alcohol to yield colorless crystals with an efficiency of 93-97%.By the optimized reaction conditions, a diversity of 1,4-DHP was prepared using various substituted aldehydes in aqueous micellar solution under MW irradiations at 80 wa s for 5 min (Table 3).

Plausible Mechanism
As per literature [32], a conceivable mechanism can be reasonably suggested for the production of pyran pyrazole 5a from the four-component reaction between aniline 1, di-ethyl acetylene dicarboxylate 2, substituted benzaldehyde 3, and malononitrile 4 (Scheme 3).The hypothesis suggests that the substituted benzaldehyde 3 may undergo activation by water, leading to increased electrophilicity of the carbonyl carbon.It is suggested that this process involves the establishment of hydrogen bonds between the oxygen atom of the carbonyl group and water molecules, while simultaneously, hydrogen bonds form between the acidic hydrogen of malononitrile and the oxygen of water.Following this, Knoevenagel condensation takes place [33], resulting in the generation of an intermediate 7. Subsequently, aniline 1 reacts with diethyl acetylene dicarboxylate 2, resulting in the generation of enolate intermediate 6. Afterward, the Michael reaction takes place between intermediate 6 and intermediate 7, which leads to the generation of transient intermediate [34] 8 which undergoes intramolecular cyclization, followed by tautomerization, ultimately resulting in the formation of the target compound, the 1,4-dihydropyridine derivative 9.

Plausible Mechanism
As per literature [32], a conceivable mechanism can be reasonably suggested for the production of pyran pyrazole 5a from the four-component reaction between aniline 1, diethyl acetylene dicarboxylate 2, substituted benzaldehyde 3, and malononitrile 4 (Scheme 3).The hypothesis suggests that the substituted benzaldehyde 3 may undergo activation by water, leading to increased electrophilicity of the carbonyl carbon.It is suggested that this process involves the establishment of hydrogen bonds between the oxygen atom of the carbonyl group and water molecules, while simultaneously, hydrogen bonds form between the acidic hydrogen of malononitrile and the oxygen of water.Following this, Knoevenagel condensation takes place [33], resulting in the generation of an intermediate 7. Subsequently, aniline 1 reacts with diethyl acetylene dicarboxylate 2, resulting in the generation of enolate intermediate 6. Afterward, the Michael reaction takes place between intermediate 6 and intermediate 7, which leads to the generation of transient intermediate [34] 8 which undergoes intramolecular cyclization, followed by tautomerization, ultimately resulting in the formation of the target compound, the 1,4dihydropyridine derivative 9.

Anti-Microbial Activity
The antimicrobial activities of the synthesized compounds 5a-n were evaluated using the Minimum Inhibitory Concentration (MIC) method.The results were compared to the reference drugs Fluconazole and Amoxicillin, with concentrations of 4 g/mL and 2 g/mL, respectively, in their respective areas of application.Table 4 shows that compound 5a-n exhibits moderate to excellent resistance against the tested strains.1,4-DHPs with polar electron-withdrawing groups (5b, 5c, 5i, and 5j) a ached to the phenyl ring at position 4 exhibited effectiveness against all the tested strains, comparable to the standard drugs Amoxicillin (MIC 4 µg/mL) and Fluconazole (MIC 2 µg/mL) this may be due the formation of H-bonds with the different parts of the protein of microbes.However, the resistance efficiency against the tested microbes decreases if the same phenyl ring is substituted with less polar groups such as Me (5f and 5m), OMe (5g and 5n), and Halogens Scheme 3. Plausible mechanism for synthesis of 1,4-DHPs using Al(DS) 3 in water under microwave radiations.

Anti-Microbial Activity
The antimicrobial activities of the synthesized compounds 5a-n were evaluated using the Minimum Inhibitory Concentration (MIC) method.The results were compared to the reference drugs Fluconazole and Amoxicillin, with concentrations of 4 g/mL and 2 g/mL, respectively, in their respective areas of application.Table 4 shows that compound 5a-n exhibits moderate to excellent resistance against the tested strains.1,4-DHPs with polar electron-withdrawing groups (5b, 5c, 5i, and 5j) attached to the phenyl ring at position 4 exhibited effectiveness against all the tested strains, comparable to the standard drugs Amoxicillin (MIC 4 µg/mL) and Fluconazole (MIC 2 µg/mL) this may be due the formation of H-bonds with the different parts of the protein of microbes.However, the resistance efficiency against the tested microbes decreases if the same phenyl ring is substituted with less polar groups such as Me (5f and 5m), OMe (5g and 5n), and Halogens (5d, 5e, 5k, and 5l) this is due to their less or no ability of formation of H-Bonds.

Conclusions
An environmentally green procedure was performed for the synthesis of a novel 1,4-DHP scaffold via a one-pot, four-component reaction using aqueous micellar solution under microwave irradiation by treatment of a mixture of substituted benzaldehyde, diethyl acetylene dicarboxylate, and either ammonium acetate or aniline, along with malononitrile in an equivalent ratio.In summary, the outlined procedure illustrates remarkable efficacy in generating 1,4-DHP derivatives from easily accessible starting materials in a single step, utilizing a micellar solution of Al(DS) 3 in water.The resulting crops are swiftly attained with adaptability and variety, achieving outstanding yields and purity.This approach demonstrates efficiency in terms of labor, cost-effectiveness, and minimal waste generation while operating under mild reaction conditions.Furthermore, all synthesized 1,4-DHPs display potent activity against the evaluated microbial strains.On a note, it finds that the presence of polar groups such as NO 2 on the phenyl ring imparts comparable resistance to standard drugs such as Amoxicillin and Fluconazole.

Table 1 .
Impact of solvents on production of the

5a-h. Solvent Time (min) Yield a (%)
a Reactions supported using various solvents.Reactions occurred at 100 • C in MW irradiation.

Table 2 .
Impact of pressure on the percentage yield of 5a-h using Al(DS) 3 in water.