One-pot Multicomponent synthesis of 3-(substitutedphenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c] Pyrazole-2(1H)-carbaldehyde Derivatives Act as Antibacterial Agents

Introduction

Microbial infection is still one of the most dangerous side effects in several industries, including those that deal with medical devices, pharmaceuticals, water purification systems, healthcare tools, fabrics, packaged food, and food storage. Due to their potential to improve the quality and safety of numerous materials, antimicrobials are attracting more and more attention through both academic and industrial research.¹ The process of antibacterial activity is extremely complicated since it involves living things whose basic needs, such as nourishment, respiration, metabolism, and capacity for reproduction, could be impacted if harmful substances are present.²

In synthetic anion receptor systems, the ring of imidazole functions as an efficient donor moiety for hydrogen bonds. Due to the great therapeutic effectiveness of medications related to imidazole, medicinal chemists have been inspired to create five-membered heterocyclic structures with an imidazole nucleus that display a range of qualities, such as antifungal and antibacterial, anti-inflammatory and analgesic, antitubercular, anti-depressant, anti-cancer, antiviral, and antileishmanial activities.^3-4 Multi-component processes are also used to synthesize tetra-substituted imidazoles.^5-7 Tetra-substituted imidazoles are important in biochemical and pharmacological processes.^8-11

Different configurations with the pyrazole nucleus allow for a variety of uses in fields like engineering, pharmacy, and agriculture. They are specifically described as potent activities  ^12-13 anti-tuberculosis.^14-15 Imidazopyrazole plays a vital role in medicinal chemistry and is a crucial element in the chemistry of heterocyclics because of its extensive spectrum of bioactivities,^16-23 The therapeutic potential of pyrazole-carbaldehydes and related pharmaceuticals has drawn medicinal chemists’ attention, leading them to develop a variety of pyrazole-carbaldehyde derivatives.^24-27 Pyrazole-carbaldehyde derivatives may be important in anticancer ²⁸ and antibacterial activity ^29–30 because of their significant biological and pharmacological benefits.

To further our research, we synthesized 3-(substitutedphenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde motifs, a multicomponent synthesis that can be completed in one pot. Because of step and atom economy, avoiding protective group tactics, and a quick purification process, MCR transforms into a potent chemical tool for the creation of complex molecules. However, a number of heterocyclic compounds can be synthesized with considerably greater efficiency using the multicomponent reaction technique.³¹

Material and Methods

Instrumentation

Melting points have been established in nursing associates who are uncorrected and seem to have open capillaries. The solvents and chemicals utilized were pure and of laboratory standard. By using TLC, the reaction was observed. Shimadzu spectrometers were used to record FT-IR spectra in KBr. The ¹H NMR measurements were made in DMSO-d6 by means of TMS as an internal standard with an advanced spectrometer at a frequency of 300 MHz, DMSO-d₆ was used to record ¹³C NMR, and a GC-MS (EI-Shimadzu QP2010PLUS) system was used to record mass spectra.

A general method for preparation of compounds 5(a-l).

In this section, we describe the synthesis of 3-(substitutedphenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde derivatives a one-pot multicomponent reaction, which an equimolar quantity of imidazolidine-2,4-dione(1)is reacted with substituted aromatic aldehydes 2(a-g). Bleaching Earth Clay (pH 12.5 wt%) was used as a catalyst, and PEG-400 was used as a sustainable solvent while stirring at 70-80^°C for one hour. Hydrazine hydrate (3) was then added to this mixture, and it was shaken for nearly an hour. An excess of formic acid (4) was added to the reaction mixture. After the reaction is over (monitored by TLC). The mixture was placed on ice-cold water once the reaction was finished, and it was shaken for five minutes. Diluted HCl was used to neutralize if necessary. The solid crude material was separated using a water aspirator, washed with ice-cold water, and recrystallized from ethanol. The prepared products 5(a-l) were identified by spectroscopic investigation and comparison of their melting points.

Spectral data of some synthesized compounds.

3-(3-Chlorophenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde 5a: C₁₁H₉ClN₄O₂,m.p. 220 °C, this compound was obtained in 90% yield; IR υ_max: 3334 (N-H), 1769 (C=O, CHO), 1700 (C=O).¹H NMR (δ, ppm); 10.60 (s, 1H, CHO), 8.20-8.23 (d, 2H, Ar-H), 7.56-7.73 (d, 2H, Ar-H), 7.50 (s, 1H, NH), 6.00 (s, 1H, NH), 5.03 (s, 1H, NH), 4.20 (s, 1H).¹³C NMR (δ, ppm); 173.8, 158.1, 149.2, 146.4, 143.1, 127.8, 126.8, 125.2, 124.0, 70.3, 63.2.MS (EI): m/z (M+) 264.5.

3-(3-Bromophenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde 5b: C₁₁H₉BrN₄O₂, m.p.218 °C, this compound was obtained in 92% yield; IR υ_max: 3334 (N-H), 1765 (C=O, CHO), 1703 (C=O), 1599. ¹H NMR (δ, ppm); 10.57 (s, 1H, CHO), 8.22-8.23 (d, 2H, Ar-H), 7.60-7.69 (d, 2H, Ar-H), 7.55 (s, 1H, NH), 6.09 (s, 1H, NH), 5.07 (s, 1H, NH), 4.32 (s, 1 H). ¹³C NMR (δ, ppm); 173.8, 158.0, 149.6, 146.1, 145.7, 143.3, 127.8, 126.8, 123.0, 70.3, 63.4.MS (EI): m/z (M+) 307.99.

3-(3-Nitrophenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde 5c: C₁₁H₉N₅O₄, m.p.223 °C,  this compound was obtained in 85% yield;  IR υ_max: 3343 (N-H), 1768 (C=O, -CHO), 1702 (C=O). ¹H NMR (δ, ppm); 10.59 (s, 1H, CHO), 8.22-8.27 (d, 2H, Ar-H), 7.60-7.66 (d, 2H, Ar-H), 7.51 (s, 1H, NH), 6.00 (s, 1H, NH), 5.01 (s, 1H, NH), 4.29 (s, 1H). ¹³C NMR (δ, ppm); 173.0, 158.0, 149.4, 146.7, 143.1, 127.8, 125.2, 123.0, 124.3, 70.2, 63.5. MS (EI): m/z (M+) 275.

3-(4-Fluorophenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde 5d: C₁₁H₉FN₄O₂, m.p.230 °C, this compound was obtained in 90% yield; IR cm^-1 υ_max: 3333 (N-H), 1770 (C=O, -CHO), 1700 (C=O). ¹H NMR (δ, ppm); 10.63 (s, 1H, -CHO), 8.20-8.23 (d, 2H, Ar-H), 7.66-7.69 (d, 2H, Ar-H), 7.57 (s, 1H, -NH), 6.09 (s, 1H, -NH), 5.06 (s, 1H, -NH), 4.34 (s, 1 H). ¹³C NMR (δ, ppm); 173.8, 158.0, 149.1, 146.4, 145.4, 142.9, 127.8, 125.5, 124.3, 70.3, 63.2. MS (EI): m/z (M+) 247.

Antibacterial activity against different bacteria by Well plate diffusion Method

The bacterial cultures were used to prepare the microorganism’s inoculum. Clean, sanitized Petri plates received nutrient agar (15 mL) (Hi media), which was then allowed to cool and harden. A spreading stick was used to uniformly distribute bacterial strain broth (100 μL) across the medium until it had fully dried; using a sterile cork borer, 6 mm-diameter wells were drilled. All of the substances were produced as DMSO solutions (10 mg/mL). Plant extract solutions containing 100μl were poured into the wells. Petri plates were incubated for 24 hours at 37°C. Positive control was constructed using streptomycin. It was used with DMSO as a negative control.³² All measurements were made in triplicates, and the zone of inhibition (ZI) widths served as a measure of antibacterial activity.

Result and Discussion

3-(Substitutedphenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde derivatives 5(a-l)were synthesised as shown in Scheme. The synthesis of the target molecule was achieved by a one-pot multicomponent reaction by adding equimolar quantities of imidazolidine-2,4-dione(1)with substituted aromatic aldehydes 2(a-g),hydrazine hydrate (3), and an excess of formic acid (4). This method is efficient and green because all reactions are preferred in the presence of Bleaching Earth Clay (pH 12.5 wt%) as a catalyst, and greener PEG-400 was used to afford the title 5(a-l) compounds in good yields as shown in Table 1within a very short reaction time.

Scheme 1: Synthesis of 3-(substitutedphenyl)-5-oxo-3,4,5,6-tetrahydroimidazo [4,5-c] pyrazole-2(1H)-carbaldehyde derivatives. Click here to View scheme

The structures of the prepared products 5a-l were determined using FT-IR, ¹H NMR, ¹³C NMR spectroscopy, and mass spectrometry. The FT-IR spectra of compounds 5a-l appear in bands at 3334-3333 cm^-1 due to the N-H vibration. The characteristic absorption of the aldehyde hydrogen appeared between 1770 cm^-1 and 1768 cm^-1. Also, the C=O stretching of the imidazolone carbonyl group at 1701-1700 cm^-1. In ¹H NMR, the NH protons appeared in three sharp singlets: 5.06, 6.11, and 7.57 ppm. The characteristic aldehyde proton appears in a more deshielded region at 10.62 ppm, and the aromatic protons are found to be in the expected region between 7.66 and 8.23 ppm for four protons. The most distinguishing signal in the ¹³C NMR spectrum data of the title compounds is associated with the carbon of the carbonyl group and is located at 173 ppm due to the aldehydic group, and 158 ppm belongs to the ketonic group. The signal at 63 ppm is due to -CH, which confirms that there is one hydrogen atom at that carbon. While all aromatic carbons of the compounds exhibit signals between 123 and 149 ppm. The anticipated M⁺ peak, which corresponds to the actual molecular mass, can be found in the mass spectra of all substances. Table 1 displays the physiochemical properties of all synthesized derivatives (5a-l)

Table 1: Physiochemical properties of synthesized derivatives (5a-l).

Compound number	Structure	Time (hours)	Yield (%)	M.P. (°C)
5a	3-Cl	2.5	90	220-222
5b	3-Br	2.5	92	230-232
5c	3-NO2	3	85	225-227
5d	4-F	3	90	223-225
5e	3,4-Cl	3.5	80	228-230
5f	4-Cl	2.5	92	221-223
5g	4-Br	2.5	90	228-230
5h	4- NO2	3	89	226-228
5i	4-OH	3.5	85	235-238
5j	4-CH3	3	82	200-202
5k	4-OCH3	3	87	232-234
5l	3-Cl, 4-OCH3	3.5	82	237-239

 

Antibacterial activity

From the information provided above, it can be seen that the synthetic molecule was developed to suppress the growth of a number of examined species of microbes that are both gram-positive and gram-negative, such as S. aureus, E. coli, B. subtilis, and S. Typhi. According to the findings, compounds 5d and 5g have demonstrated substantial activity against S. Typhe. Against E. coli and B. subtilis, compound 5g showed impressive antibacterial activity. Against S. aureus, compounds 5a, 5f, and 5h demonstrated remarkable antibacterial activity, as indicated in Table 2 and in graphical form in Figure 1. Generally, we can conclude that the synthetic compound (5a-l) exhibits good to moderate antibacterial activity.

Table 2: Antibacterial Screening of the synthesized derivatives (5a-l).

Compound number	Mean zone inhibition (MZI)a
	S. Typhi	E. coli	B. subtilis	S. aureus
5a	14	12	16	17
5b	16	14	18	15
5c	13	13	13	16
5d	18	11	17	16
5e	11	10	12	13
5f	15	13	19	17
5g	18	16	21	15
5h	13	12	14	17
5i	12	13	13	15
5j	11	10	12	14
5k	12	12	12	13
5l	12	12	11	13
Standard (Streptomycin)	20	19	29	22

^a Values are mean (n=3), Salmonella Typhi =S. Typhi, Escherichia coli = E. coli, Bacillus subtilis = B. subtilis and Staphylococcus aureus = S. aureus

Figure 1: Graphical form of antibacterial activity Click here to View figure

Conclusion

The synthesized 3-(substitutedphenyl)-5-oxo-3,4,5,6-tetrahydroimidazo[4,5-c]pyrazole-2(1H)-carbaldehyde compounds have been screened for notable antibacterial activity. The inclusion of halo-substituted phenyl groups may be significant in imparting the growth inhibitory action against Gram-positive and Gram-negative microbes, as indicated by the substantial antibacterial activity of compounds 5a, 5b, 5d, 5f, and 5g.

Acknowledgment

We appreciate the research facilities provided by the School of Chemical Sciences at S.R.T.M. University, Nanded.

Conflict of Interest

No authors have revealed any conflicts of interest.

Funding Sources

This research received no specific grants from any agencies.

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