Synthesis and Biological Evaluation of 1-Naphthyl and 4-Biphenyl Derivatives of 2, 4, 5-Trisubstituted-1H-Imidazoles as Antibacterial and Antifungal Agents.

S.D. Sawant¹*, R.N. Patil² and A.A. Baravkar²

¹Sinhgad Technical Education Society’s Smt. Kashibai Navale College of Pharmacy,

Department of Pharmaceutical Chemistry. Kondhwa (Bk), Pune, India. 411 048.

²SVPM’s College of Pharmacy, Malegaon (Bk), Baramati, Pune, India. 413 115.

²Karpagam University, Coimbatore, Tamil Nadu

*Corresponding Author E-mail: sawant99@rediffmail.com

ABSTRACT:

In current study, 2,4,5-trisubstituted-1H-imidazoles have been synthesized by starting from acid chloride preparation of various aromatic acetic acids (Ar’) such as 1-naphthyl acetic acid and 4-biphenyl acetic acid, which are used as such for the Friedel–Craft acylation (FCA) using various substituted aromatic nucleus (1a-b) (Ar) such as anisole, thioanisole etc and anhydrous aluminium trichloride to get various ethanone derivatives (2c-d). Methylsulfanyl group is converted into methylsulfonyl by refluxing former with glacial acetic acid and hydrogen peroxide to get ethanone derivatives containing methylsulfonyl moiety (2e-f). These ethanone derivatives (2a-f) were subjected to microwave irradiation for 90 secs to obtain diketones or ethanediones (3a-f). Ethanediones, in presence of acetic acid, ammonium acetate and benzaldehyde offered targeted compounds (4a-f). The structural elucidation of synthesized compounds was done using IR as well as ¹H NMR spectroscopy and biological screening for antibacterial action using E. coli and S. aureus and antifungal action using C. albicans and C. fumigatus was done using cup plate agar diffusion method.

KEYWORDS: 1H-imidazole, Microwave irradiation, Antibacterial, Antifungal..

INTRODUCTION:

Imidazole ring system has wide applicability in industry, medicinal chemistry and in many other fields. The substituted imidazoles are well known therapeutic agents. Imidazole heterocycle is one of the important structures present in large number of natural products and biologically active compounds¹. The following imidazole derivatives are applied in medicine: Tolbutamide (antidiabetic agent)², econazole (antifungal agent)³, ketoconazole, itraconazole and clotrimazole (potent cytochrome P450 (CYP)3A inhibitors)⁴, fluconazole (antifungal agent)⁵, carnosine (antioxidant agent)⁶, ketoconazole (antisteroidogenic effects)⁷. Literature surveys indicate that there are various methods reported for synthesis of imidazole which are trisubstituted as well as tetrasubstituted^8,9. First triaryl imidazole synthesis was reported in 1882 by Japp and Radziszewski¹⁰.

Among the family of heterocyclic compounds, nitrogen containing heterocycles especially piperidin-4-ones presumably gaining considerable importance owing to their varied biological properties such as antiviral, antitumour¹¹, local anaesthetic¹², antimicrobial, bactericidal, fungicidal, herbicidal, insecticidal, antihistaminic, anti-inflammatory, anticancer, CNS stimulant and depressant activities^{13, 14, 15}.

In present research work an attempt has been made to synthesize 1-naphthyl and 4- biphenyl derivatives of 1H-imidazoles (4a-f) in order to get new compounds with antibacterial activity and antifungal activity. Different fused or joined ring systems were selected keeping in mind the aromaticity and number of carbon atoms of the fused ring system. Non-linear relationships between lipophilicity and biological activity are very common. Biological activities most commonly increase with increasing lipophilicity. But such an increase is no longer obtained if the limit of lipophilicity is surpassed (crossed). In such cases biological activity remain constant or decrease more or less rapidly with further increase of lipophilicity¹⁶. Synthesis of unsymmetrical 1H-imidazoles with fused or joined ring system was not yet found to be reported, hence was attempted and made possible.

MATERIALS AND METHODS:

All required material was purchased are of Aldrich Chemical Company or Merck and were used without further purification. Melting points were determined using Biotechnics melting point apparatus and are uncorrected. Infrared (IR) (in cm^-1) spectra were recorded on Schimatzu FT-IR spectrometer. Nuclear Magnetic Resonance (¹H NMR) spectra was recorded on Varian spectrometer (300 MHz) using CDCl3 as solvent with TMS as internal standard. The purity of compounds was confirmed by TLC using different mobile phases. Analytical TLC was performed on aluminium backed pre-coated silica gel plates having 0. 25 nm thickness containing pf₂₅₄ indicator (E. Merck). Visualization of spots was effected by exposure to either UV_254nmlamp or iodine vapours.

General procedure for synthesis of 1, 2-diaryl ethanones (2a-d) :

Aryl acetic acids such as 1-naphthyl acetic acid and 4-biphenyl acetic acid (1 mol) was dissolved in excess quantity of thionyl chloride (2 mol) and allowed to reflux on steam bath for 3 h. The excess of thionyl chloride was recovered under vacuum. The resulting acid chloride solution was cooled and added drop wise into the cooled mixture of anhydrous AlCl₃ (1. 5 mol) and substituted aromatic compounds (1a-b). The reaction mixture was stirred for 45 min at room temperature. The reaction mixture is quenched with cold-HCl, extracted with chloroform (3x20 ml). The combined organic extracts were washed with sodium bicarbonate solution, water and dried over anhydrous sodium sulfate. Recrystallization from methanol after solvent removal gave the ethanone derivatives¹⁷ (2a-d).

Compound 2a: 1(4-methoxyphenyl)-2(1-naphthyl) ethanone :

C₁₈H₁₆O₂, Mol. Wt. =264; 56. 84 % yield; mp 128-130 °C; R_f 0. 52; IR (KBr) ν_max : 1672 (C=O), 1261 (Ar-O-Me, asym), 1029 (Ar-O-Me, sym) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm) : 3. 84 (s, 3H, CH₃), 4. 67 (s, 2H, CH₂), 6. 90-8. 07 (m, 11H, Ar).

Compound 2b: 1(4-methoxyphenyl)-2(4-biphenyl) ethanone :

C₁₉H₁₈O_2;Mol. Wt. =278; 54. 33 % yield; 174-176 °C; R_f0. 73; IR (KBr) ν_max : 1678 (C=O), 1267 (Ar-O-Me, asym), 1022 (Ar-O-Me, sym), 1568 (str. of biphenyl ring) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm) : 3. 81 (s, 3H, CH₃), 4. 64 (s, 2H, CH₂), 6. 91-8. 11 (m, 13H, Ar).

Compound 2c: 1(4-methylsulfanylphenyl)-2(1-naphthyl) ethanone :

C₁₈H₁₆OS, Mol. Wt. =280; 54. 46 % yield; mp 170-172 °C; R_f 0. 48; IR (KBr) ν_max : 1674 (C=O) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm); ¹H NMR (CDCl₃, 300 MHz, δ, ppm) : 2. 45 (s, 3H, CH₃), 4. 64 (s, 2H, CH₂), 6. 89-8. 04 (m, 11H, Ar).

Compound 2d: 1(4-methylsulfanylphenyl)-2(4-biphenyl) ethanone :

C₁₉H₁₈OS, Mol. Wt. =294; 49. 09 % yield; 186-188 °C; R_f 0. 44; IR (KBr) ν_max : 1672 (C=O), 1567 (str. of biphenyl ring) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm) : 2. 61 (s, 3H, CH₃), 4. 62 (s, 2H, CH₂), 6. 90-8. 09 (m, 13H, Ar).

General procedure for synthesis of 1(4-methylsulfonylphenyl)-2-aryl ethanones (2e-f) :

Compound (2e-f) were synthesized by heating a mixture of 1, 2-diaryl ethanones (2c-d) (3. 08 mmol) and glacial acetic acid (7 mL) on oil bath. When temperature rises to 60 °C, hydrogen peroxide (2 mL, 30%) was added and heated further for 2. 5 h at 80 °C. The reaction mixture was cooled to room temperature and poured onto crushed ice. Solids obtained was filtered, dried and crystallized from methanol to afford the compounds¹⁸ (2e-f).

Compound 2e: 1(4-methylsulfonylphenyl)-2(1-naphthyl) ethanone :

C₁₈H₁₆O₃S, Mol. Wt. =312; 90. 90 % yield; mp 193-195 °C; R_f 0. 83; IR (KBr) ν_max : 1679 (C=O), 1315 (SO₂, asym), 1157 (SO₂, sym) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm) : 3. 08 (s, 3H, CH₃), 4. 63 (s, 2H, CH₂), 6. 59-8. 11 (m, 11H, Ar).

Compound 2f: 1(4-methylsulfonylphenyl)-2(4-biphenyl) ethanone :

C₁₉H₁₈O₃S, Mol. Wt. =326; mp 198-199 °C; R_f 0. 81; IR (KBr) ν_max : 1679 (C=O), 1315 (SO₂, asym), 1160 (SO₂, sym), 1570 (str. of biphenyl ring) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm) : 3. 11 (s, 3H, CH₃), 4. 62 (s, 2H, CH₂), 6. 65-8. 15 (m, 13H, Ar).

General Procedure for synthesis of 1, 2-diaryl ethandiones (3a-f) :

Selenium dioxide (0. 15 mol) was added into the solution of ethanone derivatives (2a-f) (0. 1 mol) in DMSO (15 mL) and irradiated in the microwave oven (domestic household oven 1000 W) for 90 secs. The hot mixture was filtered to remove the selenium metal and filtrate was poured over crushed ice. The resulting precipitate was filtered, dried and recrystallized from methanol to get the ethanedione derivatives¹⁹ (3a-f).

Compound 3a: 1(4-methoxyphenyl)-2(1-naphthyl) ethanedione :

C₁₈H₁₄O₃, Mol. Wt. =278; 96. 15 % yield; mp 115-118 °C; R_f 0. 59; IR (KBr) ν_max : 1666 (C=O), 1652(C=O), 1259 (Ar-O-Me, asym), 1026(Ar-O-Me, sym) cm^-1;(CDCl₃, 300 MHz, δ, ppm): 3. 87 (s, 3H, CH₃), 6. 98-8. 11 (m, 11H, Ar).

Compound 3b: 1(4-methoxyphenyl)-2(4-biphenyl) ethanedione :

C₁₉H₁₆O₃, Mol. Wt. =292; 86. 53 % yield; mp 108- 112 °C; R_f 0. 59; IR (KBr) ν_max : 1664 (C=O), 1654 (C=O), 1263 (Ar-O-Me, asym), 1024 (Ar-O-Me, sym), 1572 (str. Of biphenyl ring) cm^-1;(CDCl₃, 300 MHz, δ, ppm): 3. 83 (s, 3H, CH₃), 6. 89-8. 14 (m, 13H, Ar).

Compound 3c: 1(4-methylsulfanylphenyl)-2(1-naphthyl) ethanedione :

C₁₈H₁₄O₂S, Mol. Wt. =294; 89. 33 % yield; mp 116-118 °C; R_f 0. 63; IR (KBr) ν_max : 1665 (C=O), 1655 (C=O), 1574 (str. of biphenyl ring) cm^-1;(CDCl₃, 300 MHz, δ, ppm): 2. 48 (s, 3H, CH₃), 6. 90-8. 11 (m, 11H, Ar).

Compound 3d: 1(4-methylsulfanylphenyl)-2(4-biphenyl) ethanedione :

C₁₉H₁₆O₂S, Mol. Wt. =308; 80. 76 % yield; mp 105-108 °C; R_f 0. 44; IR (KBr) ν_max : 1674 (C=O), 1656 (C=O), 1573 (str. of biphenyl ring) cm^-1;(CDCl₃, 300 MHz, δ, ppm): 2. 79 (s, 3H, CH₃), 6. 82-8. 13 (m, 13H, Ar).

Compound 3e: 1(4-methylsulfonylphenyl)-2(1-naphthyl) ethanedione :

C₁₈H₁₄O₄S, Mol. Wt. =326; 96. 15 % yield; mp 154-156 °C; R_f 0. 76; IR (KBr) ν_max : 1683(C=O), 1666(C=O), 1296 (SO₂, asym), 1147 (SO₂, sym) cm^-1;(CDCl₃, 300 MHz, δ, ppm): 3. 07 (s, 3H, CH₃), 6. 87-8. 08 (m, 11H, Ar).

Compound 3f: 1(4-methylsulfonylphenyl)-2(4-biphenyl) ethanedione :

C₁₉H₁₆O₄S, Mol. Wt. =340; 83. 69 % yield; mp 155-158 °C; R_f 0. 62; IR (KBr) ν_max : 1314 (SO₂, asym), 1159 (SO₂, sym), 1570 (str. of biphenyl ring), 1671 (C=O), 1656 (C=O) cm^-1;(CDCl₃, 300 MHz, δ, ppm): 3. 07 (s, 3H, CH₃), 6. 79-8. 07 (m, 13H, Ar).

General Procedure for synthesis of 2, 4, 5-triaryl-1H-imidazoles (4a-f) :

In around bottom flask fitted with condenser and a magnetic stirrer, ethanediones (3a-f) (0. 05 mol), benzaldehyde (50 mmol) and ammonium acetate (150 mmol) were dissolved in acetic acid (100 mL). The mixture was heated under reflux in an oil bath for 2½ h with stirring. After this time, the mixture was cooled to room temperature and filtered to remove any precipitate. Water (500 mL) was added to the filtrate and the precipitate formed was collected. Recrystallization from ethanol afforded the title compounds²⁰ (4a-f).

Compound 4a: 2-phenyl-4(1-naphthyl)-5(4-methoxyphenyl)-1H-imidazole :

C₂₆H₂₀N₂O, Mol. Wt. =376; 89. 84 % yield; mp 257-259 °C; R_f 0. 81; IR (KBr) ν_max : 1260 (Ar-O-Me, asym), 1029 (Ar-O-Me, sym), 3452 (N-H), 3053 (C-H), 1588 (C=C), 1584 (C=N) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm): 3. 85 (s, 3H, CH₃), 6. 89-8. 03 (m, 16H, Ar), 9. 45 (br s, NH).

Table-1 : Antibacterial and antifungal activity of synthesized 1H-imidazoles (4a-f).

Compound No.	Antibacterial activity		Antifungal activity
	Zone of inhibition in mm
	Escherichia coli	Staphylococcus aureus	Candida albicans	Candida fumigatus
4a	13	11	17	19
4b	14	12	16	17
4c	19	17	21	22
4d	17	16	20	18
4e	20	18	22	20
4f	21	19	20	19
Amikacin	22	23	---	---
Fluconazole	---	---	25	26
DMF	---	---	---	---

Compound 4b: 2-phenyl-4(4-biphenyl)-5(4-methoxyphenyl)-1H-imidazole :

C₂₈H₂₂N₂O, Mol. Wt. =402; 81. 33 % yield; mp 235-238 °C; R_f 0. 67; IR (KBr) ν_max : 1270 (Ar-O-Me, asym), 1034 (Ar-O-Me, sym), 1572 (Str. of biphenyl ring), 3451 (N-H), 3060 (C-H), 1591(C=C), 1587 (C=N) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm): 3. 87 (s, 3H, CH₃), 7. 11-8. 12 (m, 18H, Ar), 9. 23 (br s, NH).

Compound 4c: 2-phenyl-4(1-naphthyl)-5(4-methylsulfanylphenyl)-1H-imidazole :

C₂₆H₂₀N₂S, Mol. Wt. =392; 92. 21 % yield; mp 229-233 °C; R_f 0. 76; IR (KBr) ν_max : 1261 (Ar-O-Me, asym), 1027 (Ar-O-Me, sym), 3449 (N-H), 3051 (C-H), 1587 (C=C), 1581 (C=N) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm): 2. 49 (s, 3H, CH₃), 6. 86-8. 0 (m, 16H, Ar), 9. 33 (br s, NH).

Compound 4d: 2-phenyl-4(4-biphenyl)-5(4-methylsulfanylphenyl)-1H-imidazole :

C₂₈H₂₂N₂S, Mol. Wt. =418; 92. 65 % yield; mp 240-242 °C; R_f 0. 79; IR (KBr) ν_max : 3450 (N-H), 3056 (C-H), 1570 (Str. of biphenyl ring), 1589 (C=C), 1582 (C=N) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm): 2. 82 (s, 3H, CH₃), 7. 09-8. 08 (m, 18H, Ar), 9. 54 (br s, NH).

Compound 4e: 2-phenyl-4(1-naphthyl)-5(4-methylsulfonylphenyl)-1H-imidazole :

C₂₆H₂₀N₂O₂S, Mol. Wt. =424; 91. 10 % yield; mp 276-278 °C; R_f 0. 72; IR (KBr) ν_max : 1260 (Ar-O-Me, asym), 1027 (Ar-O-Me, sym), 3447 (N-H), 3051 (C-H), 1587 (C=C), 1580 (C=N) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm): 3. 09 (s, 3H, CH₃), 6. 85-8. 1 (m, 16H, Ar), 9. 43 (br s, NH).

Compound 4f: 2-phenyl-4(4-biphenyl)-5(4-methylsulfonylphenyl)-1H-imidazole :

C₂₈H₂₂N₂O₂S, Mol. Wt. =450; 90. 90 % yield; mp 256-258 °C; R_f 0. 92; IR (KBr) ν_max : 1314 (SO₂, asym), 1158 (SO₂, sym), 1569 (Str. of biphenyl ring), 3449 (N-H), 3056 (C-H), 1589 (C=C), 1581 (C=N) cm^{-1; 1}H NMR (CDCl₃, 300 MHz, δ, ppm): 3. 10 (s, 3H, CH₃), 7. 05-8. 01 (m, 18H, Ar), 9. 34 (br s, NH).

Biological activity:

Synthesized compounds have been screened for in vitro antibacterial and antifungal activity using cup plate agar diffusion method^{21, 22}by measuring zone of inhibition in mm. The compounds were taken at a concentration of 0. 5 mg/mL using dimethyl formamide (DMF) as solvent. Amikacin (100 mcg/mL) was used as a standard for antibacterial activity and fluconazole (100 mcg/mL) was used as a standard for testing antifungal activity of synthesized compounds. Escherichia coli (gram negative) and Staphylococcus aureus (gram positive) are used for testing antibacterial activity and Candida albicans and Candida fumigatus are used for testing antifungal activity in potato dextrose agar medium. The sterilized agar medium was poured in petri dishes and allowed to solidify. On the surface of the media microbial suspensions were spread with the help of sterilized triangular loop. A stainless steel cylinder of 8 mm diameter (pre-sterilized) was used to bore the cavities. In to these wells were added 0. 1 ml portions of the test compounds in solvent. The drug solution was allowed to diffuse for about an hour into the medium. The plates were incubated at 37 °C for 24 h and 30 °C for 48 h for antibacterial and antifungal activities, respectively. The zone of inhibition observed around the cups after respective incubation was measured. The results are presented in Table-1.

RESULTS AND DISCUSSION:

The novel synthesized 1H-imidazoles, exhibits comparable antibacterial and antifungal activities against the organisms tested. It has been observed that some of 1H-imidazole derivatives exhibited interesting microbial activities. 4e and 4f exhibited most significant activity against bacteria. While 4c and 4f are most active against fungi. All other compounds exhibited low to moderate activity (Table-1). Compound 4a is showing less antibacterial action as compared to other compounds.

CONCLUSION:

1H-imidazoles containing 1-naphthyl and 4-biphenyl ring system can be explored for antibacterial and antifungal activity. From the study it is also evident that sulphur containing compounds shows higher activity than others.

ACKNOWLEDGMENT:

Authors are thankful to Board of College and University Development (BCUD), Pune University for providing research grant to this research project.

Future plans:

It is planned to synthesize 1-naphthyl, 2-naphthyl and 4-biphenyl derivatives of 1H-imidazoles using various aromatic aldehydes such as benzaldehyde, furfuraldehyde, anisaldehyde etc. and to evaluate them for anti-inflammatory and analgesic activity.

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