Synthesis, Antifungal and Anti-Tubercular Activity Evaluation of Series of Imidazo [2,1-b]1,3,4-Thiadiazole Derivatives

Pranali Dhapodkar¹*, Anjali Ganjare¹, Kalyani Lanjewar², Pranita Rathod¹, Shweta Nawghare¹

¹Associate Professor, G H Raisoni Institute of Pharmacy, Nagpur, Maharashtra, India.

²Assistant Professor, G H Raisoni Institute of Pharmacy, Nagpur, Maharashtra, India.

*Corresponding Author E-mail: pranali.dhapodkar@raisoni.net, anjali.ganjare@raisoni.net, kalyanithombre22271994@gmail.com, pranita.rathod@raisoni.net, shweta.nawghare@raisoni.net

ABSTRACT:

Antimicrobial drugs are the greatest contribution to the 20^th century to therapeutics. The methodology indicates and characterizes various aspects like melting point, solubility, TLC, and IR. The experimental work comprises of various methods like preparation of 2-amino-5-trifluromethyl,1,3,4, thiadiazole preparation. According to this, we synthesized a series of sixteen derivatives of selected compound. The synthesized compounds were evaluated for antimicrobial and antifungal activity by disc diffusion method. The results of the antimicrobial screening studies clearly shows moderate to mild antimicrobial activity.

KEYWORDS: Antimicrobial Drugs, Sixteen Derivatives, 2-Amino-5-Trifluromethyl,1,3,4, Thiadiazole, Disc Diffusion Method.

INTRODUCTION:

Antimicrobial drugs are the greatest contributions of the 20^th century to therapeutics. Their advent changed the outlook of the physician about the power drugs can have on disease. They are one of the few curative drugs. Their importance is magnified in the developing countries, where infective diseases predominate. As a class they are one of the most frequently used as well as misused drugs.

The clinical value of antimicrobial agents cannot be assumed simply because they inhibit or kill microbial pathogens in vitro. Pharmacodynamic studies, which show how pharmaceutical agents interact with their therapeutic targets, have found that the success of antimicrobial agents at eliminating bacterial pathogens is affected by the local milieu of infection. Pharmacodynamic principles are demonstrated by studies in animal models, in which local milieu of infection is controlled by adjusting inoculums density, duration of infection before treatment and other factors.

Synthetic antimicrobial agents can be classified as:

Sulfonamides

Quinolones

Antitubercular agents

Miscellaneous agents

Sulfonamides:

Sulfonamides were discovered in the mid-1930. Protonsil rubrum a red dye was one of a series of dyes examined by Gerhard Domagk of Bayer of germany in the belief that it might be taken up selectively by certain pathogenic bacteria and not by human cells work in the manner analogous to Gram-stain works and serves as a selective position to kill these cells. The dye, indeed, proved active in vivo against streptococcal infections in mice. Trefouel and Bovet in France soon showed that the urine of protonsil rubrum treated animals was bioactive in-vitro. As poor as the potency of sulfonamide is when compared with that of modern agents that begin to succeed it shortly there after, its impact on medicine was enormous. Once mainstaiys of anti-microbial chemotherapy, the Sulfonamides have decreased enormously in popularity and are now comparatively minor drugs. The cheapeness of sulfonamides is one of their most attractive features and accounts for much of their persistence on the market.

EXPERIMENTAL:

The identification and characterization of the compound were carried out by the following procedure to ascertain that all prepared compounds were of different chemical nature, than the respective parent compound.

1. Melting Point

2. Solubility

3. Thin Layer Chromatography

4. I.R.

5. N.M.R.

The experimental work comprises of:

1. Preparation of 2-amino-5-trifluoromethyl-1,3,4-thiadiazole.

2. Preparation of 4-substituted phenacyl bromide.

3. Preparation of 2-(trifluoromethyl)-6-(4-substituted phenyl)-imidazo(2,1-b)-1,3,4-thiadiazole.

4. Preparation of 2-(trifluoromethyl)-5-substituted-6-(4-substituted phenyl)-imidazo(2,1-b)-1,3,4-thiadiazole

1. General method of synthesis of 2-amino-5- trifluoromethyl -1,3,4-thiadiazole (PR1):

Trifluoroacetic anhydride (20ml) was added dropwise to powdered thiosemicarbazide (8g) kept at ice-salt mixture temperature. At the end of a vigorous reaction, the mixture was warmed, first at room temperature then at 40 ^oC and kept one hour in a boiling water bath. The reaction mixture was cooled, diluted with water & made alkaline with ammonia. The crystalline precipitate was recrystallized in dilute alcohol, giving the rectangular plates .( m.p.=235 ^oC)

(See Table1)

General structure:

2. General Procedure for synthesis of 4-substituted phenacyl bromibe:

To a solution of various p-substituted acetophenone (0.1 M) in acetic acid (20 ml) was added, drop wise bromine (0.1M) in acetic acid (15 ml) with stirring at 0-10^οC during 1hour. It was further stirred for 4hour at room temperature and poured on to crushed ice (100gm). The solid that separated was filtered, washed with water and dried. The crude p-substituted phenacyl bromide was purified by recrystallization twice from methanol. (See Table 2)

General Structure:

Where, R = Cl, Br, CH₃, OCH₃.

3. General method for synthesis of 2- trifluoromethyl -6-(4-substituted phenyl)-imidazo(2,1-b)-1,3,4-thiadiazoles (PRA1-PR1D): A mixture of equimolar quantities of 2-amino-5-alkyl/aryl-1,3,4-thiadiazole (0.01 M) and bromoacetyl compound (0.01 M) was refluxed in dry ethanol for 8 h. Excess of solvent was distilled off and the solid hydrobromide that separated was collected by filtration suspended in water & neutralized by aqueous Na₂CO₃ solution to get free base. It was filtered wash with water dried, & recrystallized from suitable solvent.

General structure:

4. General method for synthesis of 2-( trifluoromethyl)-5-carbaldehyde-6-(4-substituted phenyl)-imidazo(2,1-b)-1,3,4-thiadiazole (PR1AV-PR1DV): Vilsmeier–Haack reagent was prepared by adding phosphoryl chloride (3 ml) in dimethylformamide (20 ml) at 0 ^οC with stirring. Then appropriately substituted 2-alkyl/aryl-6-arylimidazo[2,1-b] [1,3,4] thiadiazole (0.01 mol) was added to the reagent and stirred at 0 ^οC for 30 min. The mixture was further stirred for 2 h at room temperature and at 60 ^οC for additional 2 h. The reaction mixture was then poured in sodium carbonate solution and stirred at 90 ^οC for 2 h. After cooling, the mixture was diluted with water, extracted with chloroform, and the collective extract was washed with water and dried over anhydrous sodium sulfate. The residue obtained after the removal of chloroform was recrystallized from a suitable solvent to get the crystalline solid.

General structure:

Table 2: Physical parameters of different synthesized phenacyl bromides:

Sl. No.	R^’	Nature ofcrystal	Percent-ge Yield (%)	M. P. (^oC)	M.F.	M.W.
1	Cl	Colorless	70-75	92-94(97)	C8H6OBrCl	233.49
2	--	Whitish yellow	65-70	98-105(108-110)	C8H6OBr2	77.94
3	CH3	Colourless	70-75	42-44(48-51)	C9H9OBr	213.96
4	OCH3	Brown	60-65	48-52(50-55)	C9H9O2Br	229.96

SPECTRAL DATA:

Infra red spectral study of the synthesized compounds:

Compound code	Spectral peaks (cm^-1)	Molecular nature
	3271.64 3034.44 2944.77 1565.92 1696.09 755.00 575.64	N-H (stretching) Ar. C-H (stretching) Al. C-H (stretching) Ar. C=C C=N CS C-Cl
	3167.51 3120.26 2938.02 1691.27 1570.00 1350.89	N-H (stretching) Ar. C-H (stretching) Al. C-H (stretching) C=N Ar. C=C NO2 (stretching)
	3147.26 2921.63 1670.05 1092.48 1476.24 733.78	Ar. C-H (stretching) Al. C-H (stretching) C=N C-N Ar. C=C C-Cl
	3149.19 2927.41 1674.87 1475.28 1181.19	Ar. C-H (stretching) Al. C-H (stretching) C=N Ar. C=C C-N
	3146.29 2938.98 1667.16 1481.06 1347.03 1303.64	Ar. C-H (stretching) Al. C-H (stretching) C=N Ar. C=C C-N C-O
	3143.40 2960.20 1666.20 1460.81 1343.18 736.67	Ar. C-H (stretching) Al. C-H (stretching) C=N Ar. C=C C-N C-Cl

¹H NMR Spectral data:

Compound code	Chemical shift value (in ppm)	Proton value
	7.94 7.72-7.25 4.27	1H, CH 8H, Ar-H 2H, CH2
	7.96 7.68-7.25 4.27	1H, CH 8H, Ar-H 2H, CH2
	7.74 7.73-6.92 4.26 3.84	1H, CH 8H, Ar-H 2H, CH2 3H, OCH3
	7.92 7.70-7.20 4.26 2.37	1H, CH 8H, Ar-H 2H, CH2 3H, CH3
	7.10 7.50 8.20 4.30	2H, NH2 2H, Ar-H 2H, Ar-H 2H, CH2

RESULTS AND DISCUSSION:

We have synthesized a series of sixteen derivatives of 2-(trifluoromethyl)-6-(4-substituted phenyl) imidazo[2,1-b] [1,3,4]-thiadiazole by reacting 2-amino-5-( trifluoromethyl)-1,3,4-thiadiazole with 4-substituted phenacyl bromide as depicted in scheme.

The intermediate compounds i.e. 2-amino-5- trifluoromethyl -1,3,4-thiadiazole were prepared by reacting trifluoromthyl acetic anhydried with thiosemicarbazide. The compounds were confirmed by IR, and ¹H NMR. (Fig.1, 2 and 17). Four 4-substituted Phenacyl bromides were prepared by reacting 4-substituted acetophenones with bromine according to literature and confirmed by physical constants.

The compound 2-(trifluoromethyl)-6-(4-substituted phenyl) imidazo[2,1-b] [1,3,4]-thiadiazoles was confirmed by IR, ¹H NMR and other physical parameters. The compounds showed absorption bands ranging from 3149-3034 cm^-1 for C-H aromatic stretching, 2960-2845 cm^-1 for aliphatic stretching, 1521-1342 cm^-1 for NO₂ group. In ¹H NMR spectra the presence of methylene proton and methyl protons between δ 4.27-4.26ppm and 3.80-2.30ppm was observed respectively. For aromatic protons multiplets were observed between δ 7.94-7.25ppm.

The synthesized compounds were evaluated for antimicrobial by disc diffusion method. The antimicrobial activity as calculated by the zones of inhibition against S. aureus (Gram positive) and Klebsiella (Gram negative) as compared to that of standard drug ciprofloxacin are given in the Table 8.

The results of the antibacterial screening studies clearly show moderate to mild antimicrobial activity. The compounds SKS-07 and SKS-23 are the compounds which exhibited activity against S. aureus, while only compound SKN-1 showed moderate activity against Kelbsiella at all five concentrations. The SKN-03 showed moderate activity against Kelbsiella at concentration of 75µg.

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Received on 07.03.2025      Revised on 29.03.2025

Accepted on 16.04.2025 Published on 12.07.2025

Available online from July 21, 2025

Asian Journal of Pharmaceutical Analysis. 2025; 15(3):181-184.

DOI: 10.52711/2231-5675.2025.00028

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