Design, synthesis, spectral analysis and in vitro microbiological evaluation of 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones

Abstract

A series of novel 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 were synthesized, and studied for their in vitro antibacterial and antifungal activities against clinically isolated strains. Generally compounds possessing electron donating groups showed good antibacterial activity. Compound 31, which contain both electron withdrawing chloro and electron donating methyl groups showed potent activity against all the tested Gram positive and Gram negative bacterial strains whereas compounds 32 and 33 which contain electron donating methoxy functional group at the para position of the phenyl ring attached to pyrimidine ring showed promising activity against S.aureus, S.typhii and E.coli. Compounds 32 and 33, both containing electron withdrawing groups (-Cl, -F) showed excellent activities against all the tested A. flavus, Mucor, Rhizopus and M.gypsuem fungal strains. while against Mucor, compound 27 which contains an electron donating methyl group at the para position of the phenyl ring attached to pyrimidine ring showed promising activity. Also compound 31, which contains both electron withdrawing chloro and electron donating methyl groups showed potent activity against A. flavus and Rhizopus.

Keywords::

• 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones
• 2-amino-4,6-diaryl-pyrimidines
• thioglycolic acid
• antibacterial activity
• antifungal activity

Introduction

Various 4-thiazolidinones have attracted considerable attention as they are endowed with wide range of pharmacological activities. Peptidoglycan is an essential component of the cell wall of both Gram-positive and Gram-negative bacteria. 4-Thiazolidinones have been reported as novel inhibitors of the bacterial enzyme Mur B which is a precursor, acting during the biosynthesis of peptidoglycan [1]. A wide variety of biological properties such as hypolipidaemic [2], antidegenerative [3], muscarinic receptor 1 agonist [4], antiproteolytic[5], anti-inflammatory [6], antiviral [7], antifungal [8], antibacterial [9], antitubercular [10], anticonvulsant [11], respiratory [12] and hypnotic [13] activities have been reported for 4-thiazolidinones.

Pyrimidines are the basic nucleus in nucleic acids and have been associated with a number of biological activities. Substituted aminopyrimidine nuclei are common in marketed drugs such as anti-atherosclerotic aronixil, anti-histaminic thonzylamine, anti-anxielytic buspirone, anti-psoriatic enazadrem, and other medicinally relevant compounds. Some notable biological activity of pyrimidine derivatives include adenosine receptor antagonists [14], kinase inhibitors [15], analgesic [16], anti-inflammatory [16], inhibitors of cyclin-Dependent kinases 1 and 2 [17], calcium channel antagonist [18], antihistaminic [19], antitubercular [20] activities.

Recently, we exploited the synthesis of 6-aryl-1,2,4,5-tetrazinane-3-thiones [21], fused indazoles [22], 2,6-diarylpiperidin-4-one derivatives [23–25] with a view to incorporate various other bioactive heterocyclic nucleus such as 1,2,3-selenadiazoles, 1,2,3-thiadiazoles, diazepans intact for evaluation of associated antibacterial and antifungal activities.

Among the various substituted thiazolidin-4-one compounds, pyrimidine moiety present thiazolidine-4-one compound exhibit strong tuberculosis [26] and antimicrobial activity [27]. In the interest of above, we planned to synthesize a system, which comprises both 4-thiazolidinones and 2-amino-4,6-diarylpyrimidine components together to give a compact structure like title 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones (Figure 1).

Figure 1.  Synthetic compounds having the core structure with antituberculosis and antimicrobial activity.

Experimental

Chemistry

TLC was performed to assess the reactions and the purity of the products. All the reported melting points were taken in open capillaries and were uncorrected. IR spectra were recorded in KBr (pellet forms) on a Nicolet-Avatar–330 FT-IR spectrophotometer and noteworthy absorption values (cm⁻¹) alone are listed. ¹H and ¹³C NMR spectra were recorded at 400 MHz and 100 MHz respectively on Bruker AMX 400 NMR spectrometer using DMSO-d as solvent. The ESI +ve MS spectra were recorded on a Bruker Daltonics LC-MS spectrometer. Satisfactory microanalysis was obtained on Carlo Erba 1106 CHN analyzer.

Adopting the literature reports 1,3-diaryl-prop-2-en-1-ones 1-11 [28] and 2-amino-4,6-diarylpyrimidines 12-22 [29], were synthesized.

General method for the synthesis of 2-phenyl-3- (4,6-diphenylpyrimidin-2-yl)thiazolidin-4-one 23

To an ice cold stirred solution of 2-amino-4,6-diphenylpyrimidine in dry dichloromethane was added benzaldehyde in drops followed by dicyclohexylcarbodiimide (DCC). After five min. thioglycolic acid was added and stirring was continued for additional 5 h at 0°C. Then the reaction mixture was filtered to remove dicyclohexyl urea followed by washing with 5% citric acid, 10% sodium bicarbonate, brine solution, finally with water and dried over anhydrous sodium sulfate. After evaporation of the solvent under reduced pressure, a gummy mass was obtained, which was solidified on treatment of petroleum ether (bp40°–60°). Final purification of 2-phenyl-3-(4,6-diphenylpyrimidin-2-yl)thiazolidin-4-one 23 was done by column chromatography using silica gel (100-200 mesh), with ethyl acetate -Petroleum ether (bp40–60) in the ratio (2:8) as eluent. IR (KBr) (cm⁻¹): 3125, 3033, 2927, 2851, 1716, 1627, 1576, 1350, 710, 698, 649; ¹H NMR (δ ppm): 3.21 (d, 1H, CH_2a at H_5a, J = 15.37Hz), 3.38 (d, 1H, CH_2e at H_5e, J = 15.37Hz), 5.25 (s, 1H, CH at H₂), 7.19-8.37 (m, 16H, H_arom), A singlet for CH proton at position 5^′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.0 C-5, 62.5 C-2, 108.1 C-5′, 131.4 C-2^‴, 125.9-128.8 –C_arom, 139.1 C-4′, 139.1 C-6″, 161.3 C-4′, 161.3 C-6′, 163.8 C-2′, 170.6 C-4.

The compounds 24-33 were similarly synthesized.

3-(4-(4″-chlorophenyl)-6′-phenylpyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 24 IR (KBr) (cm⁻¹): 3120, 3033, 2927, 2851, 1696, 1627, 1575, 1310, 894, 710, 650, 647; ¹H NMR (δ ppm): 3.22 (d, 1H, CH_2a at H_5a, J = 15.37Hz), 3.39 (d, 1H, CH_2e at H_5e, J = 15.34Hz), 5.27 (s, 1H, CH at H₂), 7.31-8.44 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 33.9 C-5, 62.6 C-2, 108.8 C-5′, 127.5-133.1 –C_arom, 131.4 C-2^‴, 135.9 ipso C, 139.1 C-4^″, 139.7 C-6″, 164.9 C-4″, 165.0 C-6′, 162.9 C-2′, 170.6 C-4.

3-(4′-(3″-chlorophenyl)-6′-phenylpyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 25 IR (KBr) (cm⁻¹): 3115, 3033, 2927, 2850, 1714, 1627, 1575, 1344, 894, 767, 690, 648; ¹H NMR (δ ppm): 3.22 (d, 1H, CH_2a at H_5a, J = 15.36Hz), 3.39 (d, 1H, CH_2e at H_5e, J =15.37Hz), 5.27 (s, 1H, CH at H₂), 7.21-8.24 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 33.9 C-5, 62.4 C-2, 108.9 C-5′, 124.6-129.0 –C_arom, 130.6 ipso C, 131.5 C-2^‴, 139.0 C-4^″, 141.8 C-6″, 164.9 C-4′, 165.5 C-6′, 162.7 C-2′, 170.6 C-4.

3-(4′-(4″-methoxyphenyl)-6′-phenylpyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 26 IR (KBr) (cm⁻¹): 3065, 3038, 2927, 2851, 1714, 1627, 1577, 1351, 700, 650, 649; ¹H NMR (δ ppm): 3.23 (d, 1H, CH_2a at H_5a, J = 15.35Hz), 3.39 (d, 1H, CH_2e at H_5e, J = 15.27Hz), 3.84 (s, 3H, OCH₃), 5.28 (s, 1H, CH at H₂), 7.21-8.21 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.5 C-5, 54.9 OCH₃ on aryl ring, 62.5 C-2, 108.7 C-5′, 126.0-128.6 –C_arom, 129.1 C-2^‴, 139.1 C-4^″, 141.5 C-6″, 164.0 C-4′, 165.0 C-6′, 162.3 C-2′, 170.6 C-4.

3-(4′-(4″-methylphenyl)-6′-phenylpyrimidin-2′-yl)-2-phenylthiazolidin-4-one. (27) IR (KBr) (cm⁻¹): 3060, 3033, 2926, 2852, 1715, 1627, 1579, 1350, 714, 700, 643.; ¹H NMR (δ ppm): 2.32 (s, 3H, CH₃), 3.23 (d, 1H, CH_2a at H_5a, J = 15.34Hz), 3.40 (d, 1H, CH_2e at H_5e, J = 15.36Hz), 5.27 (s, 1H, CH at H₂), 7.20-8.24 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 24.5 CH₃ on aryl ring, 34.1 C-5, 62.6 C-2, 108.4 C-5′, 126.0-131.4 –C_arom, 133.1 C-2^‴, 135.9 ipso C, 138.7 C-4^″, 139.1 C-6″, 164.9 C-4′, 165.5 C-6′, 162.6 C-2′, 170.8 C-4.

3-(4′-(4″-fluorophenyl)-6′-phenylpyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 28 IR (KBr) (cm⁻¹): 3071, 3027, 2928, 2852, 1712, 1626, 1575, 1352, 836, 769, 698; ¹H NMR (δ ppm): 3.20 (d, 1H, CH_2a at H_5a, J = 15.24Hz), 3.37 (d, 1H, CH_2e at H_5e, J = 15.28Hz), 5.26 (s, 1H, CH at H₂), 6.64-8.19 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.5 C-5, 62.9 C-2, 108.9 C-5′, 127.3-143.1 –C_arom, 143.6 C-2^‴, 145.1 C-4^″, 146.1 C-6″, 166.8 C-4′, 167.0 C-6′, 163.9 C-2′, 171.4 C-4.

3-4′-phenyl-(6′-(4″-chlorophenyl)pyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 29 IR (KBr) (cm⁻¹): 3071, 3027, 2926, 2852, 1721, 1627, 1576, 1398, 782, 730, 693, 582; ¹H NMR (δ ppm): 3.21 (d, 1H, CH_2a at H_5a, J = 15.33Hz), 3.38 (d, 1H, CH_2e at H_5e, J = 15.32Hz), 5.25 (s, 1H, CH at H₂), 7.15-7.93 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.1 C-5, 62.9 C-2, 108.9 C-5′, 126.5-128.6 –C_arom, 129.2 C-2^‴, 139.1 C-4^″, 141.9 C-6″, 164.4 C-4′, 165.3 C-6′, 162.5 C-2′, 170.8 C-4.

3-4′-phenyl-(6′-(4″-methoxyphenyl)pyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 30 IR (KBr) (cm⁻¹): 3065, 3033, 2928, 2851, 1715, 1627, 1590, 1370, 835, 770, 699, 656; ¹H NMR (δ ppm): 3.20 (d, 1H, CH_2a at H_5a, J = 15.08Hz), 3.37 (d, 1H, CH_2e at H_5e, J = 15.34Hz), 3.86 (S, 3H, OCH₃), 5.26 (s, 1H, CH at H₂), 6.97-8.20 (m, 15H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.1 C-5, 55.2 OCH₃ on aryl ring, 62.5 C-2, 108.6 C-5′, 114.1-129.1 –C_arom, 129.5 C-2^‴, 130.2 ipso C, 139.2 C-6″, 146.1 C-4^″, 163.8 C-4′, 164.4 C-6′, 161.2 C-2′, 170.7 C-4.

3-(4′-(4″-chlorophenyl)-6′-(p-tolylpyrimidin-2′-yl))-2-phenylthiazolidin-4-one. 31 IR (KBr) (cm⁻¹): 3060, 3027, 2927, 2851, 1721, 1626, 1576, 1398, 781, 728, 694, 650; ¹H NMR (δ ppm): 2.40 (s, 3H, CH₃), 3.20 (d, 1H, CH_2a at H_5a, J = 15.04Hz), 3.37 (d, 1H, CH_2e at H_5e, J = 15.11Hz), 5.26 (s, 1H, CH at H₂), 7.18-8.33 (m, 14H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 25.2 CH₃ on aryl ring, 34.5 C-5, 62.9 C-2, 108.9 C-5′, 131.4 C-2^‴, 126.1-130.4 –C_arom, 133.1 ipso C, 138.7 C-6″, 139.1 C-4^″, 164.8 C-4′, 165.0 C-6′, 161.2 C-2′, 170.8 C-4.

3-(4′,6′-bis(p-chlorophenyl)pyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 32 IR (KBr) (cm⁻¹): 3060, 3027, 2927, 2852, 1727, 1627, 1575, 1400, 897, 787, 730, 693; ¹H NMR (δ ppm): 3.20 (d, 1H, CH_2a at H_5a, J = 15.21Hz), 3.36 (d, 1H, CH_2e at H_5e, J = 15.23Hz), 5.26 (s, 1H, CH at H₂), 7.28-8.20 (m, 14H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.5 C-5, 62.5 C-2, 108.7 C-5′, 129.1 C-2^‴, 126.0-128.6 –C_arom, 139.1 C-6″, 141.8 C-4^″, 164.1 C-4′, 161.3 C-6′, 165.3 C-2′, 170.9 C-4.

3-(4′-(p-chlorophenyl)-6′-(p-fluorophenyl)pyrimidin-2′-yl)-2-phenylthiazolidin-4-one. 31 IR (KBr) (cm⁻¹): 3065, 3027, 2926, 2853, 1719, 1627, 1576, 1394, 897, 833, 776, 728, 695; ¹H NMR (δ ppm): 3.18 (d, 1H, CH_2a at H_5a, J = 14.92Hz), 3.35 (d, 1H, CH_2e at H_5e, J = 14.92Hz), 5.26 (s, 1H, CH at H₂), 7.18-8.19 (m, 14H, H_arom), A singlet for CH proton at position 5′ of pyrimidine moiety is merged with aromatic protons. ¹³C NMR (δ ppm): 34.1 C-5, 62.6 C-2, 110.1 C-5′, 133.2 C-2^‴, 127.3-143.1 –C_arom, 145.1 C-6″, 146.8 C-4^″, 166.8 C-4′, 167.0 C-6′, 163.4 C-2′, 171.8 C-4.

Microbiology

Materials

All the clinically isolated bacterial strains namely Staphylococcus aureus, β-Haemolytic streptococcus, Vibreo cholerae, Salmonella typhii, Shigella felxneri, Escherichia coli, Klebsiella pneumonia, Pseudomonas aeruginosa and fungal strains namely Aspergillus flavus, Mucor, Rhizopus and Microsporum gypsuem were obtained from Faculty of Medicine, Annamalai University, Annamalainagar-608 002, Tamil Nadu, India.

In vitro antibacterial and antifungal activity

Minimum inhibitory concentration (MIC) in μg/mL values is carried out by two-fold serial dilution method [30]. The respective test compounds 23-33 were dissolved in dimethyl sulphoxide (DMSO) to obtain 1 mg mL⁻¹ stock solution. Seeded broth (broth containing microbial spores) was prepared in NB from 24 h old bacterial cultures on nutrient agar (Hi-media, Mumbai) at 37 ± 1 °C while fungal spores from 1 to 7 days old Sabourauds agar (Hi-media, Mumbai) slant cultures were suspended in SDB. The colony forming units (cfu) of the seeded broth were determined by plating technique and adjusted in the range of 10⁴-10⁵ cfu/mL. The final inoculums size was 10⁵cfu/mL for antibacterial assay and 1.1-1.5 × 10² cfu/mL for antifungal assay. Testing was performed at pH 7.4 ± 0.2 for bacteria (NB) and at a pH 5.6 for fungi (SDB). Exactly 0.4 mL of the solution of test compound was added to 1.6 mL of seeded broth to form the first dilution. One milliliter of this was diluted with a further 1 mL of seeded broth to give the second dilution and so on till six such dilutions were obtained. A set of assay tubes containing only seeded broth was kept as control. The tubes were incubated in BOD incubators at 37 ± 1°C for bacteria and 28 ± 1°C for fungi. The minimum inhibitory concentrations (MICs) were recorded by visual observations after 24 h (for bacteria) and 72-96 h (for fungi) of incubation. Ciprofloxacin was used as standard for bacteria studies and Fluconazole was used as standards for fungal studies.

Results and discussion

Chemistry

Initially, attempted conversion of 2-amino-4,6-diarylpyrimidines 12-22 to 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 was carried out in the absence of dicyclohexylcarbodiimide (DCC) but yields were achieved. Instead if DCC was used as a dehydrating agent, the yield of the product has been improved significantly (i.e., c. 65%) by stirring at about 0°C. The Claisen-Schmidt condensation of equimolar quantities of various p-substituted acetophenone with different m- and p- substituted benzaldehydes in the presence of sodium hydroxide base as a catalyst yields 1,3-diaryl-prop-2-en-1-ones 1-11. When 1,3-diaryl-prop-2- en-1-ones 1-11 are treated with guanidine nitrate in the presence of potassium hydroxide alkali in refluxing ethanol for 8 h gives 2-amino-4,6-diarylpyrimidines 12-22. Novel title compounds, 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 are synthesized by the addition of benzaldehyde, followed by thioglycolic acid to 2-amino-4,6- diphenylpyrimidine in dry dichloromethane at 0°C catalyzed by dicyclohexylcarbodiimide (DCC). The schematic representation and the analytical data for compounds 23-33 are given in Scheme-1 and Table-1, respectively. The importance of the title compounds is due to their diverse potential, broad-spectrum biological activity. The structure of the newly synthesized compounds 23-33 was confirmed by melting point, elemental analysis, MS, FT-IR, one- dimensional NMR (¹H & ¹³C) spectroscopic data.

Scheme 1.  Synthesis of nove 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones.

Table 1.  Physical and analytical data of 2-phenyl-3-(4,6-diarylpyrimidin-2-yl) thiazolidin-4-ones 23-33.

Entry	X	Y	Z	Yield (%)	m.p° C	Elemental analysis (%)			m/z (M+1)^+ Molecular formula
						C Found (calculated)	H Found (calculated)	N Found (calculated)
23	H	H	H	65	145	73.31	4.60	10.23	410
						(73.35)	(4.64)	(10.26)	C25H19N3OS
24	H	H	Cl	52	162	67.65	4.04	9.41	444
						(67.66)	(4.06)	(9.46)	C25H18Cl N3OS
25	H	Cl	H	49	130	67.62	4.01	9.44	444
						(67.66)	(4.06)	(9.46)	C25H18Cl N3OS
26	H	H	OCH3	60	110	71.03	4.72	9.51	440
						(71.08)	(4.78)	(9.56)	C26H21N3O2S
27	H	H	CH3	65	162	73.72	4.92	9.89	424
						(73.76)	(4.96)	(9.92)	C26H21N3OS
28	H	H	F	70	104	70.23	4.18	9.79	428
						(70.27)	(4.21)	(9.83)	C25H18FN3OS
29	Cl	H	H	65	114	67.64	4.01	9.41	444
						(67.66)	(4.05)	(9.46)	C25H18Cl N3OS
30	OCH3	H	H	58	137	71.02	4.72	9.55	440
						(71.08)	(4.78)	(9.56)	C26H21N3O2S
31	Cl	H	CH3	55	125	68.17	4.32	9.15	458
						(68.21)	(4.36)	(9.17)	C26H20ClN3OS
32	Cl	H	Cl	60	130	62.71	3.52	8.76	479
						(62.78)	(3.55)	(8.78)	C25H17Cl2N3OS
33	Cl	H	F	65	103	64.98	3.65	9.04	462
						(65.02)	(3.68)	(9.09)	C25H17ClFN3OS

Antibacterial activity

Novel 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 were tested for their antibacterial activity in vitro against S.aureus, β-H.streptococcus, V.cholerae, S.typhii, S.felxneri, E.coli, K.pneumonia and P. aeruginosa. Ciprofloxacin was used as standard drug. Minimum inhibitory concentration (MIC) in μg/mL values is reproduced in Table 2. Compound 23 which has no substitution at the para position of phenyl rings only exerted moderate activities against all the used bacterial strains. Compounds 25 and 28 which contain electron withdrawing chloro and fluoro functional group respectively at the para position of phenyl ring attached to pyrimidine ring did not promote much activity against β-H.streptococcus, S.felxneri and E.coli. Structure-activity relationship results for the synthesized compounds have shown that 2-amino-4,6-diarylpyrimidino thiazolidinones with electron donating methoxy and methyl functional group at the para position of the phenyl ring attached to the pyrimidine ring 26, 27 and 30 exerted strong antibacterial activity against all the tested bacterial strains. Also compound 31, which contain both electron withdrawing chloro and electron donating methyl groups shows potent activity against all the tested bacterial strains whereas compounds 32 and 33 which contain electron donating methoxy functional group at the para position of phenyl ring attached to pyrimidine ring shows promising activity against S.aureus, S.typhii and E.coli.

Table 2.  In vitro antibacterial activity (MIC) values for compounds 23-33.

Compounds			Minimum	Inhibitory	Concentration	(MIC)	in μg/mL
	S.aureus	β-H streptococcus	V.cholerae	S.typhii	S.felxneri	E.coli	K.pneumonia	P.aeruginosa
23	200	100	100	50	100	50	100	200
24	100	200	50	50	100	100	200	100
25	50	-^a	50	100	-^a	-^a	100	50
26	50	6.25	25	6.25	25	6.25	12.5	25
27	25	12.5	12.5	12.5	6.25	12.5	6.25	12.5
28	50	-^a	25	50	-^a	-^a	50	100
29	200	100	100	100	200	200	50	50
30	25	25	50	50	100	12.5	25	12.5
31	12.5	25	12.5	6.25	6.25	25	25	6.25
32	6.25	50	50	12.5	100	12.5	200	50
33	12.5	100	50	12.5	100	12.5	100	100
Ciprofloxacin	25	50	25	50	50	25	50	12.5

^a No inhibition even at higher concentration i.e., at 200 μg/mL

Antifungal activity

The in vitro antifungal activity of 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 was studied against the fungal strains viz., A.flavus, Mucor, Rhizopus and M.gypsuem. Fluconazole was used as a standard drug. Minimum inhibitory concentration (MIC) in μg/mL values is reproduced in Table 3. Compound 23 did not exhibited antifungal activity against A.flavus and M.gypsuem. Further introduction of electron withdrawing chloro functional group at the para position of phenyl ring attached to pyrimidine ring in compounds 24 and 29 also did not promote much activity against A.flavus and M.gypsuem while against Mucor and Rhizopus, it has registered maximum activity at 25 μg/mL. Instead if the chloro group is attached at the meta position of phenyl ring attached to pyrimidine ring in 25 promotes pronounced activity against Mucor at 12.5 μg/mL. Compounds 32 and 33 shows excellent activitites against all the tested clinically isolated fungal strains, while against Mucor, compound 27 which contain electron donating methyl group at the para position of phenyl ring attached to pyrimidine ring showed promisable activities. The antifungal activities of these two compounds are due to the presence of two electron withdrawing chloro and fluoro functional groups attached to para position of the two phenyl rings. Compounds 26 and 30 which contain electron donating methoxy functional group at the para position of phenyl ring attached to pyrimidine ring did not promote much activity against Mucor and Rhizopus. Also compound 31, which contain both electron withdrawing chloro and electron donating methyl groups shows potent activity against A. flavus and Rhizopus.

Table 3.  In vitro antifungal activity (MIC) values for compounds 23-33

Compounds	Minimum	Inhibitory	Concentration	(MIC) in μg/mL
	A.flavus	Mucor	Rhizopus	M. gypsuem
23	-^a	100	200	-^a
24	200	25	25	200
25	100	12.5	50	200
26	50	200	-^a	100
27	100	12.5	200	200
28	50	50	100	50
29	200	25	25	200
30	100	-^a	-^a	50
31	12.5	50	6.25	100
32	6.25	12.5	25	12.5
33	12.5	6.25	12.5	25
Fluconazole	25	25	50	50

^a No inhibition even at higher concentration i.e., at 200 μg/mL

Conclusion

The microbiological screening studies carried out to evaluate the antibacterial and antifungal potencies of the newly synthesized 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 are clearly known from Table 2 and Table 3. A close inspection of the in vitro antibacterial and antifungal activity profile in differently electron donating (CH₃ and OCH₃) and electron withdrawing (-Cl and -F) functional group substituted phenyl rings of novel 2-aminopyrimidino thiazolidinones 23-33 exerted strong anti-bacterial activity against all the tested bacterial strains. Compound 31, which contain both electron withdrawing chloro and electron donating methyl groups shows potent activity against all the tested bacterial strains whereas compounds 32 and 33 which contain electron donating methoxy functional group at the para position of phenyl ring attached to pyrimidine ring shows promising activity against S.aureus, S.typhii and E.coli. Results of the anti-fungal activity study show that the nature of substituents on the phenyl ring viz., methyl, fluoro and chloro functions at the para positions of the aryl moieties are determinant for the nature and extent of the anti-fungal activity of all the synthesized compounds 23-33 over fungal strains namely A.flavus, Mucor, Rhizopus and M.gypsuem. Compounds 26 and 30 which contain electron donating methoxy functional group at the para position of phenyl ring attached to pyrimidine ring did not promote much activity against Mucor and Rhizopus. Also compound 31, which contain both electron withdrawing chloro and electron donating methyl groups shows potent activity against A. flavus and Rhizopus. The mode of action of these compounds is unknown. Further development of this group of 2-amino-4,6-diarylpyrimidino thiazolidinones may lead to compounds with better pharmacological profile than standard antibacterial and antifungal drugs.

Acknowledgement

Authors are thankful to NMR Research Centre, Indian Institute of Science, Bangalore for recording spectra. One of the authors (V. Kanagarajan) is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi, Republic of India for providing financial support in the form of CSIR-Senior Research Fellowship (SRF) in Organic Chemistry. J.Thanusu wishes to thank Annamalai University authorities for providing financial support in the form of Research Fellowship.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.