INTRODUCTION

Antibacterial resistance is growing tremendously throughout the world due to the misuse and overuse of the antibiotics. Thus, it is a serious threat for the successful treatment of the infectious diseases and becomes a foremost clinical and public health problem. According to the World Health Organization (WHO), approximately 700 000 deaths occur annually due to drug-resistant pathogens, and this number is likely to reach 10 million by 2050 if the existing trend continues [1–3]. Hence, it is essential to develop new classes of highly effective antimicrobial agents with no side effects to fight against pathogenic microorganisms that developed resistance to the antibiotics used in the current regimen [4–6].

Thiazolopyrimidine derivatives have received a significant interest from the medicinal and synthetic chemists because of their extensive pharmacological applications, such as antimicrobial [7–9], antibiofilm [10, 11], anticancer [12, 13], antiviral [14], anti-inflammatory [15–18], antitubercular [8], anti-tumoural [19], antimalarial [20], anti-HIV [20, 21], antioxidant [7, 22], and calcium channel blocking [23] activities. They are also reported as inhibitors of acetylcholinesterase (AchE) [24, 25], CDC25B phosphatase [26], and xanthine oxidase [27] enzymes, and Bcl-2 family proteins [28]. These are also considered as potential purine antagonists [29, 30]. The structures of some of the thiazolopyrimidine derivatives and their biological properties are specified in Fig. 1 [31].

Fig. 1.
figure 1

Some of the biologically active thiazolo[3,2-a]pyrimidine derivatives.

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In view of the remarkable pharmacological properties of thiazolo[3,2-a]pyrimidines, numerous methods have been accounted for their synthesis. One of the simplest method is the cyclization of dihydropyrimidine-thiones (Biginelli product) with halogen- (bromo- or chloro-) containing electrophiles like 2‑bromo-ketones [32], chloroacetyl chloride [26], chloroacetic acid [12–14], 1,2-dichloroethane [33], and methyl chloroacetate [34] to yield various C2–N3 linked various thiazolo[3,2-a]pyrimidines. Motivated by the aforementioned findings, and in continuation of our investigation towards the development of highly potent antimicrobial agents [35–37], herein we report the synthesis, characterization, and in vitro antimicrobial investigation of fused thiazolopyrimidine derivatives.

RESULTS AND DISCUSSION

In our previous report [7], we synthesized several fused thiazolo[3,2-a]pyrimidines starting from 1‑tetralone and investigated their in vitro antibacterial potency against different bacteria. We observed that the analogs bearing aryl substitutions on the thiazole ring had nearly 1.3 fold potency against a gram-negative bacteria Pseudomonas aeruginosa when compared with an antibiotic penicillin. Keeping this in mind and to examine the effect of fused ring on antimicrobial activity, we designed to synthesize a small library of thiazolo[3,2-a]pyrimidines starting from 1,3-indandione. Biginelli product, 4-phenyl-2-thioxo-3,4-dihydro-1H-indeno[1,2-d]pyrimidin-5(2H)-one (III) was prepared via a three-component condensation reaction of 1,3-indandione, benzaldehyde, and thiourea utilizing a catalyst, poly(4-vinylpyridinium)hydrogen sulfate [P(4-VPH)HSO4]. The reaction mixture was subjected to heat at 120°C under neat conditions. The intermediate (III) was obtained in 94% yield in short reaction time (15 min). The expected products (IVa–l), were obtained in 80–90% yields via the cyclization of an intermediate (III) with a variety of bromine derived electrophiles, such as phenacyl bromides and 3-(bromoacetyl)coumarins (IIa–e) under conventional refluxing in acetic acid (Scheme 2). 3-(Bromoacetyl)coumarin derivatives (IIa–e) were prepared following the earlier literature procedure (Scheme 1) [38].

scheme 1

Scheme 1. Synthesis of 3-(2-bromoacetyl)-2H-chromen-2-ones and 2-(2-bromoacetyl)- 3H-benzo[f]chromen-3-one (IIa–e).

scheme 2

Scheme 2. Synthesis of 5-phenylindeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one derivatives.

Formation of the target compounds (IVa–l) from the intermediate dihydropyrimidine-thione (III) was confirmed by FT-IR, 1H, 13C NMR, mass spectrometry and elemental analyses data. The disappearance of –NH bands at 3406 and 3280 cm–1 of the dihydropyrimidin-thione (III) and the existence of imine ‒C=N– band at 1604–1647 cm–1 in the FT-IR spectra confirm the product formation. In case of compounds (IVh–l), a strong band at 1732–1738 cm–1 signifies the existence of pyranone (–O–C=O) group. The disappearance of singlets at 9.78 and 11.68 ppm from the 1H NMR spectra and a signal at 174.32 ppm from 13C NMR spectra of the starting material, as well as the presence of a peak –of –C=N (imine carbon) at 157.47–158.64 ppm indicate the construction of the product. Elemental analyses and mass spectral data further strengthened the formation of products.

Antibacterial activity of all the synthesized molecules (IVa–l) was measured against gram-positive Streptococcus pyogene and Staphylococcus aureus and gram-negative Klebsiella pneumonia and Pseudomonas aeruginosa bacterial strains. Streptomycin was used as a standard antibacterial drug. Almost all the compounds (except for compound (IVa)) showed moderate activity against a gram-negative bacterium P. aeruginosa in terms of ZOI (14–18 mm) and MIC (50–100 µg/mL) values comparing with a standard antibiotic drug streptomycin. Compound (IVa) showed weak activity against P. aeruginosa. But the compound derived from 4-methoxyphenacyl bromide (IVf) showed equipotent activity against S. aureus (gram-positive) in terms of ZOI (22 mm) MIC (25 µg/mL) values (Table 1, Fig. 2). Replacing the 4-methoxyphenyl group of thiazole ring with 4-chlorophenyl (IVb), 4-bromophenyl (IVc), 4-nitrophenyl (IVd), 4-methylphenyl (IVe), and biphenyl (IVg) groups did not improve the antibacterial activity against S. aureus. Introducing coumarinyl groups (IVh–l) into thiazole ring also did not enhance the activity. Weak activity was observed for all the synthesized molecules against S. pyogene and K. pneumonia bacterial strains comparing with the reference drug.

Table 1.   Antibacterial activity results of substituted indeno-thiazolo-pyrimidinone derivatives (IVa–l)
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Fig. 2.
figure 2

Preliminary antibacterial (a) and antifungal (b) activities of 3-substituted-5-phenylindeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-ones (IVa–l).

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Antifungal activity was assayed against Candida glabrata, Candida albicans, Aspergillus parasiticus, and Aspergillus niger fungal strains. Clotrimazole was used as a reference drug. Activity results reveal that the maximum ZOI (20 mm) has observed in the case of compound (IVf) against A. niger. Compounds (IVa), (IVc), (IVf), and (IVk) showed good activity (ZOI 11–20 mm) against all the tested fungal strains (Table 2, Fig. 2). Moderate-to-weak activity was observed in the case of the rest of the molecules.

Table 2.   Antifungal activity results of substituted indeno-thiazolo-pyrimidinone derivatives (IVa–l)
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CONCLUSION

3-Aryl/Heteryl substituted 5-phenylindeno-thiazolopyrimidinone derivatives (IVa–l) were prepared with promising yields under conventional method. All the synthesized molecules (IVa–l) were investigated for their antimicrobial potency against different microbes. The activity data revealed that the compound containing the 4-methoxyphenyl group on the thiazole ring (IVf) has broad-spectrum antibacterial and antifungal activities against all the microbes used for the investigation. Compounds (IVa), (IVc), and (IVk) also showed good antifungal activity. Taking compound (IVf) as a lead, further modifications by substituents on the pyrimidine ring are under investigation in our laboratory.

EXPERIMENTAL

All of the solvents and reagents used were obtained commercially from Aldrich, SRL, and Qualigens chemicals, and are used as is unless otherwise mentioned. Thin layer chromatography (silica gel 60F254 pre-coated aluminum sheets obtained from Aldrich) was used to examine the development of the reaction. Column chromatography (silica-gel, 230–400 mesh procured from Aldrich) with 80% hexanes and ethyl acetate as eluent was used for the purification of the final molecules. Buchi (R-100) rotary evaporator was used to remove the solvents. Stuart SMP30 melting point apparatus was used to find out the melting point of the compounds. Perkin-Elmer 100S IR spectrophotometer was used to record the IR spectra (νmax, cm–1) in KBr. Bruker 400MHz spectrometer was used to record the NMR spectra in DMSO-d6 at 400 (1H) and 100 (13C) MHz; chemical shifts values are given in ppm (parts per million). Jeol JMSD-300 spectrometer and Carlo Erba EA1108 analytical unit were used to record mass spectra and elemental analysis data.

4-Phenyl-2-thioxo-3,4-dihydro-1H-indeno[1,2-d]pyrimidin-5(2H)-one (III). To a mixture of 1,3-indandione (10 mmol, 1.461 g), benzaldehyde (10 mmol, 1.02 mL), and thiourea (12 mmol, 0.913 g), 15 mg of P(4-VPH)HSO4 was added. The mixture was heated at 120°C for 15 min under neat conditions. Dichloromethane (50 mL) was added to the reaction mixture and stirred at ambient temperature for additional 5 min, and the catalyst was filtered off. The crude product was purified by crystallization using ethanol to obtain pure intermediate (III) as a red solid (2.748 g, 94%). mp 224–226°C; 1H NMR: 5.36 (s, 1H, H4pyrimidine), 7.25–7.39 (m, 8H, Ar-H), 7.86–7.91 (m, 1H, Ar-H), 9.77 (bs, 1H, NH), 11.68 (bs, 1H, NH); MS (ESI, m/z): 293 [M + H]+; Anal. calcd. for C17H12N2OS: C, 69.84; H, 4.14; N, 9.58. Found: C, 69.67; H, 4.31; N, 9.41.

3,5-Diphenylindeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one (IVa). To the stirred intermediate compound (III) (1 mmol, 292.35 mg) in glacial AcOH (4 mL) phenacyl bromide (1 mmol, 199.05 mg) was added. The temperature of the reaction mixture was raised to 120°C and the mixture was stirred at that same temperature for 3 h, then kept a side for 12 h. Precipitate separated out was filtered and washed with small amount of cold acetic acid. The crude material obtained was subjected to column chromatography using 20% ethyl acetate in hexanes to obtain the pure product (IVa) as an orange solid (83%, 325.73 mg). mp 261–263°C; IR: 1707 (C=O), 1600 (C=N), 1562 (C=C). 1H NMR: 6.35 (s, 1H, H4pyrimidine), 6.63–6.65 (m, 2H, Ar–H and H2thiazole), 7.10 (t, J = 7.2 Hz, 4H, Ar–H), 7.22–7.24 (m, 3H, Ar–H), 7.32 (d, J = 7.2 Hz, 1H, Ar–H), 7.36–7.46 (m, 4H, Ar–H), 7.55 (d, 3H, Ar–H, J = 7.6 Hz); MS (ESI, m/z): 393 [M + H]+; Anal. calcd. for C25H16N2OS: C, 76.51; H, 4.11; N, 7.14. Found: C, 76.40; H, 4.25; N, 7.33.

Compounds (IVb–l) were synthesized similarly as compound (IVa).

3-(4-Chlorophenyl)-5-phenylindeno[1,2-d]thiazolo-[3,2-a]pyrimidin-6(5H)-one (IVb). 4-Chlorophenacyl bromide (1 mmol, 233.49 mg) was used as reagent. Orange solid (90%, 384.22 mg); mp 254–256°C; IR: 1717 (C=O), 1605 (C=N), 1575 (C=C), 749 (C–Cl). 1H NMR: 6.34 (s, 1H, H4pyrimidine), 6.68–6.71 (m, 2H, Ar–H and H2thiazole), 7.11–7.15 (m, 7H, Ar–H), 7.26 (t, 3H, J = 8.4 Hz, Ar–H), 7.43 (t, 2H, J = 8.4 Hz, Ar–H); MS (ESI, m/z): 427 [M + H]+; Anal. calcd. for C25H15ClN2OS: C, 70.33; H, 3.54; N, 6.56; Found: C, 70.15; H, 3.67; N, 6.39.

3-(4-Bromophenyl)-5-phenylindeno[1,2-d]thiazolo-[3,2-a]pyrimidin-6(5H)-one (IVc). 4-Bromophenacyl bromide (1 mmol, 277.94 mg) was used as reagent. Orange solid (88%, 414.8 mg); mp 264–266°C; IR: 1714 (C=O), 1604 (C=N), 1576 (C=C), 616 (C–Br). 1H NMR: 6.34 (s, 1H, H4pyrimidine), 6.69–6.71 (m, 2H, Ar–H and H2thiazole), 7.11–7.23 (m, 7H, Ar–H), 7.31 (d, 1H, J = 7.2 Hz, Ar–H), 7.40 (t, 2H, J = 7.6 Hz, Ar–H), 7.58 (d, 2H, J = 8.8 Hz, Ar–H); MS (ESI, m/z): 472 [M + H]+; Anal. calcd. for C25H15BrN2OS: C, 63.70; H, 3.21; N, 5.94. Found: C, 63.57; H, 3.39; N, 5.80.

3-(4-Nitrophenyl)-5-phenylindeno[1,2-d]thiazolo-[3,2-a]pyrimidin-6(5H)-one (IVd). 4-Nitrophenacyl bromide (1 mmol, 244.04 mg) was used as reagent. Yellow solid (81%, 354.35 mg); mp 271–273°C; IR: 1715 (C=O), 1604 (C=N), 1574 (C=C), 1548, 1384 (NO2). 1H NMR: 6.39 (s, 1H, H4pyrimidine), 6.69–6.71 (m, 2H, Ar–H and H2thiazole), 7.07–7.11 (m, 3H, Ar–H), 7.22–7.34 (m, 3H, Ar–H), 7.40 (t, 2H, J = 6.8 Hz, Ar–H), 7.56 (d, 2H, J = 8.8 Hz, Ar–H), 8.21 (d, 2H, J = 8.8 Hz, Ar–H); MS (ESI, m/z): 438 [M + H]+; Anal. calcd. for C25H15N3O3S: C, 68.64; H, 3.46; N, 9.61. Found: C, 68.47; H, 3.31; N, 9.47.

5-Phenyl-3-(p-tolyl)indeno[1,2-d]thiazolo[3,2-a]-pyrimidin-6(5H)-one (IVe). 4-Methylphenacyl bromide (1 mmol, 213.07 mg) was used as reagent. Orange solid (82%, 333.33 mg); mp 267–269°C; IR: 1714 (C=O), 1647 (C=N), 1578 (C=C). 1H NMR: 2.34 (s, 3H, CH3), 6.36 (s, 1H, H4pyrimidine), 6.67 (s, 1H, H2thiazole), 7.13 (t, 6H, J = 8.0 Hz, Ar–H), 7.19 (d, 3H, J = 7.6 Hz, Ar–H), 7.33 (t, 2H, J = 7.2 Hz, Ar–H), 7.42 (t, 2H, J = 7.2 Hz, Ar–H); 13C NMR: 188.85, 157.47, 141.47, 139.62, 132.19, 130.51, 129.25, 128.92, 128.39, 128.19, 125.88, 125.37, 120.89, 119.46, 105.42, 58.89, 20.88; MS (ESI, m/z): 407 [M + H]+; Anal. calcd. for C26H18N2OS: C, 76.82; H, 4.46; N, 6.89; Found: C, 76.68; H, 4.29; N, 6.73.

3-(4-Methoxyphenyl)-5-phenylindeno[1,2-d]thiazolo-[3,2-a]pyrimidin-6(5H)-one (IVf). 4-Methoxyphenacyl bromide (1 mmol, 229.07 mg) was used as reagent. Orange solid (85%, 359.12 mg); mp 248–250°C; IR: 1715 (C=O), 1647 (C=N), 1578 (C=C), 1190 (C–O–C). 1H NMR: 3.79 (s, 3H, OCH3), 6.36 (s, 1H, H4pyrimidine), 6.69 (s, 1H, H2thiazole), 6.93 (d, 3H, J = 8.4 Hz, Ar–H), 7.12–7.17 (m, 6H, Ar–H), 7.25–7.46 (m, 4H, Ar–H); MS (ESI, m/z): 423 [M + H]+; Anal. calcd. for C26H18N2O2S: C, 73.91; H, 4.29; N, 6.63. Found: C, 73.79; H, 4.13; N, 6.51.

3-([1,1'-Biphenyl]-4-yl)-5-phenylindeno[1,2-d]-thiazolo[3,2-a]pyrimidin-6(5H)-one (IVg). 4-Phenylphenacyl bromide (1 mmol, 275.14 mg) was used as reagent. Yellow solid (80%, 374.85 mg); mp 276–278°C; IR: 1711 (C=O), 1645 (C=N), 1571 (C=C). 1H NMR: 6.45 (s, 1H, H4pyrimidine), 6.72–6.74 (m, 2H, Ar–H and H2thiazole), 7.11 (t, 3H, J = 7.2 Hz, Ar–H), 7.31 (t, 5H, J = 8.4 Hz, Ar–H), 7.42–7.51 (m, 5H, Ar–H), 7.67–7.73 (m, 4H, Ar–H); 13C NMR: 188.98, 157.84, 141.35, 141.12, 140.47, 139.01, 138.34, 133.98, 132.11, 130.41, 129.93, 129.05, 128.40, 128.18, 128.04, 127.49, 126.79, 126.54, 125.96, 120.71, 119.35, 108.94, 105.32, 58.99; MS (ESI, m/z): 469 [M + H]+; Anal. calcd. for C31H20N2OS: C, 79.46; H, 4.30; N, 5.98. Found: C, 79.31; H, 4.47; N, 5.81.

3-(2-Oxo-2H-chromen-3-yl)-5-phenylindeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one (IVh). 3-(2-Bromoacetyl)-2H-chromen-2-one (1 mmol, 267.07 mg) was used as reagent. Yellow solid (84%, 386.82 mg); mp 238–240°C; IR: 1737 (C=Olactone) 1702 (C=Oindanone), 1646 (C=N), 1581 (C=C); 1H NMR: 5.76 (s, 1H, H4pyrimidine), 6.82 (s, 1H), 6.91 (t, 3H, J = 8.0 Hz), 7.13 (d, 2H, J = 8.0 Hz), 7.25 (d, 1H, J = 6.8 Hz), 7.29 (s, 1H, H2thiazole), 7.36 (t, 2H, J = 6.4 Hz, Ar–H), 7.51 (d, 2H, J = 8.4 Hz, Ar–H), 7.61 (d, 1H, J = 7.6 Hz, Ar–H), 7.82 (d, 1H, J = 7.6 Hz, Ar–H), 8.34 (s, 1H, H4coumarin); 13C NMR: 189.19, 170.86, 157.64, 153.33, 142.26, 138.95, 137.06, 132.83, 132.78, 130.71, 129.53, 128.42, 128.33, 127.86, 127.75, 127.60, 127.11, 125.29, 124.53, 121.43, 120.00, 118.16, 115.49, 109.08, 55.77; MS (ESI, m/z): 461 [M + H]+; Anal. calcd. for C28H16N2O3S: C, 73.03; H, 3.50; N, 6.08. Found: C, 73.23; H, 3.31; N, 6.27.

3-(8-Methoxy-2-oxo-2H-chromen-3-yl)-5-phenyl-indeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one (IVi). 3-(2-Bromoacetyl)-8-methoxy-2H-chromen-2-one (1 mmol, 297.1 mg) was used as reagent. Orange solid (81%, 397.33 mg); mp 242–244°C; IR: 1736 (C=Olactone), 1703 (C=Oindanone), 1644 (C=N), 1584 (C=C), 1243 (C–O–C); 1H NMR: 3.94 (s, 3H, OCH3), 6.34 (s, 1H, H4pyrimidine), 6.79 (s, 1H, H2thiazole), 6.86 (d, 1H, J = 7.2 Hz, Ar–H), 6.97 (t, 2H, J = 7.2 Hz, Ar–H), 7.14 (t, 3H, J = 7.6 Hz, Ar–H), 7.29–7.40 (m, 5H, Ar–H), 7.88 (s, 1H, Ar–H), 8.29 (s, 1H, H4coumarin); 13C NMR: 189.25, 170.83, 157.27, 146.31, 145.99, 142.89, 142.24, 132.99, 132.56, 130.49, 128.23, 127.48, 127.00, 125.59, 124.57, 124.38, 121.13, 120.57, 119.69, 118.78, 115.11, 108.84, 56.23, 55.62; MS (ESI, m/z): 491 [M + H]+; Anal. calcd. for C29H18N2O4S: C, 71.01; H, 3.70; N, 5.71. Found: C, 71.23; H, 3.55; N, 5.57.

3-(6-Bromo-8-methoxy-2-oxo-2H-chromen-3-yl)-5-phenylindeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one (IVj). 6-Bromo-3-(2-bromoacetyl)-8-methoxy-2H-chromen-2-one (1 mmol, 275.99 mg) was used as reagent. Green solid (86%, 489.7 mg); mp 279–281°C; IR: 1732 (C=Olactone), 1703 (C=Oindanone), 1644 (C=N), 1558 (C=C), 1243 (C–O–C), 723 (C–Br); 1H NMR: 3.97 (s, 3H, OCH3), 6.35 (s, 1H, H4pyrimidine), 6.98–7.00 (m, 2H, Ar–H and H2thiazole), 7.16 (t, 3H, J = 7.2 Hz, Ar–H), 7.23 (d, 1H, J = 7.2 Hz, Ar–H), 7.32 (t, 2H, J = 8.0 Hz, Ar–H), 7.40–7.55 (m, 3H, Ar–H), 7.56 (s, 1H, Ar–H), 7.84 (s, 1H, H4coumarin); MS (ESI, m/z): 570 [M + H]+; Anal. calcd. for C29H17BrN2O4S: C, 61.17; H, 3.01; N, 4.92. Found: C, 61.05; H, 3.28; N, 4.79.

3-(6,8-Dinitro-2-oxo-2H-chromen-3-yl)-5-phenyl-indeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one (IVk). 3-(2-Bromoacetyl)-6,8-dinitro-2H-chromen-2-one (1 mmol, 357.07 mg) was used as reagent. Yellow solid (82%, 451.41 mg); mp 287–289°C. IR: 1738 (C=Olactone), 1697 (C=Oindanone), 1619 (C=N), 1588 (C=C), 1564, 1348 (NO2); 1H NMR: 6.40 (s, 1H, H4pyrimidine), 7.00–7.02 (m, 2H, Ar–H and H2thiazole), 7.15 (d, 2H, J = 6.8 Hz, Ar–H), 7.23 (d, 1H, J = 6.8 Hz, Ar–H), 7.31 (d, 2H, J = 7.2 Hz, Ar–H), 7.41 (d, 2H, J = 7.6 Hz, Ar–H), 7.69 (d, 1H, J = 9.2 Hz, Ar–H), 8.14 (s, 1H, H4coumarin), 8.51 (s, 1H, Ar–H), 8.65 (s, 1H, Ar–H); MS (ESI, m/z): 551 [M + H]+; Anal. calcd. for C28H14N4O7S: C, 61.09; H, 2.56; N, 10.18. Found: C, 61.21; H, 2.37; N, 10.01.

3-(3-Oxo-3H-benzo[f]chromen-2-yl)-5-phenylindeno[1,2-d]thiazolo[3,2-a]pyrimidin-6(5H)-one (IVl). 2-(2-Bromoacetyl)-3H-benzo[f]chromen-3-one (1 mmol, 317.13 mg) was used as reagent. Red solid (83%, 423.77 mg); mp 282–284°C; IR: 1737 (C=Olactone), 1657 (C=Oindanone) 1625 (C=N), 1561 (C=C); 1H NMR: 6.44 (s, 1H, H4pyrimidine), 6.97 (s, 1H, H2thiazole), 7.06 (t, 2H, J = 7.2 Hz, Ar–H), 7.23 (d, 1H, J = 7.2 Hz, Ar–H), 7.33–7.44 (m, 4H, Ar–H), 7.62–7.74 (m, 5H, Ar–H), 7.84 (t, 1H, J = 8.0 Hz, Ar–H), 8.11 (t, 1H, J = 7.2 Hz, Ar–H), 8.24–8.38 (m, 1H, Ar–H), 8.40 (s, 1H, H4coumarin); 13C NMR: 189.12, 169.49, 158.65, 153.77, 142.36, 141.09, 139.07, 135.08, 134.99, 134.33, 131.95, 130.29, 129.96, 128.98, 128.82, 128.74, 128.64, 128.26, 126.49, 126.20, 122.40, 120.46, 119.20, 116.51, 115.75, 112.53, 112.20, 104.54, 59.37; MS (ESI, m/z): 511 [M + H]+; Anal. calcd. for C32H18N2O3S: C, 75.28; H, 3.55; N, 5.49. Found: C, 75.43; H, 3.39; N, 5.31.

Antimicrobial Activity

Antimicrobial screening was carried out using the agar well diffusion method [39, 40]. Minimum inhibitory concentration (MIC, µg/mL) values for antibacterial activity and zone of inhibition (ZOI, mm) values for antifungal activity were measured, and the data were compared with the standard drugs streptomycin and clotrimazole respectively. ZOI values were measured at 150 µg/mL for compounds (IVa–l) and 30 µg/mL for standard drug. Gram-positive strains included Streptococcus pyogenes and Staphylococcus aureus; gram-negative, Klebsiella pneumonia and Pseudomonas aeruginosa. Fungal strains included Candida glabrata, Candida albicans, Aspergillus parasiticus, and Aspergillus niger.