Synthesis and Evaluation of Some Novel Chromone Based Dithiazoles as Antimicrobial Agents

Novel substituted 1,2,4-dithiazolylchromones 3a–j were synthesized by the reaction of 3-formylchromones (1a–j) with two equivalents of p-chlorothiobenzamide (2) in dry xylene and characterized spectroscopically (IR, 1H and 13C NMR, mass) and elemental analysis. All synthesized compounds were screened for in vitro antimicrobial activity against various pathogenic bacterial and fungal strains and were found to possess good to moderate inhibitory potential against all tested strains. Antimicrobial results reveal that compounds bearing lipophilic electron withdrawing groups such as chloro and bromo displayed significant inhibitory potential against both bacterial and fungal strains. Particularly, compound 3c displayed significant inhibitory against bacterial strains and compound 3h exhibits significant inhibitory potential in comparison to standard drug fluconazole against fungal strain S. cerevisiae.


Introduction
Fungal and bacterial infections are affecting millions of people worldwide, and are associated with high rates of mortality and morbidity [1]. Resistance to antimicrobial agents is becoming a major problem; microbes acquire the ability to resist antimicrobial drugs by undergoing genetic changes either by mutation or gene transfer within or between species that allow microbes to defend themselves against the antimicrobial agents [2]. Therefore, the discovery of new antimicrobials has assumed critical importance to combat the fungal and bacterial infections. Heterocycles containing N and S are found to display variety of biological activities; amongst them, dithiazoles both 1,2,3-and 1,2,4-are endowed with interesting biological activities, in particular, antimicrobial activity [3]. Similarly, chromone moiety constitutes the basic nucleus of flavones, which are most important and widespread natural product of plants and display a large number of biological activities such as antifungal [4], antibacterial [5], anticancer [6], antiviral [7], antioxidant [8] antimalarial [9] neuroprotective [10], and HIV inhibitory [11].
Recently, we have reported the synthesis of 3-(5-phenyl-3H- [1,2,4]-dithiazol-3-yl) chromen-4-ones with significant inhibitory potential against microbial strains; particularly, compound having both electron withdrawing groups such as chloro and fluoro linked with chromone ring showed more inhibitory potential against fungal strains than standard drug [12]. Therefore, it was decided in the present study to incorporate chloro group at the phenyl ring of the 1,2,4dithiazolylchromones in order to observe the effect of novel substitution on antimicrobial activity.

Result and Discussion
Substituted 3-formylchromones (1a-j) were reacted with two equivalents of p-chlorothiobenzamide (2) in dry xylene for 3-4 hours leading to the formation of substituted 3-[5 -(p-chloro)-phenyl-3H-[1,2,4]-dithiazol-3 -yl]-4H-chromen-4-one (3a-j). The crude mixtures were purified by column chromatography over silica gel (60-120 mesh) and eluted with 1% ethyl acetate in hexane. The purified products were characterized by spectroscopic techniques such as IR, 1 H 2 International Journal of Medicinal Chemistry and 13 C NMR, and mass and elemental analysis (Scheme 1, Table 1). 1 H NMR spectrum of compound 3c showed a doublet at 8.21 ( = 0.9 Hz) assigned to C3 -H and another doublet at 7.75 ( = 0.9 Hz) attributed to C2-H of chromone moiety. These NMR spectral features are indicative of an intact chromone ring. 13 C NMR spectrum showed resonances at 174.8 (C=O), 156.2 (C3 ), 140.3 (C3), and at 82.8 corresponding to (C5 ), besides the other anticipated carbon resonances of chromone as well as dithiazole ring of compound 3c. Its IR spectrum showed a characteristic band at 1645.7 cm −1 attributed to (C=O) group and band at 771.4 cm −1 corresponding to the disulphide linkage (-S-S-). Mass spectrum revealed a molecular ion peak at m/z 361.5(M + ) corresponding to molecular mass 361.5 of compound 3c and its elemental composition is also corroborated by the microanalytical data.

Antimicrobial Evaluation
3.1. Antibacterial Activity. In vitro antibacterial activity of compounds (3a-j) were established against five human pathogenic bacteria: Staphylococcus aureus (MTCC96), Bacillus subtilis (MTCC 2451), Escherichia coli (MTCC 82), Pseudomonas aeruginosa (MTCC 2642), and Salmonella typhimurium (MTCC 1251). The antibacterial activity of compounds was determined, with comparison to standard antibiotic discs of amoxicillin and gentamycin. Inhibitory potential of the synthesized compounds was determined in terms of minimum inhibitory concentration (MIC) in g/mL ( Table 2) by using turbidity method.
All synthesized compounds (3a-j) were found to be active against all pathogenic bacterial strains. Compound 3c showed excellent inhibitory activity against B. subtilis with MIC 0.78, which is comparable with positive control gentamycin, followed by compound 3h with MIC 1.56, wherein compound 3f showed inhibitory activity with MIC 3.12. Compounds 3a, 3c, and 3e displayed promising activity against S. aureus with MIC 6.25. Compound 3h exhibited great inhibitory activity against E. coli with MIC 1.56 followed by compounds 3c and 3g with MIC 3.12, wherein compounds 3f and 3i with MIC 6.25. Compounds 3h and 3c showed good inhibitory activity against P. aeruginosa and S. typhi with MIC 6.25 and 12.5, respectively.

Antifungal
Activity. In vitro antifungal activity of various synthesized compounds (3a-j) was carried out against three fungal strains, that is, Saccharomyces cerevisiae (MTCC 172), Candida albicans (MTCC 3018), and Cryptococcus gastricus (MTCC 1715). Fluconazole was used as a positive control. Inhibitory potential of the synthesized compounds was determined in terms of minimum inhibitory concentration (MIC) in g/mL using turbidity method (Table 3).
In the case of fungal strain S. cerevisiae, compound 3h displayed very high inhibitory activity with MIC 0.78, which is more than that of fluconazole used as positive control under similar conditions, followed by compound 3g with MIC 3.12 and compound 3a with MIC 6.25. Compound 3c exhibited promising inhibitory activity against C. albicans with MIC 3.12, which is higher than standard positive control, followed by compounds 3a, 3e, and 3h with MIC 6.25. Compounds 3h showed comparable inhibitory activity against C. gastricus (MIC 6.25) with standard drug fluconazole, followed by 3c with MIC 12.5, respectively. The structure activity relationship of all the synthesized compounds was developed on the basis of obtained in vitro antimicrobial results. The substitution of -chloro group at para position of phenyl ring of the 1,2,4dithiazole enhanced the inhibitory activity against various bacterial and fungal strains comparison to our previously synthesized compounds [12]. It was observed that substitution of electron withdrawing groups at C 6 and C 7 positions of chromone ring leads to an increase in both antifungal and antibacterial activities. Therefore, efforts are made to perform substitution with different electron withdrawing groups at C 6 , C 7 , and C 8 positions of the chromone ring. C 6 position of chromone moiety also has been substituted with electron donating substitution that is, -methyl, to compare the antimicrobial activity with electron withdrawing substitutions and results revealing that electron donating substitution decreases both antifungal and antibacterial activities. Substitution at C 7 with electron withdrawing groups fluoro and bromo leads to an increase in both antifungal and antibacterial activities, especially, against B. subtilis, E. coli, and S. cerevisiae.

Conclusion
Various substituted 3-[5-(p-chloro)-phenyl-3H-[1,2,4]-dithiazol-3 -yl]-4H-chromen-4-one (3a-j) were synthesized and characterized by rigorous spectroscopic techniques such as 1 H and 13 C NMR, IR, and mass and elemental analysis. All synthesized compounds (3a-j) were screened for in vitro antimicrobial activity against various pathogenic bacterial and fungal strains. Compound 3c exhibited excellent inhibitory activity against B. subtilis with MIC 0.78 followed by compound 3h which displayed promising inhibitory activity against B. subtilis and E. coli with MIC 1.56. Compound 3h showed excellent inhibitory activity against S. cerevisiae with MIC 0.78 as compared to standard drug fluconazole and Scheme 1: Synthesis of 1,2,4-dithiazolylchromones 3a-j.  International Journal of Medicinal Chemistry compound 3c showed promising inhibitory activity against C. albicans with MIC 3.12. These compounds can be taken as "lead" to develop more potent antimicrobial agents.

Chemistry.
Starting materials and reagents were purchased and used after further purification (crystallization/ distillation). JEOL AL-300FT (300 MHz) NMR spectrometer was used to record 1 H and 13 C NMR (75 MHz) spectra and chemical shifts ( ) are reported as downfield displacements from tetramethylsilane (TMS) used as internal standard and coupling constants ( ) are reported in Hz. IR spectra were recorded with Shimadzu FT-IR-8400S on KBr pellets. Mass spectra (ESI method) were recorded on Bruker Daltonics esquire 300 mass spectrometer and HR-MS (ESI-TOF). Elemental analyses were carried out on a Thermoelectron EA-112 elemental analyser and reported in percent abundance.

Biological Activity
Antibacterial and antifungal activities of synthesized compounds were evaluated using the broth macro dilution method to determine the minimum inhibitory concentration (MIC) [13,14].

MIC Determination.
The broth dilution test was performed in test tubes. In twofold serial dilutions of the compounds, a standardised suspension (McFarland turbidity standard) of test bacteria (100 L) was added to obtain a final concentration. A growth control tube and sterility control tube were used in each test. After overnight incubation at 37 ∘ C, the MIC was determined by measuring optical density at 600 nm as the lowest concentration that inhibits growth, evidenced by the absence of turbidity.

MIC Determination.
The broth dilution test was performed in test tubes. The conidial suspension gave the final concentration. A growth tube and sterility control tube were used in each test. After 48 h incubation at 26 ∘ C, the MIC was determined visually as the lowest concentration that inhibits growth, evidenced by the absence of turbidity.