A Mini Library of Novel Triazolothiadiazepinylindole Analogues: Synthesis, Antioxidant and Antimicrobial Evaluations

A new series of novel triazolothiadiazepinylindole analogues were synthesized with an aim to examine possible antioxidant and antimicrobial activities. The titled compounds (3a–z) were obtained in good yield by reacting 5-(5-substituted-3-phenyl-1H-indol-2-yl)-4-amino-4H-1,2,4-triazole-3-thiols 1a–c with 3-(2,5-disubstituted-1H-indol-3-yl)-1(4-substituted phenyl)prop-2-en-1-ones 2a–i. All the newly synthesized compounds were characterized by IR, 1H NMR, mass spectroscopic and analytical data. The synthesized analogues were tested for antioxidant and antimicrobial potency. Among the tested compounds 3a–c and 3j–l have shown very promising free radical scavenging activity and total antioxidant capacity. Compounds 3d–f, 3m–o, and 3s–z have shown excellent ferric reducing antioxidant activity. An outstanding antimicrobial activity is observed with compounds 3a–c and 3j–l.


Introduction
Antioxidants [1][2][3] act as "free radical scavengers" hence to prevent or slow the damage done by the free radicals [4][5][6]. Free-radical-induced oxidative stress associated with several cellular toxic processes including oxidative damage to protein, and DNA, membrane lipid oxidation, enzyme inactivation, and gene mutation leads to carcinogenesis [7]. Antioxidants are involved in processes such as immunity, protection against tissue damage, and reproduction and prevent growth or development caused by free radicals [8][9][10]. Antioxidants are useful in the prevention and treatment of Parkinson's and Alzheimer's disease [11][12][13].
Heterocycles constitute one of the major areas of organic chemistry and play important roles in drug discovery. Many of the best selling drugs currently in use contain one or more heterocyclic rings. Several fused heterocycles as well as biheterocycles are referred to as privileged structures [14]. Among them, sulfur-and nitrogen-containing heterocyclic compounds have maintained the interest of researchers and their unique structures led to several applications in different areas [15]. Triazoles and their derivatives constitute an important class of heterocyclic compounds and their analogues have been reported to possess various biological activities such as antimicrobial [16], anti-inflammatory [17], antihypertensive, anti-HIV [18], anticancer, and antitumor [19,20]. Several compounds containing 1,2,4-triazole rings known as drugs like fluconazole, posaconazole, alprazolam, [21] and triazolothiadiazepine analogues represent a wellknown class of drug substances at different stages of research, which possess antiviral [22] and antimicrobial properties [23].
Indole is a heterocycle of great importance in biological systems [24,25]. The indole moiety is present in a number 2 The Scientific World Journal of drugs currently [26] in the market; in our previous approaches, we have described some new indole analogues with highly potent antioxidant, DNA cleavage and antimicrobial activities [27][28][29][30].
Interestingly, we have developed a new green protocol for the synthesis of rapid and clean synthetic route towards mini library of triazolothiadiazepinylindole analogues, which showed in vitro antioxidant and antimicrobial activities.

Materials and Methods
2.1. Chemistry. All chemicals used in this investigation were of analytical grade and were purified whenever necessary. Melting points of the synthesized compounds were measured in open capillaries and are uncorrected. Reactions were monitored by thin-layer chromatography (TLC) on silica gel 60 F 254 aluminium sheets (MERCK). Iodine vapour was used as detecting agent. IR spectra were recorded in KBr on PerkinElmer and FTIR spectrophotometer (] max in cm −1 ). 1 H NMR and 13 C NMR spectra on BRUKER AVENCE II 400-MHz NMR spectrometer and the chemical shifts were expressed in ppm ( scale) downfield from TMS as an internal reference. The mass spectra were recorded on LC-MSD-Trap-SL instrument. The elemental analysis was performed by using FLASH EA 1112 SERIES instrument.

General Procedure for the Synthesis of Compounds 3a-z
(1) Conventional Method. To a solution of substituted indolyltriazole 1a-c (0.01 mol) in acetic acid substituted chalcones 2a-i (0.01 mol) were added. The reaction mixture was refluxed 3-4 hrs. The completion of the reaction was monitored by TLC. After the completion, the reaction mixture was poured to a beaker containing 100 mL of ice-cold water. The crude products thus separated were filtered and recrystallized from ethanol to yield target compounds 3a-z.
(2) Microwave Oven Method. A mixture of substituted indolyl triazole 1a-c (0.01 mol) and substituted chalcones 2ai (0.01 mol) was powdered, mixed, and introduced to borosil sample crucible containing few drops of acetic acid. This was subjected to microwave irradiation for 10 minutes with 70% microwave power. After the completion (TLC), reaction mixture was brought to room temperature, washed with ethanol, and recrystallized to get the title compounds 3a-z which were found to be in good purity (TLC) and excellent yield.

Antioxidant Activities
(1) Free Radical Scavenging Activity. Free radical scavenging activity was done by DPPH method [32]. Different concentrations (25 g, 50 g, and 100 g) of samples and butylated hydroxy anisole (BHA) were taken in different test tubes. The volume was adjusted to 100 L by adding MeOH. Five milliliters of 0.1 mM methanolic solution of DPPH was added to these tubes and shaken vigorously. The tubes were allowed to stand at 27 ∘ C for 20 min. The control was prepared as above without any samples. The absorbances of samples were measured at 517 nm. Radical scavenging activity was calculated using the following formula: (2) Total Antioxidant Capacity. Various concentrations of samples (25 g, 50 g, and 100 g) were taken in a series of test tubes. To this, 1.9 mL of reagent solution (0.6 M sulfuric acid, 28 mM sodium phosphate, and 4 mM ammonium molybdate) was added. The tubes were incubated at 95 ∘ C for 90 min and allowed to cool. The absorbance of each aqueous solution was measured at 695 nm against a blank. Antioxidant capacities are expressed as equivalents of ascorbic acid. Ascorbic acid equivalents were calculated using standard graph of ascorbic acid. The values are expressed as ascorbic acid equivalents in g per mg of samples.
(3) Ferric Reducing Antioxidant Power. Various concentrations of samples (25 g, 50 g, and 100 g) were mixed with 2.5 mL of 200 mmol/L sodium phosphate buffer (pH 6.6) and 2.5 mL of 1% potassium ferricyanide. The mixture was incubated at 50 ∘ C for 20 min. Next, 2.5 mL of 10% trichloroacetic acid (w/v) was added. From this solution, 5 mL was mixed with 5 mL of distilled water and 1 mL of 0.1% ferric chloride and absorbance was measured spectrophotometrically at 700 nm. BHA was used as standard. Candida tropicalis ATCC 8302 and Candida albicans ATCC 60193by applying the agar plate diffusion technique [33]. Dilution process was adopted at 25 g, 50 g, and 100 g/mL concentrations, respectively. The activity is compared with reference drugs gentamycin for antibacterial and fluconazole for antifungal activity. The zone of inhibition after 24 hr of incubation at 37 ∘ C in case of antibacterial activity and 48 hr in case of antifungal activity was compared with that of standards. to thiadiazepine C=N stretching, respectively. The 1 H NMR spectrum of 3a has exhibited a singlet at 12.47 ppm due to indole NH and peak at 11.63 ppm is due to indole NH which is also D 2 O exchangeable. A multiplet between 7.31-8.47 ppm corresponds to twenty aromatic protons present in the molecule and a peak at 5.65 ppm is assigned for the -CH= of thiadiazepine ring proton. The 13 C NMR spectrum of compound 3a has shown peaks at 108, 111, 113, 117, 118, 118, 118, 120, 123, 125, 126, 126, 126, 128, 128, 128,  128, 129, 129, 129, 130, 132, 133, 134, 135, 138, 138, 144, 145, and 166. The mass spectrum of compound 3a has shown molecular ion peak at m/z 712 [M] +• which is corresponding to molecular weight of the compound. The above spectral data supports the formation of compound 3a.

Results and Discussion
Various new triazolothiadiazepinylindole analogues synthesized during the present investigation are listed in (Table 1).

Biological Activities.
The compounds 3a-z were screened for their antioxidant (free radical scavenging, total antioxidant capacity, and ferric reducing antioxidant power) and antimicrobial activities.

Antioxidant Activities
(1) Free Radical Scavenging Activity. The target compounds were screened for free radical scavenging activity by DPPH method [32]. The samples were prepared at concentrations of 25, 50, and 100 g/100 L and butylated hydroxy anisole (BHA) was taken as standard. DPPH is a stable free radical in a methanolic solution. Because of the unpaired electron of DPPH, it gives a strong absorption maxima at 517 nm in the visible region (purple color). In addition, the unpaired electron of the radical becomes paired in the presence of a hydrogen donor (a free radical scavenging antioxidant), decreasing the absorption. Among the compounds tested 3ac and 3j-l have shown very promising free radical scavenging activity. The increased activity is due to the existence of halogen substitution at the five positions of both indoles. The hydrogen of indole NH could be donated to the DPPH to form DPPH free radical; by the presence of phenyl ring at the third position of indole, the DPPH free radical will be stabilized by the resonance. Compounds 3d-f, 3mo, and 3s-x containing halogen atom at five positions of indole and a methyl group at another indole ring have shown moderate activity, whereas compounds 3g-i, 3pr, and 3y-z have shown the least activity compared with the standard. The bar graph representation of percentage of free radical scavenging activity is displayed in Figures 1  and 2.

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The Scientific World Journal (2) Total Antioxidant Capacity. Total antioxidant activity was performed to all the newly synthesized compounds [34].
analogues were screened for in vitro antibacterial activity against ( Table 2) gram-negative bacteria Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae) and gram-positive bacteria Staphylococcus aureus (S. aureus) at 25 g/mL, 50 g/mL, and 100 g/mL concentrations, respectively. Gentamycin was used as standard. The zone of inhibitions was measured in mm for each concentration. Most of the screened compounds were found to have significant antibacterial activity. Compounds 3a-c and 3j-l have shown very good activity against all the three bacterial strains. Compounds 3d-f, 3m-o, and 3s-x have shown moderate activity and compounds 3g-i, 3p-r, and 3y-z have shown the least activity. Antifungal screening of the compounds was carried out in vitro against two fungi strains Candida tropicalis and Candida albicans at 25 g/mL, 50 g/mL, and 100 g/mL concentrations using fluconazole as standard. Among the tested indole analogues the majority of compounds exhibited moderate to significant antifungal activity.

Conclusions
We have synthesized titled compounds 3a-z by economic, better yield, and safer methods through the formation of compounds 1a-c and 2a-i under thermal and microwave condition. The compounds 3a-z were subjected for their antioxidant and antimicrobial screening. Very potent antimicrobial, scavenging and antioxidant activity was observed with compounds containing halogens at the fifth position of indoles. Excellent ferric reducing activity was observed with compounds containing electron donor group at five positions    The Scientific World Journal 9 of one/both indoles. Therefore, the findings will provide a great impact on chemists and biochemists for further investigations in the indole field in search of molecules possessing potent antioxidant and antimicrobial activities.