Synthesis and biological activity of 1,2,4-triazolo-[3,4-b]thiadiazole as antimicrobial agents

Some novel 6-fluoro chroman derivatives having 1,2,4-triazolo-[3,4-b]thiadiazole were synthesized and characterized by IR, NMR and mass spectral analysis. All synthesized compounds were screened for antimicrobial activity using broth dilution method. All the compounds showed good antimicrobial activity and compound 5e showed significant antibacterial activity


INTRODUCTION
Over the years, synthetic heterocyclic chemistry is providing momentum to the development of new drug scaffolds through interactive manipulation of functional groups around the basic skeleton. Among these, heterocyclic compounds have been given special importance because of a wide variety of biological properties associated with them. The importance of heterocycles in biological systems encouraged chemists to design and modify new heterocyclic compounds [1,2]. During the last two decades, the chemistry of 1,2,4triazole and 1,2,4-triazolo-[3,4-b]-1,3,4-thiadiazole and their derivatives have received considerable attention owing to their synthetic and effective biological importance [3][4][5]. 1,2,4-Triazoles and their derivatives occupy a essential position in medicinal chemistry because of their potential biological activities such as antibacterial [6], antifungal [7], antitubercular [8], anti-inflammatory [9] etc. The 1,2,4-triazole ring is an integral part found in various drugs such as rizatriptan, ribavirin, and fluconazole ( Fig.1), which find a wide range of applications in pharmaceutical industry [10][11][12].

EXPERIMENTAL
All chemicals and solvents were purchased from Spectrochem Pvt Ltd., Mumbai of LR grade and were used without further purification. Melting points were taken in open capillary method and are uncorrected. IR spectra were recorded on FTIR-8400 spectrophotometer (Shimadzu, Kyoto, Japan), using DRS probe KBr pallet. 1 HNMR spectra of the synthesized compounds were recorded on a Bruker-Avance-II (400 MHz) CDCl 3 and DMSO-d 6 solvent. Chemical shifts are expressed in δ ppm downfield from TMS as an internal standard. Mass spectra were determined using direct inlet probe on a GCMS-QP 2010 mass spectrometer (Shimadzu, Kyoto, Japan).
A mixture of potassium 2-[(6-fluorochroman-2-yl) carbonyl] hydrazine carbodithioate 3 (3.5 g, 0.1 M) in water (5 ml) and hydrazine hydrate (3.4 ml, 0.05 M) was refluxed for 6-7 h with occasional shaking. The colour of the reaction mixture changed to green with the evolution of hydrogen sulfide. A homogenous reaction mixture was obtained during the reaction process. The completion of the reaction was monitored on TLC. The reaction mixture was cooled to room temperature and diluted with water (100 ml). On acidification with concentrated hydrochloric acid, the required triazole 4 gets precipitated. Further it was filtered, washed thoroughly with cold water and recrystallized from ethanol.

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ILCPA Volume 26 separated out was filtered, washed with water and neutralized with sodium bicarbonate solution. Crude product was purified by column chromatography to give the analytical pure compounds. Physical constants of newly synthesized triazolo [3,4-b] [1,3,4]thiadiazoles derivatives 5a-5j are recorded in Table 1.

ANTI MICROBIAL ACTIVITY
All the glass apparatus used were sterilized before use. The broth dilution technique was used to determine the minimum inhibitory concentration (MIC) of the synthesized compounds. Bacterial strain of Staphylococcus aureus, Streptococcus pyogenes, Pseudomonas aeruginosa and Escherichia coli and fungal strains of Aspergillus niger, Candida albicans and Aspergillus clavatus were used in the present study. DMSO was used as the solvent for the compounds. A blank test was carried out to check the antimicrobial activity of DMSO.

International Letters of Chemistry, Physics and Astronomy Vol. 26
Ampicillin and Chloramphenicol were used as the standard drugs for antibacterial activity. Greseofulvin was used as the standard drug for antifungal activity. The synthesized 1,2,4-triazolo- [3,4-b]thiadiazole 5a-5j were screened for their antimicrobial activity by the broth dilution method to evaluate the minimum inhibitory concentration Table 2. All of the precursors (5a-j) of the title compounds showed antibacterial activity in the range of 62.5-500 μg/mL for Staphylococcus aureus, 100-500 μg/mL for Streptococcus pyogenes, 100-500 μg/mL for Pseudomonas aeruginosa, and 62.5-500 μg/mL for Escherichia coli. It was observed that compound 5e (MIC = 62.5 μg/mL) against s. aureus as well as compound 5e (MIC = 62.5 μg/mL) against E.coli have found to be better active as

CONCLUSION
In present report, we are reporting very efficient method for the synthesis of some novel 1,2,4-triazolo- [3,4-b]thiadiazole derivatives possessing 6-fluoro chroman nucleus. All synthesized compounds were obtained in good yield. From the results of antimicrobial data, compounds 5e showed excellent results against Gram positive and Gram negative bacteria while compounds 5a, 5c and 5f were found moderate active. All synthesized compounds showed minimal activity against fungi pathogens as compared to the standard drugs.