Synthesis and Antitubercular Activity of Some Novel Thiazolidinone Derivatives

Purpose : To synthesize and characterize novel thiazolidinone derivatives and screen them for antitubercular activity. Methods: A series of twelve novel thiazolidinones 4a-l have been synthesized by cyclocondensation of various Schiff bases of amino thiadiazole with thioglycollic acid. Various Schiff bases 3a-l were synthesized by condensation of 2-amino-5-aryl-5H-thiazolo[4,3-b]-l,3,4-thiadiazole with various aryl aldehydes. The synthesized compounds were characterized by FTIR, 1 H-NMR, 13 C-NMR and mass spectrometry. Docking studies were carried out for the synthesized compounds which were also evaluated for in vitro anti-tubercular activity at a concentration of 0.1 – 100.0 μg/mL by Microplate Blue Alamar Assay method. Pyrazinamide and streptomycin were used as standard antitubercular agents. Results: The synthesized compounds showed good docking score, compared to standard drugs. Two of the compounds (labelled 4f and 4i ) showed higher antitubercular activity than the standards (pyrazinamide and streptomycin) while compounds four others compounds (labeled 4b, 4c, 4e, 4h, 4k and 4l ) showed comparable activity to pyrazinamide but greater activity than streptomycin. Conclusion: We report the successful synthesis of novel thiazolidinones, as well as their spectral characterization, docking properties and in vitro antitubercular activities which, for some, are superior to currently used anti-tubercular agents.


INTRODUCTION
Despite the ready availability of effective treatments, tuberculosis remains a major public health threat worldwide. The emergence of drug-resistant strains of Mycobacterium tuberculosis, particularly multiple drug resistant strains [1][2][3][4], has complicated treatment protocols and raises the concern that tuberculosis may once again become an incurable disease. For this reason, it is critical to discover new drugs acting with a mechanism different from those presently in use.
In order to further assess the pharmacological profile of this class of compounds, it was thought worthwhile to synthesize some new congeners of heterocycles by incorporating thiadiazole and thiazolidinone moieties in a single molecular framework. The present work deals with the synthesis of these compounds as well as their anti-tubercular screening.

EXPERIMENTAL Materials
Analytical grade solvents and commercially available reagents were used without further purification. All chemicals were obtained from Spectrochem Ltd (Mumbai, India). Column chromatography was carried out over silica gel (60 -120 mesh), purchased from Sisco Research Laboratories Pvt Ltd. Mumbai, India. Melting point was determined in a programmable melting point apparatus (Servewell Instruments Pvt Ltd). Fourier transform infrared spectroscopy (FTIR) in KBr disk were recorded from 4000 to 400 cm -1 on a Shimadzu FT-IR spectrometer (model no. 00518). 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded on AMX 400-MHz and 500-MHz Bruker spectrometer in DMSO-d 6 or CDCl 3 using tetramethylsilane (TMS) as an internal standard. Chemical shifts are given in δ relative to TMS while the coupling constants are given in Hz. Mass spectra were recorded using Agilent 1100 MSD spectrometer in electrospray mode.

Synthesis
All the compounds were synthesized according to the scheme shown in Figure 1. The purity of the compounds was determined on TLC plates using silica gel G as a stationary phase and iodine vapor as visualizing agent.

General procedure for the synthesis of compounds 3a-3l
To a stirred solution of compound 1(0.01M) in ethanol (50 ml) containing sulphuric acid (2 ml) was added appropriate aromatic aldehyde (0.01M) and the mixture refluxed for 4 -6 h on a water bath. The separated solid was filtered and recrystallized from ethanol to give compounds 3a-3l.

General procedure for the synthesis of compounds 3a-3l
To a stirred solution of compound 1(0.01M) in ethanol (50 ml) containing sulphuric acid (2 ml) was added appropriate aromatic aldehyde (0.01M) and the mixture refluxed for 4 -6 h on a water bath. The separated solid was filtered and recrystallized from ethanol to give compounds 3a-3l.

Evaluation of anti-tubercular activity
The antimycobacterial activity of the synthesized compounds 3a-l and 4a-l were   [14,15]. This methodology is non-toxic, uses thermally stable reagent, and shows good correlation with proportional and BACTEC radiometric methods [16], The activity was expressed as minimum inhibitory concentration (MIC) in μg/mL. The drug concentration tested were in the range 0.1 -100.0 μg/mL. A blue color in the well was interpreted as absence of bacterial growth, and pink color was scored as growth. MIC (minimal inhibition concentration) was defined as the lowest drug concentration, which prevented a color change from blue to pink. Streptomycin and pyrazinamide were used as reference standards.

Data analysis
Docking was carried out for the synthesized compounds 3a-l, and 4a-l using Hex 5.1 software Scotland, UK. Molecular docking involves the following steps using Hex 5.

RESULTS
The physicochemical parameters of the synthesized compounds, including melting point, are shown in Table 1 while docking score and anti-tubercular results are listed in Tables 2 and 3, respectively. In Schiff base series compounds 3f, 3h and 3i showed good antitubercular activity compared to standard streptomycin. In Thiazolidinone series compounds 4f and 4i showed good antitubercular activity compared to both standards pyrazinamide and streptomycin and compounds 4b, 4c, 4e, 4h, 4k, 4l showed similar activity compared to standard pyrazinamide but showed better than standard streptomycin. The detail spectral data of the compounds are as follows.                and N (10.14).

Docking
The synthesized compounds showed good docking score compared to the standard drugs, streptomycin and pyrazinamide. Tabulation of ligands docking score for 3a-l and 4a-l are shown in Tables 2 and 3, respectively.

Anti-tubercular activity
The synthesized compounds showed higher in vitro activity than the standard drugs, streptomycin and pyrazinamide. In the Schiff base series 3a-l, compounds 3f, 3h and 3i showed good antitubercular activity compared to streptomycin. In the Thiazolidinone series 4a-l, compounds 4f and 4i showed higher antitubercular activity than both pyrazinamide and streptomycin; compounds 4b, 4c, 4e, 4h, 4k, 4l showed similar activity to pyrazinamide but they all exhibited greater activity than streptomycin.

DISCUSSION
FTIR, 1 H-NMR, 13 C-NMR and mass spectra are in agreement with the proposed structures. All synthesized compounds showed good docking score compared to the standard drugs while the thiazolidinone series showed better docking scores than the Schiff base series. This suggests that cyclisation increases the docking scores. A closer look at the anti-tubercular results reveals that in Schiff base series (3a-l), increasing the substitution on nitrogen gave better activity (4a-l), perhaps due to increase in hydrophobicity resulting in better penetration of the Mtb cell wall. The structure -activity relationship of the compounds show that the presence of pharmacophoric moieties such as 1,3,4-thiadiazole nucleus (thiazolidinone moiety), increases antitubercular activity. A substituent, such as 4-methyl, 4-hydroxy group, attached to the phenyl ring and increases the substitution on nitrogen (NH 2 group) with thiazolidinone moiety to give higher activity, perhaps due to increase in hydrophobicity resulting in better penetration into the Mtb cell wall..

CONCLUSION
The overall outcome of these results reveals that thiazolidinone ring is a satisfactory backbone for antitubercular activity. These preliminary buy encouraging anti-tubercular results could offer an excellent framework in the field that may lead to the discovery of potent antitubercular agents.