Synthesis, Characterization, and Biological Evaluation of Novel 7-Oxo-7H-thiazolo[3,2-b]-1,2,4-triazine-2-carboxylic Acid Derivatives

Design, synthesis and antibacterial activity of novel 7H-thiazolo[3,2-b]-1,2,4-triazin-7-one derivatives

Based on the observed biological activity of 1,2,4-triazin-5-one derivatives and their cyclic analogues, a novel series of 7H-thiazolo[3,2-b]-1,2,4-triazin-7-one derivatives that contain ester moiety compounds 3a-3g, carboxylic acid moiety compounds 4a-4g and piperazine amide moiety compounds 5a-5k at position-3 of the thiazolotriazinone scaffold were synthesized. The intermolecular cyclization occurred regioselectively at N2-position of 1,2,4-triazine ring was characterized by X-ray single-crystal diffraction analysis. The in vitro biological activities of the target compounds were assayed against some bacterial strains. Compared with ciprofloxacin, compounds 3g and 4g exhibited more excellent antibacterial activity, especially the activity against Staphylococcus aureus and Escherichia coli, showing that the fluorine at the para position of the benzyl group would be the best choice. In addition, compounds 4e-4g with carboxylic acid moiety can enhance the antibacterial activity. Compounds 5g-5k containing bulky 1-(substituted phenyl)piperazine moiety were found with slightly less biological activity. Similar to ciprofloxacin, the docking result of target compounds with DNA topoisomerase II indicates the carboxyl group of the target compounds with carboxylic acid moiety has a crucial salt bridge interaction with Mg2+ in the protein.

Computational Insight into Substrate-Induced Conformational Changes in Methionyl-tRNA Synthetase of Mycobacterium Tuberculosis

Tuberculosis caused by Mycobacterium tuberculosis (M.tb) has killed millions worldwide. Antibiotic resistance leads to the ineffectiveness of the current therapies. Aminoacyl tRNA synthetase (aaRS) class of proteins involved in protein synthesis are promising bacterial targets for developing new therapies. Here, we carried out a systematic comparative study on the aaRS sequences from M.tb and human. We listed important M.tb aaRS that could be explored as potential M.tb targets alongside the detailed conformational space analysis of methionyl-tRNA synthetase (MetRS) in apo- and substrate-bound form, which is among the proposed targets. Understanding the conformational dynamics is central to the mechanistic understanding of MetRS, as the substrate binding leads to the conformational changes causing the reaction to proceed. We performed the most complete simulation study of M.tb MetRS for 6 microseconds (2 systems × 3 runs × 1 microsecond) in the apo and substrate-bound states. Interestingly, we observed differential features, showing comparatively large dynamics for the holo simulations, whereas the apo structures became slightly compact with reduced solvent exposed area. In contrast, the ligand size decreased significantly in holo structures possibly to relax ligand conformation. Our findings correlate with experimental studies, thus validating our protocol. Adenosine monophosphate moiety of the substrate exhibited quite higher fluctuations than the methionine. His21 and Lys54 were found to be the important residues forming prominent hydrogen bond and salt-bridge interactions with the ligand. The ligand-protein affinity decreased during simulations as computed by MMGBSA analysis over the last 500 ns trajectories, which indicates the conformational changes upon ligand binding. These differential features could be further explored for designing new M.tb inhibitors.

Synthesis, Structure and Stereochemistry of Dispirocompounds Based on Imidazothiazolotriazine and Pyrrolidineoxindole

Methods for the synthesis of two types of isomeric dispirocompounds based on imidazothiazolotriazine and pyrrolidineoxindole, differing in the structure of imidazothiazolotriazine fragment, namely, linear dispiro[imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazine-6,3'-pyrrolidine- 4',3″-indolines] and angular dispiro[imidazo[4,5-e]thiazolo[2,3-c][1,2,4]triazine-7,3'-pyrrolidine-4',3″-indolines] were proposed. The first method relies on a 1,3-dipolar cycloaddition of azomethine ylides generated in situ from paraformaldehyde and N-alkylglycine derivatives to the corresponding oxindolylidene derivatives of imidazothiazolotriazine. The cycloaddition leads to a mixture of two diastereomers resulted from anti- and syn-approaches of azomethine ylide in approximately a 1:1 ratio, which were separated by column chromatography. Another method consists in rearrangement of linear dispiro[imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazine-6,3'-pyrrolidine-4',3″-indolines] into hitherto unavailable angular dispiro[imidazo[4,5-e]thiazolo[2,3-c]-[1,2,4]triazine-7,3'-pyrrolidine-4',3″-indolines] upon treatment with KOH. It was found that the anti-diastereomer of linear type underwent rearrangement into the isomeric angular syn-diastereomer, while the rearrangement of the linear syn-diastereomer gave the angular anti-diastereomer.

Synthesis of functionalized imidazo[4,5-e]thiazolo[3,2-b]triazines by condensation of imidazo[4,5-e]triazinethiones with DMAD or DEAD and rearrangement to imidazo[4,5-e]thiazolo[2,3-c]triazines

Two series of functionalized imidazothiazolotriazine derivatives were synthesized via the condensation of imidazo[4,5-e]-1,2,4-triazine-3-thiones with acetylenedicarboxylic acid dimethyl and diethyl esters (DMAD and DEAD) and subsequent base-catalyzed rearrangement of the obtained imidazo[4,5-e]thiazolo[3,2-b]-1,2,4-triazines into regioisomeric imidazo[4,5-e]thiazolo[2,3-c]-1,2,4-triazine derivatives.

Inhibitors of aminoacyl-tRNA synthetases as antimycobacterial compounds: An up-to-date review

Aminoacyl-tRNA synthetases (aaRSs) are crucial for the correct assembly of amino acids to cognate tRNA to maintain the fidelity of proteosynthesis. AaRSs have become a hot target in antimicrobial research. Three aaRS inhibitors are already in clinical practice; antibacterial mupirocin inhibits the synthetic site of isoleucyl-tRNA synthetase, antifungal tavaborole inhibits the editing site of leucyl-tRNA synthetase, and antiprotozoal halofuginone inhibits proline-tRNA synthetase. According to the World Health Organization, tuberculosis globally remains the leading cause of death from a single infectious agent. The rising incidence of multidrug-resistant tuberculosis is alarming and urges the search for new antimycobacterial compounds, preferably with yet unexploited mechanism of action. In this literature review, we have covered the up-to-date state in the field of inhibitors of mycobacterial aaRSs. The most studied aaRS in mycobacteria is LeuRS with at least four structural types of inhibitors, followed by TyrRS and AspRS. Inhibitors of MetRS, LysRS, and PheRS were addressed in a single significant study each. In many cases, the enzyme inhibition activity translated into micromolar or submicromolar inhibition of growth of mycobacteria. The most promising aaRS inhibitor as an antimycobacterial compound is GSK656 (compound 8), the only aaRS inhibitor in clinical trials (Phase IIa) for systemic use against tuberculosis. GSK656 is orally available and shares the oxaborole tRNA-trapping mechanism of action with antifungal tavaborole.