Synthesis, Characterization, and Biological Evaluation of New Derivatives Targeting MbtI as Antitubercular Agents

Tuberculosis (TB) causes millions of deaths every year, ranking as one of the most dangerous infectious diseases worldwide. Because several pathogenic strains of Mycobacterium tuberculosis (Mtb) have developed resistance against most of the established anti-TB drugs, new therapeutic options are urgently needed. An attractive target for the development of new antitubercular agents is the salicylate synthase MbtI, an essential enzyme for the mycobacterial siderophore biochemical machinery, absent in human cells. A set of analogues of I and II, two of the most potent MbtI inhibitors identified to date, was synthesized, characterized, and tested to elucidate the structural requirements for achieving an efficient MbtI inhibition and a potent antitubercular activity with this class of compounds. The structure-activity relationships (SAR) here discussed evidenced the importance of the furan as part of the pharmacophore and led to the preparation of six new compounds (IV–IX), which gave us the opportunity to examine a hitherto unexplored position of the phenyl ring. Among them emerged 5-(3-cyano-5-(trifluoromethyl)phenyl)furan-2-carboxylic acid (IV), endowed with comparable inhibitory properties to the previous leads, but a better antitubercular activity, which is a key issue in MbtI inhibitor research. Therefore, compound IV offers promising prospects for future studies on the development of novel agents against mycobacterial infections.

S4 concentrated under reduced pressure to give a dark brown residue, which was purified through flash column chromatography.
Procedure K. Ethyl 2-isocyanoacetate (2.0 mmol) was added to a solution of the appropriate acetophenone (1.0 mmol) and I2 (1.6 mmol) in DMSO (3 mL). The mixture was stirred at 130 °C for 3 h; after completion, the reaction was quenched with H2O (50 mL) and extracted with EtOAc (3 × 50 mL). The combined organic layers were washed with brine, dried over anhydrous Na2SO4, and concentrated under reduced pressure. The crude was purified by flash column chromatography on silica gel to afford the desired product.
Procedure L. SeO2 (2.0 mmol) was dissolved in dry 1,4-dioxane (1 mL) and H2O (2 drops). Then, the suitable acetophenone (1.0 mmol) was added to the solution and the mixture was stirred at reflux for 7 h. After cooling to room temperature, 5 mL of DCM were added, and the reaction mixture was filtered over a pad of celite, washing with DCM. The solution was concentrated in vacuo, and the residue was diluted with H2O (2 mL) and heated at reflux for 10 min. The resulting mixture was cooled in an ice bath affording a precipitate, which was filtered and used in the following reaction without further purification.
Procedure M. Polymerized ethyl 2-oxoacetate in 50% toluene (3.0 mmol) was heated to 60 °C for 15 min before it was added to a solution of NH4OAc (3.0 mmol) in H2O (0.65 mL) and CH3CN (1.3 mL) at 0 °C. Then, a solution of the suitable 2,2-dihydroxyethanone (1.0 mmol) in CH3CN (1.3 mL) was added dropwise at 0 °C; the resulting mixture was stirred between 0-5 °C for 30 min and then at room temperature for 1.5 h. After the removal of the solvent under reduced pressure, the residue was diluted with H2O (10 mL) and extracted with EtOAc (3 × 10 mL). The organic layer was washed with brine and dried over anhydrous Na2SO4, filtered, and concentrated in vacuo. The residue was purified by flash column chromatography and crystallized from EtOAc and hexane to afford the desired product.
Procedure N. NaN3 (1.5 mmol) was added to a solution of the appropriate boronic acid (1 mmol) and Cu(OAc)2 (0.1 mmol) in dry MeOH (5 mL), under a nitrogen atmosphere. The mixture was stirred at 55 °C for 1.5 h (4 h for 6a). Then, ethyl propiolate (3 mmol) and (+)sodium L-ascorbate (0.1 mmol) were added, and the stirring was continued at room temperature overnight (24 h for 6a). Air was bubbled into the solution for 2 h to oxidize the residual organoboron compound, and then the mixture was diluted with EtOAc (5 mL) and filtered over a pad of celite. The solution was concentrated in vacuo to remove the solvent and then washed with H2O. The organic phase was dried over anhydrous Na2SO4, filtered, and concentrated in vacuo; the crude product was purified by flash column chromatography.