Synthesis and Development of N-2,5-Dimethylphenylthioureido Acid Derivatives as Scaffolds for New Antimicrobial Candidates Targeting Multidrug-Resistant Gram-Positive Pathogens

Abstract

The growing antimicrobial resistance to last-line antimicrobials among Gram-positive pathogens remains a major healthcare emergency worldwide. Therefore, the search for new small molecules targeting multidrug-resistant pathogens remains of great importance. In this paper, we report the synthesis and in vitro antimicrobial activity characterisation of novel thiazole derivatives using representative Gram-negative and Gram-positive strains, including tedizolid/linezolid-resistant S. aureus, as well as emerging fungal pathogens. The 4-substituted thiazoles 3h, and 3j with naphthoquinone-fused thiazole derivative 7 with excellent activity against methicillin and tedizolid/linezolid-resistant S. aureus. Moreover, compounds 3h, 3j and 7 showed favourable activity against vancomycin-resistant E. faecium. Compounds 9f and 14f showed broad-spectrum antifungal activity against drug-resistant Candida strains, while ester 8f showed good activity against Candida auris which was greater than fluconazole. Collectively, these data demonstrate that N-2,5-dimethylphenylthioureido acid derivatives could be further explored as novel scaffolds for the development of antimicrobial candidates targeting Gram-positive bacteria and drug-resistant pathogenic fungi.

Keywords: 2-aminothiazoles; antibacterial; anticancer activity; antifungal; benzimidazole; hydrazone; sulphanilamide.

Scheme 1

Synthesis of thiazole derivatives 2–7. R¹ = (a) H; (b) CH₃; (c) C₆H₅; (d) 4-F-C₆H₄; (e) 4-CN-C₆H₄; (f) 4-Cl-C₆H₄; (g) 4-NO₂-C₆H₄; (h) 3,4-diCl-C₆H₄; (i) 4-Br-C₆H₄; (j) naphthalen-2-yl; (k) chromenon-3-yl. Reagents and conditions: (i) ClCH₂COOH, 10% K₂CO₃, r.t. 24 h, AcOH to pH 6; (ii) a 50% ClCH₂CHO, acetone, reflux, 12 h; b ClCH₂COCH₃, water, rt, 24 h, AcONa, heated to boil; c–k α-bromoacetophenone, acetone, reflux, 2–5 h, (iii) PPA, 120 °C, 2–3 h, crushed ice; (iv) 3-chloropentane-2,4-dione, acetone, reflux, 2 h; (v) 2,3-dichloroquinoxaline or (vi) 2,3-dichloro-1,4-naphthoquinone, AcOH, AcONa, rt, 24 h, 70–80 °C, 10 h, water.

Scheme 2

Chemical transformations of carboxylic acids 3c, f, h and hydrazide 9f. (3, 16, 17c) R¹ = H; (3, 16, 17f) R¹ = 4-Cl; (3, 16, 17h) R¹ = 2,4-diCl; (10f) R² = 5-nitrothiophen-2-yl; (11f) R² = 5-nitrofuran-2-yl; (12f) R² = indol-3-yl; (14f) R³ = Et; (15f) Hex. Reagents and conditions: (i) MeOH, a few drops of conc. H₂SO₄, reflux, 4 h; 5% Na₂CO₃ (ii) N₂H₄·H₂O, 1,4-dioxane, reflux, 5 h; (iii) corresponding aldehyde, 1,4-dioxane, a few drops of glacial acetic acid, reflux, 2–12 h; (iv) cyclopentanone, 1,4-dioxane, a few drops of conc. acetic acid, reflux, 3 h; (v) corresponding ketone, 1,4-dioxane, a few drops of conc. acetic acid, reflux, 4 (14) or 8 (15) h; (vi) o-phenylenediamine, 15% HCl, reflux 72 h, water, 10% K₂CO₃; (vii) sulphanilamide, TEA, DMF, rt, 0.5 h, HBTU, DMF, argon, r.t. 72 h, 10% K₂CO₃.

Figure 1

The in vitro anticancer activity of compounds 1–17 on A549 (A) and Caco-2 (B) adenocarcinoma cell line. Cells were exposed with fixed 100 µM of compounds 1–17 or cisplatin (CP) that served as a cytotoxicity control for 24 h. After the treatment, the remaining viability was determined by using MTT assay. Data represent the mean ± SD of triplicate experiments (n = 3). Statistical differences were determined using the Kruskal–Wallis test. * p < 0.05; ** p < 0.01, *** p < 0.001, **** p < 0.0001.