Síntese de compostos bioativos via condensação de Knoevenagel e reações sequenciais utilizando quitosana como organocatalisador reciclável

Abstract

The Knoevenagel condensation reaction is a widely explored variant of aldol reactions, used to enhance the functionality of carbon chains in the search for new bioactive molecules. As it is a catalyst dependent process, the pursuit of innovative catalytic methodologies capable of promoting faster, more selective, and more efficient reactions has intensified. In this context,

low molecular weight commercial chitosan (LWC) and EDTA-modified chitosan (LWC- EDTA-72h) were applied in the synthesis of 3-coumarin-carboxylic acids, Knoevenagel

adducts derived from aromatic aldehydes or isatin, as well as xanthene and tetraketone derivatives. The obtained compounds were characterized by 1H and 13C NMR, melting point analysis, and evaluated for antileishmanial, antimicrobial, antitumor in vitro, and larvicidal activity against Aedes aegypti. LWC-EDTA-72h acted as a bifunctional heterogeneous organocatalyst at room temperature in reactions between aldehydes or isatins and malononitrile in ethanol:water (3:7, v/v), producing the desired adducts (1a–l and 2a–h) in 0.5–120 min with yields of 83–96%. The catalyst maintained its activity over six recovery and reuse cycles. In larvicidal assays, the Knoevenagel adducts 1c (R = 4-NO2) and 1g (R = 3-OCH3) exhibited LC50 values of 3.39 and 5.13 ppm, respectively, altering larval hemolymph without significant increases in nitric oxide (NO). As antimicrobials, 1a (R = H), 1f (R = 3-NO2), and 1h (R = 2- NO2) inhibited all evaluated strains (MIC = 512–1024 μg/mL). Against Leishmania infantum, the isatin-derived Knoevenagel adducts 2b (R = 5-Cl; R' = H), 2c (R = 5-Br; R' = H), and 2g (R = 5-CH3; R' = N-benzyl) showed IC50 values of 15.05, 18.38, and 12.7 μM (promastigote) and EC50 values of 5.74, 4.64, and 3.8 μM (axenic amastigote), with no cytotoxicity toward human erythrocytes. In PBMCs, they exhibited CC50 values of 25.2–34.41 μM, with SI values of 4.39– 7.42. No isatin-derived Knoevenagel adduct showed antimicrobial activity at the tested concentrations (2–1024 μg/mL). In the sequential synthesis of 3-coumarinocarboxylic acid 3a (R = H), commercial LWC was the most efficient catalyst in water or ethanol:water (3:7, v/v) at 75 °C, providing the product in short reaction times and good yields. With substituted salicylaldehydes, new coumarin derivatives (3b–i) were obtained in 10–45 min with yields of 77–88%, and up to four catalyst reuse cycles. Acids 3c–e, 3h, and 3i inhibited microbial growth from 1024 μg/mL, with compound 3d (R = 5-Br) being considered bactericidal and fungicidal. Xanthene (4a–g) and tetraketone (5a–g) derivatives were obtained through sequential reactions of aromatic aldehydes and dimedone (1:2 equivalents) in water at 75 °C using commercial LWC. The processes proved simple, sustainable, and efficient. Xanthene derivatives were obtained in 10–60 min (73–93%) and tetraketone in 10–20 min (82–94%), maintaining their structure and catalytic activity after six recycling cycles. In the biological assays, xanthene derivatives showed greater cytotoxicity than tetraketone. Compound 4f (R = 4-OH) was the most active (>94% inhibition) against chronic myeloid leukemia (K-562), human colon cancer (HCT-116), and lung carcinoma (A-549) cell lines, with IC50 values of 12.08, 6.61, and 5.54 μM, respectively, but exhibited high cytotoxicity toward non-tumor cells (MRC-5, 96.07%), resulting in a low SI. Nevertheless, it stands out as a lead compound for future modifications aiming at improved selectivity.