Synthesis and Biological Activity of Usnic Acid Enamine Derivatives

Bioactivities and Structure-Activity Relationships of Usnic Acid Derivatives: A Review

Usnic acid has a variety of biological activities, and has been widely studied in the fields of antibacterial, immune stimulation, antiviral, antifungal, anti-inflammatory and antiparasitic. Based on this, usnic acid is used as the lead compound for structural modification. In order to enhance the biological activity and solubility of usnic acid, scholars have carried out a large number of structural modifications, and found some usnic acid derivatives to be of more potential research value. In this paper, the structural modification, biological activity and structure-activity relationship of usnic acid were reviewed to provide reference for the development of usnic acid derivatives.

Antibacterial quaternary ammonium agents: Chemical diversity and biological mechanism

Bacterial infections have seriously threatened public health especially with the increasing resistance and the cliff-like decline of the number of newly approved antibacterial agents. Quaternary ammonium compounds (QACs) possess potent medicinal properties with 95 successfully marketed drugs, which also have a long history as antibacterial agents. In this review, we summarize the chemical diversity of antibacterial QACs, divided into chain-like and aromatic ring, reported over the past decade (2012 to mid-2022). Additionally, the structure-activity relationships, mainly covering hydrophobicity, charges and skeleton features, are discussed. In the cases where sufficient information is available, antibacterial mechanisms including biofilm, cell membrane, and intracellular targets are presented. It is hoped that this review will provide sufficient information for medicinal chemists to discover the new generation of antibacterial agents based on QACs.

Microwave-Assisted Extraction and HPLC-UV-CD Determination of (S)-usnic Acid in Cladonia foliacea

During the years, many usnic acid (UA) conjugates have been synthesized to obtain potent endowed with biological properties. Since (S)-UA is less abundant in nature than (R)-enantiomer, it is difficult to source, thus precluding a deeper investigation. Among the lichens producing UA, Cladonia foliacea is a valuable (S)-UA source. In the present work, we report on a rapid HPLC-UV/PAD-CD protocol suitable for the analysis and the identification of the main secondary metabolites present in C. foliacea extract. Best results were achieved using XBridge Phenyl column and acetonitrile and water, which were both added with formic acid as mobile phase in gradient elution. By combining analytical, spectroscopical, and chiroptical analysis, the most abundant analyte was unambiguously identified as (S)-UA. Accordingly, a versatile microwave-assisted extractive (MAE) protocol, assisted by a design of experiment (DoE), to quantitatively recover (S)-UA was set up. The best result in terms of UA extraction yield was obtained using ethanol and heating at 80 °C under microwave irradiation for 5 min. Starting from 100 g of dried C. foliacea, 420 mg of (S)-UA were achieved. Thus, our extraction method resulted in a suitable protocol to produce (S)-UA from C. foliacea for biological and pharmaceutical investigation or commercial purposes.

Synthesis and antiproliferative activity of novel (+)- usnic acid analogues

Twenty one novel (+)- usnic acid-based analogues belonging to three classes such as enamines, imines, and pyrazoles were synthesized. All the synthesized compounds were characterized by their spectral data (¹H NMR, ¹³C NMR, IR, and HRMS). The synthesized compounds were evaluated for their antiproliferative activity against a panel of four human cancer cell lines including HeLa (cervix), MDA-MB-231 (breast), A549 (lung), and MiaPaca (pancreas) by employing SRB cell proliferation assay. Screening results indicated that all synthesized compounds showed enhanced activity than the parent compound. Most significantly, compounds 2e and 4a showed potent antiproliferative activity against all the cancer cell lines tested. Compounds 2e and 4a arrested the cell cycle in G2/M phase and induced apoptosis in HeLa cells. In view of significant antiproliferative activity, compounds 2e and 4a can be considered as lead molecules for further development.[Formula: see text].

Derivatives of usnic acid cause cytostatic effect in Caco-2 cells

Usnic acid has anti-cancer activity, however, low solubility and toxicity limit the potential. To investigate biological activity of usnic acid derivatives, enantiopure derivatives were synthesised by reacting usnic acid with ethylenediamine, which yielded one dimer product ((+)-1), and two tetra cyclic compounds ((+)-2 and (-)-2). The products were characterised with NMR, and evaluated in vitro in human colon cancer cell line Caco-2 by cell count, phase-contrast microscopy, MTT-assay, measurement of DNA content and cell cycle distribution. All compounds tested showed cytostatic effect in Caco-2 cells, but each compound had a distinct cellular effect. Compound (+)-1 showed anti-proliferative activity by increasing the percentage of cells in S-phase with 25% compared to the control. Compounds (+)-2 and (-)-2 induced paraptosis, but only compound (+)-2 modulated cell cycle distribution by accumulating cells in G₂/M-phase by 47% and reduced DNA content by 60%. All compounds express interesting cellular and potential anti-proliferative activity.

Synthesis and cytotoxic evaluation of usnic acid benzylidene derivatives as potential anticancer agents

A series of usnic acid benzylidene derivatives (groups I-V) were designed, synthesized and evaluated for their anticancer activity in the search for potentially new anticancer agents. Compounds 1a, 5b, 2b, 2e and 2f exhibited the most potent cytotoxcity against K562 cell line with IC₅₀ values of 10.0 ± 3.6, 5.6 ± 0.4, 8.8 ± 1.0, 4.5 ± 0.1 and 8.4 ± 0.4 μM, respectively. It is noteworthy that compound 2e displayed potent cytotoxicity against K562 cells without any cytotoxic effect on HEK293 normal cell line.

Synthesis and antimycobacterial activity of (+)-usnic acid conjugates

New therapeutics are urgently needed to fight tuberculosis and mycobacteria-related diseases that are a major health hazard especially in poor countries. Natural products have been the source of important antitubercular drugs in the past and still need to receive attention as a potent reservoir of chemical structures. Fifteen known and two new (+)-usnic acid (a benzofurandione formerly isolated from lichens) enamines and hydrazones are here described and tested against sensitive and multidrug-resistant strains of mycobacteria. Among several (+)-usnic acid conjugates, PS14 and PS18 showed potent activity against both susceptible and resistant Mycobacterium tuberculosis strains (MIC values of 1-32 and 2-32 mg/L, respectively) comparable with MIC of other antitubercular drugs already in use for tuberculosis treatment.

Structure of a new usnic acid derivative from a deacylating Mannich reaction: NMR studies supported by theoretical calculations of chemical shifts

In a conventional Mannich reaction using piperidine, hydroxypiperidines, morpholine, and N-methylpiperazine with usnic acid, a deacetylation was observed resulting in a substitution at C-2, a loss of an acetyl group, and a Mannich base with a stabilized enol. The enol has a hydrogen bond to the nitrogen of the secondary amine. The structure was investigated by nuclear magnetic resonance and deuterium isotope effects on ¹³ C chemical shifts as well as with density functional theory calculations to study the changed hydrogen bond pattern. It was found that the hydrogen bond involving the OH-9 group in chloroform forms a strong hydrogen bond than in usnic acid itself and that this hydrogen bond becomes even stronger in the more polar solvent, dimethylsulfoxide. Tautomerism was observed in the Mannich base as demonstrated by deuterium isotope effects on chemical shifts. The position of the tautomeric equilibrium depends on the solvent, and the position of the equilibrium governs the strength of the OH-9…O═C hydrogen bond.

Synthesis of acyl-hydrazone from usnic acid and isoniazid and its anti-Mycobacterium tuberculosis activity

Se evaluó el ácido úsnico (1), aislado del liquen Evernia prunastri (Cajamarca-Perú), producto natural conocido por sus actividades biológicas, y, del mismo modo, se evaluó la síntesis de su derivado acil-hidrazona (2), obtenido a partir de una reacción de condensación entre el ácido úsnico y la isoniazida en solución etanólica a reflujo, con un rendimiento global de 95%. Ambos compuestos fueron evaluados y comparados con la isoniazida según su actividad anti-Mycobacterium tuberculosis basada en el ensayo de susceptibilidad mediante el método TEMA. Los resultados mostraron que el compuesto 1 presenta valores de MIC de 16,0 μg/mL frente a las cepas H37Rv, TB DM 97 y MDR DM 1098, mientras que el compuesto 2 presenta valores de MIC de 2,0; 64,0 y 64,0 μg/mL respectivamente.

Tyrosyl-DNA Phosphodiesterase 1 Inhibitors: Usnic Acid Enamines Enhance the Cytotoxic Effect of Camptothecin

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a repair enzyme for stalled DNA-topoisomerase 1 (Top1) cleavage complexes and other 3'-end DNA lesions. TDP1 is a perspective target for anticancer therapy based on Top1-poison-mediated DNA damage. Several novel usnic acid derivatives with an enamine moiety have been synthesized and tested as inhibitors of TDP1. The enamines of usnic acid showed IC₅₀ values in the range of 0.16 to 2.0 μM. These compounds revealed moderate cytotoxicity against human tumor MCF-7 cells. These new compounds enhanced the cytotoxicity of the established Top1 poison camptothecin by an order of magnitude.