Two New α-Glucosidase Inhibitory Depsidones from the Lichen Parmotrema cristiferum (Taylor) Hale.

A bioactivity-guided investigation of the lichen Parmotrema cristiferum (Taylor) Hale (Parmeliaceae) led to the isolation of two new depsidones, cristifones A and B (1 and 2). The structures of the isolated compounds were identified by spectroscopic methods and comparison with the literature data. Compound 1 showed the initial combined structures of depsidone and depside cores. The two isolated compounds were then evaluated for α-glucosidase inhibition. Compounds 1 and 2 were confirmed as potent, with IC50 values of 21.5 and 18.4 µM, respectively. Compound 2 was a non-competitive inhibitor against α-glucosidase, as indicated by the intersect in the second quadrant of each respective plot.

Biological Activities of Lichen-Derived Monoaromatic Compounds.

Lichen-derived monoaromatic compounds are bioactive compounds, associated with various pharmacological properties: antioxidant, antifungal, antiviral, cytotoxicity, and enzyme inhibition. However, little is known about data regarding alpha-glucosidase inhibition and antimicrobial activity. Very few compounds were reported to have these activities. In this paper, a series of monoaromatic compounds from a lichen source were isolated and structurally elucidated. They are 3,5-dihydroxybenzoic acid (1), 3,5-dihydroxybenzoate methyl (2), 3,5-dihydroxy-4-methylbenzoic acid (3), 3,5-dihydroxy-4-methoxylbenzoic acid (4), 3-hydroxyorcinol (5), atranol (6), and methyl hematommate (7). To obtain more derivatives, available compounds from the previous reports such as methyl ß-orsellinate (8), methyl orsellinate (9), and D-montagnetol (10) were selected for bromination. Electrophilic bromination was applied to 8-10 using NaBr/H2O2 reagents to yield products methyl 5-bromo-ß-orsellinate (8a), methyl 3,5-dibromo-orsellinate (9a), 3-bromo-D-montagnetol (10a), and 3,5-dibromo-D-montagnetol (10b). Compounds were evaluated for alpha-glucosidase inhibition and antimicrobial activity against antibiotic-resistant, pathogenic bacteria Enterococcus faecium, Staphylococcus aureus, and Acinetobacter baumannii. Compound 4 showed stronger alpha-glucosidase inhibition than others with an IC50 value of 24.0 µg/mL. Synthetic compound 9a exhibited remarkable activity against Staphylococcus aureus with a MIC value of 4 µg/mL. Molecular docking studies were performed to confirm the consistency between in vitro and in silico studies.

α-Glucosidase inhibitory derivatives of protocetraric acid.

Lichen-derived depsidones have been a successful source for alpha-glucosidase inhibitory agents with numerous advantages. In this article, derivatives of protocetraric acids were designed and synthesised. Diels-Alder reaction, esterification, and Friedel-Crafts alkylation of protocetraric acid with different reagents under Lewis acid were performed. Eleven products were prepared, including 10 new compounds and parmosidone A. Among them, compounds 2-4 and 6 had the novel skeletons. The newly synthetic products were evaluated for alpha-glucosidase inhibition. Among tested compounds, 9 showed the strongest activity, with an IC50 value of 5.9 µM. The molecular docking model indicated the consistency between in vitro and in silico data of alpha-glucosidase inhibition.

Two new phenolic compounds from Boerhavia erecta collected in Vietnam.

Boerhavia erecta is a tropical plant that is widely used in Asian folk medicine. Little is known about the alpha-glucosidase inhibition and antimicrobial properties of compounds from this plant. In the present study, the phytochemical study of the aerial parts of B. erecta collected in Vietnam was conducted using multiple chromatographic methods. The chemical structures of isolated compounds were identified by comprehensive spectroscopic methods. Two new compounds, berectone C (1) and (E)-tetracosyl 3-(3-hydroxy-4-methoxyphenyl)acrylate (4), together with the known compounds boeravinone C (2), liquiritigenin (3), bis(1H-indol-3-yl)methanone (5), and indole-3-carboxylic acid (6) were isolated and structural elucidated. Compounds 1 and 4 were evaluated for alpha-glucosidase inhibition and antimicrobial activity against antibiotic-resistant, pathogenic bacteria Enterococcus faecium, Staphylococcus aureus, and Acinetobacter baumannii. Compound 1 showed strong inhibition of the alpha-glucosidase enzyme (IC50 43 µg/mL). Only compound 1 exhibited antimicrobial property against A. baumannii, forming an inhibition zone of 11 mm.

Reticulatin, a novel C43-spiroterpenoid from the lichen Parmotrema reticulatum growing in Vietnam.

A novel C43-spiroterpenoid, reticulatin (1), was isolated from the lichen Parmotrema reticulatum (Taylor) M. Choisy (Parmeliaceae). Five previously-reported compounds were also isolated: zeorin (2), leucotylin (3), lupeol (4), betulinic acid (5), and dihydroreynosin (6). The structures were elucidated by 1D, 2D NMR, and HRESIMS spectroscopy and comparison with the literature. We propose that reticulatin is a biosynthetic product of fusicoccadiene and vinapraesorediosic acid A via Diels-Alder addition. Reticulatin is the first C43-spiroterpenoid identified from lichen metabolites. All compounds were evaluated for inhibition of α-glucosidase. Compound 1 showed the most potent inhibition, with an IC50 value of 3.90 µM, much lower than that of the acarbose positive control (IC50 165 µM).

A new diphenyl ether from Parmotrema indicum Hale growing in Vietnam.

Chemical investigation of the lichen Parmotrema indicum Hale led to the isolation of one new diphenyl ether, parmetherine D (1), along with eight known compounds (2-9). The structures were determined by analysis of MS and NMR data and by comparison with the literature. Compounds 1, 2, and 7 were evaluated for α-glucosidase inhibition. Only compound 1 exhibited significant inhibition.[Formula: see text].

Nervione, a new benzofuran derivative from Nervilia concolor.

A new benzofuran derivative, nervione (1), was isolated from Nervilia concolor (Blume) Schltr. (Orchidaceae). Eight previously reported compounds were also isolated: 5,7-dimethoxyflavone (2), 3,5,7-trimethoxyflavone (3), 7-methoxyflavone (4), 3,7-dimethoxy-5-hydroxyflavone (5), tetramethylscutellarein (4',5,6,7-tetramethoxyflavone) (6), 5,7-dimethoxy-4'-hydroxyflavone (7), rhamnetin (8), and 5,7-dihydroxy-3',4'-dimethoxyflavone (9). The structures were elucidated by 1D, 2D NMR, and HRESIMS spectroscopy in addition to the literature. The relative configuration of 1 was defined using DP4+ probability while its absolute configuration was defined by comparison of the ECD spectrum of 1 with those of previously reported compounds. All isolated compounds were evaluated for alpha-glucosidase inhibition, revealing weak or no activity.[Formula: see text].