Two New Benzoquinone Derivatives from Vietnamese Knema globularia Stems.

The chemical investigation of the stems of Knema globularia led to the isolation of two new benzoquinones derivatives, embenones A and B (1 and 2), along with three known compounds (3-5). The structures of the isolated compounds were determined using spectroscopic techniques, including HRESIMS, 1D and 2D NMR, in conjunction with comparison to existing literature data. Compounds 1 and 2 represent new carbon skeletons in nature. Furthermore, all isolated compounds were evaluated for their α-glucosidase inhibitory activity, with compounds 1-3 exhibiting superior potency relative to the positive control (acarbose, IC50 331 µM). Their IC50 values ranged from 1.40 to 96.1 µM.

New Halogenated Flavonoids from Adenosma bracteosum and Vitex negundo and Their α-Glucosidase Inhibition.

Adenosma bracteosum and Vitex negundo are natural sources of methoxylated flavonoids. Little is known about the α-glucosidase inhibition of multi-methoxylated flavonoid derivatives. Eighteen natural flavonoids were isolated from A. bracteosum and V. negundo. Seven halogenated derivatives were synthesized. Their chemical structures were elucidated by extensive NMR analysis and high-resolution mass spectroscopy as well as comparisons in literature. All compounds were evaluated for their α-glucosidase inhibition. Most compounds showed good activity with IC50 values ranging from 16.7 to 421.8 µM. 6,8-Dibromocatechin was the most active compound with an IC50 value of 16.7 µM. A molecular docking study was conducted, indicating that those compounds are potent α-glucosidase inhibitors.

A New Carvotacetone Derivative from the Aerial Part of Sphaeranthus africanus.

Sphaeranthus africanus L. is native in Vietnam. Little is known about α-glucosidase inhibition of Sphaeranthus africanus and its isolated compounds. A bioactive-guided isolation was applied to the Vietnamese Sphaeranthus africanus to find α-glucosidase inhibitory components. Eight compounds were detected and structurally elucidated. They are 3-angeloyloxy-5-[2'',3''-epoxy-2''-methylbutanoyloxy]-7-hydroxycarvotacetone, 3-angeloyloxy-5-[3''-chloro-2''-hydroxy-2''-methylbutanoyloxy]-7-hydroxycarvotacetone, 3-angeloyloxy-5-[2''R,3''R-dihydroxy-2''-methyl-butanoyloxy]-7-hydroxycarvotacetone, 3-angeloyloxy-5-[2''S,3''R-dihydroxy-2''-methylbutanoyloxy]-7-hydroxycarvotacetone, 3-angeloyloxy-5-[2''S,3''S-dihydroxy-2''-methylbutanoyloxy]-7-hydroxycarvotacetone, 5-angeloyloxy-7-hydroxy-3-tigloyloxycarvotacetone, 3-O-methylquercetin, and chrysosplenol D. Their chemical structures were elucidated by extensive 1D and 2D NMR analysis and high-resolution mass spectroscopy as well as comparisons in literature. 3-Angeloyloxy-5-[2''S,3''S-dihydroxy-2''-methylbutanoyloxy]-7-hydroxycarvotacetone is a new compound. Isolated compounds were evaluated for the α-glucosidase inhibition. Isolated compounds showed moderate activity with IC50 values ranging from 128.9-274.3 µM while others are weak. A molecular docking study was conducted, indicating that isolated compounds are potent α-glucosidase inhibitory compounds.

α-Glucosidase Inhibitory Cycloartanes from the Vietnamese Combretum quadrangulare.

Leaves of Combretum quadrangulare Kurz showed potent α-glucosidase inhibition. Two new cycloartane-type triterpenes, combretic acids D and E were isolated from the bioactive fraction. The chemical structures were determined using NMR and MS methods. Combretic acid D represents for the first cycloartane having a dihydrofuran ring in the side chain. Combretic acids D and E showed significant α-glucosidase inhibition, with IC50 values of 13.9 and 30.7 µM, respectively. Combretic acid D was determined to be a non-competitive type in the kinetic study. The docking study in combination with dynamic simulations of this compound provided the molecular understanding of α-glucosidase inhibition.

Alpha-glucosidase inhibitors from Nervilia concolor, Tecoma stans, and Bouea macrophylla.

Tecoma stans (L.) Juss. Ex Kunth is widely used in folk medicine. In ethnomedicine, it is applied as a cardioprotective, hepatoprotective, antiarthritic, antinociceptive, anti-inflammatory, and antimicrobial. The aqueous extract is considered antidiabetic, and is used as a traditional remedy in Mexico. More than 120 chemical constituents have been identified in its leaves, barks, and roots. However, less is known about the phytochemical properties of T. stans flower extracts. The herbal plant Nervilia concolor (Blume) Schltr. is native to Vietnam, and is used in traditional Chinese medicine to treat diseases such as bronchitis, stomatitis, acute pneumonia, and laryngitis. Only two previous reports have addressed the chemical content of this plant. Bouea macrophylla Griff., commonly known as marian plum or plum mango, is a tropical plant that is used to treat a range of illnesses. Phytochemical analysis of B. macrophylla suggests the presence of volatile components and flavonoids. However, existing data have been obtained from screening without isolation. As part of our ongoing search for alpha-glucosidase inhibitors from Vietnamese medicinal plants, we conducted bioactive-guided isolation of the whole plant N. concolor, the flowers of T. stans, and the leaves of B. macrophylla. We isolated and structurally elucidated five known compounds from T. stans: ursolic acid (TS1), 3-oxours-12-en-28-oic acid (TS2), chrysoeriol (TS3), ferulic acid (TS4), and tecomine (TS5). Three known compounds were isolated from Nervilia concolor: astragalin (NC1), isoquercitrin (NC2), and caffeic acid (NC3). From B. macrophylla, betullinic acid (BM1), methyl gallate (BM2), and 3-O-galloyl gallic acid methyl ester (BM3) were isolated. All compounds showed promising alpha-glucosidase inhibition, with IC50 values ranging from 1.4 to 143.3 µM. The kinetics of enzyme inhibition showed BM3 to be a competitive-type inhibitor. An in silico molecular docking model confirmed that compounds NC1, NC2, and BM3 were potential inhibitors of the α-glucosidase enzyme. Molecular dynamics simulations were carried out with compound BM3 demonstrating the best docking model during simulation up to 100 ns to explore the stability of the complex ligand-protein.

Antimelanogenic Activity of Ocotillol-Type Saponins from Panax vietnamensis.

The ocotillol (OCT)-type saponins have been known as a tetracyclic triterpenoid, possessing five- or six-membered epoxy ring in the side chain. Interestingly, this type saponin was mostly found in Panax vietnamensis Ha et Grushv., Araliaceae (VG), hence making VG unique from the other Panax spp. Five OCT-type saponins, majonoside R2, vina-ginsenoside R2, majonoside R1, pseudoginsenoside RT4, vina-ginsenoside R11, together with three protopanaxadiol (PPD)-type saponins and four protopanaxatriol (PPT)-type saponins from VG were evaluated for their antimelanogenic activity. All of isolates were found to be active. More importantly, the five OCT-type saponins inhibited melanin production in B16-F10 mouse melanoma cells, without showing any cytotoxicity. Besides ginsenoside Rd and ginsenoside Rg3 in PPD and notoginsenoside R1 in PPT-type saponins, majonoside R2 was the most potent melanogenesis inhibitory activity in OCT-type saponins. In this article, we highlighted antimelanogenic activity of OCT-type saponins and potential structure-activity relationship (SAR) of ginsenosides. Our results suggested that OCT-type saponins could be used as a depigmentation agent.