Hydnocarpin-Type Flavonolignans: Semisynthesis and Inhibitory Effects on <i>Staphylococcus aureus</i> Biofilm Formation

A new, efficient, and general semisynthesis of hydnocarpin-type flavonolignans was developed and optimized, enabling gram-scale production of hydnocarpin D (<b>2</b>). Moreover, the syntheses of optically pure hydnocarpin isomers [(10<i>R</i>,11<i>R</i>)-hydnocarpin (<b>1a</b>), (10<i>R</i>,11<i>R</i>)-hydnocarpin D (<b>2a</b>), and (10<i>S</i>,11<i>S</i>)-hydnocarpin D (<b>2b</b>)], as well as the synthesis of isohydnocarpin (<b>8</b>), were achieved for the first time utilizing this new method. The synthesis is based on the two-step transformation of the readily available flavonolignans from milk thistle (<i>Silybum marianum</i>), accessible by isolation from the commercial extract silymarin. The first step relies on the regioselective formylation of the C-3 hydroxy group of the dihydroflavonol-type precursor using the Vilsmeier–Haack reagent, followed by formic acid elimination by triethylamine in the second step. The synthesized compounds were effective inhibitors of <i>Staphylococcus aureus</i> biofilm formation, with (10<i>S</i>,11<i>S</i>)-hydnocarpin D (<b>2b</b>) being the most potent inhibitor. Furthermore, the effect of glucose on biofilm formation was tested, and glucose decreased the biofilm inhibitory activity of <b>2b</b>. Moreover, <b>2b</b> increased the susceptibility of <i>Staph. aureus</i> to enrofloxacin

A Novel Semisynthetic Flavonoid 7‑<i>O</i>‑Galloyltaxifolin Upregulates Heme Oxygenase‑1 in RAW264.7 Cells via MAPK/Nrf2 Pathway

Quercetin and gallic acid are natural activators of the transcription factor Nrf2, which regulates the expression of many cytoprotective enzymes including heme oxygenase-1 (HO-1). We developed procedures for the synthesis of monogalloyl esters of quercetin and taxifolin (dihydroquercetin), namely, 3-<i>O</i>-galloylquercetin and 7-<i>O</i>-galloyltaxifolin, and examined their effect on the Nrf2 pathway in RAW264.7 cells. Unlike quercetin and free gallic acid, 3-<i>O</i>-galloylquercetin and natural quercetin derivatives isoquercitrin (quercetin-3-<i>O</i>-β-d-glucoside) and taxifolin had no effect on the expression of HO-1. In contrast, 7-<i>O</i>-galloyltaxifolin increased both mRNA and protein levels of HO-1 at concentrations of 25 μM and above. The induction of HO-1 by 7-<i>O</i>-galloyltaxifolin was primarily associated with the production of reactive oxygen species and phosphorylation of mitogen-activated protein kinases (MAPKs), including p38 MAPKs and ERKs, followed by nuclear accumulation of Nrf2 and downregulation of Keap1, a negative regulator of Nrf2. We conclude that 7-<i>O</i>-galloyltaxifolin upregulates HO-1 via activation of the MAPK/Nrf2 signaling pathway

Synthesis and Antiangiogenic Activity of New Silybin Galloyl Esters

The synthesis of various silybin monogalloyl esters was developed, and their antiangiogenic activities were evaluated in a variety of in vitro tests with human umbilical vein endothelial cells (HUVECs). A structure–activity relationship (SAR) study found the regioselectivity of the silybin galloylation to be highly significant. Silybin (as an equimolar mixture of two diastereomers A and B) exhibited quite poor antiangiogenic activities, whereas its B stereoisomer is more active than silybin A. The galloylation of phenolic OH groups of natural silybin (a mixture of both isomers) leads to increases in their antiangiogenic activities, which is more apparent with the 7-OH than the 20-OH. In contrast, gallates at aliphatic OH groups either had a comparable activity to the parent compound or are even worse than silybin, which was observed in the case of 3-<i>O</i>-galloylsilybin. The most effective compound from this series (7-<i>O</i>-galloylsilybin) has also been prepared from stereochemically pure silybins A and B to evaluate the effect of stereochemistry on the activity. As with silybin itself, the B isomer of 7-<i>O</i>-galloylsilybin was more active than the A isomer

Effects of 2,3-Dehydrosilybin and Its Galloyl Ester and Methyl Ether Derivatives on Human Umbilical Vein Endothelial Cells

The effects in vitro of 2,3-dehydrosilybin and several galloyl esters and methyl ethers on the viability, proliferation, and migration of human umbilical vein endothelial cells (HUVECs) were evaluated. The monogalloyl esters were synthesized by a chemoselective esterification method or by Steglich esterification of suitably protected 2,3-dehydrosilybin (<b>1</b>) with protected gallic acid. 2,3-Dehydrosilybin (<b>1</b>) displayed more potent cytotoxic, antiproliferative, and antimigratory activities (IC₅₀ 12.0, 5.4, and 12.2 μM, respectively) than silybin. The methylated derivatives were less active, with the least potent being 3,7-di-<i>O</i>-methyl-2,3-dehydrosilybin (<b>6</b>). On the other hand, galloylation at C-7 OH and C-23 OH markedly increased the cytotoxicity and the effects on the proliferation and migration of HUVECs. The most active derivative was 7-<i>O</i>-galloyl-2,3-dehydrosilybin (<b>13</b>; IC₅₀ value of 3.4, 1.6, and 4.7 μM in the cytotoxicity, inhibition of proliferation, and antimigratory assays, respectively). Overall, this preliminary structure–activity relationship study demonstrated the importance of a 2,3-double bond, a C-7 OH group, and a galloyl moiety in enhancing the activity of flavonolignans toward HUVECs