Unconventional application of the Mitsunobu reaction: Selective flavonolignan dehydration yielding hydnocarpins

7-O-tyrosyl Silybin Derivatives as a Novel Set of Anti-Prostate Cancer Compounds

Silybin is a natural compound extensively studied for its hepatoprotective, neuroprotective and anticancer properties. Envisioning the enhancement of silybin potential by suitable modifications in its chemical structure, here, a series of new 7-O-alkyl silybins derivatives were synthesized by the Mitsunobu reaction starting from the silybins and tyrosol-based phenols, such as tyrosol (TYR, 3), 3-methoxytyrosol (MTYR, 4), and 3-hydroxytyrosol (HTYR, 5). This research sought to explore the antioxidant and anticancer properties of eighteen new derivatives and their mechanisms. In particular, the antioxidant properties of new derivatives outlined by the DPPH assay showed a very pronounced activity depending on the tyrosyl moiety (HTYR > MTYR >> TYR). A significant contribution of the HTYR moiety was observed for silybins and 2,3-dehydro-silybin-based derivatives. According to the very potent antioxidant activity, 2,3-dehydro-silybin derivatives 15ab, 15a, and 15b exerted the most potent anticancer activity in human prostate cancer PC-3 cells. Furthermore, flow cytometric analysis for cell cycle and apoptosis revealed that 15ab, 15a, and 15b induce strong G1 phase arrest and increase late apoptotic population in PC-3 cells. Additionally, Western blotting for apoptotic marker cleaved caspase-3 confirmed apoptosis induction by these silybin derivatives in PC-3 cells. These findings hold significant importance in the investigation of anticancer properties of silybin derivatives and strongly encourage swift investigation in pre-clinical models and clinical trials.

Enantioselective Total Synthesis of (+)-Pepluanol A

Herein, we report an enantioselective and convergent total synthesis of (+)-pepluanol A, a structurally intriguing Euphorbia diterpenoid natural product featuring a 5/6/7/3-fused tetracyclic skeleton, from known building blocks in 11 steps. The successful strategy relies on a phenyl selenide-mediated Morita-Baylis-Hillman type reaction as a connective step, forging the precursor for the key intramolecular Diels-Alder reaction to construct the congested 5/6/7-tricyclic framework. A diastereoconvergent cascade starting with an acid-induced removal of the C1-MOM protecting group followed by a retro-aldol/aldol reaction resulted in the formation of a single diastereomer. This stereoconvergency allowed for the successful substrate-controlled diastereoselective cyclopropanation of an advanced intermediate to establish the full carboskeleton of (+)-pepluanol A (1).

Chirality Matters: Biological Activity of Optically Pure Silybin and Its Congeners

This review focuses on the specific biological effects of optically pure silymarin flavo-nolignans, mainly silybins A and B, isosilybins A and B, silychristins A and B, and their 2,3-dehydro derivatives. The chirality of these flavonolignans is also discussed in terms of their analysis, preparative separation and chemical reactions. We demonstrated the specific activities of the respective diastereomers of flavonolignans and also the enantiomers of their 2,3-dehydro derivatives in the 3D anisotropic systems typically represented by biological systems. In vivo, silymarin flavonolignans do not act as redox antioxidants, but they play a role as specific ligands of biological targets, according to the "lock-and-key" concept. Estrogenic, antidiabetic, anticancer, antiviral, and antiparasitic effects have been demonstrated in optically pure flavonolignans. Potential application of pure flavonolignans has also been shown in cardiovascular and neurological diseases. Inhibition of drug-metabolizing enzymes and modulation of multidrug resistance activity by these compounds are discussed in detail. The future of "silymarin applications" lies in the use of optically pure components that can be applied directly or used as valuable lead structures, and in the exploration of their true molecular effects.

Inhibitory effect of hydnocarpin D on T-cell acute lymphoblastic leukemia via induction of autophagy-dependent ferroptosis

Hydnocarpin D (HD) is a bioactive flavonolignan compound that possesses promising anti-tumor activity, although the mechanism is not fully understood. Using T cell acute lymphoblastic leukemia (T-ALL) cell lines Jurkat and Molt-4 as model system, we found that HD suppressed T-ALL proliferation in vitro, via induction of cell cycle arrest and subsequent apoptosis. Furthermore, HD increased the LC3-II levels and the formation of autophagolysosome vacuoles, both of which are markers for autophagy. The inhibition of autophagy by either knockdown of ATG5/7 or pre-treatment of 3-MA partially rescued HD-induced apoptosis, thus suggesting that autophagy enhanced the efficacy of HD. Interestingly, this cytotoxic autophagy triggered ferroptosis, as evidenced by the accumulation of lipid ROS and decrease of GSH and GPX4, while inhibition of autophagy impeded ferroptotic cell death. Our study suggests that HD triggers multiple cell death processes and is an interesting compound that should be evaluated in future preclinical studies.

Diversity-Oriented Syntheses of β-Substituted α-Amino γ-Lactam Peptide Mimics with Constrained Backbone and Side Chain Residues

α- N-(Fmoc)Amino-γ-lactam dipeptides with a variety of β-substituents were synthesized stereoselectively with minimal β-elimination by routes employing, respectively, Mitsunobu chemistry and cyclic sulfamidate nucleophilic ring opening from trans- and cis-β-hydroxy-α-amino-γ-lactam precursors. This diversity-oriented method provides stereochemically pure dipeptide mimics bearing Cys, Ser, Thr, Dap, Dab, His, and other amino acid residues with constrained backbone and side chain conformations.

Regioselective synthesis of 7-O-esters of the flavonolignan silibinin and SARs lead to compounds with overadditive neuroprotective effects

A series of neuroprotective hybrid compounds was synthesized by conjugation of the flavonolignan silibinin with natural phenolic acids, such as ferulic, cinnamic and syringic acid. Selective 7-O-esterfication without protection groups was achieved by applying the respective acyl chlorides. Sixteen compounds were obtained and SARs were established by evaluating antioxidative properties in the physicochemical FRAP assay, as well as in a cell-based neuroprotection assay using murine hippocampal HT-22 cells. Despite weak activities in the FRAP assay, esters of the α,β-unsaturated acids showed pronounced overadditive effects at low concentrations greatly exceeding the effects of equimolar mixtures of silibinin and the respective acids in the neuroprotection assay. Cinnamic and ferulic acid esters (5a and 6a) also showed overadditive effects regarding inhibition of microglial activation, PC12 cell differentiation, in vitro ischemia as well as anti-aggregating abilities against Aβ42 peptide and τ protein. Remarkably, the esters of ferulic acid with silybin A and silybin B (11a and 11b) showed a moderate but significant difference in both neuroprotection and in their anti-aggregating capacities. The results demonstrate that non-toxic natural antioxidants can be regioselectively connected as esters with medium-term stability exhibiting very pronounced overadditive effects in a portfolio of biological assays.

A chiral template-driven synthesis of a 3'-deoxy-3'-18F-fluorothymidine precursor

An asymmetric synthesis of a 3'-deoxy-3'-¹⁸F-fluorothymidine (¹⁸F-FLT) precursor has been developed wherein the deoxysugar moiety was synthesized using a novel Ga-mediated allylation of (R)-2,3-cyclohexylideneglyceraldehyde as the key step. The synthesis deviates significantly from the previous syntheses of the ¹⁸F-FLT precursors wherein the expensive starting material, thymidine was used.