Synthesis, characterization, and structure of trans-3,4-dihydroxy-1-selenolane {DHS(OH)2} substituted derivatives

Controlling the Redox Catalytic Activity of a Cyclic Selenide Fused to 18-Crown-6 by the Conformational Transition Induced by Coordination to an Alkali Metal Ion

trans-3,4-Dihydroxyselenolane (DHS), a water-soluble cyclic selenide, exhibits selenoenzyme-like unique redox activities through reversible oxidation to the corresponding selenoxide. Previously, we demonstrated that DHS can be applied as an antioxidant against lipid peroxidation and a radioprotector by means of adequate modifications of the two hydroxy (OH) groups. Herein, we synthesized new DHS derivatives with a crown-ether ring fused to the OH groups (DHS-crown-n (n = 4 to 7), 1-4) and investigated their behaviors of complex formation with various alkali metal salts. According to the X-ray structure analysis, it was found that the two oxygen atoms of DHS change the directions from diaxial to diequatorial by complexation. The similar conformational transition was also observed in solution NMR experiments. The ¹H NMR titration in CD₃OD further confirmed that DHS-crown-6 (3) forms stable 1:1 complexes with KI, RbCl and CsCl, while it forms a 2:1 complex with KBPh₄. The results suggested that the 1:1 complex (3·MX) exchanges the metal ion with metal-free 3 through the formation of the 2:1 complex. The redox catalytic activity of 3 was evaluated using a selenoenzyme model reaction between H₂O₂ and dithiothreitol. The activity was significantly reduced in the presence of KCl due to the complex formation. Thus, the redox catalytic activity of DHS could be controlled by the conformational transition induced by coordination to an alkali metal ion.

An Efficient Method for Selective Syntheses of Sodium Selenide and Dialkyl Selenides

The studies on the selective synthesis of dialkyl selenide compounds 1 were presented. Overcoming the complexity and difficulty of selenides (R-Se-R) and/or multiselenides (R-Se_n-R; n ≥ 2), we aimed to optimize the reaction condition for the tolerable preparation of sodium selenide (Na₂Se) by reducing Se with NaBH₄, and then to achieve selective syntheses of dialkyl selenides 1 by subsequently treating the obtained sodium selenide with alkyl halides (RX). Consequently, various dialkyl selenides 1 were efficiently synthesized in good-to-moderate yields. The investigations on reaction pathways and solvent studies were also described.

Synthesis of 4-Selenothreofuranose Derivatives via Pummerer-Type Reactions of trans-3,4-Dioxygenated Tetrahydroselenophenes Mediated by a Selenonium Intermediate

Selenosugars are interesting targets of organic synthesis as they would possess potential biological activities. However, 4-selenotherofuranose derivatives, which have trans configuration for the two dihydroxy substituents at the 2,3-positions and a glycoside bond at the anomeric position, are not available in the current selenosugar library. In this study, racemic 4-selenothreofuranose derivatives were synthesized from trans-3,4-dioxygenated tetrahydroselenophenes in 77-99% yields with the α/β selectivity about 7:3 via oxidation and subsequent seleno-Pummerer rearrangement. The acetoxy group introduced at the anomeric position was then substituted with various nucleophiles, including activated 6-chloropurine, which afforded 4'-selenothreonucleoside derivatives, in the presence of BF₃·OEt₂ or TMSOTf. The stereochemistry of the selenosugar products was established by ¹H NMR spectroscopy as well as X-ray analysis. The similar α/β selectivity observed in the latter glycosylation reaction to that in the former seleno-Pummerer rearrangement suggested the mediation of a common selenonium intermediate (-Se⁺=C<). It was also suggested that an unexpected interaction between the ester protecting group at the 3-position of the selenofuranose ring and the anomeric carbon atom decreases the α/β selectivity.

Mimicking the lipid peroxidation inhibitory activity of phospholipid hydroperoxide glutathione peroxidase (GPx4) by using fatty acid conjugates of a water-soluble selenolane

A series of fatty acid conjugates of trans-3,4-dihydroxy-1-selenolane (DHS) were synthesized by reacting DHS with appropriate acid chlorides. The obtained monoesters were evaluated for their antioxidant capacities by the lipid peroxidation assay using a lecithin/cholesterol liposome as a model system. The observed antioxidant capacities against accumulation of the lipid hydroperoxide (LOOH) increased with increasing the alkyl chain length and became saturated for dodecanoic acid (C₁₂) or higher fatty acid monoesters, for which the capacities were much greater than those of DHS, its tridecanoic acid (C₁₃) diester, and PhSeSePh. On the other hand, the bacteriostatic activity of myristic acid (C₁₄) monoester, evaluated through the colony formation assay using Bacillus subtilis, indicated that it has higher affinity to bacterial cell membranes than parent DHS. Since DHS-fatty acid conjugates would inhibit lipid peroxidation through glutathione peroxidase (GPx)-like 2e⁻ mechanism, higher fatty acid monoesters of DHS can mimic the function of GPx4, which interacts with LOOH to reduce it to harmless alcohol (LOH). Importance of the balance between hydrophilicity and lipophilicity for the design of effective GPx4 mimics was suggested.