Natural-Antioxidant-Inspired Benzo[b]selenophenes: Synthesis, Redox Properties, and Antiproliferative Activity

Recent Advances in the Synthesis and Antioxidant Activity of Low Molecular Mass Organoselenium Molecules

Selenium is an essential trace element in living organisms, and is present in selenoenzymes with antioxidant activity, like glutathione peroxidase (GPx) and thioredoxin reductase (TrxR). The search for small selenium-containing molecules that mimic selenoenzymes is a strong field of research in organic and medicinal chemistry. In this review, we review the synthesis and bioassays of new and known organoselenium compounds with antioxidant activity, covering the last five years. A detailed description of the synthetic procedures and the performed in vitro and in vivo bioassays is presented, highlighting the most active compounds in each series.

Palladium-Catalyzed Direct Selanylation of Chalcogenophenes and Arenes Assisted by 2-(Methylthio)amide

The direct and selective conversion of a C-H bond into a C-Se bond remains a significant challenge, which is even more intricate with substrates having an innate regioselectivity under several reaction conditions, such as chalcogenophenes. We overrode their selectivity toward selanylation using palladium, copper, and the 2-(methylthio)amide directing group. This chelation-assisted direct selanylation was also suitable for mono and double ortho functionalization of arenes. The mechanistic studies indicate high-valent Pd(IV) species in the catalytic cycle, a reversible C-H activation step, and Cu(II) as a sequestering agent for organoselenide byproducts.

Crystal structures of 3-halo-2-organochalcogenylbenzo[b]chalcogenophenes

The structure of the title compounds 3-bromo-2-(phenyl-sulfan-yl)benzo[b]thiophene (C14H9BrS2; 1), 3-iodo-2-(phenyl-sulfan-yl)benzo[b]thio-phene (C14H9IS2; 2), 3-bromo-2-(phenyl-selan-yl)benzo[b]seleno-phene (C14H9BrSe2; 3), and 3-iodo-2-(phenyl-selan-yl)benzo[b]seleno-phene (C14H9ISe2; 4) were determined by single-crystal X-ray diffraction; all structures presented monoclinic (P21/c) symmetry. The phenyl group is distant from the halogen atom to minimize the steric hindrance repulsion for all structures. Moreover, the structures of 3 and 4 show an almost linear alignment of halogen-selenium-carbon atoms arising from the intra-molecular orbital inter-action between a lone pair of electrons on the halogen atom and the anti-bonding σ*Se-C orbital (n halogen→σ*Se-C). This inter-action leads to significant differences in the three-dimensional packing of the mol-ecules, which are assembled through π-π and C-H⋯π inter-actions. These data provide a better comprehension of the inter-molecular packing in benzo[b]chalcogenophenes, which is relevant for optoelectronic applications.

Recent Advances in the Synthesis of Selenophenes and Their Derivatives

The selenophene derivatives are an important class of selenium-based heterocyclics. These compounds play an important role in prospecting new drugs, as well as in the development of new light-emitting materials. During the last years, several methods have been emerging to access the selenophene scaffold, employing a diversity of cyclization-based synthetic strategies, involving specific reaction partners and particularities. This review presents a comprehensive discussion on the recent advances in the synthesis of selenophene-based compounds, starting from different precursors, highlighting the main differences, the advantages, and limitations among them.

Synthesis of 2-Organylchalcogenopheno[2,3-b]pyridines from Elemental Chalcogen and NaBH4 /PEG-400 as a Reducing System: Antioxidant and Antinociceptive Properties

An alternative method to prepare 2-organylchalcogenopheno[2,3-b]pyridines was developed by the insertion of chalcogen species (selenium, sulfur or tellurium), generated in situ, into 2-chloro-3-(organylethynyl)pyridines by using the NaBH₄ /PEG-400 reducing system, followed by an intramolecular cyclization. It was possible to obtain a series of compounds with up to 93 % yield in short reaction times. Among the synthesized products, 2-organyltelluropheno[2,3-b]pyridines have not been described in the literature so far. Moreover, the compounds 2-phenylthieno[2,3-b]pyridine (3 b) and 2-phenyltelluropheno[2,3-b]pyridine (3 c) exhibited significant antioxidant potential in different in vitro assays. Further studies demonstrated that compound 3 b exerted an antinociceptive effect in acute inflammatory and non-inflammatory pain models, thus indicating the involvement of the central and peripheral nervous systems on its pharmacological action. More specifically, our results suggest that the intrinsic antioxidant property of compound 3 b might contribute to attenuating the nociception and inflammatory process on local injury induced by complete Freund's adjuvant (CFA).

Chain Breaking Antioxidant Activity of Heavy (S, Se, Te) Chalcogens Substituted Polyphenols

Polyphenols are probably the most important family of natural and synthetic chain-breaking antioxidants. Since long ago, chemists have studied how structural (bioinspired) modifications can improve the antioxidant activity of these compounds in terms of reaction rate with radical reactive oxygen species (ROS), catalytic character, multi-defence action, hydrophilicity/lipophilicity, biodistribution etc. In this framework, we will discuss the effect played on the overall antioxidant profile by the insertion of heavy chalcogens (S, Se and Te) in the phenolic skeleton.

Synthesis of benzo[d]imidazo[2,1-b]benzoselenoazoles: Cs2CO3-mediated cyclization of 1-(2-bromoaryl)benzimidazoles with selenium

The synthesis of benzimidazo[2,1-b]benzoselenoazoles is described. The novel ring-closure reaction of 1-(2-bromoaryl)benzimidazoles with Se powder is promoted by Cs₂CO₃ (2 equiv) in DMF at 150 °C. Moreover, the obtained tetracyclic heterocycles are all novel compounds. Single-crystal X-ray analysis of the parent benzimidazo[2,1-b]benzoselenoazole revealed that the tetracyclic ring is almost planar. Absorption spectroscopy data of the benzimidazo[2,1-b]benzoselenoazoles showed the λ_max was dependent on the number of rings.

A Three-Component Strategy for Benzoselenophene Synthesis under Metal-Free Conditions Using Selenium Powder

An efficient three-component benzoselenophenes formation has been developed from substituted indoles, acetophenones, and selenium powder under metal-free conditions. 2-Aryl indoles played an important role to promote benzoselenophene formation from acetophenone derivatives and selenium powder. One C-C and two C-Se bonds were selectively formed to provide 40 new benzoselenophenes in good yields.

Palladium-catalyzed successive C–H bond arylations and annulations toward the π-extension of selenophene-containing aromatic skeletons

A modular approach for the synthesis of planar π-extended selenium containing molecules from selenophene has been developed.

Synthesis of 2,3-bis-organochalcogenyl-benzo[b]chalcogenophenes promoted by Oxone®

We report here an alternative and tunable metal-free synthesis of benzo[b]chalcogenophenes via the electrophilic cyclization of 2-functionalized chalcogenoalkynes promoted by Oxone®.

Resveratrol-Inspired Benzo[b]selenophenes Act as Anti-Oxidants in Yeast

Resveratrol is a natural (poly)phenol primarily found in plants protecting them against pathogens, as well as harmful effects of physical and chemical agents. In higher eukaryotic cells and organisms, this compound displays a remarkable range of biological activities, such as anti-oxidant, anti-inflammatory, anti-cancer, anti-aging, cardio- and neuro-protective properties. Here, biological activities of synthetic selenium-containing derivatives of resveratrol—benzo[b]selenophenes—have been studied in lower eukaryotes Saccharomyces cerevisiae. Their toxicity, as well as DNA damaging and reactive oxygen species (ROS) inducing potencies, manifested through their ability to act as redox active anti-microbial agents, have been examined. We show that some benzo[b]selenophenes can kill yeast cells and that the killing effects are not mediated by DNA damage types that can be detected as DNA double-strand breaks. These benzo[b]selenophenes could potentially be used as anti-fungal agents, although their concentrations relevant to application in humans need to be further evaluated. In addition, most of the studied benzo[b]selenophenes display redox-modulating/anti-oxidant activity (comparable or even higher than that of resveratrol or Trolox) causing a decrease in the intracellular ROS levels in yeast cells. Therefore, after careful re-evaluation in other biological systems these observations might be transferred to humans, where resveratrol-inspired benzo[b]selenophenes could be used as supra-anti-oxidant supplements.