Chemistry of biologically important synthetic organoselenium compounds

3-chymotrypsin-like protease in SARS-CoV-2

Coronaviruses constitute a significant threat to the human population. Severe acute respiratory syndrome coronavirus-2, SARS-CoV-2, is a highly pathogenic human coronavirus that has caused the coronavirus disease 2019 (COVID-19) pandemic. It has led to a global viral outbreak with an exceptional spread and a high death toll, highlighting the need for effective antiviral strategies. 3-Chymotrypsin-like protease (3CLpro), the main protease in SARS-CoV-2, plays an indispensable role in the SARS-CoV-2 viral life cycle by cleaving the viral polyprotein to produce 11 individual non-structural proteins necessary for viral replication. 3CLpro is one of two proteases that function to produce new viral particles. It is a highly conserved cysteine protease with identical structural folds in all known human coronaviruses. Inhibitors binding with high affinity to 3CLpro will prevent the cleavage of viral polyproteins, thus impeding viral replication. Multiple strategies have been implemented to screen for inhibitors against 3CLpro, including peptide-like and small molecule inhibitors that covalently and non-covalently bind the active site, respectively. In addition, allosteric sites of 3CLpro have been identified to screen for small molecules that could make non-competitive inhibitors of 3CLpro. In essence, this review serves as a comprehensive guide to understanding the structural intricacies and functional dynamics of 3CLpro, emphasizing key findings that elucidate its role as the main protease of SARS-CoV-2. Notably, the review is a critical resource in recognizing the advancements in identifying and developing 3CLpro inhibitors as effective antiviral strategies against COVID-19, some of which are already approved for clinical use in COVID-19 patients.

Practice of green chemistry strategies in synthetic organic chemistry: a glimpse of our sincere efforts in green chemistry research

This feature article summarises our recent contributions (2019-2023) in designing and developing a handful of promising organic transformations for accessing several diversely functionalised biologically relevant organic scaffolds, following the green chemistry principles, particularly focusing on the application of low-energy visible light, electrochemistry, ball-milling, ultrasound, and catalyst- and additive-free synthetic strategies.

Visible-light induced selenocyclization of 2-ethynylanilines under ambient conditions: simple FeBr3 as a dual-functional catalyst

We report herein a visible-light induced, Fe-catalyzed selenocyclization of 2-ethynylanilines with diselenides under ambient conditions, employing ethyl acetate as a benign solvent with no stoichiometric additive required. The simple iron salt FeBr3 serves as both a photo-induced LMCT (Ligand-to-Metal Charge Transfer) catalyst and a Lewis acid catalyst to promote the desired transformation in a sustainable manner, enabling the facile synthesis of diverse 3-selenylindoles with extended substitution patterns. Moreover, gram-scale reactions and late-stage functionalization of bioactive molecules further highlight the synthetic practicality of this method.

Electrochemical selenylative ipso-annulation of N-benzylacrylamides to construct seleno-azaspiro[4.5]decadienones

Herein, we present the electrochemical synthesis of selenylated azaspiro[4.5]decadienones through domino selenylation/ipso-annulation of N-benzylacrylamides with diselenides. The method showed a wide substrate scope under mild and external oxidant-free reaction conditions, involving the construction of C-Se and C-C bonds. Gram-scale synthesis and further functional group conversion of the product are also demonstrated.

Bismuth(III)-Catalyzed Regioselective Selenation of Indoles with Diaryl Diselenides: Synthesis of 3-Selanylindoles

Heterocyclic aryl selenides have recently attracted considerable research interest owing to their applications in biological and pharmaceutical fields. Herein, we describe a simple and general synthesis of 3-selanylindoles via a novel regioselective C-H selenation of indoles using a bismuth reagent as a catalyst. The reactions of indoles with diselenides in the presence of 10 mol% BiI3 at 100 °C in DMF afforded the corresponding 3-selanylindoles in moderate-to-excellent yields. The reaction proceeded efficiently under aerobic conditions by adding only a catalytic amount of BiI3, which was non-hygroscopic and less toxic, and both selanyl groups of the diselenide were transferred to the desired products.

Interplay between diphenyl diselenide and copper: Impact on D. melanogaster survival, behavior, and biochemical parameters

Copper (Cu2+) is a biologically essential element that participates in numerous physiological processes. However, elevated concentrations of copper have been associated with cellular oxidative stress and neurodegenerative diseases. Organo‑selenium compounds such as diphenyl diselenide (DPDS) have in vitro and in vivo antioxidant properties. Hence, we hypothesized that DPDS may modulate the toxicity of Cu2+ in Drosophila melanogaster. The acute effects (4 days of exposure) caused by a high concentration of Cu2+ (3 mM) were studied using endpoints of toxicity such as survival and behavior in D. melanogaster. The potential protective effect of low concentration of DPDS (20 μM) against Cu2+ was also investigated. Adult flies aged 1-5 days post-eclosion (both sexes) were divided into four groups: Control, DPDS (20 μM), CuSO4 (3 mM), and the combined exposure of DPDS (20 μM) and CuSO4 (3 mM). Survival, biochemical, and behavioral parameters were determined. Co-exposure of DPDS and CuSO4 increased acetylcholinesterase (AChE) activity and the generation of reactive oxygen species (ROS as determined by DFCH oxidation). Contrary to our expectation, the co-exposure reduced survival, body weight, locomotion, catalase activity, and cell viability in relation to control group. Taken together, DPDS potentiated the Cu2+ toxicity.

Regioselective 1,4-/1,3-Difunctionalization of 1,3-Enynes with Selenosulfonates in Water

1,4-/1,3-Regioselective bifunctionalization of 1,3-enynes with selenosulfonates in water under catalyst-free conditions for the construction of sulfonyl allene and 1,3-disulfonyl-conjugated dienes respectively have been developed. The reactions feature mild reaction conditions in aqueous solution and remarkable regioselectivity controlled by substrates.

Construction of 5-Amino-1,2-Selenazole Scaffolds through N-Selenocyanation/Cyclization of Enaminones Using KSeCN

A metal-free and mild approach for constructing 5-amino-1,2-selenazole skeletons by NBS/KSeCN-mediated N-selenocyanation and nucleophilic cyclization of β-enaminones has been developed. Various isoselenazole compounds and the isoselenazolyl derivatives of anti-inflammatory medicines, including isosepac, oxaprozin, and ibuprofen, have been obtained with good yields. This efficient, "one-pot", and atomic economy strategy may represent an alternative route for the construction of a 1,2-selenazole framework via the "+SeCN" pathway and provide new access to heterocycles containing a Se-N bond.

Aminoselenation and Dehydroaromatization of Cyclohexanones with Anilines and Diselenides

A three-component cascade reaction involving cyclohexanones, anilines, and diaryl diselenides under metal-free conditions is reported. The ortho-selenation of cyclohexanones with diaryl diselenides, followed by sequential dehydroaromatization with anilines, enables the preparation of a variety of o-selanyl anilines in moderate to excellent yields. This innovative transformation is notable for its excellent tolerance of functional groups and is suitable for the late-stage modification of complex pharmaceuticals.

Competing electrophilic substitution and oxidative polymerization of arylamines with selenium dioxide

This article describes the detailed analysis of the reaction between arylamines, such as aniline, o-anisidine, and methyl anthranilate, with selenium dioxide in acetonitrile. A systematic analysis of the reaction products with the help of 77Se NMR and single-crystal X-ray crystallography revealed that the reaction progress follows three major reaction pathways, electrophilic selenation, oxidative polymerization, and solvent oxidation. For aniline and o-anisidine, predominant oxidative polymerization occurred, leading to the formation of the respective polyaniline polymers as major products. For methyl anthranilate, the oxidative polymerization was suppressed due to the delocalization of amine lone pair electrons over the adjacent carboxylate function, which prompted the selenation pathway, leading to the formation of two of the isomeric diorganyl selenides of methyl anthranilate. The diaryl selenides were structurally characterized using single-crystal X-ray diffraction. Density functional theory calculations suggest that the highest occupied molecular orbital of methyl anthranilate was deeply buried, which suppressed the oxidative polymerization pathway. Due to solvent oxidation, oxamide formation was also noticed to a considerable extent. This study provides that utmost care must be exercised while using SeO2 as an electrophile source in aromatic electrophilic substitution reactions.

Selenization of Small Molecule Drugs: A New Player on the Board

There is an urgent need to develop safer and more effective modalities for the treatment of a wide range of pathologies due to the increasing rates of drug resistance, undesired side effects, poor clinical outcomes, etc. Throughout the years, selenium (Se) has attracted a great deal of attention due to its important role in human health. Besides, a growing body of work has unveiled that the inclusion of Se motifs into a great number of molecules is a promising strategy for obtaining novel therapeutic agents. In the current Perspective, we have gathered the most recent literature related to the incorporation of different Se moieties into the scaffolds of a wide range of known drugs and their feasible pharmaceutical applications. In addition, we highlight different representative examples as well as provide our perspective on Se drugs and the possible future directions, promises, opportunities, and challenges of this ground-breaking area of research.

Organoselenium Compounds in Medicinal Chemistry

The chemical and biological interest in this element and the molecules bearing selenium has been exponentially growing over the years. Selenium, formerly designated as a toxin, becomes a vital trace element for life that appears as selenocysteine and its dimeric form, selenocystine, in the active sites of selenoproteins, which catalyze a wide variety of reactions, including the detoxification of reactive oxygen species and modulation of redox activities. From the point of view of drug developments, organoselenium drugs are isosteres of sulfur-containing and oxygen-containing drugs with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. This statement is the paramount relevance considering the big number of clinically employed compounds bearing sulfur or oxygen atoms in their structures including nucleosides and carbohydrates. Thus, in this article we have focused on the relevant features of the application of selenium in medicinal chemistry. With the increasing interest in selenium chemistry, we have attempted to highlight the most significant published data on this subject, mainly concentrating the analysis on the last years. In consequence, the recent advances of relevant pharmacological organoselenium compounds are discussed.

Synthesis of Selenium Derivatives using Organic Selenocyanates as Masked Selenols: Chemical Reduction with Rongalite as a Simpler Tool to give Nucleophilic Selenides

The chemical reduction within a family of organic selenocyanates, as masked selenols, using reducing agents, such as Rongalite, sodium dithionite, and sodium thiosulfate is investigated. Using Rongalite, the corresponding diselenides were obtained quantitatively and selectively in very good to excellent yields (51-100 %) starting from alkyl, aryl, and benzyl selenocyanates. The scope of the reaction is unaffected by the electronic nature of the substituents. Furthermore, the reducing agent, Rongalite, is compatible with hydrolysable and reducing-sensitive functional groups. Additionally, a simple methodology employing the in-situ generated benzyl selenolate anion (PhCH2Se-) to promote aliphatic nucleophilic substitution, epoxide ring opening, and Michael addition reactions has been developed; thus, extending the structural diversity of the synthesized selenium derivatives.

Selenosalicylate; a little-studied heavy-element analogue of the versatile thiosalicylate ligand

Selenosalicylic acid (ortho-HSeC6H4CO2H), the heavy element congener of the widely studied thiosalicylic acid, was prepared by reaction of 2-carboxybenzenediazonium chloride (HO2CC6H4N2+Cl-) with Na2Se2, followed by reduction of the resulting diselenide (SeC6H4CO2H)2 with zinc and acetic acid. The coordination chemistry of the selenosalicylate ligand towards a variety of platinum(ii), palladium(ii), nickel(ii), gold(iii), gold(i), rhodium(iii), iridium(iii) and ruthenium(ii) centres was explored. X-ray crystal structure determinations were carried out on the complexes [Pt(SeC6H4CO2)(PPh3)2], [{(p-cym)Ru(SeC6H4CO2)}2] (p-cym = η6-p-cymene, CH3C6H4CH(CH3)2), [{Cp*Rh(SeC6H4CO2)}2] (Cp* = η5-C5Me5) and [Cp*Ir(SeC6H4CO2)(PPh3)], and comparisons are made with corresponding thiosalicylate complexes. The complexes were characterised by NMR spectroscopy as well as ESI mass spectrometry, which indicated a greater propensity for fragmentation including by selenium loss, compared to the thiosalicylate analogues. Hirshfeld surface analysis to visualise and quantify intermolecular interactions revealed the dominance of H⋯H contacts in [{(p-cym)Ru(SeC6H4CO2)}2] and [Cp*Ir(SeC6H4CO2)(PPh3)].

Transition metal-free C(sp3)-H selenation of β-ketosulfones

The installation of selenium groups has become an essential step across a number of industries such as agrochemicals, drug discovery, and materials. However, direct C(sp3)-H selenation, which is most atom economical, remains a formidable challenge, and only a few examples have been reported to date. In this article, we introduce the transition metal-free C(sp3)-H selenation with the easily available β-ketosulfones and diselenides as the material source. This benign protocol permits access to a broad spectrum of α-aryl(alkyl) seleno-β-ketosulfones in high yields with outstanding functional group compatibility. Distinct advantages of this protocol over all previous methods encompass the utilization of base and air as an oxidant, room temperature, and enhanced green chemistry matrices.

NaOAc-Assisted Aerobic Oxidation Protocol for the Synthesis of Pentacoordinate Chalcogenyl Spirophosphoranes with P-Se/P-S Bonds under Open Air

The NaOAc-assisted aerobic oxidation reaction of pentacoordinate hydrospirophosphoranes and dichalcogenyl compounds with open air as a green oxidant has been developed under mild conditions. A series of novel pentacoordinate spirophosphoranes with P-Se/P-S bonds were synthesized in excellent yields. The reaction mechanism was determined by 31P nuclear magnetic resonance tracing experiments, high-resolution mass spectrometry tracing experiments, and X-ray diffraction analysis. The method features a broad substrate scope, good functional group tolerance, and a high degree of atomic utilization and is meaningful for the synthesis of bioactive chalcogenphosphate compounds with chalcogen and phosphorus moieties.

Selective Access to Functional Fluoroenones via Palladium-Catalyzed Selenofluoroalkylacylation of Terminal Alkynes

The trifluoromethylacyl group (-COCF3) is an important motif and widely studied in catalysis, medicinal chemistry, and materials science. Herein, a novel palladium-catalyzed selenofluoroalkylacylation of terminal alkynes with commercially available fluoroalkyl anhydride and diorganyl diselenides to afford β-seleno and aryl/alkyl disubstituted enones under mild conditions is disclosed. In addition, selenodifluoroacetylations and selenoperfluoroacetylations are also suitable for this reaction. Mechanistic studies reveal that this reaction proceeds via an oxidative radical-polar crossover process.

Synthesis of Selenium-Containing N-Quinazolinyl Acroleins via a 3,3-Radical Rearrangement Cascade Reaction

An effective approach for the construction of 2-aryl-3-(3-oxo-1-aryl-2-(organoselanyl)prop-1-en-1-yl)quinazolin-4(3H)-ones was developed. Excellent to almost quantitative yields were obtained by the cascade reaction of propargyl quinazoline-4-yl ethers, diselenides, and 70% tert-butyl hydrogen peroxide aqueous solution under metal-free and mild conditions. The synthesized hybrids, with conglomeration of quinazolinone, organoselenium, aldehyde, and fully substituted alkene moieties in one molecule, will have the potential for applications in development of new drugs or drug candidates.

Base-Catalyzed One-Pot Synthesis of Selenosulfides: A Base Basicity-Controlled Approach

We developed a novel and efficient sequential one-pot synthesis of selenosulfides via a base-catalyzed methodology utilizing readily available starting compounds, under mild reaction conditions. This method eliminated the need for excess oxidants or additives and simplified the synthesis procedure. Furthermore, organic amine bases served as exceptional catalysts for synthesizing the target products. The performance of a catalytic system depends on the basicity of the bases. The selection of suitable bases, based on their pKaH values, is crucial for the selective synthesis of selenosulfides without the formation of byproducts. This method provides a direct route for the preparation of selenosulfides, which are important scaffolds in organic chemistry.

Aryl selenonium vs. aryl sulfonium counterions in polyoxometalate chemistry: the impact of Se+ cationic centers on the photocatalytic reduction of dichromate

A selenonium organic counter ion has been used in polyoxometalate chemistry to develop a new aryl selenonium polyoxometalate (POM) hybrid, and its photocatalytic properties have been explored in comparison with an aryl sulfonium POM-hybrid counterpart for the first time. The chalcogenonium counterions, namely, methyldiphenylsulfonium trifluoromethane sulfonate (MDPST) and methyldiphenylselenonium trifluoromethane sulfonate (MDPSeT), and their octamolybdate ([Mo8O26]4-) hybrids, 1 and 2, with the general formula (C13H13X)4[Mo8O26] (where X = S for 1 and Se for 2) were synthesized and characterized. Hybrids 1 and 2 vary in their chalcogenonium cationic center (S+vs. Se+), which enabled a direct comparison of their photocatalytic properties as a function of the cationic center. The photocatalytic activities of hybrids 1 and 2 were tested using the reduction of dichromate (Cr2O72-) as a model reaction under UV irradiation. A 99% photocatalytic reduction of Cr2O72- with a rate constant of 0.0305 min-1 was achieved with hybrid 2, while only a 67% reduction with a rate constant of 0.0062 min-1 was observed with hybrid 1 in 180 minutes. The better catalytic performance of hybrid 2 may be correlated to the larger atomic radii of Se than S, which helps in better stabilizing the photogenerated electron-hole (e--h+) pair on the POM cluster by polarizing its lone pair more efficiently compared to S. The catalytic recyclability was tested for up to 4 cycles using hybrid 2, and up to 98% reduction was obtained even after the 4th cycle. Recyclability tests and control experiments also indicated the generation of some elemental Se through possible cleavage of some C-Se bonds of MDPSe under prolonged UV exposure during catalysis, and the Se thus generated was found to contribute to the catalytic reduction of dichromate. This study, therefore, opens new avenues for aryl selenonium moieties and their POM hybrids for potential catalytic applications.

Selenium Dibromide Click Chemistry: The Efficient Synthesis of Novel Selenabicyclo[3.3.1]nonene-2 and -nonane Derivatives

Highly efficient and convenient methods for the preparation of 35 novel derivatives of 9-selenabicyclo[3.3.1]nonane and 9-selenabicyclo[3.3.1]nonene-2 in high yields based on the adduct of the transannular addition of SeBr2 to 1,5-cyclooctadiene were developed. The methods for the amination of the adduct made it possible to obtain both diamino selenabicyclo[3.3.1]nonane derivatives and their dihydrobromide salts in one step in 88-98% yields. The methods meet the criteria of click chemistry. Compounds with high glutathione peroxidase mimetic activity were found among water-soluble dihydrobromide salts. The selective reaction of 2,6-dibromo-9-selenabicyclo[3.3.1]nonane with acetonitrile to form 6-bromo-9-selenabicyclo[3.3.1]nonene-2 was discovered. The latter compound served as a promising starting material to give rise to the new class of selenabicyclo[3.3.1]nonene-2 derivatives, e.g., 6-alkoxy-9-selenabicyclo[3.3.1]nonenes were obtained in 94-99% yields.

Manganese-promoted reductive cross-coupling of disulfides with dialkyl carbonates

Manganese is a cheap and environmentally friendly metal on Earth. Herein, we report a manganese-promoted reductive cross-coupling using easily available and odorless disulfides as thiolating agents in an excellent 100% sulfur atom economy. The protocol featured a broad substrate scope, including various alkyl disulfides and excellent functional group compatibility, constructing diverse thioethers under simple conditions. Ultimately, thioethers can be prepared in gram-scale reactions and further transformed into structurally complex molecules.

Genotoxicity and cytotoxicity potential of organoselenium compounds in human leukocytes in vitro

In the current work, cytotoxicity and genotoxicity of different organoselenium compounds were examined using Trypan blue exclusion and alkaline comet assays with silver staining respectively. Leukocytes were subjected to a 3-hour incubation with organoselenium compounds at concentrations of 1, 5, 10, 25, 50, and 75 μM, or with the control vehicle (DMSO), at a temperature of 37 °C. The viability of the cells was evaluated using the Trypan blue exclusion method, while DNA damage was analyzed through the alkaline comet assay with silver staining. The exposure of leukocytes to different organoselenium compounds including i.e. (Z)-N-(pyridin-2-ylmethylene)-1-(2-((2-(1-((E)-pyridin-2-ylmethyleneamino)ethyl)phenyl)diselanyl)phenyl)ethanamine (C1), 2,2'(1Z,1'E)-(1,1'-(2,2'-diselanediylbis(2,1-phenylene))bis(ethane-1,1-diyl)) bis(azan-1-yl-1-ylidene)bis -methan-1-yl-1-ylidene)diphenol (C2), and dinaphthyl diselenide (NapSe)2, At concentrations ranging from 1 to 5 μM, no significant DNA damage was observed, as indicated by the absence of a noteworthy increase in the Damage Index (DI). Our results suggest that the organoselenium selenium compounds tested were not genotoxic and cytotoxic to human leukocytes in vitro at lower concentration. This study offers further insights into the genotoxicity profile of these organochalcogens in human leukocytes. Their genotoxicity and cytotoxicity effects at higher concentration are probably mediated through reactive oxygen species generation and their ability to catalyze thiol oxidation.

Photoinduced Copper-Catalyzed 1,2-Difunctionalization of 1,3-Dienes with Aryl Diselenides

Described herein is a photoinduced copper-catalyzed 1,2-difunctionalization of 1,3-dienes. The selenium atom radical was generated by the visible light irradiation of diselenides, triggering radical addition with 1,3-dienes to form allyl radical intermediate. Subsequent rapid Z/E isomerization allowed for thermodynamically favorable intermediate formation and enabled copper catalyzed stereoselective functionalization with various nucleophiles.

Electrochemical selenocyclization of 2-ethynylanilines with diselenides: facile and efficient access to 3-selenylindoles

An efficient electrochemical selenocyclization strategy for the synthesis of 3-selenylindoles from 2-ethynylanilines and diselenides has been developed in simple tube- or beaker-type undivided cells under ambient conditions. Notably, these sustainable transformations are completed within a short time with low equivalents of charges, diselenides and electrolytes, exhibiting a broad substrate scope with excellent functional group compatibility. Moreover, a gram-scale electrosynthesis and late-stage functionalization of complex molecules further demonstrate the practical synthetic potential of this facile electrochemical system.

Mechanochemical Solvent-Free One-Pot Synthesis of Poly-Functionalized 5-(Arylselanyl)-1H-1,2,3-triazoles Through a Copper(I)-Catalyzed Click Reaction

A mechanochemistry-driven practical and efficient synthetic protocol for accessing diverse series of biologically relevant poly-functionalized 5-(arylselanyl)-1H-1,2,3-triazoles through copper(I)-catalyzed click reaction between aryl/heteroaryl acetylenes, diaryl diselenides, benzyl bromides, and sodium azide has been accomplished under high-speed ball-milling. Advantages of this method include operational simplicity, avoidance of using solvent and external heating, one-pot synthesis, short reaction time in minutes, good to excellent yields, broad substrate scope, and gram-scale applications. Furthermore, synthesized organoselenium compounds were synthetically diversified to biologically promising selenones.

Silver-Mediated Intermolecular [2 + 2 + 1] Cyclization of Terminal Alkynones with Elemental Selenium: Regioselective Synthesis of 2,4- or 3,4-Dicarbonylselenophenes

An efficient and atom-economical silver-mediated [2 + 2 + 1] cyclization protocol for the selective synthesis of 2,4- or 3,4-dicarbonylselenophenes has been developed. Readily accessible substrates, commercially available elemental selenium, and good functional group tolerance make this procedure attractive for the selective synthesis of dicarbonylselenophenes. Preliminary mechanistic investigations indicated that silver acetylene species are possible intermediates for the formation of 3,4-dicarbonylselenophenes.

Selenoneine-inspired selenohydantoins with glutathione peroxidase-like activity

Chronic diseases such as cystic fibrosis, inflammatory bowel diseases, rheumatoid arthritis, and cardiovascular illness have been linked to a decrease in selenium levels and an increase in oxidative stress. Selenium is an essential trace element that exhibits antioxidant properties, with selenocysteine enzymes like glutathione peroxidase being particularly effective at reducing peroxides. In this study, a series of synthetic organoselenium compounds were synthesized and evaluated for their potential antioxidant activities. The new selenohydantoin molecules were inspired by selenoneine and synthesized using straightforward methods. Their antioxidant potential was evaluated and proven using classical radical scavenging and metal-reducing methods. The selenohydantoin derivatives exhibited glutathione peroxidase-like activity, reducing hydroperoxides. Theoretical calculations using Density Functional Theory (DFT) revealed the selenone isomer to be the only one occurring in solution, with selenolate as a possible tautomeric form in the presence of a basic species. Cytocompatibility assays indicated that the selenohydantoin derivatives were non-toxic to primary human aortic smooth muscle cells, paving the way for further biological evaluations of their antioxidant activity. The results suggest that selenohydantoin derivatives with trifluoro-methyl (-CF3) and chlorine (-Cl) substituents have significant activities and could be potential candidates for further biological trials. These compounds may contribute to the development of effective therapies for chronic diseases such cardiovascular diseases.

Visible Light and Triselenium Dicyanide (TSD): New Horizons in the Synthesis of Organic Selenocyanates

Herein, we describe a new method for the synthesis of α-carbonyl selenocyanates by reacting triselenium dicyanide (TSD) and styrenes under blue light irradiation and O2 atmosphere. The reactions are triggered by the formation of Se-centered radical species, followed by the addition/oxidation of the styrene π-bond. α-Carbonyl selenocyanates and α-hydroxy selenocyanates were obtained in moderate to excellent yields from aryl- and alkyl-substituted alkenes, respectively. It was demonstrated that α-carbonyl selenocyanates could be used as a synthetic platform in a multicomponent reaction strategy to prepare 2-phenylimidazo[1,2-a]pyridine derivatives, which were evaluated for their photophysical properties. Overall, this new method provides a useful tool for synthesizing α-carbonyl selenocyanates, and demonstrates their potential for use in the synthesis of other compounds, thus giving new synthetic opportunities to construct organic selenocyanate compounds.

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.

Regioselective Radical Cascade Cyclizations of Alkyne-Tethered Cyclohexadienones with Chalcogenides under Visible-Light Catalysis

Herein, we demonstrated a silver/K2S2O8-mediated highly regio- and diastereoselective 6/5-exo trig radical cascade cyclization of alkyne-tethered cyclohexadienones with sulfonyl hydrazides or sodium sulfinates and subsequent selenation to access 6,6-dihydrochromenone and 6,5-fused tetrahydro benzofuranone derivatives. This reaction protocol features high functional group compatibility and has a wide substrate scope providing a variety of dihydrochromenones and tetrahydro benzofuranone derivatives in good to excellent yields. The reaction proceeds via the attack of a sulfonyl radical to alkyne over the activated Michael acceptor. The TEMPO quenching experiment implies the presence of a radical intermediate. Further synthetic versatility of 6,6- and 5,6-fused derivatives is also showcased.

Electrochemical direct C-H mono and bis-chalcogenation of indolizine frameworks under oxidant-free conditions

Herein, we disclosed a sustainable electrochemical approach for site-selective C-H mono and bis-chalcogenation (sulfenylation or selenylation) of indolizine frameworks. Diversely functionalized disulfides and diselenides possessing EDGs and EWGs were successfully reacted with a variety of indolizines to directly access sulfenylated/selenylated indolizines in 40-96% yields. A mechanistic radical pathway was also validated with control experiments and cyclic voltammogram data.

Antiviral Effect of 5'-Arylchalcogeno-3-aminothymidine Derivatives in SARS-CoV-2 Infection

The understanding that zidovudine (ZDV or azidothymidine, AZT) inhibits the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 and that chalcogen atoms can increase the bioactivity and reduce the toxicity of AZT has directed our search for the discovery of novel potential anti-coronavirus compounds. Here, the antiviral activity of selenium and tellurium containing AZT derivatives in human type II pneumocytes cell model (Calu-3) and monkey kidney cells (Vero E6) infected with SARS-CoV-2, and their toxic effects on these cells, was evaluated. Cell viability analysis revealed that organoselenium (R3a-R3e) showed lower cytotoxicity than organotellurium (R3f, R3n-R3q), with CC50 ≥ 100 µM. The R3b and R3e were particularly noteworthy for inhibiting viral replication in both cell models and showed better selectivity index. In Vero E6, the EC50 values for R3b and R3e were 2.97 ± 0.62 µM and 1.99 ± 0.42 µM, respectively, while in Calu-3, concentrations of 3.82 ± 1.42 µM and 1.92 ± 0.43 µM (24 h treatment) and 1.33 ± 0.35 µM and 2.31 ± 0.54 µM (48 h) were observed, respectively. The molecular docking calculations were carried out to main protease (Mpro), papain-like protease (PLpro), and RdRp following non-competitive, competitive, and allosteric inhibitory approaches. The in silico results suggested that the organoselenium is a potential non-competitive inhibitor of RdRp, interacting in the allosteric cavity located in the palm region. Overall, the cell-based results indicated that the chalcogen-zidovudine derivatives were more potent than AZT in inhibiting SARS-CoV-2 replication and that the compounds R3b and R3e play an important inhibitory role, expanding the knowledge about the promising therapeutic capacity of organoselenium against COVID-19.

Organoselenium Compounds: Chemistry and Applications in Organic Synthesis

Selenium, originally described as a toxin, turns out to be a crucial trace element for life that appears as selenocysteine and its dimer, selenocystine. From the point of view of drug developments, selenium-containing drugs are isosteres of sulfur and oxygen with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. In this article, we have focused on the relevant features of the selenium atom, above all, the corresponding synthetic approaches to access a variety of organoselenium molecules along with the proposed reaction mechanisms. The preparation and biological properties of selenosugars, including selenoglycosides, selenonucleosides, selenopeptides, and other selenium-containing compounds will be treated. We have attempted to condense the most important aspects and interesting examples of the chemistry of selenium into a single article.

Silver-Mediated [2 + 2 + 1] Cyclization of ortho-Propioloylbenzonitriles with Elemental Selenium: Synthesis of 4H-indeno[1,2-c][1,2]selenazol-4-ones

An efficient silver-mediated [2 + 2 + 1] cyclization protocol of ortho-propioloylbenzonitriles with elemental selenium for the synthesis of 4H-indeno[1,2-c][1,2]selenazol-4-ones has been developed. One C-Se bond, one N-Se bond, and one C-C bond were rapidly constructed in one step. The reaction might proceed via the formation of a highly reactive selenoketene intermediate, followed by intramolecular cyclization.

Synthesis of Benzoselenophenes via TMSCN-Enabled Radical-Mediated Tandem Reaction Involving Enamides and Elemental Selenium

This study presents a TMSCN-enabled tandem reaction involving enamides and elemental selenium to access a diverse array of benzoselenophenes. Notably, this methodology involves the direct 2-fold C(sp2)-H bond activation without the need for preinstalled halides or boronic acids as reaction handles. The protocol offers several noteworthy features, including the absence of transition metals and strong oxidants, high reaction efficiency, broad substrate scopes, and the use of stable elemental selenium as a selenium source.

Visible-Light Photocatalytic Synthesis of Difluoromethylated Selenides from Selenosulfonates through a Radical Process

A photocatalytic synthesis of difluoromethylated selenides from selenosulfonates is described here. Bench-stable difluoromethyl phosphonium salt [Ph3PCF2H]Br reacts smoothly with selenosulfonates to give a series of functionalized difluoromethylated selenides in moderate to good yields via a radical process. This protocol is free of a stoichiometric base and reductant, has tolerance of functional groups, and has successful late-stage modification of bioactive molecules, which provides facile access to molecules of pharmaceutical relevance.

2-Benzamide Tellurenyl Iodides: Synthesis and Their Catalytic Role in CO2 Mitigation

Benzamide-derived organochalcogens (chalcogen=S, Se, and Te) have shown promising interest in biological and synthetic chemistry. Ebselen molecule derived from benzamide moiety is the most studied organoselenium. However, its heavier congener organotellurium is under-explored. Here, an efficient copper-catalyzed atom economical synthetic method has been developed to synthesize 2-phenyl-benzamide tellurenyl iodides by inserting a tellurium atom into carbon-iodine bond of 2-iodobenzamides in one pot with 78-95 % yields. Further, the Lewis acidic nature of Te center and Lewis basic nature of nitrogen of the synthesized 2-Iodo-N-(quinolin-8-yl)benzamide tellurenyl iodides enabled them as pre-catalyst for the activation of epoxide with CO2 at 1 atm for the preparation of cyclic carbonates with TOF and TON values of 1447 h-1 and 4343, respectively, under solvent-free conditions. In addition, 2-iodo-N-(quinolin-8-yl)benzamide tellurenyl iodides have also been used as pre-catalyst for activating anilines and CO2 to form a variety of 1,3-diaryl ureas up to 95 % yield. The mechanistic investigation for CO2 mitigation is done by 125 Te NMR and HRMS studies. It seems that the reaction proceeds via formation of catalytically active Te-N heterocycle, an ebtellur intermediate which is isolated and structurally characterized.

Catalyst- and metal-free C(sp2)-H bond selenylation of (N-hetero)-arenes using diselenides and trichloroisocyanuric acid at room temperature

In this paper, we report an eco-friendly approach for the C(sp2)-H bond selenylation of imidazopyridines and other N-heteroarenes as well as simple arenes at ambient temperature. This new protocol consists of the reaction between (N-hetero)-arenes and the diorganyl-diselenides and trichloroisocyanuric acid (TCCA)-ethanol reagent system. In a short reaction time, the desired selenylated products were obtained regioselectively in good yields, with tolerance for a wide range of functional groups.

Antimicrobial Coatings for Medical Textiles via Reactive Organo-Selenium Compounds

Bleached and cationized cotton fabrics were chemically modified with reactive organoselenium compounds through the nucleophilic aromatic substitution (SNAr) reaction, which allowed for organo-selenium attachment onto the surface of cotton fabrics via covalent bonds and, in the case of the cationized cotton fabric, additional ionic interactions. The resulting textiles exhibited potent bactericidal activity against S. aureus (99.99% reduction), although only moderate activity was observed against E. coli. Fabrics treated with reactive organo-selenium compounds also exhibited fungicidal activities against C. albicans, and much higher antifungal activity was observed when organo-selenium compounds were applied to the cationized cotton in comparison to the bleached cotton. The treatment was found to be durable against rigorous washing conditions (non-ionic detergent/100 °C). This paper is the first report on a novel approach integrating the reaction of cotton fabrics with an organo-selenium antimicrobial agent. This approach is attractive because it provides a method for imparting antimicrobial properties to cotton fabrics which does not disrupt the traditional production processes of a textile mill.

Visible-Light-Induced Intramolecular Tandem Cyclization of Unactivated Indoloalkynes for the Synthesis of Sulfonylated and Selenylated Indolo[1,2-a]quinolines

A convenient and efficient visible-light-induced method has been developed for the construction of sulfonated and selenylated indolo[1,2-a]quinolines through sulfonyl or selenyl radical-initiated tandem cyclization of unactivated alkynes with sodium sulfinates or diaryl diselenides under mild conditions. This protocol, which simply utilizes visible light as the safe and eco-friendly energy source and an inexpensive and nontoxic organic dye as a photocatalyst without the aid of an external photocatalyst, provides various sulfonyl- and selenyl-containing indolo[1,2-a]quinolines in moderate to good yields.

Controlled Synthesis and Uniform Anchoring of Hollow CuxO Nanocubes on Carbon Nanofiber for Enhanced Se(S)-Se(S) Bond Activation

In the present study, we fabricated hollow cubic Cu_xO nanoparticles (∼23 nm) incorporated with CNF (HC-Cu_xO/CNF) through controlled thermal oxidation of solid cubic Cu₂O nanoparticles (∼21 nm) supported on carbon nanofibers (SC-Cu₂O/CNF) under airflow, exploiting the nanoscale Kirkendall effect. These hollow Cu_xO nanocubes with increased surface areas exhibited outstanding catalytic activity for unsymmetrical chalcogenide synthesis under ligand-free conditions.

Synthesis of Selenocyanates and Selenoethers of Amino Pyrazoles and Amino Uracils by In Situ Triselenium Dicyanide from Malononitrile and Selenium Dioxide

Herein, we report an efficient method for synthesis of novel selenocyanates of amino pyrazole, amino uracil, and amino isoxazole derivatives using in situ triselenium dicyanide from the combination of malononitrile and selenium dioxide in DMSO medium. Using the same combination but changing the stoichiometry of reagents and sequence of addition and temperature, symmetrical selenoethers of amino pyrazoles and amino uracils were prepared in good yields. Furthermore, selenocyanates of amino pyrazoles were utilized for the synthesis of corresponding alkynyl selenides in the presence of CuI and Cs₂CO₃. The salient features of this methodology are inexpensive starting materials, short reaction time, and good to very good yields. This method is also applicable for the gram-scale synthesis of selenocyanates of amino pyrazoles and amino uracils.

Ebselen derivatives inhibit SARS-CoV-2 replication by inhibition of its essential proteins: PLpro and Mpro proteases, and nsp14 guanine N7-methyltransferase

Proteases encoded by SARS-CoV-2 constitute a promising target for new therapies against COVID-19. SARS-CoV-2 main protease (M^pro, 3CL^pro) and papain-like protease (PL^pro) are responsible for viral polyprotein cleavage-a process crucial for viral survival and replication. Recently it was shown that 2-phenylbenzisoselenazol-3(2H)-one (ebselen), an organoselenium anti-inflammatory small-molecule drug, is a potent, covalent inhibitor of both the proteases and its potency was evaluated in enzymatic and antiviral assays. In this study, we screened a collection of 34 ebselen and ebselen diselenide derivatives for SARS-CoV-2 PL^pro and M^pro inhibitors. Our studies revealed that ebselen derivatives are potent inhibitors of both the proteases. We identified three PL^pro and four M^pro inhibitors superior to ebselen. Independently, ebselen was shown to inhibit the N7-methyltransferase activity of SARS-CoV-2 nsp14 protein involved in viral RNA cap modification. Hence, selected compounds were also evaluated as nsp14 inhibitors. In the second part of our work, we employed 11 ebselen analogues-bis(2-carbamoylaryl)phenyl diselenides-in biological assays to evaluate their anti-SARS-CoV-2 activity in Vero E6 cells. We present their antiviral and cytoprotective activity and also low cytotoxicity. Our work shows that ebselen, its derivatives, and diselenide analogues constitute a promising platform for development of new antivirals targeting the SARS-CoV-2 virus.

Small-molecule organoselenocyanates: Recent developments toward synthesis, anticancer, and antioxidant activities

Cellular redox homeostasis is very important for the overall cellular development, function, and oxidative stress often disrupts the process. Small-molecule organoselenium compounds exert key roles in maintaining the redox homeostasis during oxidative stress and cancer owing to their notable antioxidant activities. Among different organoselenium compounds, small-molecule organoselenocyanates have attracted much research attention due to their synthetic utilities and therapeutic potentials. Therefore, the development of convenient synthetic methodologies to different classes of organoselenocyanates from various precursors was explored over the years as useful synthetic building blocks. Additionally, considering their inherent redox and antioxidant properties, the development of biologically relevant organoselenocyanates upon their conjugation with the existing drugs and natural products has been chosen for enhancing the drug potencies and in ameliorating the drug-induced side-effects. In the present report, we have discussed some of the very recent and relevant developments on these aspects in a very concise manner.

Photo- or Electrochemical Cyclization of Dienes with Diselenides to Access Seleno-Benzo[b]azepines

A cascade selenylation/cyclization of dienes with diselenides has been realized under visible-light irradiation or electrolysis conditions. Employing O₂ or electricity as a "green" oxidant, this protocol provides a green and efficient method for an array of biologically important seleno-benzo[b]azepine derivatives in moderate to good yields. The direct sunlight irradiation and gram-scale reaction render the approach practical and attractive.

Redox-responsive microemulsion: Fabrication and application to curcumin encapsulation

HYPOTHESIS

Stimulus-responsive microemulsions have aroused significant attention because of their versatile and reversible switchability between stable and unstable states. However, most stimuli-responsive microemulsions are based on stimuli-responsive surfactants. We posit that the change in the hydrophilicity of a selenium-containing alcohol triggered by a mild redox reaction could also influence the stability of microemulsions and provide a new nanoplatform for the delivery of bioactive substances.

EXPERIMENTS

A selenium-containing diol (3,3'-selenobis(propan-1-ol), PSeP) was designed and used as a co-surfactant in a microemulsion with ethoxylated hydrogenated castor oil (HCO40), diethylene glycol monohexyl ether (DGME), 2-n-octyl-1-dodecanol (ODD) and water. The redox-induced transition in PSeP was characterized by ¹H NMR, ⁷⁷Se NMR, and MS. The redox-responsiveness of the ODD/HCO40/DGME/PSeP/water microemulsion was investigated through determination of a pseudo-ternary phase diagram, analysis by dynamic light scattering, and electrical conductivity, and its encapsulation performance was evaluated by determination of the solubility, stability, antioxidant activity, and skin penetrability of encapsulated curcumin.

FINDINGS

The redox conversion of PSeP enabled efficient switching of ODD/HCO40/DGME/PSeP/water microemulsions. Addition of oxidant (H₂O₂), oxidized PSeP into more hydrophilic PSeP-Ox (selenoxide), disrupting the emulsifying capacity of the combination of HCO40/DGME/PSeP, markedly reducing the monophasic microemulsion region in the phase diagram, and inducing phase separation in some formulations. Addition of reductant (N₂H₄·H₂O), reduced PSeP-Ox and restored the emulsifying capacity of the combination of HCO40/DGME/PSeP. In addition, PSeP-based microemulsions can significantly enhance the solubility in oil (by 23 times), stability, antioxidant capacity (DPPH∙ radical scavenging by 91.74 %), and skin penetrability of curcumin, showing clear potential for encapsulation and delivery of curcumin and other bioactive substances.

Thio(seleno)cyano-difluoroalkylation of Alkenes Using Visible-Light Photocatalysis

A mild and efficient three-component thio(seleno)cyano-difluoroalkylation of simple alkenes is demonstrated using an iridium(ruthenium) photocatalyst. This protocol provides a direct and regioselective installation of both C-S(Se)CN [thio(seleno)cyanation] and C-C_F (difluoroalkylation) bonds.

Chemistry Related to the Catalytic Cycle of the Antioxidant Ebselen

The antioxidant drug ebselen has been widely studied in both laboratories and in clinical trials. The catalytic mechanism by which it destroys hydrogen peroxide via reduction with glutathione or other thiols is complex and has been the subject of considerable debate. During reinvestigations of several key steps, we found that the seleninamide that comprises the first oxidation product of ebselen underwent facile reversible methanolysis to an unstable seleninate ester and two dimeric products. In its reaction with benzyl alcohol, the seleninamide produced a benzyl ester that reacted readily by selenoxide elimination, with formation of benzaldehyde. Oxidation of ebselen seleninic acid did not afford a selenonium seleninate salt as previously observed with benzene seleninic acid, but instead generated a mixture of the seleninic and selenonic acids. Thiolysis of ebselen with benzyl thiol was faster than oxidation by ca. an order of magnitude and produced a stable selenenyl sulfide. When glutathione was employed, the product rapidly disproportionated to glutathione disulfide and ebselen diselenide. Oxidation of the S-benzyl selenenyl sulfide, or thiolysis of the seleninamide with benzyl thiol, afforded a transient thiolseleninate that also readily underwent selenoxide elimination. The S-benzyl derivative disproportionated readily when catalyzed by the simultaneous presence of both the thiol and triethylamine. The phenylthio analogue disproportionated when exposed to ambient or UV (360 nm) light by a proposed radical mechanism. These observations provide additional insight into several reactions and intermediates related to ebselen.

Three-Component Electrochemical Aminoselenation of 1,3-Dienes

Azoles and organoselenium compounds are pharmacologically important scaffolds in medicinal chemistry and natural products. We developed an efficient regioselective electrochemical aminoselenation reaction of 1,3-dienes, azoles, and diselenide derivatives to access selenium-containing allylazoles skeletons. This protocol is more economical and environmentally friendly and features a broad substrate scope; pyrazole, triazole, and tetrazolium were all tolerated under the standard conditions, which could be applied to the expedient synthesis of bioactive molecules and in the pharmaceutical industry.