Synthesis of 2H,3-4-Dihydro-1,2-benzoselenazin-3-one and derivatives : A new heterocyclic ring system

Organoselenium compounds as mimics of selenoproteins and thiol modifier agents

Selenium is an essential trace element for animals and its role in the chemistry of life relies on a unique functional group: the selenol (-SeH) group. The selenol group participates in critical redox reactions. The antioxidant enzymes glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) exemplify important selenoproteins. The selenol group shares several chemical properties with the thiol group (-SH), but it is much more reactive than the sulfur analogue. The substitution of S by Se has been exploited in organic synthesis for a long time, but in the last 4 decades the re-discovery of ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) and the demonstration that it has antioxidant and therapeutic properties has renovated interest in the field. The ability of ebselen to mimic the reaction catalyzed by GPx has been viewed as the most important molecular mechanism of action of this class of compound. The term GPx-like or thiol peroxidase-like reaction was previously coined in the field and it is now accepted as the most important chemical attribute of organoselenium compounds. Here, we will critically review the literature on the capacity of organoselenium compounds to mimic selenoproteins (particularly GPx) and discuss some of the bottlenecks in the field. Although the GPx-like activity of organoselenium compounds contributes to their pharmacological effects, the superestimation of the GPx-like activity has to be questioned. The ability of these compounds to oxidize the thiol groups of proteins (the thiol modifier effects of organoselenium compounds) and to spare selenoproteins from inactivation by soft-electrophiles (MeHg⁺, Hg²⁺, Cd²⁺, etc.) might be more relevant for the explanation of their pharmacological effects than their GPx-like activity. In our view, the exploitation of the thiol modifier properties of organoselenium compounds can be harnessed more rationally than the use of low mass molecular structures to mimic the activity of high mass macromolecules that have been shaped by millions to billions of years of evolution.

The role of methoxy substituents in regulating the activity of selenides that serve as spirodioxyselenurane precursors and glutathione peroxidase mimetics

A series of o-(hydroxymethyl)phenyl selenides containing single or multiple methoxy substituents was synthesized, and the rate at which each compound catalyzed the oxidation of benzyl thiol to its disulfide with excess hydrogen peroxide was measured. This assay provided the means for comparing the relative abilities of the selenides to mimic the antioxidant selenoenzyme glutathione peroxidase. The mechanism for catalytic activity involves oxidation of the selenides to their corresponding selenoxides with hydrogen peroxide, cyclization to spirodioxyselenuranes, followed by reduction with two equivalents of thiol to regenerate the original selenide with concomitant disulfide formation. A single p-methoxy group on each aryl moiety afforded the highest catalytic activity, while methoxy groups in the meta position had little effect compared to the unsubstituted selenide, and o-methoxy groups suppressed activity. The installation of multiple methoxy groups on each aryl moiety provided no improvement. These results can be rationalized on the basis of dominating mesomeric and steric effects of the p- and o-substituents, respectively.

Synthesis and Characterization of a New Five and Six Membered Selenoheterocyclic Compounds Homologues of Ebselen

The discovery of the antioxidant activity of selenoenzyme glutathione peroxidase (GPx) has attracted growing attention in the biochemistry of selenium. Among molecules which mimic the structure of the active site of the enzyme, N-phenyl-1,2-benzisoselenazolin-3-one 1, Ebselen, exhibited useful anti-inflammatory properties. It has been extensively investigated and has undergone clinical trials as an anti-inflammatory agent. Unfortunately, Ebselen exhibits relatively poor catalytic activity, prompting attempts to design more efficacious GPx mimetics that would retain his low toxicity while manifesting improved catalytic properties. In this context, novel 1,2-benzoselenazine and 1,2-benzoselenazols, which are five and six membered homologues of Ebselen were synthesized and characterized. One structure has been proven by single crystal X-ray crystallography.

Effect of functional groups on antioxidant properties of substituted selenoethers

Selenoethers attached to functional groups through propyl chain viz., bis(3-carboxypropyl)selenide (SeBA), bis(3-hydroxypropyl)selenide (SePOH) and bis(3-aminopropyl)selenide dihydrochloride (SePAm), have been examined for their ability to inhibit peroxyl radical mediated DNA damage, peroxyl radical scavenging ability and glutathione peroxidase (GPx) like activity. The DNA damage was monitored by gel electrophoresis, bimolecular rate constants for scavenging of model peroxyl radical were determined by pulse radiolysis and the GPx activity was followed by their ability to reduce hydrogen peroxide in the presence of glutathione utilizing NADPH decay and HPLC analysis. Among these compounds, SeBA showed maximum DNA protecting activity and it was also the most efficient in scavenging peroxyl radicals with the highest GPx mimicking activity. Quantum chemical calculations confirmed that SeBA with the highest energy level of HOMO (highest occupied molecular orbital) is the easiest to undergo oxidation and therefore exhibits better radical scavenging, GPx mimicking and DNA protecting activity than SePOH or SePAm.

Design and synthesis of some biologically interesting natural and unnatural products based on organosulfur and selenium chemistry

Organosulfur and selenium chemistry has provided fertile ground for the discovery of novel synthetic methodology and for the design of bioactive molecules with potential therapeutic applications. Thus, acetylenic sulfones have been employed in novel strategies for the synthesis of nitrogen heterocycles, including several biologically active alkaloids. The conjugate addition of nitrogen nucleophiles containing ester or chloroalkyl substituents to acetylenic sulfones was followed by base-mediated intramolecular alkylation or acylation to afford variously substituted piperidines, pyrrolizidines, indolizidines, quinolizidines, decahydroquinolines, and 4-quinolones. The products include the dendrobatid alkaloids (–)-pumiliotoxin C, indolizidines (–)-167B, 207A, 209B, and 209D, as well as (–)-(ent)-julifloridine, (–)-lasubine II, myrtine, and two recently discovered alkaloids from the medicinal plant Ruta chalepensis , which had not been previously synthesized. Acetylenic sulfones were also incorporated on solid supports and employed in the types of cyclizations mentioned above, as well as for Diels–Alder reactions and a large variety of 1,3-dipolar cycloadditions. Conjugate additions of tertiary cyclic α-vinyl amines to acetylenic sulfones generated zwitterions that underwent exceptionally facile formal aza-Cope rearrangements to afford ring-expanded macrocyclic amines. An iterative version was developed and used in the synthesis of motuporamine A and B. With respect to organoselenium chemistry, two classes of compounds are described that function as novel mimetics of the selenoenzyme glutathione peroxidase (GPx), which protects cells from oxidative stress caused by the formation of peroxides during aerobic metabolism. They include cyclic seleninates and spirodioxyselenuranes, both of which efficiently catalyze the reduction of peroxides with thiols and are of potential value in the mitigation of oxidative stress. Their aromatic derivatives are generally less effective catalysts, but substituent effects can be used to modulate their activities. The mechanism of their catalytic cycles has been elucidated and Hammett plots indicate that the oxidation of Se(II) to Se(IV) is the rate-determining step for both classes. A methoxy-substituted aromatic spirodioxyselenurane provided the fastest rate for a small-molecule selenium compound that we have observed to date for the reduction of hydrogen peroxide with benzyl thiol.

Synthesis of telluroamino acid derivatives with remarkable GPx like activity

A series of modular telluroamino acid derivatives with remarkable GPx-like behavior was prepared in an efficient and short two-step synthesis.

o-Hydroxylmethylphenylchalcogens: Synthesis, Intramolecular Nonbonded Chalcogen···OH Interactions, and Glutathione Peroxidase-like Activity

[Structure: see text]. The synthesis and characterization of a series of organochalcogen (Se, Te) compounds derived from benzyl alcohol 13 are described. The synthesis of the key precursor dichalcogenides 15, 22, and 29 was achieved by the ortho-lithiation route. Selenide 18 was obtained by the reaction of the dilithiated derivative 14 with Se(dtc)2. Oxidation of 15 and 22 with H2O2 afforded the corresponding cyclic ester derivatives 17 and 24, respectively. Oxidation of selenide 18 with H2O2 affords the spirocyclic compound 19. The presence of intramolecular interactions in dichalcogenides 15 and 22 has been proven by single-crystal X-ray studies. The cyclic compounds 17 and 19 have also been characterized by single-crystal X-ray studies. GP(X)-like antioxidant activity of selenium compounds has been evaluated by the coupled bioassay method. Density functional theory calculations at the mPW1PW91 level on ditelluride 22 have identified a fairly strong nonbonding interaction between the hydroxy oxygen and tellurium atom. The second-order perturbation energy obtained through NBO analysis conveys the involvement of n(O) --> sigma(Te-Te) orbital overlap in nonbonding interaction. Post wave function analysis with the Atoms in Molecules (AIM) method identified distinct bond critical point in 15 and 22 and also indicated that the nonbonding interaction is predominantly covalent. Comparison between diselenide 15 and ditelluride 22 using the extent of orbital interaction as well as the value of electron density at the bond critical points unequivocally established that a ditelluride could be a better acceptor in nonbonding interaction, when the hydroxy group acts as the donor.

Aromatic Derivatives and Tellurium Analogues of Cyclic Seleninate Esters and Spirodioxyselenuranes That Act as Glutathione Peroxidase Mimetics

[Reaction: see text]. Several novel organoselenium and tellurium compounds were prepared and evaluated as mimetics of the selenoenzyme glutathione peroxidase, which protects cells from oxidative stress by reducing harmful peroxides with the thiol glutathione. The compounds were tested for catalytic activity in a model system wherein tert-butyl hydroperoxide or hydrogen peroxide were reduced with benzyl thiol and the rate of the reaction was measured by monitoring the formation of dibenzyl disulfide. Thus, aromatic derivatives 19, 22, 24, and 25 proved to be inferior catalysts compared to the parent cyclic seleninate ester 14 and spirodioxyselenurane 16. In the case of 19 and 22, this was the result of their rapid conversion to the relatively inert selenenyl sulfides 31 and 32, respectively. In general, hydrogen peroxide was reduced faster than tert-butyl hydroperoxide in the presence of the selenium-based catalysts. The cyclic tellurinate ester 27 and spirodioxytellurane 29 proved to be superior catalysts to their selenium analogues 14 and 16, respectively, resulting in the fastest reaction rates by far of all of the compounds we have investigated to date. Oxidation of 29 with hydrogen peroxide produced the unusual and unexpected peroxide 33, in which two hypervalent octahedral tellurium moieties are joined by ether and peroxide bridges. The structure of 33 was confirmed by X-ray crystallography. Although 33 displayed strong catalytic activity when tested independently in the model system, its relatively slow formation from the oxidation of 29 rules out its intermediacy in the catalytic cycle of 29.

An Evaluation of Various Computational Methods for the Treatment of Organoselenium Compounds

A reliable computational method for the prediction of organoselenium geometries and bond dissociation energies (BDEs) has been determined on the basis of the performance of density functional theory (DFT: B3LYP and B3PW91) and ab initio molecular orbital procedures (Hartree-Fock (HF)) in conjunction with various Pople basis sets including (but not limited to) the 6-31G(d), 6-31G(d,p), 6-311G(d), 6-311G(d,p), 6-311G(2df,p), and 6-311G(3df,3pd) sets. Predicted geometries and BDEs are compared with available experimental data and quadratic configuration interaction including single and double substitutions (QCISD) results. The B3PW91/6-311G(2df,p) level of theory is recommended for the prediction of the geometries and energetics of organoselenium compounds.

Combining benzo[d]isoselenazol-3-ones with sterically hindered alicyclic amines and nitroxides: enhanced activity as glutathione peroxidase mimics

Benzo[d]isoselenazol-3-ones N-substituted with sterically hindered diamagnetic and paramagnetic five- or six-membered nitroxides or their precursors, including ring-opened diselenides, exhibit synergism in glutathione peroxidase (GPx) activity.

Highly Efficient Dendrimer-Based Mimic of Glutathione Peroxidase

In this communication, we report the synthesis of the three generations of Fréchet-type poly(aryl ether) dendrimers with a diselenide core that demonstrate generation-dependent glutathione peroxidase (GPx) activity with initial reduction rates as high as 2431.20 muM min-1 for the third-generation product, around 1400 times faster than Ebselen. It represents a successful example of using a dendrimer as a model for a GPx mimic.

Diselenides and Allyl Selenides as Glutathione Peroxidase Mimetics. Remarkable Activity of Cyclic Seleninates Produced in Situ by the Oxidation of Allyl ω-Hydroxyalkyl Selenides

A series of aliphatic diselenides and selenides containing coordinating substituents was tested for glutathione peroxidase (GPx)-like catalytic activity in a model system in which the reduction of tert-butyl hydroperoxide with benzyl thiol to afford dibenzyl disulfide and tert-butyl alcohol was performed under standard conditions and monitored by HPLC. Although the diselenides showed generally poor catalytic activity, allyl selenides proved more effective. In particular, allyl 3-hydroxypropyl selenide (25) rapidly generated 1,2-oxaselenolane Se-oxide (31) in situ by a series of oxidation and [2,3]sigmatropic rearrangement steps. The remarkably active cyclic seleninate 31 proved to be the true catalyst, reacting with the thiol via a postulated mechanism in which the thioseleninate 32 is first produced, followed by further thiolysis to selenenic acid 33 and oxidation-dehydration to regenerate 31. In contrast to catalysis with GPx, formation of the corresponding selenenyl sulfide 34 comprises a competing deactivation pathway in the catalytic cycle of 31, as a separate experiment revealed that authentic 34 was a much less effective catalyst than 31. 1,2-Oxaselenane Se-oxide (37), the six-membered homologue of 31, was formed similarly from allyl 4-hydroxybutyl selenide (26), but proved a less effective catalyst than 31. Compounds 31 and 37 are the first examples of unsubstituted monocyclic seleninate esters.

Design, synthesis and glutathione peroxidase-Like properties of ovothiol-Derived diselenides

Eleven imidazole diselenides derived from the naturally occurring family of antioxidants, the ovothiols, were synthetised by cyclisation of selenoamides with trimethylsilyltrifluoromethanesulfonate or refluxing of cyanoamines in a selenium/sodium borohydride mixture. These compounds were assayed for their glutathione peroxidase-like (GSH Px-like) activity and their capacity to be reduced by glutathione. The most active molecules of the series were 4 times more potent in the GSH Px-like test than the widely known reference compound, ebselen. This catalytic activity was mediated by the formation of the antioxidant selenol intermediate upon partial but significant exchange reaction with glutathione.

Mechanistic studies of the tellurium(II)/tellurium(IV) redox cycle in thiol peroxidase-like reactions of diorganotellurides in methanol

Di-n-hexyl telluride (2), di-p-methoxyphenyl telluride (3), and (S)-2-(1-N,N-dimethylaminoethyl)phenyl phenyl telluride (4) catalyzed the oxidation of PhSH to PhSSPh with H(2)O(2) in MeOH. Telluride 2 displayed greater rate acceleration than the diaryltellurides 3 and 4 as determined by the initial velocities, v(0), for the rate of appearance of PhSSPh determined at 305 nm by stopped-flow spectroscopy. Rate constants for the oxidation of tellurides 2-4 (k(ox)), rate constants for the introduction of PhSH as a ligand to the Te(IV) center (k(PhSH)) of oxidized tellurides 5-7, and thiol-independent (k(1)) and thiol-dependent (k(2)) rate constants for reductive elimination at Te(IV) in oxidized tellurides 5-7 were determined using stopped-flow spectroscopy. Oxidation of the Te atom of the electron-rich dialkyl telluride 2 was more rapid than oxidation of diaryl tellurides 3 and 4. The dimethylaminoethyl substituent of 4, which acts as a chelating ligand to Te(IV), did not affect k(ox). Values of k(PhSH) for the introduction of PhSH to oxidized dialkyl tellurane 5 and oxidized diaryl tellurane 6 were comparable in magnitude, while the chelating dimethylaminoethyl ligand of oxidized telluride 7 diminished k(PhSH) by a fator of 10(3). Reductive elimination by both first-order, thiol-independent (k(1)) and second-order, thiol-dependent (k(2)) pathways was slower from dialkyl Te(IV) species derived from 2 than from diaryl Te(IV) species derived from 3. The chelating dimethylaminoethyl ligand of Te(IV) species derived from 4 diminished k(1) by a factor of 50 and k(2) by a factor of 3 (relative to the 3-derived species).

Remarkable activity of a novel cyclic seleninate ester as a glutathione peroxidase mimetic and its facile in situ generation from allyl 3-hydroxypropyl selenide

1,2-Oxaselenolane Se-oxide is a novel cyclic seleninate ester that functions as a remarkably efficient glutathione peroxidase mimetic by catalyzing the reduction of tert-butyl hydroperoxide to tert-butyl alcohol in the presence of benzyl thiol. The seleninate ester can be conveniently generated in situ by oxidation of allyl 3-hydroxypropyl selenide with tert-butyl hydroperoxide. Its catalytic activity surpasses that of several other known GPx mimetics containing cyclic selenenamide structures, which were also tested for comparison.

Synthesis of novel cyclomaltoheptaose (beta-cyclodextrin) derivatives containing the Ebselen key moiety of benzoisoselenazolone

A series of five novel cyclomaltoheptaose (beta-cyclodextrin) derivatives containing benzoisoselenazolone groups have been synthesized as glutathione peroxidase mimics.