Synthesis of 2-substituted 5,7,8-trimethyl-6-hydroxythiochromans and purported syntheses of sulfur-containing analogs of vitamin E

Kinetic study of the reaction of thiophene-tocopherols with peroxyl radicals enlightenings the role of O˙⋯S noncovalent interactions in H-atom transfer

Three new α-tocopherol thiophene derivatives were efficiently synthesized, characterized and used for the first time as chain-breaking antioxidants for the inhibition of the autoxidation of reference oxidizable substrates. The rate constant of the reaction with alkylperoxyl (ROO˙) radicals and the stoichiometry of radical trapping (n) for the thiophene-tocopherol compounds were determined by measuring the oxygen consumption during the autoxidation of styrene or isopropylbenzene, using a differential pressure transducer. The measurement of the reaction with ROO˙ radicals in an apolar solvent at 30 °C showed inhibition rate constants (kinh) in the order of 104 M-1 s-1. To rationalise the kinetic results, the effect of the thiophene ring on the H-atom donation by O-H groups of the functionalized tocopherols was investigated by theoretical calculations. The importance of noncovalent interactions (including an unusual O˙⋯S bond) for the stability of the conformers has been shown, and the O-H bond dissociation enthalpy (BDE(OH)) of these derivatives was determined. Finally, the photophysical properties of these new compounds were investigated to understand if the addition of thiophene groups changes the absorption or emission spectra of the tocopherol skeleton for their possible application as luminescent molecular probes.

The Synergy between Glutathione and Phenols-Phenolic Antioxidants Repair Glutathione: Closing the Virtuous Circle-A Theoretical Insight

Glutathione (GSH) and phenols are well-known antioxidants, and previous research has suggested that their combination can enhance antioxidant activity. In this study, we used Quantum Chemistry and computational kinetics to investigate how this synergy occurs and elucidate the underlying reaction mechanisms. Our results showed that phenolic antioxidants could repair GSH through sequential proton loss electron transfer (SPLET) in aqueous media, with rate constants ranging from 3.21 × 10⁶ M^-1 s^-1 for catechol to 6.65 × 10⁸ M^-1 s^-1 for piceatannol, and through proton-coupled electron transfer (PCET) in lipid media with rate constants ranging from 8.64 × 10⁶ M^-1 s^-1 for catechol to 5.53 × 10⁷ M^-1 s^-1 for piceatannol. Previously it was found that superoxide radical anion (O₂^•-) can repair phenols, thereby completing the synergistic circle. These findings shed light on the mechanism underlying the beneficial effects of combining GSH and phenols as antioxidants.

Brønsted acid-catalyzed solvent-controlled regioselective hydrothiolation and diastereoselective cascade cyclization of dienes

A highly regio- and diastereo-selective Brønsted acid-catalyzed tandem hydrothiolation/Friedel-Crafts reaction of linear 1,3-dienes has been developed for the first time, which provides a metal-free and atom-economic way of constructing thiochromane derivatives. Meanwhile, by changing the solvent, 4,3-addition hydrothiolation of 1,3-dienes was also discovered. The origin of the observed selectivity was explained by density functional theory calculations.

Expeditious Access to Spiro-Fused 2,5-Cyclohexadienones via Thio(seleno)cyanative ipso-Cyclization

A facile oxidative dearomatization of N-(p-methoxyaryl)propiolamides has been established for the synthesis of spiro-fused 2,5-cyclohexadienone frameworks via thio(seleno)cyanative ipso-cyclization in the presence of ceric ammonium nitrate (CAN) as the oxidant. The present method, involving the formation of C-S and C-C bonds, was also extended to (p-methoxyaryl)propiolates for thiocyanative ipso-cyclization. Furthermore, the obtained chalcogeno-spirocyclohexadienones were transformed into uniquely functionalized spirocyclohexadienone derivatives.

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.

Ditocopheryl Sulfides and Disulfides: Synthesis and Antioxidant Profile

Symmetrical ditocopheryl disulfides (Toc)₂ S₂ and symmetrical and unsymmetrical ditocopheryl sulfides (Toc)₂ S were simply prepared under remarkably mild conditions with complete control of the regiochemistry by using δ-, γ-, and β-tocopheryl-N-thiophthalimides (Toc-NSPht) as common starting materials. The roles of sulfur atom(s), H-bond and aryl ring substitution pattern on the antioxidant profile of these new compounds, which were assembled by linking together two tocopheryl units, are also discussed.

Thiophenols, Promising Scavengers of Peroxyl Radicals: Mechanisms and kinetics

The activity of 12 thiophenols as primary antioxidants in aqueous solution has been studied using density functional theory. Twelve different substituted thiophenols were tested as peroxyl radicals scavengers. Single electron transfer (SET) and formal hydrogen transfer (FHT) were investigated. The SET mechanism was found to be the main mechanism, with rate constants that are close to the diffusion limit, which means that these thiophenolic compounds have the capacity to scavenge peroxyl radicals before they can damage biomolecules. All 12 thiophenolic compounds react faster with methylperoxyl than with hydroperoxyl radicals. In addition, it was found that pH plays an important role in the reactivity of these compounds. © 2019 Wiley Periodicals, Inc.

Photocatalytic difunctionalisations of alkenes with N-SCN sulfoximines

Reacting N-Br sulfoximines with ammonium thiocyanate leads to unprecedented sulfur reagents, which add to vinyl arenes under photocatalysis via N-centered sulfoximidoyl radicals.

Vitamin E Chemistry. Studies into Initial Oxidation Intermediates of α-Tocopherol: Disproving the Involvement of 5a-C-Centered “Chromanol Methide” Radicals

Contrary to concepts handed down in the literature from the early days of vitamin E research, one-electron oxidation of vitamin E does not involve 5a-C-centered radicals. A combined approach of analytical techniques, in particular electron paramagnetic resonance spectroscopy (EPR), organic synthesis of special derivatives, isotopic labeling, kinetic studies, and computational chemistry was used to re-evaluate the one-electron and two-electron oxidation chemistry of alpha-tocopherol (alpha-toc). EPR in combination with 5a-13C-labeled compounds provided no indication of the involvement of 5a-C-centered radicals. Oxidation of special tocopherol derivatives were used to disprove the occurrence of 5a-C-centered one-electron intermediates. Additionally it was shown that those vitamin E reactions that were commonly evoked to plead for the involvement of C-centered tocopheryl radicals actually proceeded via heterolytic, i.e., non-radical, intermediates. The results will help to clear widely spread misunderstandings about the chemistry of vitamin E and will have mechanistic implications for the synthesis of tocopherol-based supramolecular structures and 5a-substituted alpha-tocopherol derivatives.

Exploring a possible way to synthesize novel better antioxidants based on vitamin E: a DFT study

The promoting effect of heterocyclic ring and heteroatom on the antioxidant properties of vitamin E has been investigated systemically by using density functional theory. The calculated results show that the heteroatom plays a more important role in promoting the antioxidant properties than the heterocyclic ring, indicating that replacing O atom by S or Se is impossible to synthesize the novel better antioxidants. Furthermore, the bond dissociation enthalpy (BDE) and ionization potential (IP) of the corresponding molecules are decreased dramatically when the O atom is replaced by NH. In addition, the calculated results also reveal that reducing the atom number of heterocyclic ring is an ideal way to synthesize novel antioxidants with better antioxidant activity than vitamin E.

Enzyme-linked immunosorbent assay for the organophosphorus insecticide fenthion. Influence of hapten structure

Novel procedures for fenthion hapten synthesis are described following three different strategies. The first one attaches the spacer arm to the oxygen atom of the aromatic fenthion ring. The second one binds it through the thiophosphate moiety and the third strategy consists on the attachment of the spacer arm to the sulfur atom of the molecule. A total of nine fenthion haptens have been synthesized and used for immunoreagent production (protein conjugates and polyclonal antibodies). The developed conjugate-coated format ELISA exhibited a detection limit of 0.03 ng/ml, an IC50 of 0.05 ng/ml and a dynamic range between 0.03 and 1 ng/ml. There was little or no cross-reactivity to similar tested compounds. The ELISA was used to determine fenthion residues in white wine samples without any purification or preconcentration steps. Recoveries ranged between 81% and 113%.

A short and efficient synthesis of the pharmacological research tool GW501516 for the peroxisome proliferator-activated receptor delta

The most potent and selective peroxisome proliferator-activated receptor delta (PPARdelta) agonist GW501516 (1) was synthesized in 4 steps and 78% overall yield starting from o-cresol by using a one-pot regiocontrolled dialkylation of mercaptophenol 5 as the key step.

Intramolecular homolytic substitution at selenium: synthesis of novel selenium-containing vitamin E analogues

Treatment of 1-(benzylselenenyl)-5-butyl-5-nonanol (10) with oxalyl chloride followed by the sodium salt of N-hydroxypyridine-2-thione afforded the corresponding pyridine-2-thione-N-oxycarbonyl (PTOC) oxalate ester which was not isolated but immediately heated to provide 2,2-dibutylselenane (7). This transformation presumably involves a tertiary alkyl radical that undergoes intramolecular homolytic substitution at selenium with loss of the benzyl radical to provide the selenium-containing ring system (7). A similar protocol, when applied to 1-(2-benzylselenenyl-5-methoxyphenyl)-3-methyl-3-heptanol (18) and 1-(2-benzylselenenyl-5-methoxyphenyl)-3,7,11,15-tetramethyl-3-hexadecanol (19), followed by deprotection, afforded the selenium-containing alpha-tocopherol analogues 4 and 1f, respectively, in moderate yields. To the best of our knowledge, these transformations represent the first examples of tertiary radicals involved in homolytic substitution chemistry at selenium.

The antioxidant profile of 2,3-dihydrobenzo[b]furan-5-ol and its 1-thio, 1-seleno, and 1-telluro analogues

A novel synthesis of 2,3-dihydrobenzo[b]thiophene-5-ol based on intramolecular homolytic substitution on sulfur was reported. The "antioxidant profile" of the series of 2,3-dihydrobenzo[b]furan-5-ol (2a) its 1-thio (2b), 1-seleno (2c) and 1-telluro (2d) analogues was determined by studies of redox properties, the capacity to inhibit stimulated lipid peroxidation, the reactivity toward tert-butoxyl radicals, the ability to catalyze decomposition of hydrogen peroxide in the presence of glutathione, and the inhibiting effect on stimulated peroxidation in liver microsomes. The one-electron reduction potentials of the aroxyl radicals corresponding to compounds 2a-2d, E degrees (ArO(*)/ArO(-)) were 0.49, 0.49, 0.49, and 0.52 V vs NHE, respectively, as determined by pulse radiolysis. With increasing chalcogen substitution the compounds become slightly more acidic (pK(a) = 10.6, 10.0, 9.9, and 9.5, respectively, for compounds 2a-2d). By using Hess' law, the homolytic O-H bond dissociation enthalpies of compounds 2a-2d (340, 337, 336, and 337 kJ mol(-)(1), respectively) were calculated. The reduction potentials for the proton coupled oxidation of compounds 2a-2d (ArOH --> ArO(*) + H(+)) as determined by cyclic voltammetry in acetonitrile were 1.35 (irreversible), 1.35 (quasireversible) 1.13 (reversible), and 0.74 (reversible) V vs NHE, respectively. As judged by the inhibited rates of peroxidation, R(inh), in a water/chlorobenzene two-phase lipid peroxidation system containing N-acetylcysteine as a thiol-reducing agent in the aqueous phase, the antioxidant capacity increases (2d > 2c = 2b > 2a) as one traverses the group of chalcogens. Whereas the times of inhibition, T(inh), were slightly reduced for the oxygen (2a) and sulfur (2b) derivatives in the absence of the thiol-reducing agent, they were drastically reduced for the selenium (2c) and tellurium (2d) derivatives. This seems to indicate that the organochalcogen compounds are continuously regenerated at the lipid aqueous interphase and that regeneration is much more efficient for the selenium and tellurium compounds. The absolute rate constants for the oxidation of compounds 2a-2b by the tert-butoxyl radical in acetonitrile/di-tert-butyl peroxide (10/1) were the same-2 x 10(8) M(-)(1) s(-)(1). Whereas the oxygen, sulfur, and selenium derivatives 2a-2c were essentially void of any glutathione peroxidase-like activity, the organotellurium compound 2d accelerated the initial reduction of hydrogen peroxide, tert-butyl hydroperoxide, and cumene hydroperoxide in the presence of glutathione 100, 333, and 213 times, respectively, as compared to the spontaneous reaction. Compounds 2a-2d were assessed for their capacity to inhibit lipid peroxidation in liver microsomes stimulated by Fe(II)/ADP/ascorbate. Whereas the oxygen, sulfur, and selenium compounds showed weak inhibiting activity (IC(50) values of approximately 250, 25, and 13 microM, respectively), the organotellurium compound 2d was a potent inhibitor with an IC(50) value of 0.13 microM.