- Optimization of the antioxidant activity of hydroxy-substituted 4-thiaflavanes: A proof-of-concept study
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The design and the synthesis of a new family of hydroxy-4-thiaflavanes, in which the reactive phenolic OH is ortho to the sulfur atom of the benzofused oxathiin ring, allowed to prepare antioxidants that show rate constants for the reaction with peroxyl radicals (kinh), and bond dissociation energies (BDE), of the ArO-H group identical to those of α-tocopherol, the main component of vitamin E and the most effective lipophilic antioxidant known in nature. The peculiar conformation of the six-membered heterocyclic ring prevents the formation of an intramolecular hydrogen bond between the OH group and the S atom, while ensuring a good stabilization by electron donation of the phenoxyl radical formed after the reaction with peroxyl radicals. The preparation of these compounds was achieved through an inverse electron demand hetero Diels-Alder reaction of styrenes with o-thioquinones, in turn prepared from accurately designed 1,3-dihydroxy arenes. Properly arranging the substitution pattern on the aromatic ring, as in derivatives 9 and 11, allowed to reach values of kinh up to 4.0× 106 M-1 s -1 and BDE(OH) of 77.2 kcal mol-1. This approach represents an innovative way to obtain highly active antioxidants without using strongly electron donating alkylamino groups which are associated with adverse toxicological profiles. Sulfur makes the difference (again): The stereoelectronic features triggered by the sulfur atom in the benzo-fused six-membered ring of 4-thiaflavanes have been exploited to design a new family of phenolic antioxidants that show an ability to react with peroxyl radicals identical to that of α-tocopherol, the main component of vitamin E and the most potent natural lipophilic antioxidant (see scheme). Copyright
- Viglianisi, Caterina,Bartolozzi, Maria Grazia,Pedulli, Gian Franco,Amorati, Riccardo,Menichetti, Stefano
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p. 12396 - 12404
(2011/11/29)
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- COMPETITION BETWEEN NUCLEAR AND SIDE-CHAIN SUBSTITUTION IN THE OXIDATION OF SOME ALKYLAROMATIC COMPOUNDS BY CERIUM(IV) AMMONIUM NITRATE AND COBALT(III) ACETATE
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The distribution between nuclear and side-chain substitution (N:S ratio) in the oxidations of m-methoxytoluene, 2-methylnaphtalene, mesitylene, and fluorene by cerium(IV) ammonium nitrate (CAN) and cobalt(III) acetate in acetic acid has been determined.The two oxidants exhibit remarkably different behaviours, the propensity for nuclear substitution being much stronger with CAN than with Co(OAc)3.For example, with m-methoxytoluene, CAN affords only products of nuclear acetoxylation, whereas Co(OAc)3 gives side-chain acetoxylation exclusively.The N:S ratio and the isomeric distribution for the CAN-induced reactions are consistent with a mechanism involving a common radical cation intermediate for the side-chain and nuclear substitution.The same mechanism might hold in the reactions with Co(OAc)3; however, in this case, the simultaneous operation of two different mechanisms is an additional possibility: a radical cation mechanism for the nuclear substitution and a hydrogen atom transfer mechanism for the side-chain reaction.
- Baciocchi, Enrico,Rol, Cesare,Sebastiani, Giovanni V.
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p. 513 - 518
(2007/10/02)
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