Optimization of the Antioxidant Activity of Hydroxy Substituted 4-Thiaflavanes: A Proof-of-Concept Study

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 benzo-fused oxathiin ring, allowed to obtain antioxidants with rate constants for the reaction with peroxyl radicals (kinh), and bond dissociation energies (BDE), of the ArO-H group identical to those of alpha-tocopherol, the main component of Vit. 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 via 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 represent an innovative way to obtain highly active antioxidants without using strongly electron donating alkyl-amino groups which are associated with adverse toxicological profiles