Linkage and Stereochemistry Characters of Phenolic Antioxidant Product Formation

The present study developed a smart and novel strategy to elucidate the linkage and stereochemistry characters during phenolic antioxidant product formation. A series of phenolic isomers or analogues were treated with 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide radical, to create 16 antioxidant dimerization reactions in aqueous solution. The products were rapidly identified by ultraperformance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass-spectrometry. Through a systematic function–structure relationship analysis of these reactions and theoretical calculations, it is concluded that the phenolic antioxidant product is formed via linear linkage or furanocyclic linkage. The linear linkage is fulfilled via a radical coupling and controlled by the O–O linkage exclusion, meta-linkage exclusion, and catechol-activated principles. However, when an exocyclic π-bond conjugates with the phenolic core and is affixed at the −OH para-position, the furanocyclic linkage may occur via a subsequent intramolecular Michael addition. The intramolecular addition always lacks Re-attack to show “α,β diastereoselectivity.” The α,β diastereoselectivity is the stereochemistry character of furanocyclic linkage during phenolic antioxidant product formation. All these novel findings can benefit not only the field food science but also other fields as well.