About the impact of oak ellagitannins on wine odoriferous thiols under acidic and oxidation conditions

The decrease of the level of characteristic odoriferous thiols in wines has been in part imputed to chemical transformations that take place during aging of wines in oak-made barrels and subsequent storage in bottles. Oxygen dissolved in wines certainly plays a role in such chemical transformations, such as the oxidation of catecholic catechins into thiol-trapping ortho-quinones. However, wines aged in barrels or supplemented with oak chips contain pyrogallolic C-glucosidic ellagitannins, which can also trap thiols in mildly acidic and oxygenated wine solutions by both nucleophilic substitution and oxidative addition reactions. The C-glucosidic ellagitannin vescalagin and methyl gallate were chosen as substrates to investigate these reactions in the presence of two well-known wine odoriferous thiols, 3-sulfanylhexan-1-ol (3SH) and 2-furanmethanethiol (2FMT). Three novel thio-vescalagin condensates were thus hemisynthesized under acidic conditions, and the FeCl3-mediated oxidation of methyl gallate furnish different thiogallate adducts under conditions applicable to the oxidative hemisynthesis of thio-ellagitannin adducts. (C) 2015 Elsevier Ltd. All rights reserved.