Modification of grape seed and wood tannins to lipophilic antioxidant derivatives

We made lipophilic plant tannins by grafting on them alkyl chains through friendly and cheap chemical modifications. For that purpose, some reactions were applied to tannins from wood (chestnut, oak and quebracho) and from grape seed: esterification by a fatty acid and oxa-Pictet–Spengler coupling. The latter was carried out with variable length methylketones in very mild and simple conditions. After modification, antioxidant properties of the resulting extracts were suitably preserved, according to measurements by inhibition of the induced oxidation of methyl linoleate and by reactivity with the free radical 2,2-diphenyl-picrylhydrazyl. All derivatives acquired a lipophilic character as proven by measurement of their partition coefficient between octanol and water. Since methyl linoleate is a well known model of lipids, these modified extracts should be useful to protect them from oxidation.