Reaction of condensed tannins with ammonia

Reaction of condensed (flavonoid) mimosa tannin extracts with concentrated aqueous ammonia has been investigated. For that purpose, catechin was also used as a flavonoid model compound and treated in similar conditions. Solid state CP-MAS 13C NMR and MALDI-ToF spectroscopy studies revealed that, unlike what was recently theorised, amination is not always regioselective and leading to the conversion of one single OH group in C4′ into NH2. In the present study, new reactions have been evidenced, clearly leading to multiamination of several phenolic hydroxyl groups, heterocycle opening, and oligomerisation and cross-linking through the formation of N bridges between flavonoid units.

► Condensed (flavonoid) tannins reacted with concentrated aqueous ammonia. ► Catechin was also used as a model compound and treated in similar conditions. ► Solid state 13C NMR and MALDI-ToF showed that amination is not always regioselective. ► Multiamination of several phenolic hydroxyl groups and heterocycle opening was detected ► Oligomerisation and cross-linking by N bridges between flavonoids were detected.