High stability organic tanning using plant polyphenols. Part 2. The mechanism of the vegetable tannin-oxazolidine tannage.

Catechin was used as a model for condensed vegetable tannins, to elucidate the reaction between the flavonoid ring system and oxazolidines in the comb ination tannage. Using H-1-NMR, C-13-NMR and paper chromatography, it is sh own that the crosslinking reaction occurs predominantly at the C-6 and C-8 of the phloroglucinol A ring of catechin. The crosslinker reacts at the C-5 of oxazolidine, by ring opening and reacting via the N-methylol intermedia te.