| Abstract: | Summary From the ranks of the colourless hydroxyflavans, four differenthydroxyflavan-3-ols(catechins and epi-catechins), as well as eight hydroxyflavan-3,4-diols, have so far been encountered in nature. These substances are also procurable by synthesis. On treatment with hot water or dilute acids, they are easily converted into soluble tannins or insoluble phlobaphenes. The mechanism of the self-condensation is explained using catechin via the isolation of a dimerie product. A probable path for the self-condensation of the diols has been indicated. In contrast to the products of their self-condensation, the monomeric polyhydroxyflavans are not real tannins. A particular accumulation of phenolhydroxyl groups in the molecule, the tendency to form supersaturated solutions and low solubility in water in the — generally not attainable — crystalline state are prerequisites for tanning properties. In nature, the self-condensation of polyhydroxyflavans proceeds (e. g. in the wood ofAcacia catechu or in Quebracho Wood) without the assistance of enzymes. Dehydrogenative polymerisation occurs with formation of brown or red phlobaphenes (e.g. in cocoa beam) which are generally insoluble. |
