178 



FLAVONOIDS AND RELATED COMPOUNDS 



Although only one purified leucoanthocyanidin has been found to be a glycoside, sev- 

 eral instances are known where they apparently exist as part of a colloidal polymer, pos- 

 sibly bound to a hemicellulose by glycosidic bonds. Polymers similar to some of the 

 natural ones have also been prepared by treating the monomer with 2N HCl in the cold 

 (14). Obviously in these no hemicellulose would be present. These polymers may be dis- 

 tinguished from the monomers by their insolubility in both water and ethyl acetate, whereas 

 the monomers (and catechins) are soluble in ethyl acetate but not in water (15). The leuco- 

 anthocyanidins, by definition, are compounds which form anthocyanidins on heating with 

 acid. The reaction is oxidative, involving loss of a hydrogen atom as well as dehydration: 



H OH 



H* 



Unfortunately their name is a misnomer since even under the most carefully controlled 

 conditions only about 25% of the theoretical yield of anthocyanidin is obtained (16). The 

 mechanism of this reaction and other aspects of leucoanthocyanidin chemistry are re- 

 viewed by Bokadia (17). The major products on heating with acid are red-brown polymers 

 loosely known as phlobaphenes or tannin-reds. The catechins under these conditions also 

 form similar brown polymers. Freudenberg and Weinges (18) have shown that compounds 

 with 4' and 7 hydroxy 1 groups are most susceptible to polymer formation. According to 

 Freudenberg et al. (19) the initial step in formation of these polymers is a condensation 

 between C-1 of one monomer and C-6 of another: 



The yield of this dimer is only about 10%, so that the formation of other types of product 

 is by no means excluded (18). Polymers of this sort involving unknown numbers of units 

 probably constitute the group of compounds known as "condensed tannins", "phlobatannins", 

 or "catechol tannins". They possess many of the typical tannin properties as described 

 for the hydrolysable tannins (Chap. 4) and are very widely distributed in nature. Accord- 

 ing to Russel (20) all tannins produced by normal physiological processes in wood, bark, 

 leaves and roots are phlobatannins. The polymeric leucoanthocyanidins may also have 

 this type of structure (21). Oxidation of these colorless polymers on heating with acid 

 forms the colored phlobaphenes, as well as some anthocyanidin if a 3, 4-diol structure is 

 present and splitting of the polymer occurs. These complex and little-understood reac- 

 tions are further complicated by the loose terminology which has been used. A rough 

 summary is diagrammed as follows: 



