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FLAVONOIDS AND RELATED COMPOUNDS 



Although not indicated by these partial formulas, hydroxyl groups are normally present 

 on the aromatic rings or they may be found combined as methoxyl groups or glycosides. 



In addition to the flavonoids other compounds such as xanthones, condensed tannins, 

 etc. seem to fit into a natural grouping; and they will also be discussed in this chapter. 



The flavonoids include many of the most common pigments and occur throughout the 

 entire plant kingdom from the fungi to the angiosperms. They are found both in the flow- 

 ers and in the vegetative parts of the higher plants. In addition to their possible function 

 as flower pigments in attracting pollinating birds and insects, various other roles have 

 been suggested for them in the plant. The growth inhibitor of dormant peach buds is a 

 flavanone, naringenin (1), which is an activator of indoleacetic acid oxidase (2). Other 

 flavonoids inhibit the same enzyme (3) so that plant growth could be controlled by the bal- 

 ance between inhibiting and activating flavonoids. A possible role of flavonoids in the 

 physiology of sexual reproduction was first indicated by the experiments of Moewus (4) 

 which showed the sex-determining action of flavonoids of algal gametes. Later Kuhn and 

 Lttw (5) showed that the inability of two Forsytliia varieties to cross -pollinate was related 

 to the presence of specific flavonoids in the pollen of each one. Much more work along 

 these lines needs to be done to determine whether the widespread occurrence of flavonoids 

 in pollen is functional or just fortuitous. Hartshorne (6) has suggested that they may be 

 related only indirectly to the biochemistry of sex. That is, a particular flavonoid syn- 

 thesis may be possible only in the environment of one or the other sex. In connection 

 with sexual biochemistry it is intriguing to find that certain isoflavones act as estrogens 

 for mammals (7), and their structures may be written so as to bear a steric resemblance 

 to those of the steroid hormones, e. g. : 



genistein 



estradiol 



Sandalwood contains similar estrogenic isoflavone derivatives, pterocarpin and homoptero- 

 carpin (8). To be estrogenic, an isoflavone must have a free 5-OH group (9). It has been 

 suggested that some flavonoids act as antibiotics to protect the plant against attack by 

 parasites. The review of Geissman (10) discusses other possible roles of the flavonoids. 



CATECHINS AND LEUCOANTHOCYANIDINS 



The catechins and leucoanthocyanidins are two groups which show many similarities, 

 differing as they do in only a single, aliphatic hydroxyl group. They are all colorless 

 compounds, existing throughout the plant kingdom but especially in the higher woody plants. 

 Leucoanthocyanidins have been found in ferns but not in plants lower than ferns. They 

 almost never exist as glycosides (hence the name "leucoanthocyanidin" is preferred to 

 "leucoanthocyanin"), but some catechins may occur as esters of gallic acid. One epi- 

 catechin glucoside has been found (11) and one leucoanthocyanidin glucoside (12). Only 



