180 



FLAVONOIDS AND RELATED COMPOUNDS 



FLAVANONES AND FLAVANONOLS 



These compounds occur in very small amounts compared with the other flavonoids. 

 They are either colorless or only slightly yellow. Because of their low concentration 

 and lack of color they have been largely neglected. Glycosides of the flavanones are well- 

 known as, for instance, hesperidin and naringin from citrus fruit peels. (The correspond- 

 ing aglycones are hesperetin and naringenin. ) The flavanonols are probably the least 

 known of the flavonoids, and it is not known whether they normally exist as glycosides. 

 Pacheco (25) has made the most extensive recent study of these compounds, but his ana- 

 lytical procedure would have hydrolyzed any glycosidic bonds if they were present. Only 

 about 8 flavanonols have been isolated. Unlike the leucoanthocyanins, they are stable to 

 hot hydrochloric acid but are decomposed by warm alkali to form chalcones. According 

 to Nord and de Stevens (26) a flavanone type of unit is probably present in bagasse lignin. 

 It is peculiar that no condensed tannins based on the flavanones have been found since 

 flavanones appear to be structurally as suited for polymerization as the catechins or leu- 

 coanthocyanidins. The fact that they are frequently 5- or 7 -glycosides may hinder con- 

 densation at position 6 to give a polymer such as is formed from the aglyconic catechins 

 and leucoanthocyanidins. 



FLAVONES, FLAVONOLS, ISOFLAVONES 



The flavones and flavonols are probably the most widely distributed of all the yellow 

 plant pigments, although the deeper yellow colors of plants are normally due to carotenoids. 

 Some of the flavones and flavonols are still economically important, and luteolin was prob- 

 ably the first dye to be used in Europe. Quercetin is one of the commonest phenolic com- 

 pounds of vascular plants, followed closely by kaempferol (27). Isoflavones are much less 

 important. Only about half a dozen of them are known (28). 



These compounds are usually soluble in hot water and alcohol although a few highly 

 methylated forms are insoluble in water. They vary in hydroxylation from flavone itself, 

 which occurs as dust on primrose {Primula spp. ) flowers, to nobiletin of tangerines 

 (Citrus nobilis). 



CH3O 



flavone 



nobiletin 



However, the most common derivatives have 5 and/or 7 hydroxylation in ring A and 4' 

 hydroxylation in ring B. Ring B is hydroxy lated in positions 3' and 5' only if the 4' position 

 is hydroxylated too. Rarely 2' hydroxylation is found. Additional variation is introduced 

 by methylation of hydroxyl groups to form ethers or methylenation of neighboring hydroxyl 

 groups to form methylenedioxy derivatives. Glycosidic sugar residues may occur in al- 

 most any position, although 4' -glycosides are rare and 6-glycosides unknown. The f la- 

 ones and isoflavones are most commonly 7-glycosides and the flavonols, 3-glycosides. 

 The usual sugars found in the glycosides are glucose, galactose and rhamnose although 



