182 



FLAVONOIDS AND RELATED CON4POUNDS 



Others occur. Disaccharides are occasionally present in the so called "biosides". In 

 contrast to the anthocyanins (see below) there are never two sugar residues attached to 

 different hydroxyl groups. A few flavone -sugar derivatives are known in which the sugar 

 is attached by a carbon-carbon bond rather than as a glycoside. The best known of these 

 is vitexin, a glucose derivative of apigenin: 



;H20H 



other such compounds are cited by Whalley (29) and Horhammer et al. (30). Bate-Smith 

 and Swain have proposed the name "glycoflavonol" for such compounds (31). They might 

 be formed by an aldol condensation between the carbonyl of the sugar and the active meth- 

 ylene group of an A-ring precursor like acetoacetate (see below under "Biosynthesis"). 

 The glycosides are naturally less soluble in organic solvents and more soluble in water 

 than the corresponding agly cones. They are also less colored than the aglycones, some 

 being colorless when in neutral or acidic solution. However, they become bright yellow 

 or orange in alkali and may be detected by exposing colorless plant parts to ammonia. 

 This appearance of color is due to salt formation and assumption of a quinoid structure 

 in ring B: 



Alkaline solutions of the flavonols (but not flavones) are oxidized in the air but not so 

 rapidly as to preclude the use of alkaline solutions in their preparation. Table 2 summa- 

 rizes the hydroxylation patterns of some well-known flavones, flavonols, and isoflavones. 

 Structures of a few other derivatives are given in Table 3 to illustrate some varieties of 

 methylation and glycosidation which are found. 



