FLAVONOIDS AND RELATED COMPOUNDS 



193 



CHq 



CH3CH=C — C=0 



calophyllolide 



Betanin, the red pigment of beets, has been the subject of controversy for many 

 years since it contains nitrogen. Recent work indicates that betanin is structurally quite 

 unrelated to the flavonoids, and it is therefore discussed in Chapter 14. Nitrogenous 

 anthocyanins have been reported to occur in several other plants, but even less is known 

 about them (see the reviews by Blank listed under General References). A 4' -amino 

 group has been suggested as a possibility (35). Edulein from the bark of Caswiiroa 

 ediilis has a structure completely analogous to that of the flavones but with nitrogen rather 

 than oxygen in the heterocyclic ring (36): 



Stilbene derivatives have been found in a few unrelated plants. Structurally they 

 represent a C6-C2-C6 group. Their hydroxylation patterns strongly suggest that, like 

 the flavonoids, one ring is formed from acetate and the other from shikimic acid (37). 

 Their chief interest lies in their high toxicity to fungi, fish, insects and mice. They 

 may also act as tannins, and oxidation of them can form condensed products similar to 

 the phlobatannins (38). Some apparently occur as glycosides, but the aglycone structures 

 are given here: 



OCH- 



rhapontigenin 

 (rhubarb) 



