194 



FLAVONOIDS AND RELATED COMPOUNDS 



)H 



CH==CH- 



\ / 



piceatannol {Plcea spp. ) 



pinosylvin (Pinus sylvestris) 



CHoO 



CH=CH- 



V / 



pterostilbene (Santalum album) 



The isocoumarin, phyllodulcin (q. v. ), may also be biogenetically related to the stilbenes. 

 Stilbenes are discussed in the review of Geissman and Hinreiner (see under General 

 References). 



Benzophenone derivatives have also been found in a few plants. They may be de- 

 scribed as a Ce-Ci-Cg group, and like the stilbenes are included in this chapter because 

 the hydroxylation patterns suggest a biogenetic relationship to the flavonoids. The most 

 important members of this group are the xanthones which have been used as dyes for 

 hundreds of years. They have the basic structure and numbering system shown below: 



Only about a dozen xanthones are known from flowering plants (39). They are all hydroxy- 

 derivatives. The best known xanthones are the yellow pigment gentisin of Gentiana lutea 

 roots: 



