TERPENOIDS 249 



ences between the above Libocedrus species are examples where 

 collaboration is essential. The chemist is sometimes led to feel that the 

 calamitous phenomena of convergence may have misled the botanists. 



Since Erdtman in the same article also calls attention to bio- 

 chemical convergence, it is pertinent to inquire how one may deter- 

 mine which form of evidence reflects convergence in cases of 

 apparently conflicting judgments. The botanist may justifiably expect 

 the chemist to provide satisfactory proof that biochemical convergence 

 is not providing him with spurious chemical indication of relationship. 



Another group of substances some of which are terpenoid in 

 character are the saponins. Some of the saponins are triterpenes 

 while others are steroids. Although saponins have been known for 

 many years there has been relatively little attention given to them in 

 comparison with the commercially more important lower terpenes. 

 However, in the last two decades there has been renewed interest in 

 the steroidal saponins particularly with regard to sources which could 

 supply substances utihzable in the synthesis of physiologically active 

 steroids for medicinal use. A number of broad surveys have now been 

 undertaken such as that of Ricardi et al. (1958) who examined 2,894 

 Chilean species for saponins. They found over 600 species to be saponin- 

 producing. 



Although there is now considerable knowledge of steroid 

 metabolism in animals, there is apparently httle known of plant 

 steroidal biosynthesis. Interestingly, Heftmann et al (1961) in a 

 study of the biosynthesis of the steroidal sapogenin, diosgenin, of 

 Dioscorea, found that mevalonic acid, a very efficient precursor of 

 animal sterols, failed to become incorporated into diosgenin. It 

 appeared that mevalonic acid was metaboHzed, however, and the full 

 significance of these results is still unclear (see footnote, p. 232 of this 

 chapter). 



A number of surveys of steroidal saponins have been con- 

 ducted (Marker et al, 1943; Marker et al, 1947; Anzaldo et al, 1956, 

 1957). Marker and his coworkers conducted an extensive survey of 

 over 400 Mexican and United States species and discovered a series of 

 apparently related sapogenins of a type such as hecogenin illustrated 

 below: 



' o c-c 



HO 



hecogenin 



