266 BIOCHEMICAL SYSTEMATICS 



A non-lactone sesquiterpene of the guajol type, partheniol, is found 

 in Parthenium argentatum. 



H3C OH 



partheniol 



The presence of two similar sesquiterpenes, parthenin and 

 ambrosin in the genera Parthenium and Ambrosia, the latter sub- 

 stance actually occurring in both genera, is suggestive of a relation- 

 ship between the two genera not readily apparent by their taxonomic 

 disposition (that is, they are often treated as belonging to different 

 tribes or sub-tribes). The suggested relationship is further strength- 

 ened by the discovery of a third substance, coronopilin (1,2-dihydro- 

 parthenin) in both genera (Herz and Hogenaur, 1961). 



Although the eudesmol and guajol types of sesquiterpenes 

 may not appear to be closely similar, the principal difference between 

 the two lies in the type of cross linkage present. In the eudesmol type 

 a C — C linkage yields a pair of six-membered rings; in the guajol type 

 a C — C linkage yields a seven-membered and five-membered pair. 

 This minor difference between the two types of sesquiterpenes may 

 indicate close biosynthetic similarity. Therefore, it is not surprising 

 to find these compounds restricted to a few rather closely related 

 genera or even together in a single genus. 



The methyl substitution at position 5 in helenalin and other 

 guajol derivatives is considered by Herz to represent a shift from 

 position 4 of a substance such as partheniol, illustrated above. 



It is important to know whether the eudesmol or the guajol 

 type is more primitive, but evidence is insufficient at this time to 

 allow even useful speculation. It seems that the Anthemideae are 

 much more versatile in terpene and sesquiterpene synthesis than 

 other tribes of the family Compositae noted. 



At least 75 different sesquiterpenes are reported from a num- 

 ber of different families, including those of the gymnosperms, dicots, 

 and monocots (Karrer, 1958). However, within any closely circum- 

 scribed, natural biological group, the sesquiterpenes present fall 

 similarly into more or less natural chemical sub-types. Thus, only the 

 eudesmol and guajol groups of sesquiterpenes are encountered among 

 the plant genera discussed in this section. Sesquiterpenes of the bis- 

 abolen type, which may be considered more simple in chemical terms, 

 correspondingly have a broader and more complex distribution, not 

 necessarily indicative of phylogenetic relationship. 



