264 



BIOCHEMICAL SYSTEMATICS 



4. eremophilon type 



or 



5. guajol type 



Sesquiterpene derivatives of type five are present in the genus 

 Helenium wherein they are recognized by a bitterish taste. These 

 compounds occur as lactones such as helenalin, or the more complex 

 tenulin: 



O^HaC 



HsCqp^ 



H2C 



helenalin 



Herz and his coworkers (Herz et ah, 1960; Herz £ind 

 Hogenaur, 1962) have been investigating certain Helenium species 

 along with species of several related genera. Sesquiterpene lactones of 

 the helenalin type occur in all three sections of Helenium (Table 

 13-5) as well as a number of other genera. Helenalin is found in 

 Actinospermum also, and though helenalin itself has not been found 

 in Balduina, a related substance, balduilin, is present in this genus. 

 The close morphological similarity between Actinospermum and 

 Balduina indicates that the presence of these similar sesquiterpenes 

 is not coincidental. Helenium is placed in the tribe Helenieae while 

 Actinospermum and Balduina are both placed in the tribe Helian- 

 theae mainly on the basis of technical features of the capitulum. 

 However, Rock (1957) has independently suggested (on morphological 

 grounds) that the three genera are closely related, an interpretation 

 which is supported by this sesquiterpene chemistry. However, sesqui- 

 terpenes also occur in a number of other genera of the family Com- 

 positae, including Artemisia, Inula, Iva, Ambrosia, Parthenium, and 

 Balsamorhiza. Artemisia belongs to the tribe Anthemideae and Inula 

 to the tribe Inuleae, while the last four genera are placed in the tribe 

 Heliantheae. Before discussing the characteristic sesquiterpenes of 

 the various tribes it is pertinent to note that a number of lower 

 terpenes are found in the tribe Anthemideae, especially in Artemisia 

 species: 1,8-cineole {Artemisia); 1-camphor {Artemisia, Achillea); 

 fenchol {Artemisia); and thujon {Tanacetum) (Karrer, 1958). 



* The structure of tenulin is modified to conform more closely to inferences de- 

 rived from the revised formula of isotenulin, helenalin, and balduilin (Herz et al, 1961). 



