168 



TERPENOIDS AND STEROIDS 



poration of label at the circled positions in the carbon skeleton: 



A pentacyclic compound formed from squalene might be expected to have a structure and 

 label distribution as follows: 



or 



A few natural compounds of the second structure are known, and a possible mechanism of 

 rearrangement has been proposed to account for the more usual structures (112, 113). 



Other labelling experiments have shown the conversion of acetate into a-pinene (114) 

 sterols (115), solanidine, (116), and rubber (117); the conversion of mevalonic acid into 

 Q-pinene (114), sterols (118, 119), cardiac glycosides (120, 121), triterpenoids (118, 119) 

 and rubber (122); the conversion of /3-methylcrotonic acid into pulegone (123), a-pinene 

 (110), and rubber (117); and the conversion of iso-pentenyl pyrophosphate into rubber by 

 fresh latex (124). Biosynthesis of the terpenoids of peppermint (Mentha piperita) and 

 spearmint ('Me«//ia spicata) hSLS received considerable attention, and detailed pathways have 

 been presented for interconversion of the various monoterpenoids in these plants (125, 126) 

 as well as the genetic determination of the composition of the essential oils (127). 



It has been interesting to find that the lactone ring of cardiac glycosides is evidently 

 not made from mevalonic acid, but possibly is derived by the addition of two acetate units 

 to a C21 steroid (19, 121). 



