166 



TERPENOIDS AND STEROIDS 



action is illustrated for the simple case of a-pinene which forms bornyl chloride in the 

 presence of hydrochloric acid: 



H* 



In more complicated cases several different rearrangements may occur before the final 

 structure is attained so that profound modifications are introduced. The general refer- 

 ences may be consulted for critical discussion of this type of reaction. It should, however, 

 be noted here that stereochemical configuration is frequently retained, and therefore it 

 is probably an oversimplification to picture a free carbonium ion as an intermediate. 



Applications of the Wagner- Meerwein rearrar^ement to theories of terpene biosyn- 

 thesis are very common in the literature. A simple example has been suggested by 

 Enzeil and Erdtman (106) to explain the formation of cuparene from a bisabolene type 

 skeleton: 



A more complex series has been used by Wenkert (107), Valenta and Wiesner (108), and 

 Whalley (109) to account for some diterpenoid structures including the diterpenoid alka- 

 loids (see opposite page). There is tracer evidence for a small part of this scheme in 

 that Sandermann and Stockmann (110) have shown that from the labeling pattern abietane 

 type compounds are formed by rearrangement of pimarane type compounds in Pinus 

 silvestris. 



Some type of rearrangement must be involved in formation of the pentacyclic tri- 

 terpenoids since ring E and its substituents cannot be derived by any simple ring closure 

 from a squalene-type compound. Simple rearrangements, however, can account for the 

 conversion of any one of three basic types to the other two. Synthesis of a soybean sapo- 

 genin has been shown by Arigoni (111) to proceed from mevalonic acid-2-C*^ with incor- 



