157 



Terpenoids and Steroids 



CHj— OP2O6H3" 



CH2— OP2O6H3 



Farnesyl Pyrophosphate 



Two moles of farnesyl pyrophosphate then unite head-to-head 

 in what, deuterium experiments indicate/^- " is probably a re- 

 ductive process to form squalene.* All trans-squalene is formed, 

 and this is the only isomer which can cyclize to triterpenes and 

 steroids. ^^ 



The significance of the stereoisomer has been considered, and 

 a generalized scheme devised for the various modes of cycliza- 

 tion of squalene, supported by the current theories of conforma- 

 tional analysis and ionic cyclization.^*^' "• ^® 



Squalene can cyclize with no skeletal rearrangement to form 

 compounds such as the lichen substance, zeorin. It also can 

 rearrange to the lanostane skeleton found so frequently among 

 the steroids of the higher fungi. Lanosterol itself, a known 

 intermediate in the biosynthetic route to cholesterol, has been 

 found in yeast, as has squalene. 



13 H. Rilling, T. T. Tchen and Konrad Bloch, Proc. Nat. Acad. Sci. 

 44 163 (1958). 



'* H. C. Rilling and Konrad Bloch, }. Biol. Chem. 234 1424 (1959). 



* See addendum for a recent modification of this scheme. 



15 Robert G. Langdon and Konrad Bloch, ibid. 200 135 (1953). 



^^ L. Ruzicka, A. Eschenmoser and H. Heusser, Experientia 9 362 

 (1953). 



1^ A. Eschenmoser, L. Ruzicka, O. Jeger and D. Arigoni, Helv. 

 Chim. Acta 38 1890 (1955). 



1® Alexander Todd, "Perspectives in Organic Chemistry," L. Ru- 

 zicka, Bedeutung der theoretischen organischen Chemie fi'ir die 

 Chemie der Terpenverbindungen, Interscience Publishers, Inc., New 

 York, 1956, pp. 265-315; L. Ruzicka, Proc. Chem. Soc, 341-360 

 (1959); Faraday Lecture, History of the isoprene rule. 



