132 TERPENOIDS AND STEROIDS 



Additional examples of bicyclic and tricyclic sesquiterpenoids are also shown in 

 Table 8 to indicate some of the more unusual structures that are found. One of the most 

 interesting of these is iresin whose bicyclofarnesol type structure is similar to that of 

 many diterpenoids but otherwise unknown among the sesquiterpenoids (10). Acorone is 

 the only naturally occuring spirane so far discovered. 



DITERPENOIDS 



The diterpenoids are C20 compounds which may be formally regarded (with some 

 exceptions) as derived from four isoprenoid residues. Because of their high boiling 

 points they are not usually found in volatile oils of plants although a few of the lower boil- 

 ing ones may be. They are found in resins and in the resinous high boiling fractions re- 

 maining after distillation of essential oils. The rosin remaining after distilling pine tur- 

 pentine, for instance, is rich in diterpenoids. Their great complexity and difficulty of 

 separation has resulted in only a relatively few completely known structures in this group 

 compared to the vast number which probably occur in nature. As with the lower terpenes, 

 hydrocarbons, alcohols, ethers and acids are all known in this group. The only important 

 acyclic member is the alcohol phytol which forms a part of the chlorophyll molecule. 



CH,OH 



* phytol 



Many of the cyclic diterpenoids may be regarded as derived from phytol by ring 

 closures, but others do not show the head-to-tail type of linkage. Some examples are 

 given in Table 9. Diterpenoids have been reviewed by Tsutsui and Ashworth (11). 



TRITERPENOIDS 



Since C25 terpenoids are not found in nature, there is a great increase in complexity 

 on going from the diterpenoids to the C30 triterpenoids. Only a few of them have been 

 highly purified and have had their structures completely determined. Triterpenoids are 

 widely distributed in plant resins, cork, and cutin. The so-called resin acids are triter- 

 penoid acids frequently associated with polysaccharide gums in gum resins. Triterpenoid 

 alcohols occur both free and as glycosides. Many of the glycosides are classed as sapo- 

 nins (q. V. ). Triterpenoid hydrocarbons and ketones are also known. Triterpenoids are 

 reviewed by Jeger (12). 



The only important acyclic triterpenoid is the hydrocarbon squalene which was first 

 isolated from shark liver oils but is also found in some plant oils (e. g. olive oil). Since 

 it is presumed to be an intermediate in steroid biosynthesis, it must be made at least in 

 small amounts by all organisms which synthesize steroids. 



