140 



TERPENOIDS AND STEROIDS 



ergosterol (Triticum sativum) 



spinasterol (Spinacia oleracea) 



Nomenclature of the steroids is complicated by the necessity of distinguishing be- 

 tween possible sterochemical configurations. In the vast majority of plant steroids the 

 rings are all joined to one another by trans linkages (see page 122). The result of this 

 is that the entire ring system is coplanar and substituent groups extend perpendicularly 

 to the plane of the rings. The methyl group at C-10 is defined as sticking up. Any group 

 trans to it is described as a and groups cis to it /3, Steroids with the A/B ring juncture 

 trans may therefore be described as So, since the hydrogen on C-5 is below the plane. 

 All natural sterols have the C-3 hydroxyl group and the C-17 side chain j3. A greater 

 variety of configurations is found in the tetracyclic triterpenoids which closely resemble 

 the steroids in other respects (see above). Other nomenclature rules may be found in 

 the general references. 



STEROLINS AND SAPONINS 



As has been mentioned previously, many terpenoid and steroid alcohols exist in 

 nature not as free alcohols but as glycosides. Names have been assigned to certain types 

 of these glycosides — "sterolins" "saponins", "cardiac glycosides", etc. The cardiac 

 glycosides and glycosyl alkaloids will be considered in later sections. 



Sterolins or sterol glycosides are widespread in unrelated plant species. They are 

 found along with free sterols in the unsaponifiable lipid fraction but may be distinguished 

 from free sterols by their much higher melting points and low solubility in such fat sol- 

 vents as ethyl ether. They are distinguished from saponins (see below) by their insolu- 

 bility in water and lack of toxicity to animals. The first sterolin to be discovered was 

 ipuranol oilpomoea purpurea. It is a /3-sitosterol glycoside. Similar glycosides of the 

 higher plant sterols are also known; but as /B-sitosterol is the most widely distributed 

 plant sterol, so its glycosides are the commonest sterolins. 



