Chapter 8 



TERPENOIDS AND 

 STEROIDS 



The diverse compounds covered in this chapter are not traditionally grouped together 

 but are usually put under such categories as essential oils, sterols, alkaloids, pigments, 

 cardiac glycosides, etc. Only from a consideration of recent biosynthetic studies is it 

 evident that they may be reasonably grouped together as compounds whose basic skeletons 

 are all derived from mevalonic acid or a closely related precursor. To be sure, this 

 biosynthetic unity does not imply any functional unity or, indeed, any detailed unity in 

 chemical properties, which depend more on functional groups than on carbon skeleton. 



As the structures of this group of compounds were elucidated, it became apparent 

 that many of them could be regarded as built up of isoprene or iso-pentane units linked 

 together in various ways and with different types of ring closures, degrees of unsatura- 

 tion, and functional groups. 



CH 



'CHo"^ CH Cn^~^ CHo 



iso-pentane unit 



The commonest arrangement appeared to be "head-to-tail" 



c 

 I 

 c — c — c — c 



c 

 I 

 c- 



c — c— 



and this head-to-tail rule was regarded as so general that the correctness of proposed 

 structures could be judged by seeing whether they conformed to it. As more compounds 

 have been discovered, both types of possible exceptions have been found to this isoprene 

 rule i.e. isoprenoid-type compounds have been found which do not contain even num- 

 bers of isoprene units, and compounds have been found where the head-to-tail arrange- 

 ment is not followed. Individual cases will be discussed below, but they do not destroy 

 the great utility of the isoprene rule as a working hypothesis. However, there is grave 

 danger in supposing that any molecule which can be shown on paper to contain an isoprene 

 residue is actually formed by reactions similar to those of terpene biosynthesis. For 

 instance, the flavonoids can be dissected into two isoprene residues plus hypothetical C3 

 and C2 units: 



117 



