118 TERPENOIDS AND STEROIDS 



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yet their biosynthesis is completely separate from that of the terpenoids and involves no 

 isoprenoid-type intermediates. Decisions as to the proper placement of such compounds 

 must await the results of biochemical studies. 



Most compounds discussed in this chapter are considered to have their basic struc- 

 tures built up entirely of isoprenoid units. Other classes of plant constituents occasionally 

 have isoprenoid side chains attached to obviously non-isoprenoid central structures. The 

 placement of such compounds is clearly arbitrary, but some examples may be seen among 

 the flavonoids (calophyllolide, rotenone), aromatics (galbanic acid) and porphyrins (chlo- 

 rophyll). 



The fundamental isoprenoid building pattern is one of the most widespread in natural 

 products. Every living organism apparently contains some compounds built on this basis. 

 All of these may be regarded as evolutionary modifications of a primeval mevalonic acid 

 pathway. The adaptive importance of these different ramifications, however, remains 

 almost completely obscure. There seems merit in the possibility that some of these com- 

 pounds merely represent ways of disposing of excess acetate. In other cases more spe- 

 cific funtions appear reasonable and will be mentioned in the appropriate places. It is a 

 general observation that lower terpenoids are rather restricted to phylogenetically young 

 plant groups while carotenoids and steroids are more widespread. 



HEMITERPENOIDS 



Isoprene itself does not occur free in nature, but several five-carbon compounds 

 are known with the isoprene skeleton. These include simple alcohols, aldehydes, and 

 acids as well as the unusual sugar apiose (q. v. ). They may occur free or as esters and 

 ethers. Some examples are given in Table 1. Their chemistry is simple and requires 

 no additional discussion here. The natural occurrence and chemistry of tiglic and angelic 

 acids has been reviewed by Buckles et al. (1). 



MONOTERPENOIDS 



The monoterpenoids appear to be built of two isoprene residues and normally have 

 ten carbon atoms, although rare examples are known of compounds which seem constructed 

 on this general principle but have lost one or more carbon atoms. Both cyclic and open- 

 chain compounds are known. In fact almost every possible arrangement of ten carbon 

 atoms seems to occur in nature. Only some of the more common examples can be given 

 here as illustrations. The term "terpenoid" is preferred for reference to all compounds 

 built of isoprene units, regardless of the functional groups present while "terpene" refers 

 specifically to hydrocarbons. Over one hundred different monoterpenoids have been iso- 

 lated from plants. They are the major components of many essential oils and as such 

 have great economic importance as flavors, perfumes and solvents. They are character- 

 istically colorless, water-insoluble, steam distillable liquids having a fragrant odor. 

 Some are optically active. Study of their chemistry is complicated by the difficulty of ob- 

 taining pure compounds from the complex mixtures in which they usually occur and by the 

 readiness with which they undergo rearrangement. 



