156 TERPENOIDS AND STEROIDS 



The parent member of the group, tropolone, has an addition a hydroxyl group--hence 

 the name ol - one: 



On the basis of biosynthesis there appear to be two classes of tropolones in nature, one 

 derived from terpenoid-like precursors and the other by ring expansion of 6-membered 

 aromatic rings. Since the common tropolones of higher plants resemble the terpenoids, 

 they are included in this chapter. Tropolones derived from aromatic acids seem to be 

 the predominant form in the fungi. In some cases no clear-cut decision regarding pre- 

 cursors is possible at present. Structures of some tropolones have already been given 

 along with related terpenoids (e. g. eucarvone, nootkatin); and there seems no reason to 

 set them apart. These compounds are of interest primarily because of their strong fun- 

 gicidal action. In this they resemble the phenols, but their toxicity may also be attributed 

 to their strong capacity for chelation (34). The alkaloid colchicine (q. v. ) may be classed 

 as a tropolone. Tropolones have been reviewed by Pauson (35), Erdtman (36) and Nozoe 

 (37). 



RUBBER AND OTHER HIGH 

 POLYMERS OF ISOPRENE 



Rubber is by far the most important isoprenoid derivative of molecular weight 

 higher than the tetraterpenoids, and almost no substances are known with a molecular 

 weight intermediate between these two. A C45 terpene alcohol, solanesol, has been iso- 

 lated from tobacco leaves (38). 



CHq CHo CHq 



,C=CH2(CH2C==CHCH2)7CH2C=CHCH20H 



CH3 



solanesol 



SporopoUenine, the chief constituent of the outer layer (exine) of pollen grains, is prob- 

 ably a polyterpene. Practically nothing is known concerning its chemistry although its 

 great resistance to breakdown is suggested by the survival of pollen grains for thousands 

 of years in peat bogs. 



Rubber is a polymer containing from 3000 to 6000 isoprene units. A small portion 

 of the molecule may be represented as: 



