120 



TERPENOIDS AND STEROIDS 



The distribution of monoterpenoids in economically important essential oils is sur- 

 veyed in the treatise of Guenther (see General References). The majority of these com- 

 pounds occur widely and are not characteristic of particular plants or plant groups. It 

 is rare for a single plant to contain only one terpenoid, but one may be predominant. Al- 

 though the presence of monoterpenes is best documented in the seed plants, there are 

 scattered reports of the occurrence of terpene-containing volatile oils throughout the 

 plant kingdom down to the bryophytes and even fungi (2, 3). More information on the ter- 

 penes of lower plants would be welcome. 



Most investigators are of the opinion that the function of most lower terpenoids in 

 plants may be described as ecological rather than physiological. Many of them inhibit 

 the growth of competing plants and may also be insecticidal. The possible ecological 

 significance of terpenoids (and other secondary plant constituents) is well demonstrated 

 by Fraenkel (4). 



Structures of some open-chain monoterpenoids are given in Table 2. They have 

 been chosen to illustrate a variety of double bond locations, functional groups, and devia- 

 tions from the usual head-to-tail isoprene rule. Terpenoid alcohols frequently are found 

 as esters rather than free alcohols. 



It will be noted that opportunity for geometrical isomerism exists in geraniol and 

 geranial. Their isomers, nerol and neral, occur in nature; and a mixture of geranial 

 and neral, known as citral, constitutes 80% of the commercially valuable lemon grass oil. 



Considerations of sterochemistry become especially important with the cyclic ter- 

 penoids and increase in complexity in the higher terpenoids and steroids. Most cyclic 

 terpenoids may be regarded as cyclohexane derivatives, so a brief summary of cyclohex- 

 ane sterochemistry will be given at this point. A full review of this subject is presented 

 in an article by Orloff (5). A cyclohexane ring whether it is free or bonded to another 

 ring cannot be planar as it is normally pictured in structural formulas, but as a result 

 of C-C bond angles assumes a puckered shape or conformation. Geometrically the con- 

 formation may be pictured either as a "boat" form or "chair" form: 



boat 



chair 



Except in special cases this geometrical possibility does not result in the appearance of 

 actual pairs of isomers since the two forms are readily interconvertible. However, the 

 chair form is considered to represent the preferred conformation. In a fixed molecule 

 the two remaining carbon valences are not equivalent. This is best observed in a molec- 

 ular model but is indicated diagrammatically as follows: 



