122 



TERPENOIDS AND STEROIDS 



The bonds which extend roughly perpendicularly to the ring are known as axial or polar. 

 The others are named equatorial and are more nearly coplanar with the ring. Experience 

 with a molecular model will show that the ring may readily be flipped so that all axial 

 bonds become equatorial and vice versa; therefore with an unsubstituted ring the distinc- 

 tion is of no significance. However, when substituent groups are present, they are more 

 easily accommodated in equatorial positions and therefore tend to stabilize the conforma- 

 tion which has the bulkiest groups equatorial. The final consequence of conformational 

 analysis to be mentioned here is that groups on adjacent carbons are described as cis if 

 one is axial and the other equatorial, or trans if they are both either axial or equatorial. 

 In the above figure a and b are trans whereas a and c are cis. These generalizations 

 become important when considering the chemistry of these compounds, the differences 

 between compounds, and possible routes of biosynthesis. The great majority of monocylic 

 monterpenoids have the so called p-menthane skeleton: 



Variations are introduced into this basic structure by double bonds and functional groups. 

 The most important compound of this group because of its widespread occurrence and com- 

 mercial value is limonene. It is the chief constituent of citrus fruit oils but occurs in 

 many other essential oils as well. A few other monocyclic compounds lacking the p- 

 menthane structure also are known. Some examples are shown in Table 3. The sources 

 given are intended only to be typical. Most of the compounds are widely distributed. 

 Asymmetric carbon atoms are pointed out by asterisks, but generally both isomers are 

 known in nature either from two different plants or sometimes both from the same plant. 

 The (+), (-) mixture of limonenes is known commercially as dipentene. The ionones and 

 irone are important perfumery materials which closely resemble the monocyclic mono- 

 terpenoids in structure but have additional carbon atoms: 







II 



rCHCCH. 



f} 



=CHCCH. 



a-ionone (Boronia megastigma) 



irone (Iris florentina) 



