PLANT STRUCTURE AND COMPOSITION 



201 



by oxidation to the aldehyde, a whole group of acyclic monoterpenes 

 is formed by the different plants. In other monoterpenes ring closure 

 occurs to form substituted cyclohexenes or cyclohexadienes, for exam- 

 ple, limonene. 



By oxidation at various positions in the ring or side chain related 

 compounds like menthol are synthesized by various plants. Some 

 species bring about a ring closure between the isopropyl side chain 

 and the six-membered ring to form bicyclic structures. a-Pinene, the 



CH3 



HoC 



H,C 



-C— O 



CH3 — C — CH3 



HoC. 



.CH, 



a-pmene 



H 



camphor 



major component of the essential oil of a number of Pinus species, has 

 such a structure. Camphor is an example of a bicyclic oxidized ter- 

 pene. Sesquiterpenes contain three isoprene equivalents, with acyclic, 

 cyclic, and polycyclic members reported from different species of 

 plants. Rose oil and oil of citronella contain the acyclic sesquiterpene 



CH3 



CH3 



CH3 



CH3 



C=CH— CH2— CH2— C=CH— CH,— CH2C=CHCH20H 



farnesol 



