TERPENOIDS AND STEROIDS 



157 



It will be noted that the sterochemistry at all the double bonds is cis. Gutta and balata 

 are also high molecular weight polyisoprenes but have an all-trans structure. Gutta also 

 has a lower average molecular weight than rubber. Rubber may be distinguished from 

 gutta by its elasticity and incomplete solubility in aromatic hydrocarbons. 



Although only a very few plants (e.g. Hevea brasiliensis. Taraxacum spp. , guayule) 

 offer possibilities for commercial production, rubber occurs in many dicotyledons. It 

 is found in some plants as a component of latex and may be obtained by tapping the latex 

 vessels. In other plants it is found throughout the tissues and can only be extracted after 

 grinding up the plant. The plant kingdom has not been surveyed for gutta as extensively 

 as it has for rubber, but it seems to be a general rule that no plants have both. The chief 

 commercial sources of gutta are East Indian plants of the genera Payeiia and Palaquiiiui . 



MIXED TERPENOIDS 



The mixed terpenoids are a miscellaneous group of compounds which seem to be 

 built predominantly from isoprene residues but which contain additional carbon atoms or 

 lack the required number. In some cases they may come from strictly isoprenoid pre- 

 cursors as the result of extensive rearrangement and/or loss of carbon atoms. The most 

 general category to be placed in this group are the naturally occurring furans, which ac- 

 cording to Aneja et al. (39), may all be regarded as derived from isoprenoid units by the 

 loss of three carbon atoms. Some of the other compounds in this group are among the 

 most interesting natural products from a physiological point of view. 



Gibberellic acid is regarded by Birch et al. (40) as derived from a diterpenoid by 

 loss of one carbon atom and rearrangement. The structure shown for this important 

 growth substance is that produced by the fungus Gibberella fujikuroi. Higher plants ap- 

 parently have as many as nine similar gibberellins with somewhat different structures 

 (41,42,43). 



gibberellic acid 



The tocopherols, or various forms of vitamin E, are important antioxidants found 

 in various seed oils (e. g. wheat germ). The predominant member of this group is Q-to- 

 copherol. Others differ from it by having fewer methyl groups. They are all apparently 

 interconvertible in the plant (44) and may be involved in the flowering process (45) or in 

 growth responses (46). 



CH' 



CH2(CH2CH2CHCH2)3H 



a-tocopherol 



