TERPENOIDS AND STEROIDS 



153 



BITTER PRINCIPLES 



The various bitter substances distributed through the plant kingdom constitute no 

 chemically homogeneous group. It is commonly believed that alkaloids are usually the 

 cause of bitterness in plants, but in many cases terpenoids have been found to be respon- 

 sible. For example, both the saponins and cardiac glycosides are bitter. Bitter princi- 

 ples generally have been reviewed by Korte et al. (27) and Courtney (28). Many of the 

 bitter terpenoids contain ketone or lactone groupings, but bitterness does not seem to be 

 ascribable to any particular functional groups. Some bitter principles are known to be 

 terpenoids but the complete structures are not known. The bitter principles of the Cucur- 

 bitaceae, cucurbitacins, are known to be triterpenoid glycosides with a ring structure like 

 lanosterol and the following side chain at C17 (29, 30). 



I 3 



-C— CHo-CH 



I ^ 



OH 



CH. 



c=CH-C- 



I 

 CH, 



' II 

 OCCH. 



The bitterest natural product known is a diterpenoid, amarogentin, of unknown structure. 

 Structures and occurrence of some other compounds of this nature are given in Table 13. 

 Plumieride is shown as an example of a group of glucosides with pronounced structural 

 similarities which are believed to be biogenetically related although they are found in quite 

 unrelated plants (31). Other members of the group are genipin, loganin, asperuloside, 

 aucubin, catalposide, nepetalactone, and iridodial. They often have physiological effects 

 on mammals and microorganisms. Aucubin is responsible for the darkening observed on 

 grinding certain plants, especially Plantago spp. (32). 



In contrast to the terpenoid bitter principles, it should be mentioned that the sweet- 

 est natural product known is also a diterpenoid. This is a glycoside known as stevioside 

 found in Stevia rebaudiana. On hydrolysis it yields three molecules of glucose. The com- 

 plete structure of the aglycone, steviol, is unknown; but a tentative suggestion is as fol- 

 lows (33). 



q9- 



COOH 



TROPONES 



The basic tropone nucleus is a seven-membered ring containing a double bond sys- 

 tem conjugated with a keto group: 



