94 SOME MISCELLANEOUS UNSAPONIFIABLE U?IDS 



The most familiar of such compounds are the flavonoids discussed in Chapter 9. Accord- 

 ing to the hypothesis of Birch (1, 2) all compounds containing this nucleus may be regarded 

 as derived from three molecules of acetic acid, and this scheme has been corroborated 

 using labelled acetate for the phloroglucinol nucleus of the flavonoids. 



OH 



II II I 



/°" /\ /\ 



CHg CH3 CHo CHo V HC Cn 



HO. 



As indicated above phloroglucinol may be represented as a tautomeric triketone, and many 

 reactions of phloroglucinol derivatives are best understood by reference to this ketone 

 structure. 



Old reports of the widespread occurrence of free phloroglucinol in plants have not 

 been confirmed, and most such reports are questionable since the tests used give a posi- 

 tive result for complex phloroglucinol derivatives as well as for the compound itself. The 

 simplest, natural phloroglucinol derivatives are a group of cyclic triketones (phlorogluci- 

 nol tautomers) which are widespread in ferns of the family Pteridaceae but also found in 

 essential oils of some angiosperms. Plants containing such compounds have been used 

 for centuries as antihelmintic drugs, and the compounds have also been shown to possess 

 insecticidal and bacteriocidal activity (13), The bitter substances found in hops (HhhiuUis 

 lupulus) have isoprenoid side chains attached to the phloroglucinol nucleus, or to a cyclo- 

 pentatrione ring (14) which can also be regarded as derived from 3 acetate units. These 

 compounds are important for their antimicrobial action as well as the flavor which they 

 give to beer. A general review on natural phloroglucinol derivatives has been presented 

 by Hassall (15). Some examples of these compounds and their natural occurrence are 

 given in Table 2. 



The essential oil of Backhoitsia angustifolia contains compounds obviously related 

 to the above structures but lacking one phenolic hydroxyl group, e.g.: 



angustione 



Several generalizations can be made regarding these structures: 



1. All three hydroxyl groups of the phloroglucinol nucleus are never free. In 

 some cases loss of available hydrogens stabilizes the keto form; frequently 

 one hydroxyl group is methylated. 



