106 



SOME MISCELLANEOUS UNSAPONIFLABLE LIPIDS 



Enzymatic (or chemical) hydrolysis of a C-9 glycoside of anthranol is followed by oxida- 

 tion of the anthrone to an anthroquinone if oxygen is present. If the sugar is linked at 

 some other position, anthranol glycosides may be directly oxidized to anthraquinone 

 glycosides. 



Although aloe-emodin occurs in Rheum spp. as an ordinary glycoside, in aloes it is 

 found as barbaloin, an unusual compound in which a glucose-like group is linked by a 

 carbon-carbon bond to a partially reduced anthraquinone (anthrone). 



OH 



CH2OH 



barbaloin 



Other compounds apparently similar to barbaloin occur in other species of aloes. Unlike 

 glycosides they are stable toward acid hydrolysis, but may be split with ferric chloride to 

 form aloe-emodin (28). 



Still more complex are the sennosides, the active cathartics of senna, 

 pounds are dianthrones. One of them, sennidin, has the following structure: 



These com- 



G-LUCOSYL-O 



GLUCOSYL-0 



COOH 



COOH 



The chemistry of the sennosides has been reviewed by Stoll and Becker (29). It seems at 

 least possible that other anthraquinones are derived from such natural precursors by exi- 

 dative splitting. For instance the native glycosides oi Rheum frangula bark (cascara) may 

 be dianthrone glycosides which are oxidized on storage to form aloe-emodin and its glyco- 

 sides (30). 



