SOME MISCELLANEOUS UNSAPONIFL\BLE LIPIDS 



103 



ANTHRAQUINONES 



The largest group of natural quinones is made up of anthraquinones. Some of them 

 have been important as dyestuffs and others as purgatives. The plant families richest in 

 this type of compound are the Rubiaceae, Rhamnaceae and Polygonaceae. The fundamental 

 anthraquinone structure is shown below with the ring-numbering system; 



3 



Most anthraquinones from higher plants are hydroxy lated at C-1 and C-2, although anthra- 

 quinone itself has been reported to occur in various plant tannin extracts (27), and 2- 

 methylanthraquinone is known to be a constituent of teak wood. The hydroxylated anthra- 

 quinones probably do not often occur in plants as such but rather glycosides. Treatment 

 of the plants to obtain the commercially desirable products has the effect of hydrolyzing 

 the glycosides, and in some cases producing additional oxidative changes. Since the free 

 anthraquinones are the products usually dealt with, some typical structures will be con- 

 sidered in Table 4 followed by a general discussion of the native glycosides. All of these 

 anthraquinones are high-melting crystalline compounds soluble in the usual organic sol- 

 vents. They are usually red in color but different ones range from yellow to brown. They 

 dissolve in aqueous alkali with the formation of red-violet colors. 



The problem of the form in which these anthraquinones actually exist in plants re- 

 mains a knotty one, and there are apparently several possibilities. Since the native pre- 

 cursors generally break down readily under the influence of enzymes or extraction proce- 

 dures, reports of the appearance of free anthraquinones must be regarded cautiously. 

 However, in some cases sufficient pains have been taken to assure that the simple com- 

 pounds are true natural products. Many of the anthraquinones occur as glycosides with 

 the sugar residue linked through one of the phenolic hydroxyl groups. Several different 

 sugars are found in such glycosides. Thus, alizarin occurs as a 2-primoveroside (rube- 

 rythric acid); rubiadin from madder {Riibia tincluria) as a 3-gIucoside and from Galium 

 spp. as a 3-primoveroside; morindone from Copros»ia aiislralis as a 6-rutinoside and 

 from Moriiida persicaefolia as a 6-primoveroside. 



In many cases it appears that the native glucosides have as their aglycones a reduced 

 form of the anthraquinone, known as anthrone. The sugars in these reduced glycosides 

 may be linked as usual through phenolic oxygens in the outside rings or they may be at- 

 tached at C-9 to the enol form of anthrone, anthranol: 



anthraquinone 



H H 



anthrone 



anthranol 



