SOME MISCELLANEOUS UNSAPONIFLABLE LIPIDS 



109 



OTHER POLYNUCLEAR QUINONES 



A very few phenanthraquinones are known to occur in higher plants, and their struc- 

 tures are not established with certainty. Three substances known as tanshinones have 

 been isolated from the root of Salvia miltiorrliiza and shown to have carbon skeletons 

 similar to the diterpenoid, abietic acid. They may therefore be isoprenoid rather than 

 polyacetate derivatives. Another phenanthrene compound is denticulatol from the root 

 of Ru})iex chineiisis. Suggested structures for these compounds are given below. More 

 details may be found in the book of Thomson listed under general references for this 

 chapter. 



\J 



'CH, 



denticulatol 



tanshinone I 



One of the most interesting quinones both chemically and physiologically is the sub- 

 stance hypericin which is found in petals, stems, and leaves of St. John's Wort (Hyperi- 

 cu))i perforatum) and other members of the Hypericaceae. Animals eating these plants 

 become highly sensitized to light as a result of this compound becoming concentrated in 

 the skin. The biochemistry of this sensitization is unknown. Possibly the actual irritants 

 are peroxides formed in the presence of light. Other species of Hypericnni contain a sec- 

 ond, similar pigment named pseudohypericin; and buckwheat (Fagopyrum) also contains 

 a related photosensitizing pigment named fagopyrin. All of these compounds have been 

 reviewed by Brockman (39, 40). The structure of hypericin is given below: 



hypericin 



Pseudohypericin apparently has - CHOHCH3 groups rather than methyl attached to the 

 2 and 2' positions. Fagopyrin is more complex. On hydrolysis it forms hypericin and 



