AROMATIC COMPOUNDS 



55 



destroy the two heterocylic rings to form simple phenols. Structures of some representa- 

 tive compounds are given in Table 4. It will be noted that all pyranocoumarins have an 

 isoprenoid carbon skeleton in the pyran ring as do some of the furanocoumarins. It has 

 been suggested (14) that any which do not have an intact isoprene skeleton have been de- 

 rived by secondary reactions. 



The furanocoumarins are of some economic importance as the active ingredients of 

 fish poisons used by some primitive peoples. In higher animals they may show spasmoly- 

 tic and vasodilating effects. Psoralen derivatives taken orally have been used to promote 

 suntanning of the skin (15). 



LIGNANS 



The lignans may be regarded as formed by the union of two phenylpropanes through 

 their aliphatic side chains. The usual basic structure is: 



\ / 



\ / 



•c— c-c 



c— c— c 



The aromatic rings are always oxygenated. Additional ring closures may also be present. 

 All of the natural lignans contain one or more asymmetric carbon atoms and are optically 

 active. Rarely a lignan with the 2-phenylpropane structure may be encountered as in 

 pinastric acid from Lepraria flava (16); 



The lignans are colorless, crystalline solids which resemble other simple aromatic com- 

 pounds in their chemical behavior. They are widespread in the plant kingdom, occurring 

 in heartwood, leaves, resinous exudates, and other plant parts. Occasionally they are 

 found as glycosides. 



Examples of some natural lignans are given in Table 5. About three dozen are 

 known at present. Some have shown limited commercial success as antioxidants in food. 

 Sesamin has some importance as synergistic ingredient in pyrethrum insecticides. Lig- 

 nans are also the active constituents in certain medicinal plants. Podophyllin, a resinous 



