AROMATIC COMPOUNDS 49 



which cannot conveniently be described at all as derivatives of cinnamic acid. Table 2 

 illustrates some of the various compounds of this class. The hydroxycinnamic acids have 

 been reviewed by Herrmann (6). 



Caffeic acid is one of the most widespread of all plant phenolic compounds (7), fol- 

 lowed closely by ferulic and p-coumaric acids. Caffeic acid, however, frequently occurs 

 as esters rather than the free acid. The commonest caffeic acid ester is chlorogenic 

 acid in which the alcohol portion is supplied by quinic acid. Several other complex esters 

 are known in which one or more molecules of one of the hydroxycinnamic acids are ester- 

 fied to one molecule of quinic acid. Such esters involving two hydroxy acids are known 

 as "depsides". Chicoric acid of chicory has two molecules of caffeic acid ester if ied with 

 one molecule of tartaric acid. Phaseolic acid of Phaseolus vulgaris is caffeyl malic acid 

 (8). Sugar esters of caffeic acid are widespread in plants (9). 



The simple phenylpropanes are colorless, crystalline solids whose chemical reac- 

 tivity may be understood by reference to the particular functional groups which are pre- 

 sent. Those members having several free phenolic hydroxyls are readily oxidized in the 

 air especially under alkaline conditions. The oxidation products are dark-colored poly- 

 mers. Green substances are formed by oxidation of caffeic acid esters in the presence 

 of ammonia or amino acids. This formation of a green substance accounts for the name 

 of chlorogenic acid. The more volatile members of this group are very important com- 

 mercially since they contribute the characteristic flavors and odors to many valuable 

 herbs and spices. Cinnamaldehyde of cinnamon, eugenol of cloves, and apiol of parsley 

 and celery are but three of the best-known examples of phenylpropane flavor compounds. 



TABLE 2. SOME OPEN CHAIN PHENYLPROPANE DERIVATIVES, 

 STRUCTURES AND OCCURRENCE 



H0-\\ /rCH=CHCOOH feruUc acid (widespread) 



p-coumaric acid (widespread) 



H0-<\ />CH=CHCOOH 



.-^CH=( 



HO A HO 0" 



> I, \ / chlorogenic acid (widespread) 



HO-^ ^CH=CHCO 



COOH 



