AROMATIC COMPOUNDS "'■ 



The simple aromatic constituents of plants are often dismissed by physiologists as 

 uninteresting "waste products" of metabolism or assigned a vague function in helping to 

 resist parasitic attacks or attracting insects to pollinate flowers. A more specific func- 

 tion may be indicated by the observation of Sondheimer and Griffin (10), that indoleacetic 

 oxidase is activated by p-coumaric acid and inhibited by caffeic and chlorogenic acids. 

 In general monophenols activate and diphenols inhibit. The interplay of these various 

 compounds by controlling enzyme activity could exercise important control over the 

 growth of plant tissues. 



COUMARINS AND ISOCOUMARINS 



The coumarins are lactones of o-hydroxycinnamic acid. This basic nucleus with 

 its ring -numbering is as follows: 



Almost all natural coumarins have oxygen (hydroxyl or alkoxyl) at C-7. Other positions 

 may also be oxygenated, and alkyl side-chains are frequently present. Isoprenoid side- 

 chains are especially common. Some coumarins are found as glycosides. Coumarins 

 may also be artifacts which arise from enzymatic hydrolysis of glycosyl-O-hydroxycin- 

 namic acid and immediate cyclization to the lactone: 



CH=CHCOOH 



-> 



0-GlYCOSYL ^''^u-^^OH ^'^:^^^^0''''^0 



Melilotoside Coumarin 



Ring closure to the lactone occurs only with O-hydroxy-CiS-cinnamic acids (coumarinic 

 acids). Ortho-hydroxy-^raws-cinnamic acids (coumaric acids) do not form lactones di- 

 rectly. However, isomerization to the cis form can be brought about by treatment with 

 ultraviolet light whereupon immediate ring closure ensues. The lactone ring of coumarins 

 is opened by hydrolysis with warm alkali, but immediately reforms on acidification. Fu- 

 sion with alkali splits off the alkyl group forming simple phenols (e, g. resorcinol from 

 umbelliferone). 



Structures of several natural coumarins are given in Table 3. They occur in all 

 parts of plants and are widely distributed in the plant kingdom but especially common in 

 grasses, orchids, citrus fruits and legumes. Scopoletin is the most common coumarin 

 of higher plants. 



Much rarer than the coumarins are isocoumarins or 3, 4-benzopyrones. A dihydro- 

 isocoumarin, phyllodulcin, is the sweet principle of Hydrangea macrophylla (11) while 

 another is responsible for the bitter taste occasionally found in carrots (12): 



