KENNETH V. THIMANN 



25 



CHjCOOH 



CH = NOOH 



m 



yh 



CHjCOOH 



coon 



vnr 



IX 



XI 



:CH 



COOH 



'COOH 



I 



CH: 



:CH 



XE 



TTTT 



by the cinnamic acids; the aV-form (XII) is active and the trans-iorm 

 (XIII), inactive. The /^-methyl and the o-methoxy derivatives of these 

 two acids behave similarly. 



Putting all these facts together, Koepfli, Thimann, and Went ten 

 years ago (3) concluded that the minimum structural requirements for 

 primary activity are: i) a ring system as nucleus; 2) a double bond in 

 this ring; 3) a side chain; 4) a carboxyl group (or a structure readily 

 convertible to a carboxyl) on this sidechain at least one carbon atom 

 removed from the ring; and 5) a particular space relationship between 

 the ring and the carboxyl group. 



The term carboxyl should be broadened, however, because of the 

 activity of naphthalene- i-nitromethane, which in the ad-iorm (\ II) 

 has an acid group isosteric with a carboxyl (13). Indican also has weak 

 activity (13), though no sulfonic acid yet tested is active. It may thus 

 be that the requirement is merely for an acid group which is not too 

 highly dissociated. 



The Ring. — The nature of the ring is important. As a rule 5-membered 

 rings do not confer activity. Pyrroleacetic acid, in spite of its resemblance 

 to the indole structure, is inactive, as are the acetic derivatives of 

 imidazole, furane, and thiazole. Neither A^-cyclopenteneacetic nor 

 A--cyclopenteneacetic acid is active, which is of interest in comparison 

 with auxin-a (I), (11). The fusion of a 6-membered ring with the 5- 



