Naphthyloxy compounds as plant growth regulators 



the atoms joining the ring and the — COOH group, including the C atom of 

 the latter, are situated almost in the plane of the ring forming a pseudo-ring 

 with the — COOH group close to the original ring and near the centre of the 

 extended ring system thus formed. 



(v) At least one side of the plane of the extended ring system must be free 

 of atoms other than hydrogen and one of the O atoms of the — COOH group 

 projecting out from it. 



(vi) An optical configuration corresponding to the D-series of the amino- 

 acids. 



This formulation is essentially an elaboration of the views of Veldstra 

 (1944a,b) in which his 'certain spatial relationship' between the ring and the 

 side-chain now appears as an 'extended ring system.' It will be convenient 

 to view the results given in Tables 3 and 4 against this background. 



Table 4 

 Substituted 2-naphthyloxyacetic acids 



In general it may be said that nuclear chlorine substitution increases the 

 activity of phenoxyacetic and benzoic acid growth substances and in certain 

 positions, especially 4, 5, and 6, with 3-indolylacetic acid activity is only 

 slightly diminished or is even enhanced (Veldstra, 1953). By contrast no 

 increase in activity of 2-naphthyloxyacetic acid can be obtained by nuclear 

 substitution of chlorine. On Veldstra's hypothesis (Veldstra and Booij, 1949) 

 this could be explained by assuming that in 2-naphthyloxyacetic acid the 

 lipophilic/hydrophilic character (L/// ratio) of the molecule is at its optimum 

 for biological activity, chlorination merely serving to increase the LjH ratio 

 beyond the optimum, resulting in a reduction in activity. The same argu- 

 ment would apply to the effect of lengthening the side-chain in the a-acids, 

 but not to the co-acids where the side-chain can be broken by /:?-oxidation. 



Considering the effect of chlorine substitution at specific positions in the 

 ring (see Table 4) it is immediately obvious that the important ring positions 

 are 1, 4, and 5. When any of these are blocked with CI there is a complete or 



201 



