Ultraviolet absorption spectra and stereochemical structure of plant growth substances of the cis cinnamic acid type and of stilboestrol

Ultraviolet absorption spectra are recorded of cis‐ and trans‐ cinnamic acid, of cis‐ and trans‐1,2,3,4,‐tetrahydronaphthylidene (1) acetic acid and of cis‐ and trans‐β‐(naphthyl‐1) acrylic acid. The cis isomers all show a lower extinction coefficient and a shift of the maximum of absorption to shorter wave‐length in comparison with the trans isomers. It is concluded that as a result of steric hindrance in the case of the cis compounds the conjugation between aromatic ring system and carboxyl group is hindered, the molecules not possessing a plane structure. The fact that all these cis compounds show marked activity as plant growth promoter, whilst the trans isomers are totally inactive, is in accordance with the conclusions about their stereochemical structure. Comparison of the ultraviolet absorption spectra of stilboestrol and of p,p′‐dihydroxystilbene leads to the conclusion that the stilboestrol molecule does not have a plane structure and that it is doubtful whether the explanation of the oestrogenic activity based on its often suggested „formal”︁ similarity to the natural oestrogenic hormones is justified.