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In the course of an investigation into the synthesis and properties of 3‐aryl‐4‐hydroxycoumarins a new direct synthesis of these compounds was found. In an acid medium 4‐hydroxycoumarin can be coupled directly to a diazotized aniline to form the desired 4‐hydroxycoumarin derivative.

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Among the alkyl substituted 3‐phenyl‐4‐hydroxycoumarins, synthesized to investigate the relationship between chemical structure and biological activity in this class of compounds 2, eleven possess one or more ortho substituents. As the steric effects of ortho substitution can manifest themselves in the polarographic behaviour, the half wave potentials of these compounds were measured and attempts were made to find relationships between the values obtained and to determine the steric effect in each case. In addition, an equation was derived for the first reduction wave in the polarogram and a reaction mechanism for this polarographic reduction is suggested.