Persulphate oxidation of carboxylic acids. Part III. Oxidation of cis-cinnamic and biphenyl-2-carboxylic acids
3,4-Benzocoumarins were obtained by oxidative cyclisation of biphenyl-2-carboxylic acids. The parent benzocoumarin was also formed by oxidation of 2′-substituted acids with elimination of the substituent (OMe, NO2, and CO2H and in low yield Me and Cl) but 2′-benzoylbiphenyl-2-carboxylic acid gave 5-benzoyl-3,4-benzocoumarin and 2′-cyanobiphenyl-2-carboxylic acid yielded fluorenone and phenanthridine-1,10-carbolactone. Similar oxidations of cis-cinnamic acids gave poor yields of coumarins, markedly increased by the presence of an o-methoxy-group. The mechanisms of these reactions are discussed.