Synthesis of 9-oxoxanthen-2-carboxylic acids
Abstract
7-Substituted 9-oxoxanthen-2-carboxylic acids can be prepared in good yield from hydroxylated benzophenones. Oxidative coupling of 4-hydroxy-3-(3-hydroxybenzoyl)benzoic acid using potassium ferricyanide gave 7-hydroxy-9-oxoxanthen-2-carboxylic acid (8; R = H). Cyclisation of 3-(5-chloro-2-methoxybenzoyl)-4-hydroxy-benzoic acid took place in alkaline solution to give 7-chloro-9-oxoxanthen-2-carboxylic acid. Both these ring closure reactions when carried out in potassium hydrogen carbonate or in potassium hydroxide solutions respectively gave the conveniently isolated insoluble potassium salt of the 9-oxoxanthen-2-carboxylic acid.