O-Phenoxybenzoic acid

The cyclodehydration of o-phenoxybenzoic acids has been widely used to synthesize xanthones carrying variety of substituents. An Ullmann reaction provides a useful route to the required acids a 2-chlorobenzoic acid and a phenol react in an inert solvent such as nitrobenzene in the presence of copper bronze and potassium carbonate (53JCS1348). In a modified procedure sodium methoxide is used without solvent at 200 °C (79JA665). Cyclization is accomplished in concentrated sulfuric acid or in acetyl chloride containing a little sulfuric acid (61JCS2312). 

The procedure is based on the directions of Graebe,1 which outline the most convenient method of preparing xanthone. Somewhat similar is that of Perkin,2 in which salicylic acid is first converted by boiling with acetic anhydride into salicylide, which on distillation is partially converted into xanthone. A practically quantitative yield of xanthone is stated to be obtainable by warming o-phenoxybenzoic acid with concentrated sulfuric acid,3 but the preparation of this intermediate product is less satisfactory. 

Xanthones.—An efficient and convenient method of preparing xanthone in 99% yield is to heat o-phenoxybenzoic acid for 20 minutes at about 155 °C in molten sodium tetrachloroaluminate, obtained by fusing sodium chloride and aluminium chloride. Xanthones (283) were formed by cyclization of diphenyl ethers such as (282), which were prepared by addition of p-cresol to the quinone esters (281) followed by reductive methylation. ... 

Thermolysis of diphenyl carbonate (as a model system for bisphenol A-polycarbon-ate) at 360 °C results in the formation of significant amounts of carbon dioxide, phenol, xanthone, diphenyl ether, and phenyl o-phenoxybenzoate. These products are formed by the transformation of diphenyl carbonate to o-phenoxybenzoic acid, which in turn forms xanthone (by self-condensation), diphenyl ether (by decarboxylation) and phenyl o-phenoxybenzoate (by esterification with DPC) [673],... 

We noted previously that aldehydes such as benzaldehyde can be prepared by direct oxidation of the side chain in the reactant. It is also possible to go from the opposite direction and reduce a carboxylic acid to the corresponding aldehyde. In both cases, side-chain halogenation using hazardous chemicals is avoided. A one-step direct hydrogenation process has been developed by Mitsubishi using a Cr modified zirconia catalyst (Yokoyama et al., 1992), which is reported to give aldehyde selectivities of 80-97% in the hydrogenation of benzoic acid, o-methylbenzoic acid, w-phenoxybenzoic acid, dimethyl terephthalate, etc. 

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