Compound can be prepared in two different ways both starting from 2,4-difluoroaniline (I): 1) The diazotation of (I) with isoamyl nitrite (B) and condensation with anisole (A) gives 4-(2,4-difluorophenyl)anisole (II), which is hydrolyzed with HI in refluxing acetic acid yielding 4-(2,4-difluorophenyl)phenol (III). Finally, this compound is carbonated with K2CO3 and CO2 at 175 C and 1300 p.s.i. 2) The diazotation of (I) with isoamyl nitrite (B) and then condensation with benzene by means of powdered Cu gives 2,4-difluorobiphenyl (IV), which is acetylated with acetic anhydride by means of AlCl3 in CH2Cl2 yielding 4-(2,4-difluorophenyl)acetophenone (V). The oxidation of (V) with H2O2 in refluxing CH2Cl2 affords 4-(2,4-difluorophenyl)phenyl acetate (VI), which can be hydrolyzed with NaOH in refluxing water to give the phenol (III).
A facile preparation method for diflunisal has been reported: Reaction of 2,4-difluoroiodobenzene (I) with powdered Zn activated with 1,2-dibromoethane in DMF gives the 2,4-difluorophenylzinc iodide (II), which is condensed with 2-acetoxy-5-iodobenzoic acid methyl ester (III) by means of Pd(PPh3)4 in DMF to yield 4-acetoxy-2',4'-difluorobiphenyl-3-carboxylic acid methyl ester (IV). Finally, this compound is deacetylated and hydrolyzed with NaHCO3 in THF/water.