Treatment of N-Boc-4-bromo-L-phenylalanine (I) with ethanolic or methanolic HCl gave aminoester (II). Subsequent condensation of (II) with 2,6-dichlorobenzoyl chloride (III) produced amide (IV). Lithiation of 1,3-dimethoxybenzene (V) with n-butyllithium, followed by reaction with trimethyl borate and aqueous hydrolysis yielded boronic acid (VI). Suzuki coupling of bromide (IV) with boronic acid (VI) in the presence of palladium catalyst furnished biphenyl derivative (VII). Finally, hydrolysis of the ethyl ester of (VII) with LiOH provided the title carboxylic acid.
In an alternative procedure, N-Boc-L-tyrosine methyl ester (VIII) was treated with trifluoromethanesulfonic anhydride to afford triflate (IX). Suzuki coupling of (IX) with boronic acid (VI) produced biphenyl (X). After deprotection of the Boc group of (X) with trifluoroacetic acid, the resulting aminoester (XI) was acylated with acid chloride (III) to give amide (XII). The title compound was then obtained by hydrolysis of the methyl ester of (XII) with LiOH.