To a solution of 5-methyl-furan-2,4-dicarboxylic acid-2-methyl ester (23) (100 mg) in N,N-dimethylformamide (3.0 mL) was added O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (228 mg), diisopropylethylamine (0.56 mL) and 4′-methoxy-biphenyl-4-ylamine (120 mg). The resulting solution was stirred at room temperature for 2 hours. The reaction mixture was diluted with ethyl acetate (20 mL) and washed successively with water (2×20 mL), 0.1 M hydrochloric acid (20 mL), water (20 mL), saturated sodium hydrogen carbonate (10 mL) and brine (10 mL). This solution was dried and concentrated in vacuo. The residue (170 mg) was dissolved in dichloromethane (5 mL), treated with triethylamine (0.3 mL) and a scavenger resin PS-TsCl (0.6 g) and the mixture shaken for 3 hours at room temperature. The reaction mixture was filtered and concentrated in-vacuo. The residue was dissolved in methanol/tetrahydrofuran (1:3 by volume) (20 mL), treated with 1.0 M aqueous lithium hydroxide solution (2.0 mL) and allowed to stir at room temperature for 4 hours. The pH of the reaction mixture was adjusted to between pH 4 and pH 5 by careful addition of 1.0 M hydrochloric acid (1.0 mL) and partly concentrated in vacuo.
