4-Carbethoxyphenylboronic acid was prepared as follows: 4-carboxyphenylboronic acid (350 mg, 2.109 mmol) was placed in a 100 mL three-necked round-bottomed flask and dissolved in anhydrous ethanol (10 mL). The device configuration included a condenser tube, thermometer, stirrer, and rubber stopper. A mixed ethanol/HCl solution (10 mL, pH about 3.0) was added to the reaction system and the mixture was subsequently heated to 70°C and refluxed for 1 hour. Upon completion of the reaction, the mixture was cooled to room temperature and stirred continuously overnight. Subsequently, the reaction mixture was concentrated under reduced pressure. The concentrate was dissolved in ethyl acetate and washed with water. The organic layer was dried with potassium carbonate, filtered and concentrated under reduced pressure to give 520 mg of white solid. The crude product was initially purified by silica gel chromatography (Chromatotron, 6000 micron rotor, eluent 1:1 hexane:ethyl acetate) to give 470 mg of unreacted raw material mixed with the product. Further, a Vydac C-18 reversed-phase column was used with water (containing 0.1% trifluoroacetic acid) and acetonitrile as the mobile phases with gradient elution (5%-70% acetonitrile), and the final purification yielded 4-ethoxycarbonylphenylboronic acid (350 mg, 85.5% yield) as a white solid. The product was characterized by electrospray mass spectrometry (ES-MS) with m/z of 195.0 ([M+H]+).
