The general procedure for the synthesis of 2-carboxycinnamic acid, 2-alkenylbenzoic acid and 2-carboxyphenylglyoxal from 1,2-naphthoquinone is as follows: the electrochemical reduction reaction was carried out under constant potential conditions in a concentric cell, with two compartments separated by a porous (D4) glass material diaphragm equipped with a magnetic stirrer. A mercury cell was used as the cathode (area 20 cm2), a platinum plate (area 12 cm2) as the anode, and an Ag/AgCl (3M) electrode as the reference electrode. The supporting electrolyte was nominally anhydrous acetonitrile (60.05% water content) containing 0.1 M lithium perchlorate (99.99% purity). Electroactive quinone 1 (1.0 mmol) was dissolved in 60 mL of the supporting electrolyte solution and electrolyzed in an oxygen atmosphere (the solution was pre-bubbled 10 times with O2). Electrolysis was carried out at a constant potential of 0.5 V (relative to Ag/Ag+), which corresponds to the first reduction peak potential of these systems. After completion of the reduction, the solvent in the cathodic solution was removed under reduced pressure. The residue was extracted with an ether/water mixture and the organic phase was dried with anhydrous sodium sulfate and concentrated by evaporation. To separate the carboxylic acid product, the weakly basic aqueous phase is neutralized, extracted with ether and dried. If necessary, the resulting solid can be analyzed by mass spectrometry and separated by chromatography on a silica gel column (22 x 3 cm), using a dichloromethane/ethanol mixture as eluent. Spectral data of some of the resulting compounds are shown below.
