N-Alkanoyl-cysteine preparation from cystine by acylation and electrochemical reduction, under conditions giving high membrane surface-time yield and low specific energy consumption

摘要

N-Alkanoyl-cysteine preparation from cystine by acylation and electrochemical reduction. Preparation of N-(2-6C alkanoyl)-cysteine (I) involves (i) acylating cystine in an aqueous phase with a 2-6C alkanoic acid halide or anhydride in presence of an alkali(ne earth) metal hydroxide, while maintaining a pH of at least 8.5 and a temperature of at most 10 deg C; (ii) electrochemical reduction of the N,N'-di-(2-6C alkanoyl)-cystine contained in the reaction mixture, using an electrolysis cell with a cathodic fixed bed; and (iii) isolating (I) from the reaction mixture. The novel features are that the reaction mixture from stage (i) is used directly as catholyte in stage (i), the electrochemical reduction is carried out at a current density of 10-500 mA/cm2, the aqueous reaction mixture from stage (ii) is contacted with a strong acid cation exchanger to remove all metal ions, the 2-6C carboxylic acid and/or hydrohalic acid released by the ion exchange treatment is removed by distillation and (I) is recovered from the distillation bottoms or residue.