Electro-organic synthesis 68. Diastereoselective cathodic reduction of phenylglyoxylic acid attached to chiral auxiliaries

Two chiral auxiliaries were attached to phenylglyoxylic acid as amides in 6 7-70% yield. Cyclic voltammetry of one amide in acetonitrile-pivalic acid ( 0.117 mol l(-1)) shows that it is irreversibly reduced at --0.73 to -1.07 V (vs. Ag/AgCl) depending on the cation of the supporting electrolyte. Durin g preparative-scale electrolysis the amides afforded the corresponding mand elic acid derivatives in 64-76% yield and 52-86% de, respectively. The dias tereoselectivities depend on solvent, proton donor, supporting electrolyte, temperature and the substituent R in the 2-oxo acid amides. The results ar e in accordance with an ECE-reduction of the Zero acid amide to an enolate anion, which subsequently undergoes a face-selective protonation to the man delic acid amide.