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

摘要

Two chiral auxiliaries were attached to phenylglyoxylic acid as amides in 67-70% 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. During preparative-scale electrolysis the amides afforded the corresponding mandelic acid derivatives in 64-76% yield and 52-86% de, respectively. The diastereoselectivities depend on solvent, proton donor, supporting electrolyte, temperature and the substituent R in the 2-oxo acid amides. The results are in accordance with an ECE-reduction of the Zero acid amide to an enolate anion, which subsequently undergoes a face-selective protonation to the mandelic acid amide. [References: 31]