Optical enrichment in chiral crystal environment of [( R,S )‐1‐cyanoethyl][methyl ( S )‐alaninate]cobaloxime complex only by photoirradiation

The diastereomeric pair of the [( R,S )‐1‐cyanoethyl][methyl ( S )‐alaninate]cobaloxime complex cannot be separated by the fractional crystallization method. Crystal structure analysis revealed that there are two crystallographically independent molecules which are diastereomers to each other in a unit cell. When the crystal was irradiated with a xenon lamp, the cell dimensions gradually changed. After about 70 h of exposure, the change became insignificantly small. Crystal structure analysis after irradiation indicated that only the ( R )‐1‐cyanoethyl complex was epimerized and the ( S )‐1‐cyanoethyl complex remained unaltered. This led to an increased amount of the ( S )‐1‐cyanoethyl complex in the crystal. The optical enrichment occurred only by photoirradiation. The diastereomeric excess of the ( S )‐1‐cyanoethyl complex became 32% in the final stage. The reason why only the ( R )‐1‐cyanoethyl complex was epimerized was clearly explained by the size and shape of the reaction cavity for the 1‐cyanoethyl group. Copyright © 2000 John Wiley & Sons, Ltd.