Lewis Acid-Catalyzed Racemization and Recycling of the Undesired (R)-Ketamine

The first detailed description of the Lewis acid-catalyzed racemization of (R)-ketamine is reported. A process for racemization of the undesired (R)-ketamine enantiomer produced from the resolution for preparing the NMDA receptor antagonist (S)-ketamine was developed in quantitative yield with 99% chemical purity in the presence of a Lewis acid at 150 °C. Varying degrees of racemization were observed in the presence of various frequently used Lewis acids separately, and the catalytic efficiencies were arranged as follows: MgCl2 ≈ AlCl3 > FeCl3 > ZnCl2 > BF3 > CaCl2. The racemized ketamine was subsequently resolved using l-(+)-tartaric acid to obtain (S)-ketamine in 41% yield with 99.5% ee. Such a concise and cost-efficient approach for the racemization can be industrially useful to recycle the waste (R)-ketamine enantiomer into the resolution process to obtain (S)-ketamine.