ONE-POT PALLADIUM-CATALYZED RACEMIZATION OF (S)-PRAZIQUANAMINE: A KEY INTERMEDIATE FOR THE ANTHELMINTIC AGENT (R)-PRAZIQUANTEL

An one-pot palladium-catalyzed procedure for racemization of (S)-praziquanamine, which is the undesired enantiomer and produced during the resolution step for preparing the anthelmintic drug (R)-praziquantel, has been developed through dehydrogenation of (S)-praziquanamine into 6,7-dihydro-4H-pyrazino[2,1-a]isoquinolin-4-one and then hydrogenation of the double bonds. And a superior racemization method was also developed that (S)-praziquanamine was directly treated with Pd/C under H-2 atmosphere to obtain a racemic mixture in quantitative yield with 95% chemical purity. Such a concise and efficient approach to the racemization of (S)-praziquanamine can be useful to recycle the waste enantiomer into the resolution process to obtain the (R)-enantiomer on a large scale.