Synthesis of 4-Fluoro-β-(4-fluorophenyl)-l-phenylalanine by an Asymmetric Phase-Transfer Catalyzed Alkylation:  Synthesis on Scale and Catalyst Stability

4-Fluoro-β-(4-fluorophenyl)-l-phenylalanine 1 was synthesized by the asymmetric phase-transfer catalyzed alkylation of tert-butyl glycinate-benzophenone Schiff base using the cinchona alkaloid derived catalyst 6. Upon scaling, it was observed that to achieve high levels of enantioselectivity, it was necessary to add the catalyst or base last. From these studies, insight into the stability of the catalyst 6 under the reaction conditions was gained.