Improved conditions for the asymmetric synthesis of α-amino acids using Cinchona-derived anthracenylmethyl ammonium salts as chiral phase-transfer organocatalysts

Cinchonidine and cinchonine-derived monomeric anthracenylmethyl ammonium salts bearing different counter-anions are used as chiral organocatalysts in the enantioselective alkylation of a benzophenone-imine tert-butyl glycinate under phase-transfer conditions. Generally, an improvement of the enantioselectivity is observed when the counterions are tetrafluoroborate and hexafluorophosphate using 50% aqueous KOH as the base and toluene/chloroform as the solvent. The enantio selectivities achieved are comparable and frequently higher, even working under simpler and less-strict reaction conditions and with lower organocatalyst loading, than those reported previously. (c) 2006 Elsevier Ltd. All rights reserved.