Convenient conditions for the enantioselective synthesis of α-methyl-α-amino acids with the use of Cinchona ammonium salts as phase-transfer organocatalysts

The enantioselective alpha-alkylation of 2-naphthalene carb aldehyde aldimine alanine tert-butyl ester is carried out by using N-anthracenylmethyl ammonium salts from the simple and unmodified cinchonidine and cinchonine alkaloids as phase-transfer organocatalysts, which affords (S)- and (R)enantioenriched alpha-methyl-alpha-amino acid (AMAA) derivatives in 71-98 % yield and 79-96 % ee. The employed reaction conditions, rubidium hydroxide as base and toluene/chloroform as solvent at -20 degrees C, allows higher enantioselectivities to be obtained than those previously obtained by using other Cinchona alkaloid-derived ammonium salts, even with a lower catalyst loading (5 mol-%). (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007.