Enantioselective quaternization of 4-substituted oxazol-5-(4H)-ones using recoverable Cinchona-derived dimeric ammonium salts as phase-transfer organocatalysts
Dimeric anthracenyldimethyl-derived Cinchona ammoniumsalts are used as chiral organocatalysts (5 mol %) for the enantioselective 4-alkylation of 4-substituted azlactones. The corresponding adducts bearing a new quaternary center were obtained with up to 80% ee when using a dimericammonium salt derived from cinchonidine, and are precursors of α,α-disubstituted α-amino acids. The catalysts can be recovered