Solvent Effects in the Enantioselective Catalytic‐Phase‐Transfer Alkylation of Polymer‐Supported Glycine–Imine tert ‐Butyl Ester: Asymmetric Solid‐Phase Synthesis of ( R )‐α‐Amino Acid Derivatives

Polymer‐supported glycine–imine tert ‐butyl esters ( 3 , 8 ) were prepared from Merrifield‐ or Wang‐resin and used in the enantioselective synthesis of ( R )‐α‐amino acid derivatives by using ( S , S )‐3,4,5‐trifluorophenyl‐NAS bromide ( 5 ) as the chiral phase‐transfer catalyst. The chemical yields and enantioselectivities were found to be dramatically dependent upon the ratio of water to organic solvent. The optimal solvent was a mixture of toluene/chloroform/water (9:1:0.5). The catalytic enantioselective solid‐phase phase‐transfer alkylation of polymer‐supported glycine–imine substrate 3 with various alkyl halides with the use of 50 % aqueous CsOH in the optimal solvent system at 0 °C followed by hydrolysis and benzoylation afforded the corresponding ( R )‐ N ‐benzoyl‐α‐amino acid tert ‐butyl esters 11 in 60–80 % yield and with enantiomeric ratios ( er ) of 96.5:3.5 to 99.5:0.5.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)