Direct Asymmetric Synthesis of β-Bis-Aryl-α-Amino Acid Esters via Enantioselective Copper-Catalyzed Addition of p-Quinone Methides

A highly efficient synthetic approach to unnatural chiral beta-Ar,Ar'-alpha-amino acid esters bearing two contiguous stereogenic centers has been developed. The first enantioselective 1,6-conjugate addition of glycine Schiff bases to para-quinone methides was achieved using a Cu(CH3CN)(4)BF4/Ph-Foxap catalytic system (1 mol %). The reaction proceeded smoothly to generate corresponding products in high yields with excellent diastereoselectivities (up to 99:1) and enantioselectivities (up to >99% ee). Subsequent reduction delivered enantiopure unnatural amino alcohols, which were utilized for the synthesis of novel chiral ligands.