Combinatorial Design of Simplified High‐Performance Chiral Phase‐Transfer Catalysts for Practical Asymmetric Synthesis of α‐Alkyl‐ and α,α‐Dialkyl‐α‐Amino Acids

A very efficient, chiral phase-transfer catalyst, (S)-2 Db, was prepared by taking advantage of the combinatorial approach from the readily available (S)-1,1'-binaphthyl-2,2'-dicarboxylic acid. This catalyst exhibited high catalytic performance (0.01-0.1 mol %) in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester and N-(p-chlorophenylmethylene)alanine tert-butyl ester relative to other chiral phase-transfer catalysts in current use. This has created a general and highly practical procedure for the enantioselective synthesis of structurally diverse natural and unnatural alpha-alkyl-alpha-amino acids as well as alpha,alpha-dialkyl-alpha-amino acids. A similar simplified catalyst, (S)-2 Fb, is also applicable to the direct asymmetric aldol reaction between glycine Schiff base and aldehydes with moderate syn selectivity and high enantioselectivity.