A Multistage, One-Pot Procedure Mediated by a Single Catalyst: A New Approach to the Catalytic Asymmetric Synthesis of β-Amino Acids

Abstract
A catalytic asymmetric procedure for the preparation of β-amino acids (specifically β-substituted aspartic acid derivatives) is reported. The cinchona alkaloid catalyst benzoylquinine (BQ) mediates up to five distinct steps of a reaction pathway, all in one reaction vessel. The products of this reaction, highly optically enriched β-substituted aspartic acid derivatives, were prepared from N-acyl-α-chloroglycine esters and acid chlorides in the presence of the catalyst. This approach was also amenable to the synthesis of small polypeptides containing β-substituted aspartic acid units, including a non-natural fragment of the antibiotic lysobactin. The addition of Lewis acids to this system was found to accelerate the rate of specific steps in the reaction pathway. Mechanistic aspects of this reaction, such as imine formation and Lewis acid chelation to the β-lactam intermediate, were investigated through comparison of IR, NMR, and other physical data.

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