Practical synthesis of enantiomerically pure β²-amino acids via proline-catalyzed diastereoselective aminomethylation of aldehydes

Proline-catalyzed diastereoselective aminomethylation of aldehydes using a chiral iminium ion, generated from a readily prepared precursor, provides α-substituted-β-amino aldehydes with 85:15 to 90: 10 dr. The α-substituted-β-amino aldehydes can be reduced to β-substituted-γ-amino alcohols, the major diastereomer of which can be isolated via crystallization or column chromatography. The amino alcohols are efficiently transformed to protected β²-amino acids, which are valuable building blocks for β-peptides, natural products, and other interesting molecules. Because conditions for the aminomethylation and subsequent reactions are mild, β²-amino acid derivatives with protected functional groups in the side chain, such as β²- homoglutamic acid, β²-homotyrosine, and β²- homolysine, can be prepared in this way. The synthetic route is short, and purifications are simple; therefore, this method enables the preparation of protected β²-amino acids in useful quantities.