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 alpha-substituted-beta-amino aldehydes with 85:15 to 90:10 dr. The alpha-substituted-beta-amino aldehydes can be reduced to beta-substituted-gamma-amino alcohols, the major diastereomer of which can be isolated via crystallization or column chromatography. The amino alcohols are efficiently transformed to protected beta(2)-amino acids, which are valuable building blocks for beta-peptides, natural products, and other interesting molecules. Because conditions for the aminomethylation and subsequent reactions are mild, beta(2)-amino acid derivatives with protected functional groups in the side chain, such as beta(2)-homoglutamic acid, beta(2)-homotyrosine, and beta(2)-homolysine, can be prepared in this way. The synthetic route is short, and purifications are simple; therefore, this method enables the preparation of protected beta(2)-amino acids in useful quantities.