Stereoselective synthesis of b-amino acids

Due to their potential biomedicinal and synthetic applications, b-amino acids have attracted a great deal of attention recently [1–6]. The derived b-peptides of several amino acids have been reported to exhibit certain secondary structures and special stability against peptidase [7,8]. For example, the b-oligopeptides of six b-amino acid residues form stable helices in methanol [8]. Furthermore, a-substituted b-amino acids are often found as segments in bioactive compounds, such as paclitaxel [9] and many b-lactams [10–12]. With the increasing interest in b-peptides and other related biomedical compounds, the stereoselective synthesis of b-amino acids became desirable. Various successful methods have been developed for the synthesis of b-amino acids [1–6,13–19], including the Michael addition of amines to acrylate derivatives and homologation reactions from the corresponding a-amino acids. The efficient preparation of b-amino acids with a-substitution has also been explored [20]. The stereoselective preparation of a-substituted b-amino acids involves the alkylation of cyclic intermediates, which are either from the corresponding b-amino acids [21–23] or from the enolates generated in situ from the conjugate addition reaction of nucleophiles to a,b-unsaturated esters [24,25].