Synthesis of Optically Active β-Amino Acid <i>N</i>-Carboxyanhydrides

Methodology has been developed for the general synthesis of optically active β-amino acid N-carboxyanhydrides (β-NCAs) through cyclization of Nβ-Boc or Nβ-Cbz β-amino acids using phosphorus tribromide. The formation of β-NCAs was confirmed by spectroscopy as well as an X-ray structural determination of β-homoalanine-N-carboxyanhydride. The β-NCA molecules could be polymerized in good yield to give optically active poly(β-peptides) that adopt stable chiral conformations in solution. For example, helical oligo(l-β-homophenylalanine) was synthesized by polymerization of l-β-homophenylalanine-N-carboxyanhydride.