Synthesis of optically active beta-amino acid N-carboxyanhydrides

Methodology has been developed for the general synthesis of optically activ e beta-amino acid N-carboxyanhydrides (beta-NCAs) through cyclization of N- beta-Boc or N-beta-Cbz beta-amino acids using phosphorus tribromide. The fo rmation of beta-NCAs was confirmed by spectroscopy as well as an X-ray stru ctural determination of beta-homoalanine-N-carboxyanhydride. The beta-NCA m olecules could be polymerized in good yield to give optically active poly(b eta-peptides) that adopt stable chiral conformations in solution. For examp le, helical oligo(L-beta-homophenylalanine) was synthesized by polymerizati on of L-beta-homophenylalanine-N-carboxyanhydride.