Proline-like β-turn mimics accessed via Ugi reaction involving monoprotected hydrazines

A four-center, three-component Ugi-type reaction of a variety of keto acids, Boc- or Cbz-protected hydrazine, and isocyanides offers a simple and high yielding access to cyclic products containing an N-aminolactam unit. The latter are shown to form consistently an intramolecular hydrogen bond leading to a β-turn-like secondary structure. The possibility of integrating such N-aminolactam units (without disruption of the folded structure) into pseudotripeptide fragments is demonstrated.