Synthesis and Structural Characterization of β-Turn Mimics Containing ( Z )-Chloroalkene Dipeptide Isosteres.

Described here is the synthetic, spectroscopic, crystallographic, and computational analysis of a series of peptidomimetics containing l-Xaa-d-Yaa-type ( Z )-chloroalkene dipeptide isosteres (CADIs) that were measured in an investigation of the β-turn mimicry of this peptide bond surrogate. We found that the 1,3-allylic strain across the chloroalkene moiety engenders the hyperconjugative interactions between the chloroalkene moiety and the C-H bonding or antibonding orbitals of the C-H bonds in allylic positions. These effects contribute significantly to the stabilization of β-turn structures.