Design, Synthesis, and Conformational Analysis of a Novel Spiro-Bicyclic System as a Type II β-Turn Peptidomimetic

A novel highly constrained spiro-bicyclic system (5) has been developed to mimic the type II β-turn, a secondary structural feature found in many bioactive peptides. This system simultaneously restricts three (Φ₂, Ψ₂, and Φ₃) of the four torsion angles that characterize the type II β-turn. As a test of the design, the asymmetric synthesis and conformational analysis of derivative 6 starting from (R)-2-allylproline is reported. Temperature dependent NMR chemical shift studies in CDCl₃ suggest that the amide proton of 6 is involved in an intramolecular hydrogen bond. Also, NOE measurements place this hydrogen under the plane of the bicyclic ring system in proper proximity for this hydrogen bond to form with the acetyl carbonyl oxygen. Modeling studies of 6 produced eight minimum-energy conformations with torsion angles close to those of the classical type II β-turn. A comparison of the minimum-energy conformer of this molecule with the classical type II β-turn gave an RMS fit = 0.161 Å.