Intramolecular Hydrogen Bonding in Depsipeptides Containing Endo-3,6- Tricyclo[6.2.1.02,7]undeca-4,9-diene-3,6-endo-diol

A variety of endo-3,6-tricyclo[6.2.1.02,7]undeca-4,9-diene-3,6-endo-diol depsipeptide or bis-amino acid derivatives containing the same parallel strands were synthesised and examined for conformational preferences by NMR. The study indicates that this novel class of constrained peptides displays β-turn-like and β-sheet-like conformations. Evidences are supported by observation of the rates of proton-deuterium exchange, nuclear Overhauser effects and further by dynamic simulations, semi-empirical and ab initio calculations. The chemical shifts were measured in CDCl3 and CDCl3 with 15% DMSO-d6 solutions. It was carried out a van't Hoff analysis, which is in agreement with the theoretical studies. In chloroform-d, for the derivatives 3-6 the hydrogen-bonded species are enthalpically preferred but entropically disfavoured.