Probing opioid receptor interactions with azacycloalkane amino acids. Synthesis of a potent and selective ORL1 antagonist

Azacycloalkane turn mimics 6-9 were used to explore the relationship between conformation and biological activity of peptide ligands to the opioid receptor-like (ORL1) receptor. Three azabicyclo[x.y.0]alkane amino acids and a 5-tBuPro type VI beta-turn mimic were introduced into peptides 10-13 by solid-phase synthesis on MBHA resin. Biological examination of peptides 10-13 showed two new antagonists (10 and 12) exhibiting increased selectivity for the ORL1 receptor.