Synthesis of a β-turn mimetic suitable for incorporation in the peptide hormone LHRH

LHRH is a decapeptide hormone which plays a central role in neuroendocrinology. Conformational studies have suggested that LHRH may adopt a β-turn involving residues 5–8 when bound to its receptor. A β-turn mimetic with side chains corresponding to those of a Tyr-Gly-Leu-Orn tetrapeptide has therefore been synthesized for incorporation at positions 5–8 in LHRH. In the turn mimetic, residues i and i+1 are connected by a ψ[CH2O] isostere instead of an amide bond, while a covalent ethylene bridge replaces the hydrogen bond which is often found between residues i and i+3 in β-turns. The turn mimetic was assembled from three types of building blocks: an azido aldehyde, an Fmoc protected amino acid and a protected dipeptide amine.