Synthesis of bicyclic dipeptide mimetics for the cholecystokinin and opioid receptors

The cholecystokinin C-terminal octapeptide analogue H-Asp-Tyr-D-Phe-Gly-Trp-(N-Me)-Nle-Asp-Phe-NH₂ (SNF 9007) is a potent and selective ligand for both the CCK-B and δ-opioid receptors. To constrain the peptide into the biologically active conformation(s), bicyclic dipeptide mimetics for Nle-Gly and homoPhe-Gly were designed and synthesized from β-substituted aspartic acids. Alkylation of L-aspartic acid using lithium bis(trimethylsilyl)amide (LHMDS) in the presence of hexamethylphosphoramide (HMPA) gave β-substituted aspartic acids, with the major product being the (2S,3R) isomer. Additional isomers of Nle-Gly bicyclic dipeptide mimetic were obtained via the Kazmaier-Claisen rearrangement reaction. The stereochemistries of the bicyclic dipeptide mimetics were assigned by X-ray and NMR.