Stereoselective synthesis of novel dipeptide beta-turn mimetics targeting melanocortin peptide receptors.

[reaction: see text] The novel dipeptide beta-turn mimetic, 4,8-disubstituted azabicyclo[4.3.0]nonane amino acid ester (15), has been synthesized to serve as a peptide mimetic of the dipeptide Phe-Arg, which contains two important pharmacophore elements in melanotropin peptides. Introduction of side-chain functionality at C-8 was achieved by using beta-functionalized pyroglutamate (8) as a synthetic precursor. The side chain at C-4 was introduced by bromination of dehydroamino acid intermediate (10) followed by Suzuki cross-coupling.