Design and synthesis of novel χ²-constrained phenylalanine, naphthylalanine, and tryptophan analogues and their use in biologically active melanotropin peptides

A series of novel hydrophobic, bulky χ²-constrained phenylalanine, naphthylalanine, and tryptophan derivatives was designed and synthesized. The key steps involved asymmetric hydrogenations of α-enamides using Burk's DuPHOS-based Rh(I) catalysts to give high enantiomerically pure α-amino acid derivatives. The subsequent Suzuki cross couplings of the amino acid analogues with boronic acid derivatives afforded these aromatic substituted amino acids in high yields and with high enantioselectivity. The incorporation of these novel χ²-constrained amino acids into peptides and peptidomimetics provides fruitful information in the development of peptide and peptidomimetic ligands of melanotropins and an understanding of the interactions between ligands and receptors/acceptors.