Amino Acid Side Chain Attachment Approach and Its Application to the Synthesis of Tyrosine-Containing Cyclic Peptides

Abstract

The technique of resin loading by the attachment of the amino acid side chain represents a powerful tool for the synthesis of cyclopeptides by solid phase. We investigated the anchoring of the side chain of N-(9-fluorenylmethoxycarbonyl, Fmoc)-tyrosine methyl ester to benzyl-type resins by the Mitsunobu reaction. Satisfactory loading was obtained for HMPB−MBHA and Wang resins. The suitability of the preloaded resins for solid-phase peptide synthesis by using the Fmoc strategy, combined with the head-to-tail cyclization on the solid support, was illustrated by the preparation of three cyclic analogues of neuropeptide Y (NPY), a 36-residue peptide hormone and one of the most abundant neuropeptides in the brain. Each peptide contained the N- and C-terminal tetrapeptide segments of NPY, joined by different spacers: 6-aminohexanoic acid, β-alanine, or Ala-Aib. First the synthesis of the peptide methyl esters was performed, followed by saponification and cyclization on the resin. HOBt/DIC or HOBt/TBTU was used for the ring closure. The CD spectra of the three cyclopeptides in 30% trifluoroethanol showed a type I and III β-turns structure, which was already adopted by the (Ala-Aib)-containing cyclopeptide in water. The CD spectra, together with the biological assays, confirmed the suitability of these cyclopeptides as conformationally restricted peptides that may serve as lead structures in drug development.