Amino acid side chain attachment approach and its application to the synthesis of tyrosine-containing cyclic peptides

The technique of resin loading by the attachment of the amino acid side cha in represents a powerful tool for the synthesis of cyclopeptides by solid p hase. We investigated the anchoring of the side chain of N-(9-fluorenylmeth oxycarbonyl, Fmoc)-tyrosine methyl ester to benzyl-type resins by the Mitsu nobu reaction. Satisfactory loading was obtained for HMPB-MBHA and Wang res ins. The suitability of the preloaded resins for solid-phase peptide synthe sis by using the Fmoc strategy, combined with the head-to-tail cyclization on the solid support, was illustrated by the preparation of three cyclic an alogues of neuropeptide Y (NPY), a 36-residue peptide hormone and one of th e most abundant neuropeptides in the brain; Each peptide contained the N- a nd C-terminal tetrapeptide segments of NPY, joined by different spacers: 6- aminohexanoic acid, beta-alanine, or Ala-Aib. First the synthesis of the pe ptide methyl esters was performed, followed by saponification and cyclizati on on the resin. HOBt/DIC or HOBt/TBTU was used for the ring closure. The C D spectra of the three cyclopeptides in 30% trifluoroethanol showed a type I and III beta-turns structure, which was already adopted by the (Ala-Aib)- containing cyclopeptide in water. The CD spectra, together with the biologi cal assays, confirmed the suitability of these cyclopeptides as conformatio nally restricted peptides that may serve as lead structures in drug develop ment.