Nucleo amino acids as arginine mimetics in cyclic peptides

A general strategy for the synthesis of Fmoc protectednucleobase modified amino acids is presented. Fmoc protected nucleo amino acidsbearing a natural purine (guanine) as well as an artificial purine(isoadenine) in the side chain have been synthesized and incorporated into cyclicpentapeptides. The structure of the cyclic peptides is based on the well knownRGD peptides, which act as selective integrin antagonists. Thenucleo amino acids serve as conformationally constrained arginine mimeticswith a reduced basicity of the guanidino moiety.