The title compound was prepared by solid-phase peptide synthesis starting from N-Fmoc-D-proline linked to Rink resin (I). Deprotection of the Fmoc group by means of piperidine in DMA afforded proline-resin (II). Coupling of N-alpha-Fmoc-NG-Pmc-D-arginine (III) to proline-resin (II) using diisopropyl carbodiimide (DIC) and HOBt, followed by Fmoc deprotection with piperidine in DMA, furnished dipeptide-resin (IV). Further coupling and deprotection cycles incorporating the amino acid N2-Fmoc-N6-Boc-D-lysine (V) (twice) and the peptoid residue N-Fmoc-N-[3-(3-Pmc-guanidino)propyl]glycine (VIII) yielded the peptide resins (VI), (VII) and (IX), respectively.
Further coupling and deprotection cycles incorporating peptoid N-Fmoc-N-benzylglycine (X), and twice peptoid (VIII), yielded the peptoid-resins (XI) and (XII), respectively. Bromoacetic acid (XIII) was subsequently coupled to resin (XII) to afford the bromoacetamido peptoid-resin (XIV).
Displacement of the bromine of (XIV) with N-Boc-1,6-diaminohexane (XV) produced resin (XVI). Finally, the side-chain protecting groups of (XVI) were cleaved and the desired peptoid was liberated from the resin by treatment with trifluoroacetic acid.