The compound was prepared by solid-phase peptide synthesis on a PAM-resin. Starting from a N(alpha)-Boc-N(gamma)-Tos-L-arginine resin (I), cleavage of the Boc protecting group by means of trifluoroacetic acid provided Arg(Tos)-resin (II). Coupling of (II) with N(alpha)-Boc-L-tryptophan (III) using O-benzotriazol-1-yl-N,N,N',N'-tetramethyluronium hexafluorophosphate (HBTU) and diisopropylethylamine, followed by acid cleavage of the Boc group gave dipeptide resin (IV). This was in turn coupled and deprotected with the following amino acids N-Boc-D-cyclohexylalanine (V), N--Boc-L-proline (VII) and N(alpha)-Boc-N(delta)-Tos-L-ornithine (IX) to furnish the peptide resins (VI), (VIII), (X) and (XII), respectively.
The resin (X) was coupled with N-Boc-L-phenylalanine (XI) affording resin (XII). The acetylation of the free amino group, side-chain deprotection and cleavage from the resin (XII) employing liquid HF and p-cresol yielded the linear dipeptide (XIII). This was finally cyclized with benzotriazol-1-yloxytris (dimethylamino)phosphonium hexafluorophosphate (BOP) to produce the title cyclic peptide.