The title compound was obtained by solid-phase peptide synthesis using a methylbenzhydrylamine resin. After attachment of N-Boc-D-alanine (I) to the resin by means of diisopropylcarbodiimide (DIC), the N-Boc protecting group was cleaved by treatment with a solution of trifluoroacetic in CH2Cl2 in the presence of 1,2-ethanedithiol, yielding the alanine-bound resin (II). To this were sequentially coupled the appropriate protected amino acids using DIC, each followed by the Boc group cleavage with trifluoroacetic acid. Coupling/deprotection cycles with N-Boc-L-proline (III), N-alpha-Boc-N-epsilon-benzyloxycarbonyl-N-epsilon-isopropyl-L-lysine (V), N-Boc-L-leucine (VII), N-alpha-Boc-N-4-Fmoc-L-4-aminophenylalanine (IX), N-alpha-Boc-N-4-Fmoc-D-4-aminophenylalanine (XI) and N-Boc-O-benzyl-L-serine (XIII), furnished the peptide resins (IV), (VI), (VIII), (X), (XII) and (XIV).
Coupling/deprotection cycles with N-Boc-D-(3-pyridyl)alanine (XV), N-Boc-D-4-chlorophenylalanine (XVII) and N-Boc-D-(2-naphthyl)alanine (XIX) furnished the peptide resins (XVI), (XVIII) and (XX).
The side-chain Fmoc protecting groups of (XX) were then cleaved by treatment with piperidine in DMF, and the resulting compound (XXI) was acetylated with acetic anhydride to give (XXII). Finally, removal of the remaining protecting groups and cleavage from the resin was achieved by treatment with liquid HF and anisole.