Coupling of PhCH2OOC-Pro-OH (I) with diphenyl (1-amino-2-phenylethyl)phosphonate (II) by means of DCC in THF affords derivative (III), which is then subjected to hydrogenation followed by coupling with PhCH2OOC -Val-OH (IV) by means of DCC to provide peptide derivative (V). Finally, reaction of (V) with succinic anhydride in EtOAc under an atmosphere of H2 in the presence of Pd/C provides the target compound.