The reaction of N-(tert-butoxycarbonyl)-L-phenylalanine-N-carboxyanhydride (I) with N-methylethylenediamine (II) gives the phenyalaninamide (III), which is condensed with the N-protected chiral azetidinone (IV) by means of Cs2CO3 to yield the diaminopropionic acid derivative (V). The condensation of (V) with glycine tert-butyl ester (VI) by means of BOP and DIEA affords the peptide (VII), which, after deprotection with H2 over Pd/C, is condensed with 4-O-benzyl-L-tyrosine (VIII) by means of BOP and DIEA giving the acyclic precursor (IX). The deprotection of (IX) with TFA, followed by cyclization by means of EDC, HOAt, DIEA and DCM in DMF provides the protected cyclic precursor (X), which is finally deprotected by hydrogenation with H2 over Pd black in ethanol.