Condensation of Boc-D-Orn(COOCH2Ph)-OH (I) with the lithium salt of ethyl acetate (A) in the presence of CDI in THF affords derivative (II), which is then hydrogenated over Pd/C to yield the mixture of isomers (IIIa-b). Condensation of (IIIa-b) with benzyl isocyanate in THF, followed by simultaneous reduction and cyclization by means of NaH in THF, provides 1,3-dioxoperhydropyrido[1,2-c]pyrimidine (Va-b), whose N-Boc group is removed by treatment with TFA in CH2Cl2 to furnish amine (VI). Coupling of (VIa-b) to Boc-L-Trp-OH (VII) by means of BOP and Et3N in CH2Cl2, followed by chromatographic separation, gives derivative (VIII), which is subjected to Boc removal by treatment with TFA in CH2Cl2 to provide amine (IX). Finally, amine (IX) is condensed with t-butyl isocyanate in the presence of Et3N in THF to yield the target compound.