N-Boc-L-Valine N-hydroxysuccinimidyl ester (I) was coupled to L-proline benzyl ester (II), and the resulting Boc dipeptide (III) was subsequently deprotected with trifluoroacetic acid in CH2Cl2 to afford (IV). This was condensed with N-Boc-L-aspartic acid gamma-benzyl 1-N-hydroxysuccinimidyl diester (V) to give (VI). After Boc deprotection of (VI) with trifluoroacetic acid, tripeptide (VII) was coupled with N-Boc-L-leucine N-hydroxysuccinimidyl ester (VIII) to yield tetrapeptide (IX). Further Boc deprotection of (IX) with trifluoroacetic acid furnished peptide intermediate (X).
Condensation of o-tolyl isocyanate (XI) with 4-aminophenylacetic acid (XII) produced urea (XIII). This was coupled to tetrapeptide (X) using EDC and HOBt to give the corresponding amide (XIV). Finally, the benzyl ester groups of (XIV) were deprotected by hydrogenolysis over Pd/C.