The intermediate N-(pyridin-3-ylmethoxycarbonyl)-L-leucine (V) has been obtained as follows: L-Leucine methyl ester (I) was converted into the isocyanate (II) by conventional methods, and the resulting compound was condensed with 3-pyridylmethanol (III) to yield the N-acylated leucine methyl ester (IV). Finally, this compound was hydrolyzed with LiOH to afford the desired intermediate N-(pyridin-3-ylmethoxycarbonyl)-L-leucine (V).

3(S)-(Allyloxycarbonylamino)-1-chloro-5-phenyl-2-pentanone (VI) is linked to a carbazate resin (VII) to yield the resin-supported compound (VIII) suitable for solid-phase synthesis. The condensation of (VIII) with 3-phenylpropylmercaptan (IX) by means of DIEA in DMF affords the adduct (X), which is deprotected at the allyloxycarbonylamino group by means of Pd(PPh3)4 to provide the amino resin (XI). The condensation of (XI) with the previously described intermediate (V) by means of PyBOP, HOAt and DIEA in DMF gives dipeptide resin (XII). Finally, the cleavage of the target compound from the resin (XII) is performed by means of TFA in trifluoroethanol/water.