by reductocondensation of 4-phenyl-2-oxo butyric acid (IX) with L-alanyl-L-proline (IV) by means of sodium cyanoborohydride and NaOH in water.
The reductocondensation of N(epsilon)-tert-butoxycarbonyl-lysyl proline (I) with 2-oxobutyric acid (II) by means of sodium cyanoborohydride in water gives the protected compound (III), which is finally treated with HCl in ethyl acetate.
Treatment of alanine (I) with phosgene in THF provides N-carboxyanhydride derivative (II), which is then coupled to proline (III) by means of KOH and K2CO3 in THF/H2O or acetonitrile to provide dipeptide (IV). Coupling of (IV) to ethyl 2-oxo-4-phenylbutyrate (V) by means of H2 over Raney-Ni in EtOH (or EtOH/HOAc), followed by the corresponding work-up for isomer separation, yields derivative (VI). (In turn, ketoester (V) can be obtained by treatment of 2-phenylethyl bromide (VII) with Mg turnings in refluxing THF, followed by reaction of the resulting Grignard reagent with diethyl oxalate (VIII) in THF). Finally, the desired product is obtained by saponification of the ethyl ester group of (VI) with NaOH.