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.