Fig. 3. HPLC monitoring of the reaction (samples withdrawn after 1 h) of Ac-l-Tyr(Me)-5(4H)-oxazolone 2a (ca. 1 mM) with H-l-Leu-NH₂ (5 mM) carried out in CH₂Cl₂ (control, black) or in the presence of added carboxylic acids (2 mM): decanoic acid (red) or acetic acid (blue).