| Abstract: | During the hydrogenolytic removal of the benzyl ester from the pentapeptide Boc-Asp(OBzl)-Ala-Phe-Ile-Gly-OEt, either with hydrogen gas or ammonium formate in the presence of palladium-on-charcoal as catalyst, variable amounts (5–30%) of an unexpected by-product were always obtained. The compound was eventually identified as Boc-Asc-Ala-Phe-Ile-Gly-OEt (Asc = aminosuccinyl). The application of field desorption mass spectrometry as a diagnostic tool is reported. The study of this side reaction, carried out on the pentapeptide and two other dipeptide models, showed that: 1) palladium-on-charcoal may induce Asc formation; 2) the contemporary presence of the catalyst and a base (even in trace amounts) greatly increases the by-product formation; 3) the side reaction is sequence and solvent dependent; 4) the Asc formation is completely prevented by adding a few equivalents of acetic or formic acid. Some mechanistic considerations are also reported. |
