Carboxyl activation in peptide synthesis using 4-oximino-pyrazol-5-ones

4-Oximino-pyrazol-5-ones 2a, b and their esters with N-protected amino acids 3a, b have been studied for carboxyl activation in peptide synthesis. 2a, b and 3a, b appear to be diastereoisomeric mixtures whose configurations have been assigned on the basis of spectroscopic and X-ray data. Some peptides obtained using these pyrazolone derivatives are reported: no racemization was noted by the Weigand test.