Incorporation of unnatural amino acid derivatives into a peptide bond via an oxime ester catalysed by papain or lipase

In the presence of an oxime in the reaction solution, papain and lipase P (Pseudomonas from Amano) catalysed the stereoselective transesterification of an N-protected amino acid or peptide ester to form an active (oxime) ester which in turn underwent peptide bond formation with several natural and unnatural amino acid derivatives (proline, N-methylglycine, N-methylalanine, α-methylphenylalanine).