Benzotriazole-mediated synthesis of aza-peptides: en route to an aza-leuenkephalin analogue.

Novel N-(N-Pg-azadipeptidoyl)benzotriazoles 20a–e couple efficiently with α-amino acids 21a–e, dipeptides 22a–c, aminoxyacetic acid 23a, depsidipeptide 23b, and α-hydroxy-β-phenylpropionic acid 27 yielding, respectively, azatripeptides 24a–g, azatetrapeptides 25a,b, a hybrid azatripeptide with an oxyamide bond 26a, a hybrid azatetrapeptide with an ester bond 26b, and a hybrid azatripeptide with an ester bond 28. A new protocol for the synthesis of N-Pg-azatripeptides 33a,b and 35a,b, each containing a natural amino acid at the N-terminus, avoids the low coupling rates of the aza-amino acid residue and enables the solution-phase synthesis of an azaphenylalanine analogue of Leu-enkephalin 40.