Synthesis of L-Kynurenine peptides conducted without masking the side-chain amino group

Various free L-kynurenine peptides, including an analogue of the opiate agonist leucine-enkephalin, have been synthesized by using the Nα-t- butyloxycarbonyl derivative of L-kynurenine as a key precursor. By selecting appropriate mild conventional coupling procedures it was possible to carry out the synthetic operations without having to mask the side-chain amino group in any way, but intramolecular acylation with formation of a seven-membered lactam could be demonstrated under more vigorous conditions.