Synthesis of ω-amino acid peptides related to leucine-enkephalin

Syntheses are described of four peptides with modified leucine-enkephalin sequences in which the native glycylglycyl segment is replaced by an ω-amino acid residue. o-Nitrophenylthio-ω-amino acids were used as intermediates, and it was found that the derivatives of 4-aminobutyric and 5-amino-valeric acids undergo facile intramolecular cyclization under the influence of N,N'-dicyclohexylcarbodiimide.