Topographically constrained aromatic alpha-aza-amino acids. Part 2. New azaTic-containing peptides: Synthesis, conformation, and intramolecular NH center dot center dot center dot N interaction

The new pseudodipeptide Boc-azaTic-Leu-OMe (1), incorporating the conformat ionally and topographically constrained 3,4-dihydro-2(1H)-phthalazinecarbox ylic acid (azaTic) residue, has been synthesized together with the three re lated models Boc-azaTic-NHMe(3), MeCO-azaTic-Gly-OMe (4), and azaTic-Leu-OM e (6). Both the epimers of 1, generated by opposite absolute configuration at an azaTic nitrogen stereogenic centre, have been found in the asymmetric unit of the crystal. The conformational perturbations, deriving by the int roduction of the azaTic residue into a peptide backbone, are described and the nature of the NH ... N interaction, which gives rise to a typical backb one folding closing a 5-membered ring, has been studied. (C) 1999 Elsevier Science Ltd. All rights reserved.