Cyclopropane-derived peptidomimetics. Design, synthesis, and evaluation of novel enkephalin analogues

It is known that peptide mimics containing trans-substituted cyclopropanes stabilize extended conformations of oligopeptides, and molecular modeling studies now suggest that the corresponding cis-cyclopropane dipeptide isosteres could stabilize a reverse turn. To begin to assess this possibility, a series of cis-substituted cyclopropanes were incorporated as replacements of the Gly(2)-Gly(3) and Phe(4)-Leu(5) dipeptide subunits in Leu-enkephalin (H(2)N-Tyr-Gly-Gly-Phe-Leu-OH), which is believed to bind to opiod receptors in a conformation containing a beta-turn. General methods for the synthesis of the cyclopropane-containing dipeptide isosteres -XaaPsi[COcpCO]Yaa- and -XaaPsi[NHcpNH]Yaa-were developed by a sequence that featured the enantioselective cyclization of allylic diazoacetates catalyzed by the chiral rhodium complexes Rh(2)[(5S)-MEPY](4) and Rh(2)[(5R)-MEPY](4). A useful modification of the Weinreb amidation procedure was applied to the opening of the intermediate lactones with dipeptides, and a novel method for the synthesis of substituted diaminocyclopropanes was also developed. The Leu-enkephalin analogues were tested in a panel of binding and functional assays, and although those derivatives containing cyclopropane replacements of the Gly(2)-Gly(3) exhibited low micromolar affinity for the mu-receptor, analogues containing such replacements for the Phe(4)-Leu(5) subunit did not bind with significant affinity to any of the opioid receptors. These results are discussed.