Minimizing Side Reactions in Novel Peptide Ring Closing Metathesis Reactions

The opioid receptors, consisting of mu (µ), kappa (κ), and delta (δ) receptors, are important therapeutic targets in pain and mood disorders. Kappa opioid receptor (KOR) antagonists initially found utility as pharmacological tools, but more recently have shown potential for the treatment of depression and drug addiction. KOR selective peptide antagonists, such as the cyclized dynorphin (Dyn) A analog zyklophin, offer a promising profile for potential further development due to their short duration of action. Arodyn (Ac[Phe1,2,3,Arg4,D-Ala8]Dyn A(1-11)-NH2, Figure 1), an acetylated Dyn A analog, has shown potent and selective KOR antagonism. Novel cyclization strategies via ring closing metathesis (RCM) are being pursued to enhance the metabolic stability and potentially stabilize the bioactive conformation of arodyn. Side reactions such as olefin isomerization involving O-allyl groups limit the scope of RCM, especially in probing pharmacological effect of modifying critical aromatic residues. A model dipeptide RCM precursor was synthesized to probe strategies to minimize side reactions and enhance reaction yields. The results of these different side reaction minimization strategies and the application of the modified reaction conditions to the synthesis of novel arodyn analogs will be presented