Fumaroylamino-4,5-epoxymorphinans and Related Opioids with Irreversible μ Opioid Receptor Antagonist Effects

We have previously shown that cinnamoyl derivatives of 14β-amino-17-cyclopropylmethyl-7,8-dihydronormorphinone and 7α-aminomethyl-6,14-endoethanonororipavine have pronounced pseudoirreversible μ opioid receptor (MOR) antagonism. The present communication describes the synthesis and evaluation of fumaroylamino analogues of these cinnamoylamino derivatives together with some related fumaroyl derivatives. The predominant activity of the new ligands was MOR antagonism. The fumaroylamino analogues (<b>2a</b>, <b>5a</b>) of the pseudoirreversible antagonist cinnamoylamino morphinones and oripavines (<b>2b</b>, <b>5b</b>) were themselves irreversible antagonists in vivo. However the fumaroylamino derivatives had significantly higher MOR efficacy than the cinnamoylamino derivatives in mouse antinociceptive tests. Comparison of <b>2a</b> and <b>5a</b> with the prototypic fumaroylamino opioid β-FNA (<b>1a</b>) shows that they have similar MOR irreversible antagonist actions but differ in the nature of their opioid receptor agonist effects; <b>2a</b> is a predominant MOR agonist and <b>5a</b> shows no opioid receptor selectivity, whereas the agonist effect of β-FNA is clearly κ opioid receptor (KOR) mediated