Abstract: Provided herein is a compound of Formula I, or a salt thereof, wherein R1 is a moiety having the following structure: wherein Z is selected from the group consisting of O and N; X is selected from the group consisting of a bond, NH, and C(O)NH; and A is selected from the group consisting of substituted aryl and substituted heteroaryl. Compounds of Formula I are useful as opioid receptor agonists. Because the compounds exhibit relatively low levels of ?-arrestin recruitment, they do not produce the negative side effects commonly associated with morphine-derived compounds.