Management of moderate to severe pain relies heavily on opioid analgesics such as morphine, oxycodone, and fentanyl in clinics. However, their prolonged use was associated with undesirable side effects. Many new strategies to reduce side effects have been proposed, but not without disadvantages. Using a hot plate model as a phenotypic screening method, our studies identified (3<i>R</i>,4<i>S</i>)-<b>9d</b> with a new scaffold as a potent analgesic with ED<sub>50</sub> values of 0.54 mg/kg and 0.021 mg/kg in hot plate and antiwrithing models, respectively. Mechanistic studies showed that it elicited its analgesic effect via the active metabolite (3<i>R</i>,4<i>S</i>)-<b>10</b><b>a</b>. The mechanism of (3<i>R</i>,4<i>S</i>)-<b>10</b><b>a</b>-induced activation of the μ opioid receptor (MOR) was proposed by means of molecular dynamics (MD) simulation.