New μ-opioid receptor (MOR) agonists containing piperazine and homopiperazine moieties in the structures were synthesized and their affinities to and agonist potencies on MOR were evaluated. Among the synthesized compounds, 4-[4-(2-methoxyphenyl) piperazin-1-yl]-N, N- dimethyl-2, 2-diphenylbutanamide (20Aa) showed the highest affinity to the human MOR expressed in Chinese hamster ovary (CHO)-K1 cells, and the highest agonist potency on ...