and synthesized based on the known proteasomeinhibitor PI-1840. Most of them showed significant inhibition against the β5c subunit of human 20S proteasome, and five of them exhibited IC50 values at the sub-micromolar level, which were comparable to or even more potent than PI-1840. The most active two (1c and 1d) showed IC50 values of 0.12 and 0.18 μM against the β5c subunit, respectively, while they