A detailed structure-activity relationship of syringolin A (1), which is a promising antitumor natural product, was described. We previously developed syringolin A analog 2 as a potent proteasome inhibitor by the structure-based drug design of syringolin A. In this Letter, we synthesized a range of analogs of 2, having a different length of the lipophilic chain and substituted aryl group, and their cytotoxicity against human cancer cells was evaluated. It turned out that these modifications greatly affected the cytotoxicity. Further optimization would lead to develop a novel proteasome inhibitor.