Compound ZJ-101, a structurally simplified analog of the marine natural product superstolide A, was previously developed in our laboratory. In the subsequent structure–activity relationship study, two new analogs, ZJ-105 and ZJ-106, were designed and synthesized to probe the importance of the conjugated trienyl lactone moiety of the molecule by replacing the C2–C3 double bond in ZJ-101 with a single bond and switching the geometry of the C4–C5 double bond in ZJ-101 from Z to E, respectively. Biological evaluation showed that ZJ-105 completely loses antiproliferative activity whereas ZJ-106 is significantly less active against cancer cells in vitro than ZJ-101, suggesting that the conjugated trienyl lactone moiety of the molecule is critical for its anticancer activity.