We designed and synthesized novel 4-acetoxypentanamide derivatives of spliceostatin A, whose 4-acetoxypentenamide moiety is reduced (7), isomerized (8), or substituted with methyl at the α-position (9). The results of biological evaluation against AR-V7 and the docking analysis of each derivative suggest that the geometry of the 4-acetoxypentenamide moiety of spliceostatin A is important for its biological activity.