With an average GI50 value against the NCI panel of 60 human cancer cell lines of 0.12 nM, spongistatin 1 is among the most potent anti-proliferative agents ever discovered rendering it an attractive candidate for development as a payload for antibody-drug conjugates and other targeted delivery approaches. It is unavailable from natural sources and its size and complex stereostructure render chemical synthesis highly time- and resource-intensive, however, and its development requires more efficient and step-economical synthetic access. Using novel and uniquely enabling direct complex fragment coupling alkallyl- and crotylsilylation reactions, we have developed a 22-step synthesis of a rationally designed D-ring modified analog of spongistatin 1 that is equipotent with the natural product, and have used that synthesis to establish that the C(15) acetate may be replaced with a linker functional group-bearing ester with only minimal reductions in potency.<br><div><br></div>