Synthesis, Conformational Analysis, and Biological Evaluation of the Potent Microtubule-Stabilizing Agents, (–)-Zampanolide and (–)-Dactylolide

Zampanolide, a 20-membered marine polyketide, exhibits low nanomolar cytotoxicity (0.25 – 3 ng/mL) against multiple human cancer cell lines and induces apoptosis through microtubule bundle formation. Interestingly, Riccio and co-workers isolated a structurally related natural product, (+)-dactylolide, off the coast of the Vanuatu islands and reported dactylolide displays moderate cytotoxicity in the low μM range. Due to zampanolide's remarkable biological profile and limited supply, we embarked on a synthetic preparation of this complex polyketide to fully characterize its bioactive conformation and molecular interaction with tubulin.

Our synthesis of zampanolide required 26 steps in the longest linear sequence from commercially available (R)-aspartic acid. The scalable route employed a highly diastereoselective Sharpless epoxidation, a one pot cross-metathesis/Horner-Wadsworth Emmons olefination, and our recently developed electrophile-induced ether transfer methodology. Interestingly, the successful ether transfer depended upon the use of a p-bromobenzyl ether to minimize competitive Bartlett cyclization of a primary benzyl ether. Notably, our synthesis provided approximately 40 mg of dactylolide, the highest amount of material produced by any synthetic group to date.

With significant amounts of material in hand, we employed high-field NMR and molecular modeling experiments with dactylolide to help elucidate zampanolide and dactylolide's overall solution structure and specific conformational preferences. Our results indicated that both macrolide's interconvert between three conformational families in DMSO-d6, one of which strongly resembles zampanolide's bioactive conformation. A detailed knowledge of zampanolide and dactylolide's solution conformational preferences not only helped identify its bioactive conformation but also significantly aided in the development of simplified analogues.

Probing the biosynthesis of zampanolide and dactylolide revealed three beta branching sites: the C5Me, the C13 exocyclic alkene, and the C17Me. Molecular modeling suggested that the removal of the C17Me only minimally affects zampanolide and dactylolide's overall macrolide conformation. Fortunately, removing the C17Me significantly simplified the synthesis requiring only 15 steps in the longest linear sequence for construction of a 17-desmethyl-13-desmethylene analogue. The convergent strategy was highlighted by a Tsuji-Trost cyclization and an E-selective cross-metathesis reaction for installation of the C16-20 fragment.