Total Synthesis of Stelletins through an Unconventional Annulation Strategy

Marine ecosystems present the largest source of biodiversity on the planet and an immense reservoir of novel chemical entities. Sessile marine organisms such as sponges produce a wide range of complex secondary metabolites, many of these with potent biological activity engineered for chemical defense. That such compounds exert dynamic effects outside of their native context is perhaps not surprising, and the realm of marine natural products has attracted considerable attention as a largely untapped repository of potential candidates for drug development. Only a handful of the more than 15 000 marine natural products that have been isolated to date have advanced to the clinic, and more are to be expected. The rich chemical information encoded in the intricate three-dimensional structures of many marine natural products facilitates highly discriminating interactions with cell signaling pathways, and especially within cancer cells such nuanced effects offer an exciting opportunity for the development of targeted therapies that lack the side effects and general toxicity of conventional chemotherapeutics. The isomalabaricanes are a rare class of marine triterpenoids that have been hailed as promising cytotoxic lead compounds for the treatment of cancer, and they have attracted a flurry of excitement from researchers because of their potent cytotoxicity in certain human cancer cell lines along with a range of other antineoplastic effects. Most notably, their inhibitory activity is highly cell-selective, characterized by large deviations from their mean GI₅₀ concentrations across 3 orders of magnitude in the NCI-60 Human Tumor Cell Lines screen, suggesting mechanistic specificity rather than general and unbridled toxicity. Despite these auspicious preliminary reports, the isomalabaricane scaffold remains largely unexplored as a potential anticancer lead because of lack of material. This Account describes our recent efforts to develop a general, modular synthesis of the isomalabaricanes, as exemplified by the successful total syntheses of rhabdastrellic acid A, stelletin E, and stelletin A. The unorthodox trans-syn-trans configuration of their perhydrobenz[e]indene core severely circumscribes the synthetic methods available for its construction and required several generations of strategy to assemble. Ultimately, a series of unconventional transformations were identified that were capable of building this highly strained motif, and the syntheses of rhabdastrellic acid A and stelletin E were completed in racemic fashion. Subsequently, a second-generation approach to these natural products was developed, rendering the synthesis enantioselective as well as providing access to stelletin A. These synthetic efforts were greatly assisted by computational techniques such as ¹³C NMR prediction, which enabled structural assignments of hydrocarbon diastereomers, as well as relaxed surface scan conformational analysis, which informed a campaign for directed hydrogenation of an alkene. High-throughput experimentation methods were brought to bear during optimization of a late-stage Suzuki coupling on stelletin A. Finally, preliminary structure-activity relationship studies in glioblastoma and nonsmall cell lung cancer cell lines were conducted on stelletin A, revealing that the singular trans-syn-trans perhydrobenz[e]indene core is essential for the cytotoxic activity of the isomalabaricane triterpenoids.