Epoxide-initiated cationic polyene cyclisations

Effective In Situ Alkoxide Trapping by TMSX in the Presence of TPP Catalyst in Glycidol Acetal Rearrangements

This study demonstrates a nucleophile-catalyzed, trimethylsilyl halide-promoted rearrangement reaction of glycidol acetals to form halogenated cyclic acetals. The acetal group has been activated selectively in the presence of trimethylsilyl cation, which is used as in situ-generated alkoxide trapping reagent. Nucleophilic chloride and bromide ions participate in addition reactions with epoxides predominantly via SN1-type epoxide opening, while non-nucleophilic iodide and triflate ions induce a positive charge at the epoxide carbon. A systematic investigation of acetal-initiated polyene cyclization of epoxy polyenes has been conducted using bicyclic epoxonium ions as transient intermediates. Unfavorable orbital orientation and other stereoelectronic factors hinder the much-anticipated polyene cyclizations. The potential of this method has been showcased through its application in the total synthesis of parvistone A, a chlorinated styryllactone.

Cationic cyclization reactions with alkyne terminating groups: a useful tool in biomimetic synthesis

Cyclization reactions through cationic intermediates have become a highly valuable tool in organic synthesis. The use of alkynes as the terminating group in this type of cationic process offers wide synthetic possibilities because this group can serve as a precursor of different functionalities. This article shows relevant examples of cationic cyclization reactions with alkynes as terminating groups with the intention of demonstrating the potential of this type of process, particularly in the context of biomimetic synthesis of natural products.

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.

Mimicking Halimane Synthases: Monitoring a Cascade of Cyclizations and Rearrangements from Epoxypolyprenes

We have developed and rationalized a biomimetic transformation mimicking halimane synthases based on a Lewis acid-catalyzed cascade of cyclizations and rearrangements of epoxypolyprenes. Two rings, three stereogenic centers, and a new double bond were generated in a single chemical operation. Based on this cascade transformation, we achieved a unified strategy toward the stereoselective total syntheses of halimene-type terpenoids and analogues as a proof-of-concept study. This method has been applied to the rapid synthesis of diterpene isotuberculosinol, a virulence factor of Mycobacterium tuberculosis as a representative example.

Transition metal-promoted biomimetic steps in total syntheses

Important biomimetic steps in natural product synthesis have been promoted by transition metals, as exemplified by this beautiful ruthenium-catalyzed rearrangement of an endoperoxide into elysiapyrone A. Such reactions are supposed to occur during the biosynthesis, yet under different catalysis conditions.

Exploration of cascade cyclizations terminated by tandem aromatic substitution: total synthesis of (+)-schweinfurthin A

The termination of epoxide-initiated cascade cyclizations with a range of "protected" phenols is described. When the protecting group can be lost as a stabilized electrophile, the cascade process continues beyond ring closure to afford products which have undergone a tandem electrophilic aromatic substitution. A number of groups have proven viable in this process and the regiochemistry of their substitution reactions has been studied. Application of this methodology in the first total synthesis of (+)-schweinfurthin A, a potent antiproliferative agent, has been achieved.

Recognition and organocatalysis with a synthetic cavitand receptor

The cyclization reaction of an epoxyalcohol is catalyzed by a synthetic cavitand receptor with an inwardly directed carboxylic acid function. The receptor features a hydrophobic pocket in which the substrate is bound and positioned to react in a regioselective manner. The nature of this substrate-catalyst complex and its dynamic properties were investigated by NMR methods and with the aid of a model compound lacking the epoxide function. The kinetic parameters of the cyclization reaction were also studied. A catalytic cycle is proposed and diverse inhibition mechanisms are identified that parallel those encounterd in enzymology.

Epoxide-initiated cationic cyclization of azides: a novel method for the stereoselective construction of 5-hydroxymethyl azabicyclic compounds and application in the stereo- and enantioselective total synthesis of (+)- and (-)-indolizidine 167B and 209D

A novel and general method has been developed for the stereoselective construction of 5-hydroxymethyl azabicyclic ring skeletons based on epoxide-initiated cationic cyclization of azides. The key cyclization reaction was systematically studied with the model compound, 3-(1-oxa-spiro[2.4]hept-4-yl)propyl azide 3a, and EtAlCl(2) was found to be an ideal choice as the catalyst. The generality of this transformation was further tested with different ring sizes, where six- and seven-membered epoxyazides 3b,c underwent smooth cyclization to give 5-hydroxymethyl azepine 4b and 5-hydroxymethyl azocine 4c, respectively, as a single detectable diastereomer. This novel methodology was elegantly applied in the stereoselective total synthesis of indolizidine alkaloids 167B and 209D. Further, the enantioselective total synthesis of natural and unnatural indolizidine alkaloids 167B and 209D was accomplished by using Sharpless asymmetric dihydroxylation as a key step.