General Approaches to Structurally Diverse Isocyanoditerpenes

Unified Synthesis of 2-Isocyanoallopupukeanane and 9-Isocyanopupukeanane through a "Contra-biosynthetic" Rearrangement

Herein, we describe our synthetic efforts toward the pupukeanane natural products, in which we have completed the first enantiospecific route to 2-isocyanoallopupukeanane in 10 steps (formal synthesis), enabled by a key Pd-mediated cyclization cascade. This subsequently facilitated an unprecedented bio-inspired "contra-biosynthetic" rearrangement, providing divergent access to 9-isocyanopupukeanane in 15 steps (formal synthesis). Computational studies provide insight into the nature of this rearrangement.

An Enantiospecific Synthesis of Isoneoamphilectane Confirms Its Strained Tricyclic Structure

We describe a total synthesis of the rare isocyanoterpene natural product isoneoamphilectane and two of its unnatural diastereomers. The significantly strained ring system of the reported natural product─along with a hypothesis about a biosynthetic relationship to related family members─inspired us to consider a potential misassignment in the structure's relative configuration. As a result, we initially targeted two less strained, more accessible, stereoisomers of the reported natural product. When these compounds failed to exhibit spectroscopic data that matched those of isoneoamphilectane, we embarked on a synthesis of the originally proposed strained structure via an approach that hinged on a challenging cis-to-trans decalone epimerization. Ultimately, we implemented a novel cyclic sulfite pinacol-type rearrangement to generate the strained ring system. Additional features of this work include the application of a stereocontrolled Mukaiyama-Michael addition of an acyclic silylketene acetal, an unusual intramolecular alkoxide-mediated regioselective elimination, and an HAT-mediated alkene hydroazidation to forge the C-N bond of the tertiary isonitrile. Throughout this work, our synthetic planning was heavily guided by computational analyses to inform on key issues of stereochemical control.

Evolution of a Short and Stereocontrolled Synthesis of (+)-7,20-Diisocyanoadociane

A full account of the development of a concise and highly stereoselective synthesis of (+)-7,20-diisocyanoadociane (DICA)─a structurally complex isocyanoditerpene with potent antiplasmodial activity─is described. The strategy that evolved relies on the rapid construction of unsaturated tricyclic precursors designed to undergo stereocontrolled Birch reductions and a subsequent "bay ring" formation to generate the isocycloamphilectane core. This report is divided into three sections: (1) a description of the initial strategy and the results that focused our efforts on a single route to the DICA core, (2) a discussion of the precise choreography needed to enable a first-generation formal synthesis of (±)-DICA, and (3) the execution of a 13-step second-generation synthesis of (+)-DICA that builds on important lessons learned from the first-generation effort.

Metalated isocyanides: formation, structure, and reactivity

Metalated isocyanides are highly versatile organometallics. Central to the reactivity of metalated isocyanides is the presence of two orthogonally reactive carbons, a highly nucleophilic "carbanion" inductively stabilized by a carbene-like isocyanide carbon. The two reactivities are harnessed in the attack of metalated isocyanides on π-electrophiles where an initial nucleophilic attack leads to an electron pair that cyclizes onto the terminal isocyanide carbon in a rapid route to diverse, nitrogenous heterocycles. Harnessing the potent nucleophilicity of metalated isocyanides while preventing electrophilic attack on the terminal isocyanide carbon has largely been driven by empirical heuristics. This review provides a foundational understanding by surveying the formation, structure, and properties of metalated isocyanides. The focus on the interplay between the structure and reactivity of metalated isocyanides is anticipated to facilitate the development and deployment of these exceptional nucleophiles in complex bond constructions.

Concise Synthesis of the Antiplasmodial Isocyanoterpene 7,20-Diisocyanoadociane

The flagship member of the antiplasmodial isocyanoterpenes, 7,20-diisocyanoadociane (DICA), was synthesized from dehydrocryptone in 10 steps, and in 13 steps from commercially available material. Our previous formal synthesis was reengineered, leveraging only productive transformations to deliver DICA in fewer than half the number of steps of our original effort. Important contributions, in addition to the particularly concise strategy, include a solution to the problem of axial nucleophilic methylation of a late-stage cyclohexanone, and the first selective synthesis and antiplasmodial evaluation of the DICA stereoisomer with both isonitriles equatorial.

A Strategy for the Convergent and Stereoselective Assembly of Polycyclic Molecules

The stereoselective oxidative coupling of cyclic ketones via silyl bis-enol ethers followed by ring-closing metathesis is shown to be a general and powerful reaction sequence for the preparation of diverse polycyclic scaffolds from simple precursors. The modular strategy successfully constructs substructures prevalent in numerous bioactive natural product families, varying in substitution and carbocyclic composition. Several of the prepared compounds were shown to possess potent cytotoxic activity against a panel of tumor cell lines. The utility of this strategy was further demonstrated by a concise and highly convergent 17-step formal synthesis of the complex antimalarial marine diterpene, (+)-7,20-diisocyanoadociane.