Enantioselective Syntheses of Wickerols A and B.

The evolution of a successful strategy for the synthesis of the strained, cage-like antiviral diterpenoids wickerols A and B is described. Initial attempts to access the carbocyclic core were surprisingly challenging and in retrospect, presaged the many detours needed to ultimately arrive at the fully adorned wickerol architecture. In most cases, conditions to trigger desired outcomes with respect to both reactivity and stereochemistry were hard-won. The successful synthesis ultimately leveraged alkenes in virtually all productive bond-forming events. A series of conjugate addition reactions generated the fused tricyclic core, a Claisen rearrangement was used to install an otherwise unmanageable methyl-bearing stereogenic center, and a Prins cyclization closed the strained bridging ring. This final reaction proved enormously interesting because the strain of the ring system permitted diversion of the presumed initial Prins product into several different scaffolds.

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.

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.

General Approaches to Structurally Diverse Isocyanoditerpenes.

Since their discovery in the 1970s, the striking architectures and the unusual isonitrile functional groups of the isocyanoterpenes have attracted the interest of many organic chemists. The more recent revelation of their potent in vitro antiplasmodial activity sparked new endeavors to synthesize members of this family of secondary metabolites. In this Synopsis, we discuss three distinct strategies that each address multiple structurally different members of the isocyanoterpenes, ending with some of our group's recent contributions in this area.

A Formal Enantiospecific Synthesis of 7,20-Diisocyanoadociane.

7,20-Diisocyanoadociane (DICA) is a potent antimalarial isocyanoterpene endowed with a fascinating tetracyclic structure composed of fused chair cyclohexanes. We report a highly stereocontrolled synthesis of a late-stage intermediate, the "Corey dione", from which DICA has been made previously. This formal synthesis features a rapid buildup of much of the complexity of the target through a sequence of enone tandem vicinal difunctionalization, Friedel-Crafts cyclodehydration, and sequential stereocontrolled reductions. Most importantly, this success establishes the broader feasibility of our previously developed general synthesis approach to the isocyanoterpene family and provides a blueprint for a very direct synthesis of DICA and related natural products.

Outer-sphere direction in iridium C-H borylation.

The NHBoc group affords ortho selective C-H borylations in arenes and alkenes. Experimental and computational studies support an outer sphere mechanism where the N-H proton hydrogen bonds to a boryl ligand oxygen. The regioselectivities are unique and complement those of directed ortho metalations.

Concise Formal Synthesis of the Pseudopterosins via Anionic Oxy-Cope/Transannular Michael Addition Cascade.

An anionic oxy-Cope/transannular Michael addition cascade converts a spirocyclic architecture-readily available by Diels-Alder cycloaddition-into the hydrophenalene carbon skeleton of the pseudopterosin aglycones. Oxidation of the resulting cyclohexenone ring to the phenol that is characteristic of the targets completes a short formal synthesis.

Investigations into an anionic oxy-cope/transannular conjugate addition approach to 7,20-diisocyanoadociane.

An anionic oxy-Cope/transannular conjugate addition approach to the potent antimalarial 7,20-diisocyanoadociane is presented. The unexpected formation of undesired diastereomers in the key reaction led to the structural reassignment of previous products of this type of cascade and a reevaluation of the reversibility of the transannular ring closure. During efforts to coax the reaction toward the desired product, a transannular ene reaction provided tricyclic compounds relevant to the kempane diterpenoids.

Electronic effects in iridium C-H borylations: insights from unencumbered substrates and variation of boryl ligand substituents.

Experiment and theory favour a model of C-H borylation where significant proton transfer character exists in the transition state.