Origins of stereoselectivity in the oxido-alkylidenation of alkynes

Merging Strategy, Improvisation, and Conversation to Solve Problems in Target Synthesis

Total synthesis has long been depicted as the quest to conquer the structures created by nature, requiring an unflinching, single-minded devotion to the task. The goal is achieved by chemists with grit, strength of will, and a competitive spirit. While there is some truth to this viewpoint, it does not fully capture the rich experiences gained in this research realm. In our lab, strategic planning, improvisation, and conversation have worked in concert to enable progress. This Account summarizes our efforts to synthesize four different bioactive targets: merrilactone A, rocaglamide, phomactin A, and tetrapetalone A. Certain missteps were integral to success in these synthetic projects. As such, we include the hiccups, and their roles in the evolution of the strategies, along with the results that aligned with our expectations.Two of these projects (merrilactone A and rocaglamide) culminated in the total synthesis of the targets. The challenges presented by merrilactone A spawned a new design strategy for pentannulation using Nazarov cyclization chemistry. This work demonstrated that Lewis acid catalysis is often a viable electrocyclization strategy in activated, polarized pentadienyl cation intermediates. We sought to apply the same logic to the rocaglamide target, but precursors we prepared did not behave according to plan. This situation pushed us to adapt our approach to match the innate reactivity of the substrate, resulting in an on-the-spot improvisation that was not only integral to the success of the project but also expanded our understanding of pentadienyl cation chemistry. In the other two projects (phomactin A and tetrapetalone A), we did not complete a total synthesis but did build the polycyclic core of the target. Even though the hetero [4 + 2] cycloaddition plan for assembling the macrocyclic oxadecalin ring system of phomactin A failed, the original experimental design still enabled us to solve the problem. Through a wholly unanticipated series of events, our focus on the oxadecalin ring system primed us for the discovery of a sequential iodoaldol/oxa-Michael sequence, using the original [4 + 2] building blocks. Then, the bridging ring present in phomactin A demanded we implement this sequence in a transannular fashion. Finally, our successful synthesis of the tetrapetalone core was enabled by consultations with others in the community. Each bond formation seemed to require different expertise, and in three separate instances (C-N cross-coupling, diastereoselective ring-closing metathesis, and oxidative dearomatization) synthetic challenges were overcome through conversation and collaboration.In our experience, the amount of creative power we summon during a target synthesis project correlates directly with the magnitude of the structural challenges we face. When reactivity surprises us, we analyze the problem anew, consult with colleagues, and improvise with the tools at hand. The inevitable misbehavior of a complex system is a strong motivating force, and one that has helped to shape our research program for nearly two decades.

Synthesis of (±)-tetrapetalone A-Me aglycon

The first synthesis of (±)-tetrapetalone A-Me aglycon is described. Key bond-forming reactions include Nazarov cyclization, a ring-closing metathesis promoted with complete diastereoselectivity by a chiral molybdenum-based complex, tandem conjugate reduction/intramolecular aldol cyclization, and oxidative dearomatization.

Electrospray ionization (ESI) fragmentations and dimethyldioxirane reactivities of three diverse lactams having full, half, and zero resonance energies

Three lactams having, respectively, ~20, ~10, and 0 kcal/mol of resonance energy have been subjected to electrospray ionization mass spectrometry (ESI/MS) as well as to attempted reaction with dimethyldioxirane (DMDO). The ESI/MS for all three lactams are consistent with fragmentation from the N-protonated, rather than the O-protonated tautomer. Each exhibits a unique fragmentation pathway. DFT calculations are employed to provide insights concerning these pathways. N-Ethyl-2-pyrrolidinone and 1-azabicyclo[3.3.1]nonan-2-one, the full- and half-resonance lactams, are unreactive with DMDO. The "Kirby lactam" (3,5,7-trimethyl-1-azaadamantan-2-one) has zero resonance energy and reacts rapidly with DMDO to generate a mixture of reaction products. The structure assigned to one of these is the 2,2-dihydroxy-N-oxide, thought to be stabilized by intramolecular hydrogen bonding and buttressing by the methyl substituents. A reasonable pathway to this derivative might involve formation of an extremely labile N-oxide, in a purely formal sense, an example of the hitherto-unknown amide N-oxides, followed by hydration with traces of moisture.

Divergent reaction pathways of a cationic intermediate: rearrangement and cyclization of 2-substituted furyl and benzofuryl enones catalyzed by iridium(III)

In contrast to 2-substituted pyrrole enones, furyl and benzofuryl enones do not undergo the Nazarov electrocyclization. Instead, these furyl and benzofuryl enones exhibit unusual rearrangement sequences in the presence of catalytic amounts of [IrBr(CO)(DIM)((R)-(+)-BINAP)](SbF(6))(2) (1; DIM = diethylisopropylidene malonate) and AgSbF(6) (1:1). A 1,2-H shift followed by intramolecular Friedel-Crafts alkylation leads to synthetically valuable cyclohexanones with furanylic quaternary centers. The electrophilicity of 1 is essential for this rearrangement.

Highly substituted 2,3-dihydroisoxazoles by Et(3)N-catalyzed tandem reaction of electron-deficient 1,3-conjugated enynes with hydroxylamines

An Et(3)N-catalyzed tandem reaction of electron-deficient 1,3-conjugated enynes with hydroxylamines was developed which provided rapid, metal-free, and regioselective access to highly substituted multifunctionalized 2,3-dihydroisoxazoles under mild conditions. The reactions of 3-(2-arylethynyl)-4H-chromen-4-ones with hydroxylamines afford beta-amino enones under the same reaction conditions.

A Torquoselective Extrusion of Isoxazoline N-Oxides. Application to the Synthesis of Aryl Vinyl and Divinyl Ketones for Nazarov Cyclization

A mild, convenient reaction sequence for the synthesis of Nazarov cyclization substrates is described. The [3+2] dipolar cycloaddition of a nitrone and an electron-deficient alkyne gives an isolable isoxazoline intermediate, which upon oxidation undergoes stereoselective extrusion of nitrosomethane to give aryl vinyl or divinyl ketones.

Formal synthesis of (+/-)-roseophilin

A formal synthesis of (+/-)-roseophilin is described. Scandium(III)-catalyzed Nazarov cyclization of 2,5-disubstituted N-tosylpyrrole 19 gives a 5,5'-fused ketopyrrole, and ansa-bridge formation via pi-allyl palladium macrocyclization gives 21.