An Enantioselective Synthesis of the ABD Tricycle for (-)-Phomactin A Featuring Rawal's Asymmetric Diels-Alder Cycloaddition

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.

C-C Bond Cleavage Approach to Complex Terpenoids: Development of a Unified Total Synthesis of the Phomactins

The rearrangement of carbon-carbon (C-C) single bonds in readily available carbocyclic scaffolds can yield uniquely substituted carbocycles that would be challenging to construct otherwise. This is a powerful and often non-intuitive approach for complex molecule synthesis. The transition-metal-mediated cleavage of C-C bonds has the potential to broaden the scope of this type of skeletal remodeling by providing orthogonal selectivities compared to more traditional pericyclic and carbocation-based rearrangements. To highlight this emerging technology, a unified, asymmetric, total synthesis of the phomactin terpenoids was developed, enabled by the selective C-C bond cleavage of hydroxylated pinene derivatives obtained from carvone. In this full account, the challenges, solutions, and intricacies of Rh(I)-catalyzed cyclobutanol C-C cleavage in a complex molecule setting are described. In addition, details of the evolution of strategies that ultimately led to the total synthesis of phomactins A, K, P, R, and T, as well as the synthesis and structural reassignment of Sch 49027, are given.

Total Syntheses of Scaparvins B, C, and D Enabled by a Key C-H Functionalization

The clerodane diterpene family possesses an impressive range of bioactivities and high synthetic challenge due to their unique amalgamation of rings, stereocenters, and oxygenation. Herein, we disclose the first total syntheses of three members, scaparvins B, C, and D, through a route fueled by several chemoselective and carefully orchestrated steps. One such operation is a tailored late-stage C-H functionalization converting a carboxylic acid into a lactone through the oxidation of a tertiary C-H bond under conditions that minimize epoxidation of an alkene. This step, among others, afforded critical functionality to complete the targets. In addition, use of an appropriate chiral catalyst with a Rawal diene renders the sequence enantioselective.

Synthesis of tetrasubstituted 1-silyloxy-3-aminobutadienes and chemistry beyond Diels-Alder reactions

Electron-rich dienes have revolutionized the synthesis of complex compounds since the discovery of the legendary Diels–Alder cycloaddition reaction. This highly efficient bond-forming process has served as a fundamental strategy to assemble many structurally formidable molecules. Amino silyloxy butadienes are arguably the most reactive diene species that are isolable and bottleable. Since the pioneering discovery by Rawal, 1-amino-3-silyloxybutadienes have been found to undergo cycloaddition reactions with unparalleled mildness, leading to significant advances in both asymmetric catalysis and total synthesis of biologically active natural products. In sharp contrast, this class of highly electron-rich conjugated olefins has not been studied in non-cycloaddition reactions. Here we report a simple synthesis of tetrasubstituted 1-silyloxy-3-aminobutadienes, a complementarily substituted Rawal's diene. This family of molecules is found to undergo a series of intriguing chemical transformations orthogonal to cycloaddition reactions. Structurally diverse polysubstituted ring architectures are established in one step from these dienes.

Electron-rich dienes have been shown to have multiple uses in synthesis by means of cycloaddition reactions. Here, the authors report a series of non-cycloaddition-based reactions of these compounds, giving access to 4-, 5-, 6- and 7-membered ring systems.

One pot cascade synthesis of fused heterocycles from furan-tethered terminal alkynes and aldehydes in the presence of amines and CuBr

A novel one-pot protocol for the construction of complex heterocycles through furan tethered terminal alkynes, aldehydes, amines and CuBr upon heating has been developed, giving the cycloadducts in moderate to high yields along with moderate to good regioselectivities.

Cascade cyclizations of acyclic and macrocyclic alkynones: studies toward the synthesis of phomactin A

A study of the reactivity and diastereoselectivity of the Lewis acid promoted cascade cyclizations of both acyclic and macrocyclic alkynones is described. In these reactions, a β-iodoallenolate intermediate is generated via conjugate addition of iodide to an alkynone followed by an intramolecular aldol reaction with a tethered aldehyde to afford a cyclohexenyl alcohol. The Lewis acid magnesium iodide (MgI2) was found to promote irreversible ring closure, while cyclizations using BF3·OEt2 as promoter occurred reversibly. For both acyclic and macrocyclic alkynones, high diastereoselectivity was observed in the intramolecular aldol reaction. The MgI2 protocol for cyclization was applied to the synthesis of advanced intermediates relevant to the synthesis of phomactin natural products, during which a novel transannular cation-olefin cyclization was observed. DFT calculations were conducted to analyze the mechanism of this unusual MgI2-promoted process.

A macrocyclic β-iodoallenolate intermediate is key: synthesis of the ABD core of phomactin A

An enantioselective strategy for the synthesis of phomactin natural products is described. The Lewis acid triggered cyclization of a β-iodoallenolate embedded in a 12-membered macrocycle was used to obtain a highly functionalized bicyclo[9.3.1]pentadecane in good yield and high diastereoselectivity. This iodoenone contains the substituents of the AD ring system of the phomactin family of natural products, appropriate for further functionalization. Synthesis of the oxadecalin core of phomactin A from the AD iodoenone intermediate was achieved. In this unusual strategy, rings A and B are both fashioned within a macrocyclic precursor.

Constructing the Architecturally Distinctive ABD-Tricycle of Phomactin A through an Intramolecular Oxa-[3 + 3] Annulation Strategy

Our efforts in constructing the ABD-ring of phomactin A through an intramolecular oxa-[3 + 3] annulation strategy is described. This struggle entailed finding a practical and efficient preparation of annulation precursor, and a realization of the unexpected competing regioisomeric pathway. The success entailed accessing the A-ring through Diels-Alder cycloaddition of Rawal's diene. Furthermore, the discovery that the regioisomers from the annulation existed as atropisomers with respect to the D-ring olefin and that they could be equilibrated to the desired ABD-tricycle, allowing large quantities of tricycle to be accessed.

Total synthesis of (±)-phomactin A. Lessons learned from respecting a challenging structural topology

Our struggles and ultimate success in achieving a total synthesis of phomactin A are described. Our strategy features an intramolecular oxa-[3 + 3] annulation to construct its unique ABD-tricyclic manifold. Although the synthesis would constitute a distinctly new approach with the 12-membered D-ring of phomactin A being assembled simultaneously with the 1-oxadecalin at an early stage, the ABD-tricycle represents a unique structural topology that would pose a number of unprecedented challenges. One challenge concerned elaborating this tricycle to have oxygenation at the proper carbon atoms. To overcome this, we would utilize a Kornblum-DeLaMare ring-opening of a peroxide bridge as well as a challenging late-stage 1,3-allylic alcohol transposition. Further, the structural intricacies of the ABD-tricycle were uncovered by a conformational analysis that would be critical for the C5a-homologation.

Total synthesis of phomactin A

A total synthesis of (+/-)-phomactin A is described to highlight the final completion of a complex natural product target that had commenced with an intramolecular oxa-[3 + 3] annulation strategy in the construction of the ABD-tricycle. These efforts reveal structural intricacies of this ABD-tricycle with an illustrative example being the conformational analysis that was ultimately critical for the C5a-homolgation.