The CoQ oxidoreductase FSP1 acts parallel to GPX4 to inhibit ferroptosis.

Ferroptosis is a form of regulated cell death that is caused by the iron-dependent peroxidation of lipids1,2. The glutathione-dependent lipid hydroperoxidase glutathione peroxidase 4 (GPX4) prevents ferroptosis by converting lipid hydroperoxides into non-toxic lipid alcohols3,4. Ferroptosis has previously been implicated in the cell death that underlies several degenerative conditions2, and induction of ferroptosis by the inhibition of GPX4 has emerged as a therapeutic strategy to trigger cancer cell death5. However, sensitivity to GPX4 inhibitors varies greatly across cancer cell lines6, which suggests that additional factors govern resistance to ferroptosis. Here, using a synthetic lethal CRISPR-Cas9 screen, we identify ferroptosis suppressor protein 1 (FSP1) (previously known as apoptosis-inducing factor mitochondrial 2 (AIFM2)) as a potent ferroptosis-resistance factor. Our data indicate that myristoylation recruits FSP1 to the plasma membrane where it functions as an oxidoreductase that reduces coenzyme Q10 (CoQ) (also known as ubiquinone-10), which acts as a lipophilic radical-trapping antioxidant that halts the propagation of lipid peroxides. We further find that FSP1 expression positively correlates with ferroptosis resistance across hundreds of cancer cell lines, and that FSP1 mediates resistance to ferroptosis in lung cancer cells in culture and in mouse tumour xenografts. Thus, our data identify FSP1 as a key component of a non-mitochondrial CoQ antioxidant system that acts in parallel to the canonical glutathione-based GPX4 pathway. These findings define a ferroptosis suppression pathway and indicate that pharmacological inhibition of FSP1 may provide an effective strategy to sensitize cancer cells to ferroptosis-inducing chemotherapeutic agents.

Modular, Enantioselective Entry into Polysubstituted Shapeshifting Molecules.

Dynamic, shapeshifting hydrocarbons have emerged as enabling frameworks across drug discovery, materials science, and catalysis. Their employment, however, is often hampered by a lack of efficient synthetic methods for their preparation. Herein, we report a unified, concise, and modular synthesis of enantioenriched shapeshifting hydrocarbons (barbaralones and bullvalones) and multisubstituted bullvalenes, leveraging mild photochemical and base-induced rearrangements.

Enantioselective Total Synthesis of (-)-Caulamidine A.

Marine bryozoans continue to provide architecturally fascinating halogenated alkaloids that pose unique challenges for chemical synthesis. The antimalarial alkaloids caulamidines A and B, recently isolated from Caulibugula intermis, contain an intricate bis-amidine core and a chlorine-bearing neopentylic stereocenter. Compared to topologically similar C20 bis(cyclotryptamine) alkaloids, caulamidines possess an additional carbon atom of unknown biosynthetic origins, which renders their entire skeleton nonsymmetric and nondimeric. Herein, we report the first total synthesis of caulamidine A and confirm its absolute configuration. Key chemical findings include the exploitation of glycol bistriflate to facilitate a rapid, diastereoselective ketone-amidine annulation reaction and a highly diastereoselective hydrogen atom transfer to correctly establish the key chlorine-bearing stereogenic center.

A Shapeshifting Roadmap for Polycyclic Skeletal Evolution.

Polycyclic ring systems are ubiquitous three-dimensional (3D) structural motifs central to the function of many biologically active small molecules and organic materials. Indeed, subtle changes to the overall molecular shape and connectivity of atoms in a polycyclic framework (i.e., isomerism) can drastically alter its function and properties. Unfortunately, direct evaluation of these structure-function relationships typically requires the development of distinct synthetic strategies toward a specific isomer. Dynamic, "shapeshifting" carbon cages present a promising approach for sampling isomeric chemical space but are often difficult to control and are largely limited to thermodynamic mixtures of positional isomers about a single core scaffold. Here, we describe the development of a new shapeshifting C9-chemotype and a chemical blueprint for its evolution into structurally and energetically diverse isomeric ring systems. By leveraging the unique molecular topology of π-orbitals interacting through-space (homoconjugation), a common skeletal ancestor evolved into a complex network of valence isomers. This unusual system represents an exceedingly rare small molecule capable of undergoing controllable and continuous isomerization processes through the iterative use of just two chemical steps (light and organic base). Computational and photophysical studies of the isomer network provide fundamental insight into the reactivity, mechanism, and role of homoconjugative interactions. Importantly, these insights may inform the rational design and synthesis of new dynamic, shapeshifting systems. We anticipate this process could be a powerful tool for the synthesis of structurally diverse, isomeric polycycles central to many bioactive small molecules and functional organic materials.

Chemoproteomic Profiling Reveals that Anticancer Natural Product Dankastatin†B Covalently Targets Mitochondrial VDAC3.

Chlorinated gymnastatin and dankastatin alkaloids derived from the fungal strain Gymnascella dankaliensis have been reported to possess significant anticancer activity but their mode of action is unknown. These members possess electrophilic functional groups that can might undergo covalent bond formation with specific proteins to exert their biological activity. To better understand the mechanism of action of this class of natural products, we mapped the proteome-wide cysteine reactivity of the most potent of these alkaloids, dankastatin B, by using activity-based protein profiling chemoproteomic approaches. We identified a primary target of dankastatin B in breast cancer cells as cysteine C65 of the voltage-dependent anion-selective channel on the outer mitochondrial membrane VDAC3. We demonstrated direct and covalent interaction of dankastatin B with VDAC3. VDAC3 knockdown conferred hypersensitivity to dankastatin B-mediated antiproliferative effects in breast cancer cells, thus indicating that VDAC3 was at least partially involved in the anticancer effects of this natural product. Our study reveals a potential mode of action of dankastatin B through covalent targeting of VDAC3 and highlights the utility of chemoproteomic approaches in gaining mechanistic understanding of electrophilic natural products.

Taming Shapeshifting Anions: Total Synthesis of Ocellatusone C.

Guided by a synthetic design aimed at late-stage diversification, we report the preparation of unusual shapeshifting anions and their subsequent application to the total synthesis of the polyketide natural product ocellatusone C. Site-selective core functionalization of a readily accessible bicyclo[3.3.1]nonane architecture sets the stage for shape-selective side chain installation via a nonfluxional π-allyl Pd-complex derived from a barbaralyl-type anion. Several interesting chemical findings, including substituent-dependent bridged bicycloisomerism and the isolation of a stabilized, 3° carbon-bound Pd-ketone enolate complex, are reported.

Total Synthesis of Resiniferatoxin.

From both structural and functional perspectives, the large family of daphnane diterpene orthoesters (DDOs) represent a truly remarkable class of natural products. As potent lead compounds for the treatment of pain, neurodegeneration, HIV/AIDS, and cancer, their medicinal potential continues to be heavily investigated, yet synthetic routes to DDO natural products remain rare. Herein we report a distinct approach to this class of complex diterpenes, highlighted by a 15-step total synthesis of the flagship DDO, resiniferatoxin.

Annulative Methods in the Synthesis of Complex Meroterpene Natural Products.

From the venerable Robinson annulation to the irreplaceable Diels-Alder cycloaddition, annulation reactions have fueled the progression of the field of natural product synthesis throughout the past century. In broader terms, the ability to form a cyclic molecule directly from two or more simpler fragments has transformed virtually every aspect of the chemical sciences from the synthesis of organic materials to bioconjugation chemistry and drug discovery. In this Account, we describe the evolution of our meroterpene synthetic program over the past five years, enabled largely by the development of a tailored anionic annulation process for the synthesis of hydroxylated 1,3-cyclohexanediones from lithium enolates and the reactive ß-lactone-containing feedstock chemical diketene.First, we provide details on short total syntheses of the prototypical polycyclic polyprenylated acylphloroglucinol (PPAP) natural products hyperforin and garsubellin A, which possess complex bicyclo[3.3.1]nonane architectures. Notably, these molecules have served as compelling synthetic targets for several decades and induce a number of biological effects of relevance to neuroscience and medicine. By merging our diketene annulation process with a hypervalent iodine-mediated oxidative ring expansion, bicyclo[3.3.1]nonane architectures can be easily prepared from simple 5,6-fused bicyclic diketones in only two chemical operations. Leveraging these two key chemical reactions in combination with various other stereoselective transformations allowed for these biologically active targets to be prepared in racemic form in only 10 steps.Next, we extend this strategy to the synthesis of complex fungal-derived meroterpenes generated biosynthetically from the coupling of 3,5-dimethylorsellinic acid (DMOA) and farnesyl pyrophosphate. A Ti(III)-mediated radical cyclization of a terminal epoxide was used to rapidly prepare a 6,6,5-fused tricyclic ketone which served as an input for our annulation/rearrangement process, ultimately enabling a total synthesis of protoaustinoid A, an important biosynthetic intermediate in DMOA-derived meroterpene synthesis, and its oxidation product berkeleyone A. Through a radical-based, abiotic rearrangement process, the bicyclo[3.3.1]nonane cores of these natural products could again be isomerized, resulting in the 6,5-fused ring systems of the andrastin family and ultimately delivering a total synthesis of andrastin D and preterrenoid. Notably, these isomerization transformations proved challenging when employing classic, acid-induced conditions for carbocation generation, thus highlighting the power of radical biomimicry in total synthesis. Finally, further oxidation and rearrangement allowed for access to terrenoid and the lactone-containing metabolite terretonin L.Overall, the merger of annulative diketene methodology with an oxidative rearrangement transformation has proven to be a broadly applicable strategy to synthesize bicyclo[3.3.1]nonane-containing natural products, a class of small molecules with over 1000 known members.

Manumycin polyketides act as molecular glues between UBR7 and P53.

Molecular glues are an intriguing therapeutic modality that harness small molecules to induce interactions between proteins that typically do not interact. However, such molecules are rare and have been discovered fortuitously, thus limiting their potential as a general strategy for therapeutic intervention. We postulated that natural products bearing one or more electrophilic sites may be an unexplored source of new molecular glues, potentially acting through multicovalent attachment. Using chemoproteomic platforms, we show that members of the manumycin family of polyketides, which bear multiple potentially reactive sites, target C374 of the putative E3 ligase UBR7 in breast cancer cells, and engage in molecular glue interactions with the neosubstrate tumor-suppressor TP53, leading to p53 transcriptional activation and cell death. Our results reveal an anticancer mechanism of this natural product family, and highlight the potential for combining chemoproteomics and multicovalent natural products for the discovery of new molecular glues.

Chemical Investigations of Differentially Oxidized Polycylic Pyrroles from Bipolaris Fungi: Synthetic Entry Into the Bipolamine Alkaloids.

indolizidine alkaloids of unusual biosynthetic origin have recently been characterized from several species of fungi within the Pleosporaceae family. Possessing distinct polycyclic architectures with two embedded electron-rich pyrroles as well as reported antibacterial activity against gram positive and negative pathogens, these natural products represent attractive targets for total synthesis. Herein we survey the differential functionalization of a chemically sensitive bispyrrole framework resulting in the preparation of multiple bipolamine alkaloids, work which sheds light on their innate chemical reactivity and potential biosynthetic relationships.

Dearomative Synthetic Entry into the Altemicidin Alkaloids.

Altemicidin and related Streptomyces-derived monoterpene alkaloids possess dense, highly polar azaindane cores as well as potent cytotoxic and tRNA synthetase inhibitory properties. The congested α-amino acid motif decorating their presumed iridoid-like core structure has proven to be both a synthetic challenge and a biosynthetic mystery to date. Herein, we report a distinct, abiotic strategy to these alkaloids resulting in a concise synthesis of altemicidin from simple chemical feedstocks. Key chemical findings include the exploitation of a dearomative pyridinium addition and dipolar cycloaddition sequence to stereospecifically install the quaternary amine moiety, and a chemoselective molybdenum-mediated double reduction to establish the fully functionalized azaindane nucleus with minimal redox manipulations.

Evolution of a Synthetic Strategy for Complex Polypyrrole Alkaloids: Total Syntheses of Curvulamine and Curindolizine.

Structurally unprecedented antibacterial alkaloids containing multiple electron-rich pyrrole units have recently been isolated from Curvularia sp. and Bipolaris maydis fungi. This article documents the evolution of a synthetic program aimed at accessing the flagship metabolites curvulamine and curindolizine which are presumably a dimer and trimer of a C10N biosynthetic building block, respectively. Starting with curvulamine, we detail several strategies to merge two simple, bioinspired fragments, which while ultimately unsuccessful, led us toward a pyrroloazepinone building block-based strategy and an improved synthesis of this 10π-aromatic heterocycle. A two-step annulation process was then designed to forge a conserved tetracyclic bis-pyrrole architecture and advanced into a variety of late-stage intermediates; unfortunately, however, a failed decarboxylation thwarted the total synthesis of curvulamine. By tailoring our annulation precursors, success was ultimately found through the use of a cyanohydrin nucleophile which enabled a 10-step total synthesis of curvulamine. Attempts were then made to realize a biomimetic coupling of curvulamine with an additional C10N fragment to arrive at curindolizine, the most complex family member. Although unproductive, we developed a 14-step total synthesis of this alkaloid through an abiotic coupling approach. Throughout this work, effort was made to harness and exploit the innate reactivity of the pyrrole nucleus, an objective which has uncovered many interesting findings in the chemistry of this reactive heterocycle.

Syntheses of Complex Terpenes from Simple Polyprenyl Precursors.

From structure elucidation and biogenesis to synthetic methodology and total synthesis, terpene natural products have profoundly influenced the development of organic chemistry. Moreover, their myriad functional attributes range from fragrance to pharmaceuticals and have had great societal impact. Ruzicka's formulation of the "biogenetic isoprene rule," a Nobel Prize winning discovery now over 80 years old, allowed for identification of higher order terpene (aka "isoprenoid") structures from simple five-carbon isoprene fragments. Notably, the isoprene rule still holds pedagogical value to students of organic chemistry today. Our laboratory has completed syntheses of over two dozen terpene and meroterpene structures to date, and the isoprene rule has served as a key pattern recognition tool for our synthetic planning purposes. At the strategic level, great opportunity exists in finding unique and synthetically simplifying ways to connect the formal C5 isoprene fragments embedded in terpenes. Biomimetic cationic polyene cyclizations represent the earliest incarnation of this idea, which has facilitated expedient routes to certain terpene polycycle classes. Nonetheless, a large swath of terpene chemical space remains inaccessible using this approach.In this Account, we describe strategic insight into our endeavors in terpene synthesis published over the last five years. We show how biosynthetic understanding, combined with a desire to utilize abundant and inexpensive [C5]n building blocks, has led to efficient, abiotic syntheses of multiple complex terpenes with disparate ring systems. Informed by nature, but unconstrained by its processes, our synthetic assembly exploits chemical reactivity across diverse reaction types-including radical, anionic, pericyclic, and metal-mediated transformations.First, we detail an eight-step synthesis of the cembrane diterpene chatancin from dihydrofarnesal using a bioinspired-but not -mimetic-cycloaddition. Next, we describe the assembly of the antimalarial cardamom peroxide using a polyoxygenation cascade to fuse multiple units of molecular oxygen onto a dimeric skeleton. This three-to-four-step synthesis arises from (-)-myrtenal, an inexpensive pinene oxidation product. We then show how a radical cyclization cascade can forge the hallmark cyclooctane ring system of the complex sesterterpene 6-epi-ophiobolin N from two simple polyprenyl precursors, (-)-linalool and farnesol. To access the related, more complex metabolite 6-epi-ophiobolin A, we exploited the plasticity of our synthetic route and found that use of geraniol (C10) rather than farnesol (C15) gave us the flexibility needed to address the additional oxidation found in this congener. Following this work, we describe two strategies to access several guaianolide sesquiterpenes. Retrosynthetic disconnection to monoterpenes, carvone or (-)-linalool, coupled with a powerful allylation strategy allowed us to address guaianolides with disparate stereochemical motifs. Finally, we examine a semisynthetic approach to the illicium sesquiterpenes from the abundant 15-carbon feedstock terpene (+)-cedrol using an abiotic ring shift and multiple C-H oxidation reactions inspired by a postulated biosynthesis of this natural product class.

Harnessing the anti-cancer natural product nimbolide for targeted protein degradation.

Nimbolide, a terpenoid natural product derived from the Neem tree, impairs cancer pathogenicity; however, the direct targets and mechanisms by which nimbolide exerts its effects are poorly understood. Here, we used activity-based protein profiling (ABPP) chemoproteomic platforms to discover that nimbolide reacts with a novel functional cysteine crucial for substrate recognition in the E3 ubiquitin ligase RNF114. Nimbolide impairs breast cancer cell proliferation in-part by disrupting RNF114-substrate recognition, leading to inhibition of ubiquitination and degradation of tumor suppressors such as p21, resulting in their rapid stabilization. We further demonstrate that nimbolide can be harnessed to recruit RNF114 as an E3 ligase in targeted protein degradation applications and show that synthetically simpler scaffolds are also capable of accessing this unique reactive site. Our study highlights the use of ABPP platforms in uncovering unique druggable modalities accessed by natural products for cancer therapy and targeted protein degradation applications.

Total Synthesis of (-)-Curvulamine.

Curvulamine and related polypyrrole alkaloids represent a fascinating new class of natural products with unprecedented chemical structures, intriguing biological activities, and mysterious biosynthetic origins. Herein we report the first studies toward these molecules, resulting in a 10-step total synthesis of (-)-curvulamine, a dimeric member with promising Gram-positive and -negative antibiotic activity. A number of interesting chemical findings, including exploitation of the heteroaromatic pyrrolo[1,2-a]azepinone nucleus and an efficient stereodivergent reduction, are reported.

Target Identification of Bioactive Covalently Acting Natural Products.

There are countless natural products that have been isolated from microbes, plants, and other living organisms that have been shown to possess therapeutic activities such as antimicrobial, anticancer, or anti-inflammatory effects. However, developing these bioactive natural products into drugs has remained challenging in part because of their difficulty in isolation, synthesis, mechanistic understanding, and off-target effects. Among the large pool of bioactive natural products lies classes of compounds that contain potential reactive electrophilic centers that can covalently react with nucleophilic amino acid hotspots on proteins and other biological molecules to modulate their biological action. Covalently acting natural products are more amenable to rapid target identification and mapping of specific druggable hotspots within proteins using activity-based protein profiling (ABPP)-based chemoproteomic strategies. In addition, the granular biochemical insights afforded by knowing specific sites of protein modifications of covalently acting natural products enable the pharmacological interrogation of these sites with more synthetically tractable covalently acting small molecules whose structures are more easily tuned. Both discovering binding pockets and targets hit by natural products and exploiting druggable modalities targeted by natural products with simpler molecules may overcome some of the challenges faced with translating natural products into drugs.

Total Synthesis of (+)-6-epi-Ophiobolin†A.

The ophiobolin sesterterpenes are notable plant pathogens which have recently elicited significant chemical and biological attention because of their intriguing carbogenic frameworks, reactive functionalities, and emerging anticancer profiles. Reported herein is a total synthesis of (+)-6-epi-ophiobolin A in 14 steps, a task which addresses construction of the synthetically challenging spirocyclic tetrahydrofuran motif as well as several other key stereochemical problems. This work demonstrates a streamlined synthetic platform to complex ophiobolins leveraging disparate termination modes of a radical polycyclization cascade for divergent elaboration and functionalization.

Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae.

With hundreds of unique members isolated to date, guaianolide lactones represent a particularly prolific class of terpene natural products. Given their extensive documented therapeutic properties and fascinating chemical structures, these metabolites have captivated the synthetic chemistry community for many decades. As a result of divergent biosynthetic pathways, which produce a wide array of stereochemical and oxidative permutations, a unifying synthetic pathway to this broad family of natural products is challenging. Herein we document the evolution of a chiral-pool-based synthetic program aimed at accessing an assortment of guaianolides, particularly those from the plant family Apiaceae as well as Asteraceae, members of which possess distinct chemical substructures and necessitate deviating synthetic platforms. An initial route employing the linear monoterpene linalool generated a lower oxidation state guaianolide but was not compatible with the majority of family members. A double-allylation disconnection using a carvone-derived fragment was then developed to access first an Asteraceae-type guaianolide and then various Apiaceae congeners. Finally, using these findings in conjunction with a tandem polyoxygenation cascade, we developed a pathway to highly oxygenated nortrilobolide. A variety of interesting observations in metal-mediated aldehyde allylation and alkene polyoxygenation are reported and discussed.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes.

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp3)-H bond functionalization reactions in complex polycyclic architectures are reported.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products.

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.