Two-Phase Synthesis of Taxol

Heavy Atom Quantum Mechanical Tunneling in Total Synthesis

Contributions from quantum mechanical tunneling to the rates of several radical coupling reactions between carbon sp2 centers used as key steps in natural product total syntheses were computed using density functional theory. Contributions ranging from ∼15-52% from tunneling were predicted at room temperature, thereby indicating that tunneling plays an important role in the rates of these reactions and should perhaps be considered when designing complex synthetic schemes.

Divergent Synthesis of Scabrolide A and Havellockate via an exo-exo-endo Radical Cascade

Here we report a concise and divergent synthesis of scabrolide A and havellockate, representative members of polycyclic marine natural product furano(nor)cembranoids. The synthesis features a highly efficient exo-exo-endo radical cascade. Through the generation of two rings, three C-C bonds, and three contiguous stereocenters in one step, this remarkable transformation not only assembles the bowl-shaped, common 6-5-5 fused ring system from simple building blocks but also precisely installs the functionalities at desired positions and sets the stage for further divergent preparation of both target molecules. Further studies reveal that the robust and unusual 6-endo radical addition in the cascade is likely facilitated by the rigidity of the substrate.

Biosynthesis of the highly oxygenated tetracyclic core skeleton of Taxol

Taxol is a widely-applied anticancer drug that inhibits microtubule dynamics in actively replicating cells. Although a minimum 19-step biosynthetic pathway has been proposed and 16 enzymes likely involved have been characterized, stepwise biosynthetic reactions from the well-characterized di-oxygenated taxoids to Taxol tetracyclic core skeleton are yet to be elucidated. Here, we uncover the biosynthetic pathways for a few tri-oxygenated taxoids via confirming the critical reaction order of the second and third hydroxylation steps, unearth a taxoid 9α-hydroxylase catalyzing the fourth hydroxylation, and identify CYP725A55 catalyzing the oxetane ester formation via a cascade oxidation-concerted acyl rearrangement mechanism. After identifying a acetyltransferase catalyzing the formation of C7-OAc, the pathway producing the highly-oxygenated 1β-dehydroxybaccatin VI with the Taxol tetracyclic core skeleton is elucidated and its complete biosynthesis from taxa-4(20),11(12)-diene-5α-ol is achieved in an engineered yeast. These systematic studies lay the foundation for the complete elucidation of the biosynthetic pathway of Taxol.

Deciphering magnesium binding site and structure-function insights in a class II sesquiterpene cyclase

Magnesium ions (Mg2+) are crucial in class II terpene cyclases that utilize substrates with diphosphate groups. Interestingly, these enzymes catalyze reactions without cleaving the diphosphate group, instead initiating the reaction through protonation. In our recent research, we discovered a novel class II sesquiterpene cyclase in Streptomyces showdoensis. Notably, we determined its crystal structure and identified Mg2+ within its active site. This finding has shed light on the previously elusive question of Mg2+ binding in class II terpene cyclases. In this chapter, we outline our methods for discovering this novel enzyme, including steps for its purification, crystallization, and kinetic analysis.

Cytochrome P450-Mediated Skeleton Rearrangement of Taxadiene in an Engineered Escherichia coli System

In this study, we constructed a taxadiene overproduction platform and identified a cytochrome P450, CYP701A8, that activates the inert C-H bonds in taxadiene to produce three oxidized products (1-3). Compound 1 possesses a newly identified 1 (15→11) abeotaxane skeleton, while 3 features a distinctive 6/10-fused carbocyclic core with an α,β-unsaturated ketone moiety. Our quantum computations suggested a carbocation-driven rearrangement in the formation of 1. These results support CYP701A8 as a promising biocatalyst for the generation of novel taxane diterpenoids.

Total Synthesis of Taxol Enabled by Intermolecular Radical Coupling and Pd-Catalyzed Cyclization

Taxol (1) is a clinically used antineoplastic diterpenoid. The tetracyclic ring system comprises a 6/8/6-membered carbocycle (ABC-ring) and a fused oxetane ring (D-ring) embedded with a bridgehead double bond and decorated with multiple oxygen functionalities. Here, we report a convergent total synthesis of this exceedingly complex natural product. The C-ring fragment was designed to possess a bromocyclohexenone and an extra tetrahydrofuran ring to control the reactivity and selectivity, as well as to minimize functional group manipulations en route to 1. The α-alkoxyacyl telluride of the A-ring served as a radical precursor, and intermolecular radical coupling with the C-ring realized the installation of the C2- and C3-stereocenters and reductive removal of the bromide. After the C8-quaternary stereocenter was constructed by exploiting the three-dimensional shape of the intermediate, the C11-vinyl triflate of A-ring and the C8-methyl ketone of C-ring were utilized for Pd(0)-catalyzed cyclization of the central eight-membered B-ring with the bridgehead olefin. Adjustment of the oxidation level and attachment of the oxetane D-ring completed the total synthesis of 1 (28 steps, as the longest linear sequence). The fragment design principle and implementation of the powerful radical coupling reaction described in the present synthesis provide valuable information for planning and executing syntheses of diverse densely oxygenated terpenoids.

Progress toward the Total Synthesis of Jogyamycin Using a Tandem Ichikawa/Winstein Rearrangement

Jogyamycin is a densely functionalized aminocyclopentitol that displays potent antiprotozoal activity. Herein, we report a route toward this natural product that utilizes an unprecedented transformation involving a tandem Ichikawa-Winstein rearrangement to install the C-1/C-2 diamine core. Attempts to further functionalize the C-3/C-4 alkene en route to jogyamycin are also discussed.

Research Advances in Clinical Applications, Anticancer Mechanism, Total Chemical Synthesis, Semi-Synthesis and Biosynthesis of Paclitaxel

Paclitaxel, a natural secondary metabolite isolated and purified from the bark of the Taxus tree, is considered one of the most successful natural anticancer drugs due to its low toxicity, high potency and broad-spectrum anticancer activity. Taxus trees are scarce and slow-growing, and with extremely low paclitaxel content, the contradiction between supply and demand in the market is becoming more and more intense. Therefore, researchers have tried to obtain paclitaxel by various methods such as chemical synthesis, artificial culture, microbial fermentation and tissue cell culture to meet the clinical demand for this drug. This paper provides a comprehensive overview of paclitaxel extraction, combination therapy, total synthesis, semi-synthesis and biosynthesis in recent years and provides an outlook, aiming to provide a theoretical basis and reference for further research on the production and application of paclitaxel in the future.

Diastereoselective Ring Expansion of Cyclic Ketones Enabled by HAT-Initiated Radical Cascade

Herein we disclose an iron-catalyzed method for stereoselective synthesis of multisubstituted cyclic ketones containing a synthetically challenging quaternary carbon from readily accessible β-vinyl keto esters in good yields. This cascade reaction is initiated by a hydrogen atom transfer (HAT) process, after which a Dowd-Beckwith-type ring-expansion reaction occurs. This strategic transformation offers access to synthetically valuable cyclic ketones bearing two contiguous stereocenters, including quaternary stereocenters, which hold paramount significance within the realm of synthetic chemistry.

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

A computational investigation towards substitution effects on 8π electrocyclisation of conjugated 1,3,5,7-octatetraenes

A computational investigation using M06-2X/6-31+G(d) method is reported for the substitution effects on 8π electrocyclisation of conjugated octatetraene. This systematic study describes the mono- and di-substitution effect across the 1,3,5,7-octatetraene skeleton. A general preference of the outward substitution over the inward, at C1 position of the monosubstituted system is observed. However, mesomerically electron donating group (-NH2 and -OH) display an opposite effect with respect to secondary orbital interaction (SOI) between the lone pair on the substituent and the orbital. A comparative evaluation on the computed activation energies for the 1-, 2-, 3-, and 4-monosubstituted system showed an insignificant impact on the rate of the reaction, in contrast to the electrocyclic ring closure of the unsubstituted compound. Computations of disubstituted system are more pronounced, where a remarkable acceleration is observed for 2-NO2-7-NO2 substituted octatetraene at 4.9 kcal mol-1, and a noticeable deceleration for 4-CH3-5-CH3 substituted octatetraene at 25.4 kcal mol-1 from the parent molecule, 17.0 kcal mol-1. A visible accelerated effects are commonly exhibited by the substitution on the terminal double bonds (C1, C2, C7, and C8), that are 1,2-, 1,7-, 1,8-, and 2,7-patterns, in regard to the greater orbital interaction for the new σ-bond formation. Despite the unfavourable steric clashes of the substituents in the 1,8-system, an apparent reduction in the energy barrier up to 7.4 kcal mol-1 is computed for 1-NH2-8-NO2 system from 17.0 kcal mol-1. This is due to the synergistic effect of the electron donor and electron acceptor, enhancing the stability of the transition structure. The electrocyclic ring closure involving vicinal substitution patterns, such as 1,2-, 2,3-, 3,4-, and 4,5-systems are critically dominated by steric crowding between the adjacent functional groups. In certain cases of the 1,2-substituted system, a noticeable accelerated effects are found for 1-NH2-2-NH2-substituted compound (9.7 kcal mol-1) due to an increased in electronic density on the substituted terminal double bond (C1-C2), hence favouring the formation of the new σ-bond.

Synthesis of portimines reveals the basis of their anti-cancer activity

Marine-derived cyclic imine toxins, portimine A and portimine B, have attracted attention because of their chemical structure and notable anti-cancer therapeutic potential1-4. However, access to large quantities of these toxins is currently not feasible, and the molecular mechanism underlying their potent activity remains unknown until now. To address this, a scalable and concise synthesis of portimines is presented, which benefits from the logic used in the two-phase terpenoid synthesis5,6 along with other tactics such as exploiting ring-chain tautomerization and skeletal reorganization to minimize protecting group chemistry through self-protection. Notably, this total synthesis enabled a structural reassignment of portimine B and an in-depth functional evaluation of portimine A, revealing that it induces apoptosis selectively in human cancer cell lines with high potency and is efficacious in vivo in tumour-clearance models. Finally, practical access to the portimines and their analogues simplified the development of photoaffinity analogues, which were used in chemical proteomic experiments to identify a primary target of portimine A as the 60S ribosomal export protein NMD3.

Biomimetic Approach to Diverse Coral Diterpenes from a Biosynthetic Scaffold

Thousands of coral terpenes originate from simple scaffolds that undergo oxidative tailoring. While corals are excellent sources of drug leads, the challenge of supplying structurally complex drug leads from marine organisms has sometimes slowed their development. Making this even more challenging, in comparison to other organisms, such as plants and microbes, for which the terpene literature is substantial, very little is known about how the unique coral terpenes are biosynthesized and elaborated in nature. In this study, we used a semisynthetic strategy to produce at gram scale in yeast the eunicellane scaffold that underlies >200 coral compounds. Synthetic oxidation reactions were explored, generating key scaffolds that reflect three of the four structural classes derived from eunicellane and enabling the first asymmetric syntheses of the natural products solenopodin C and klysimplexin Q. Biomimetic methods and detailed mechanistic studies of synthetic reactions shed light on potential enzymological reactivity, including the role of epoxide rearrangement in eunicellane biosynthesis.

Streamlined Strategy for Scalable and Enantioselective Total Syntheses of the Eburnane Alkaloids

A general, concise, and efficient strategy for the enantioselective synthesis of the eburnane alkaloid family of natural products is disclosed. Specifically, 13 members of the natural product family were prepared from commercially available and inexpensive starting materials. The brevity and modularity of the route are largely on account of a two-phase synthesis logic and a key catalytic enantioconvergent cross-coupling to establish the C20 stereogenic center. The strategies described here are expected to facilitate in-depth biological studies and provide access to new anticancer eburnane analogues.

Reactivity of the Iboga Skeleton: Oxidation Study of Ibogaine and Voacangine

The iboga alkaloids scaffold shows great potential as a pharmacophore in drug candidates for the treatment of neuropsychiatric disorders. Thus, the study of the reactivity of this type of motif is particularly useful for the generation of new analogs suitable for medicinal chemistry goals. In this article, we analyzed the oxidation pattern of ibogaine and voacangine using dioxygen, peroxo compounds, and iodine as oxidizing agents. Special focus was placed on the study of the regio- and stereochemistry of the oxidation processes according to the oxidative agent and starting material. We found that the C16-carboxymethyl ester present in voacangine stabilizes the whole molecule toward oxidation in comparison to ibogaine, especially in the indole ring, where 7-hydroxy- or 7-peroxy-indolenines can be obtained as oxidation products. Nevertheless, the ester moiety enhances the reactivity of the isoquinuclidinic nitrogen to afford C3-oxidized products through a regioselective iminium formation. This differential reactivity between ibogaine and voacangine was rationalized using computational DFT calculations. In addition, using qualitative and quantitative NMR experiments combined with theoretical calculations, the absolute stereochemistry at C7 in the 7-hydroxyindolenine of voacangine was revised to be S, which corrects previous reports proposing an R configuration.

Diversity of Endophytic Microbes in Taxus yunnanensis and Their Potential for Plant Growth Promotion and Taxane Accumulation

Taxus spp. are ancient tree species that have survived from the Quaternary glacier period, and their metabolites, such as taxol, have been used as anticancer drugs globally. Plant-endophytic microbial interaction plays a crucial role in exerting a profound impact on host growth and secondary metabolite synthesis. In this study, high-throughput sequencing was employed to explore endophytic microbial diversity in the roots, stems, and leaves of the Taxus yunnanensis (T. yunnanensis). The analysis revealed some dominant genera of endophytic bacteria, such as Pseudomonas, Neorhizobium, Acidovorax, and Flavobacterium, with Cladosporium, Phyllosticta, Fusarium, and Codinaeopsis as prominent endophytic fungi genera. We isolated 108 endophytic bacteria and 27 endophytic fungi from roots, stems, and leaves. In vitro assays were utilized to screen for endophytic bacteria with growth-promoting capabilities, including IAA production, cellulase, siderophore production, protease and ACC deaminase activity, inorganic phosphate solubilization, and nitrogen fixation. Three promising strains, Kocuria sp. TRI2-1, Micromonospora sp. TSI4-1, and Sphingomonas sp. MG-2, were selected based on their superior growth-promotion characteristics. These strains exhibited preferable plant growth promotion when applied to Arabidopsis thaliana growth. Fermentation broths of these three strains were also found to significantly promote the accumulation of taxanes in T. yunnanensis stem cells, among which strain TSI4-1 demonstrated outstanding increase potentials, with an effective induction of taxol, baccatin III, and 10-DAB contents. After six days of treatment, the contents of these metabolites were 3.28 times, 2.23 times, and 2.17 times the initial amounts, reaching 8720, 331, and 371 ng/g of dry weight of stem cells, respectively. These findings present new insight into the industrialization of taxol production through Taxus stem cell fermentation, thereby promoting the conservation of wild Taxus resources by maximizing their potential economic benefits.

Deuterodehalogenation Under Net Reductive or Redox-Neutral Conditions Enabled by Paired Electrolysis

Interest in deuterated active pharmaceutical ingredients (APIs) is increasing as deuteration holds promise for kinetic isotope effect (KIE) regulated fine-tuning of API performance. Moreover, deuterium isotope labeling is frequently carried out to study organic and bioorganic reaction mechanisms and to facilitate complex target synthesis. As such, methods for highly selective deuteration of organic molecules are highly desirable. Herein, we present an electrochemical method for the selective deuterodehalogenation of benzylic halides via a radical-polar crossover mechanism, using inexpensive deuterium oxide (D2 O) as the deuterium source. We demonstrate broad functional group compatibility across a range of aryl and heteroaryl benzylic halides. Furthermore, we uncover a sequential paired electrolysis regime, which permits switching between net reductive and overall redox-neutral reactions of sulfur-containing substrates simply by changing the identity of the sacrificial reductant employed.

Strategies and Lessons Learned from Total Synthesis of Taxol

Taxol (paclitaxel), the most well-known taxane diterpenoid, is the best-selling natural-source anticancer drug ever produced and one of the most common prescriptions in the treatment of breast, lung, and ovarian cancers, saving countless lives around the world. Structurally, Taxol possesses a highly oxygenated [6-8-6-4] core bearing 11 stereocenters, seven of which are contiguous chiral centers. Moreover, the extremely strained bicyclo[5.3.1] undecane ring system with a bridgehead double bond is a unique structural feature. All these features make Taxol a highly challenging synthetic target. Tremendous synthetic efforts from more than 60 research groups around the world have already culminated in ten total syntheses and three formal syntheses, as well as more than 60 synthetic model studies of Taxol. This review is intended to provide a long-overdue appraisal of the great achievements in the total syntheses of Taxol reported in the last few decades. In doing so, we summarize the development of synthesis toward Taxol from 1994 to 2022, including the evolution of synthetic strategy for accessing this complex molecular scaffold and key lessons learned from such endeavors. Finally, we briefly discuss the future of the research in this area.

Total Synthesis of Taxol Enabled by Inter- and Intramolecular Radical Coupling Reactions

Taxol is a clinically used drug for the treatment of various types of cancers. Its 6/8/6/4-membered ring (ABCD-ring) system is substituted by eight oxygen functional groups and flanked by four acyl groups, including a β-amino acid side chain. Here we report a 34-step total synthesis of this unusually oxygenated and intricately fused structure. Inter- and intramolecular radical coupling reactions connected the A- and C-ring fragments and cyclized the B-ring, respectively. Functional groups of the A- and C-rings were then efficiently decorated by employing newly developed chemo-, regio-, and stereoselective reactions. Finally, construction of the D-ring and conjugation with the β-amino acid delivered taxol. The powerful coupling reactions and functional group manipulations implemented in the present synthesis provide new valuable information for designing multistep target-oriented syntheses of diverse bioactive natural products.

Total Synthesis of (+)-Coriamyrtin via a Desymmetrizing Strategy Involving a 1,3-Cyclopentanedione Moiety

We describe the total synthesis of (+)-coriamyrtin, which bears a highly functionalized cis-hydrindane skeleton and is a widely known neurotoxin of the Coriariaceae family. Our synthetic strategy involves the highly stereoselective construction of the cis-hydrindane skeleton via a desymmetrizing strategy involving a 1,3-cyclopentanedione moiety using an intramolecular aldol reaction and the formation of the 1,3-diepoxide moiety of coriamyrtin through the elaborate functionalization of the cyclopentane ring in the bicyclic structure.

Enzyme-like polyene cyclizations catalyzed by dynamic, self-assembled, supramolecular fluoro alcohol-amine clusters

Terpene cyclases catalyze one of the most powerful transformations with respect to efficiency and selectivity in natural product (bio)synthesis. In such polyene cyclizations, structurally highly complex carbon scaffolds are built by the controlled ring closure of linear polyenes. Thereby, multiple C,C bonds and stereocenters are simultaneously created with high precision. Structural pre-organization of the substrate carbon chain inside the active center of the enzyme is responsible for the product- and stereoselectivity of this cyclization. Here, we show that in-situ formed fluorinated-alcohol-amine supramolecular clusters serve as artificial cyclases by triggering enzyme-like reactivity and selectivity by controlling substrate conformation in solution. Because of the dynamic nature of these supramolecular assemblies, a broad range of terpenes can be produced diastereoselectively. Mechanistic studies reveal a finely balanced interplay of fluorinated solvent, catalyst, and substrate as key to establishing nature's concept of a shape-selective polyene cyclization in organic synthesis.

Asymmetric Total Synthesis of Twin Bufogargarizins A and B

The first and asymmetric total synthesis of bufogargarizins A and B, two unusual and highly oxygenated twin steroids with rearranged A/B rings, was achieved. The synthetically challenging [7-5-6-5] tetracyclic ring system of bufogargarizin A was efficiently constructed by the first intramolecular Ru-catalyzed [5 + 2] cycloaddition reaction of a vinyl ether cyclopropane-yne. Notably, the interesting [5-7-6-5] tetracyclic skeleton of bufogargarizin B was diastereoselectively reassembled by unique retro-aldol/transannular aldol cascade reactions from the [7-5-6-5] tetracyclic framework.

Photoinduced β-fragmentation of aliphatic alcohol derivatives for forging C-C bonds

Alcohols are ubiquitous in chemistry and are native functionalities in many natural products and bioactive molecules. As such, a strategy that utilizes hydroxy-containing compounds to develop bond disconnection and bond formation process would achieve molecular diversity. Herein we utilize bench-stable N-alkoxyphthalimides prepared from alcohols to couple with glycine derivatives via radical process under visible light irradiation, providing a variety of unnatural amino acid (UAA) and peptide derivatives. The approach allows to rapidly deconstruct molecular complexity via β-fragmentation such as saclareolide, β-pinene and camphor and provides products with unique scaffolds, which show inhibition toward the pathogenic fungi growth.

General Synthetic Approach to Diverse Taxane Cores

Chemical synthesis of natural products is typically inspired by the structure and function of a target molecule. When both factors are of interest, such as in the case of taxane diterpenoids, a synthesis can both serve as a platform for synthetic strategy development and enable new biological exploration. Guided by this paradigm, we present here a unified enantiospecific approach to diverse taxane cores from the feedstock monoterpenoid (S)-carvone. Key to the success of our approach was the use of a skeletal remodeling strategy which began with the divergent reorganization and convergent coupling of two carvone-derived fragments, facilitated by Pd-catalyzed C-C bond cleavage tactics. This coupling was followed by additional restructuring using a Sm(II)-mediated rearrangement and a bioinspired, visible-light induced, transannular [2 + 2] photocycloaddition. Overall, this divergent monoterpenoid remodeling/convergent fragment coupling approach to complex diterpenoid synthesis provides access to structurally disparate taxane cores which have set the stage for the preparation of a wide range of taxanes.

Unified Total Syntheses of Benzenoid Cephalotane-Type Norditerpenoids: Cephanolides and Ceforalides

Detailed herein are our synthetic studies toward the preparation of the C₁₈- and C₁₉-benzenoid cephalotane-type norditerpenoids. Guided by chemical network analysis, the core structure of this natural product family was constructed in a concise manner using an iterative cross-coupling, followed by a formal inverse-electron-demand [4 + 2] cycloaddition. Initial efforts to functionalize an alkene group in the [4 + 2] cycloadduct using a Mukaiyama hydration and a subsequent olefination led to the complete C₁₈-carbon framework. While effective, this approach proved lengthy and prompted the development of a direct alkene difunctionalization that relies on borocupration to advance the cycloadduct to the natural products. Late-stage peripheral C-H functionalization facilitated access to all of the known cephanolides in 6-10 steps as well as five recently isolated ceforalides in 8-13 steps.

Concise Total Synthesis of Salimabromide

We achieved a concise total synthesis of salimabromide by using a novel intramolecular radical cyclization to simultaneously construct the unique benzo-fused [4.3.1] carbon skeleton and the vicinal quaternary stereocenters. Other notable transformations include a tandem Michael/Mukaiyama aldol reaction to introduce most of the molecule's structural elements, along with hidden information for late-stage transformations, an intriguing tandem oxidative cyclization of a diene to form the bridged butyrolactone and enone moieties spontaneously, and a highly enantioselective hydrogenation of a cycloheptenone derivative (97% ee) that paved the way for the asymmetric synthesis of salimabromide.

Deconstructive Synthesis of Bridged and Fused Rings via Transition-Metal-Catalyzed "Cut-and-Sew" Reactions of Benzocyclobutenones and Cyclobutanones

Bridged and fused rings are commonly found in biologically important molecules. Current tactics to construct these ring systems are primarily based on stepwise ring formation (i.e., making one ring first followed by making another) and cycloaddition reactions (e.g., Diels-Alder reaction). To seek a complementary and perhaps more unified ring-forming approach, a deconstructive strategy based on C-C bond activation of cyclic ketones has been conceived. The named "cut-and-sew" reaction uses cyclic ketones with a tethered unsaturated moiety as substrates, which involves oxidative addition of a transition metal into the ketone C-C bond followed by intramolecular insertion of the unsaturated unit. This strategy has proved successful to access diverse ring scaffolds that are nontrivial to construct otherwise.This Account offers a concise summary of our laboratory's systematic efforts in developing transition metal-catalyzed cut-and-sew reactions for the synthesis of bridged and fused rings over the past 10 years. In particular, we will focus on the reactions using readily available benzocyclobutenones and cyclobutanones. To date, the scope of the cut-and-sew reactions has been greatly expanded. First, diverse unsaturated moieties can serve as suitable coupling partners, such as alkenyl, alkynyl, allenyl, carbonyl, and iminyl groups. Second, a variety of reaction modes have been uncovered. In this account, (4 + 2), (4 + 2 - 1), and (4 + 1) cycloadditions that lead to a range of bridged or fused scaffolds will be summarized. Third, enantioselective transformations have been realized to efficiently construct chiral scaffolds, which are enabled by two strategies: enantio-determining migratory insertion and desymmetrization of cyclobutanones. Fourth, the synthetic applications have been demonstrated in streamlined total syntheses of a number of complex natural products. Compared to conventional synthetic logics, the cut-and-sew reaction allows the development of new bond-disconnecting strategies. Thus, the syntheses of (-)-cycloclavine, (-)-thebainone A, penicibilaenes, and the proposed cycloinumakiol are discussed in more detail.In addition to the narrative of the development of the cut-and-sew chemistry, this Account also aims to provide core guiding foundations and inspirations toward broader deconstructive synthetic applications through C-C bond cleavage. It is anticipated that more classes of cyclic compounds could serve as the substrates beyond benzocyclobutenones and cyclobutanones, and more diverse unsaturated moieties could be coupled. It can also be envisaged that more innovative utilization of this cut-and-sew strategy in complex organic syntheses will be revealed in the near future.

A Two-Phase Approach to Fusicoccane Synthesis To Uncover a Compound That Reduces Tumourigenesis in Pancreatic Cancer Cells

Alterbrassicicene D (1) and 3(11)-epoxyhypoestenone (2) were synthesised via a two-phase approach featuring concise construction of the 5-8-5 tricyclic intermediate and a tandem base-mediated epoxide opening-transannular oxa-Michael addition cascade to forge the complex skeleton of 2. The route is scalable and we generated 15 g of the tricyclic intermediate in 8 steps from (R)-limonene and 720 mg of the penultimate bioactive intermediate in a protecting-group-free manner. Our synthesis enabled the structural determination of 2 and provided materials for preliminary anticancer evaluation. The penultimate intermediate showed therapeutic potential in terms of its ability to dramatically reduce the tumourigenic potential of PANC-1 pancreatic cancer cells according to a limiting dilution tumour-initiating assay. Our synthetic approach will facilitate unified access to naturally occurring fusicoccanes and their derivatives for anticancer evaluation.

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.

Dimerizing cascades of enallenamides reveal the visible-light-promoted activation of cumulated C-C double bonds

The visible-light-promoted activation of conjugated C-C double bonds is well developed, while that of cumulated systems is underexplored. We present the feasibility of this challenging approach. The localization of a triplet on an allenamide arm can be favored over that on a conjugated alkene. Allenamides with an arylacryloyl arm dimerize at room temperature in the presence of visible light and an iridium(iii) photocatalyst. Two orthogonal polycyclizations took place and their outcome is entirely dictated by the substitution of the alkene partner. Both cascades afford complex molecular architectures with high selectivity. Products form through the ordered rearrangement of twelve π electrons, providing a [3.2.0] bicyclic unit tethered to a fused tricycle, whose formation included an aryl C-H functionalization step, using disubstituted alkenes. The outcome was reverted with trisubstituted ones, which gave rise to taxane-like bridged tricycles that had two six-membered lactams flanking a cyclooctane ring, which was established through the creation of four alternate C-C bonds.

IMB5476, a novel microtubule inhibitor, induces mitotic catastrophe and overcomes multidrug resistance in tumors

IMB5046 is a nitrobenzoate microtubule inhibitor we reported previously. During screening of its structural analogues, we identified a novel compound IMB5476 with increased aqueous solubility. Here, its antitumor activity and the underlying mechanism were investigated. IMB5476 disrupted microtubule networks in cells and arrested cell cycle at G2/M phase. It inhibited purified tubulin polymerization in vitro. Competition assay indicated that it bound to tubulin at the colchicine pocket. Further experiments proved that it induced cell death by mitotic catastrophe and apoptosis. Notably, it was a poor substrate of P-glycoprotein and exhibited potent cytotoxicity against drug-resistant tumor cells. In addition, IMB5476 could inhibit angiogenesis in vitro. IMB5476 also inhibited the growth of drug-resistant KB_V200 xenografts in mice. Conclusively, our data reveal a novel nitrobenzoate microtubule inhibitor with improved aqueous solubility and can overcome multidrug resistance.

Total Synthesis and Late-Stage C-H Oxidations of ent-Trachylobane Natural Products

Herein, we present our studies to construct seven ent-trachylobane diterpenoids by employing a bioinspired two-phase synthetic strategy. The first phase provided enantioselective and scalable access to five ent-trachylobanes, of which methyl ent-trachyloban-19-oate was produced on a 300 mg scale. During the second phase, chemical C-H oxidation methods were employed to enable selective conversion to two naturally occurring higher functionalized ent-trachylobanes. The formation of regioisomeric analogs, which are currently inaccessible via enzymatic methods, reveals the potential as well as limitations of established chemical C-H oxidation protocols for complex molecule synthesis.

Total Synthesis and Late-Stage C-H Oxidations of ent-Trachylobane Natural Products

Herein, we present our studies to construct seven ent-trachylobane diterpenoids by employing a bioinspired two-phase synthetic strategy. The first phase provided enantioselective and scalable access to five ent-trachylobanes, of which methyl ent-trachyloban-19-oate was produced on a 300 mg scale. During the second phase, chemical C-H oxidation methods were employed to enable selective conversion to two naturally occurring higher functionalized ent-trachylobanes. The formation of regioisomeric analogs, which are currently inaccessible via enzymatic methods, reveals the potential as well as limitations of established chemical C-H oxidation protocols for complex molecule synthesis.

Total Synthesis of Paclitaxel

The total synthesis of paclitaxel (Taxol) is described. Double Rubottom oxidation of the bis(silyl enol ether) derived from a tricarbocyclic diketone effectively installed a bridgehead olefin and C-5/C-13 hydroxy groups in a one-step operation. The novel Ag-promoted oxetane formation smoothly constructed the tetracyclic framework of paclitaxel.

Chiral oxazolidines acting as transient hydroxyalkyl-functionalized N-heterocyclic carbenes: an efficient route to air stable copper and gold complexes for asymmetric catalysis

Optically pure oxazolidines were synthesized in nearly quantitative yields from chiral hydroxyalkyl-functionalized imidazolinium salts. Acting as transient chiral diamino N-heterocyclic carbenes (NHCs), these oxazolidines allowed the efficient formation of well-defined copper(i) and gold(i) hydroxyalkyl-NHC complexes, which could be isolated, for the first time, as air stable complexes after silica gel chromatography. Interestingly, X-ray analysis of gold complexes revealed that the hydroxyl-function is not chelated to the metal. Computational studies suggested that both cyclisation to produce oxazolidine and O–H bond elimination to form the transient carbene (prior to coordination) occur through a concerted mechanism. The novel chiral copper-catalysts, as well as oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation with high regio- and enantio-selectivities (up to 99% ee).

Well-defined chiral Cu(i) and Au(i) hydroxyalkyl NHC complexes were synthesized from oxazolidines. The copper-catalysts and oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation.

A brief overview of classical natural product drug synthesis and bioactivity

This manuscript briefly overviewed the total synthesis and structure–activity relationship studies of eight classical natural products, which emphasizes the important role of total synthesis in natural product-based drug development.

Total Synthesis of Dalesconol A by Pd(0)/Norbornene-Catalyzed Three-Fold Domino Reaction and Pd(II)-Catalyzed Trihydroxylation

Herein, we describe a concise total synthesis of dalesconol A through a "polycyclization/oxidation" approach. In the polycyclization stage, a Pd(0)/NBE-catalyzed 3-fold domino reaction and a subsequent intramolecular Michael addition have been utilized for the one-step assembly of the heptacyclic molecular skeleton. In the late stage of oxidation state adjustments, a stepwise sequence including site-selective benzylic oxidation, Pd(II)-catalyzed oxime ether directed trihydroxylation, and desaturation has been adopted to introduce the oxygen functionalities and furnish the synthesis of dalesconol A. With the advantage of the late-stage amidation of three C-H bonds in a single step, the amino analogue of dalesconol A has also been obtained with high efficiency.