A concise ring-expansion route to the compact core of platensimycin

Asymmetric Synthesis and Biological Evaluation of Platensilin, Platensimycin, Platencin, and Their Analogs via a Bioinspired Skeletal Reconstruction Approach

Platensilin, platensimycin, and platencin are potent inhibitors of β-ketoacyl-acyl carrier protein synthase (FabF) in the bacterial and mammalian fatty acid synthesis system, presenting promising drug leads for both antibacterial and antidiabetic therapies. Herein, a bioinspired skeleton reconstruction approach is reported, which enables the unified synthesis of these three natural FabF inhibitors and their skeletally diverse analogs, all stemming from a common ent-pimarane core. The synthesis features a diastereoselective biocatalytic reduction and an intermolecular Diels-Alder reaction to prepare the common ent-pimarane core. From this intermediate, stereoselective Mn-catalyzed hydrogen atom-transfer hydrogenation and subsequent Cu-catalyzed carbenoid C-H insertion afford platensilin. Furthermore, the intramolecular Diels-Alder reaction succeeded by regioselective ring opening of the newly formed cyclopropane enables the construction of the bicyclo[3.2.1]-octane and bicyclo[2.2.2]-octane ring systems of platensimycin and platencin, respectively. This skeletal reconstruction approach of the ent-pimarane core facilitates the preparation of analogs bearing different polycyclic scaffolds. Among these analogs, the previously unexplored cyclopropyl analog 47 exhibits improved antibacterial activity (MIC80 = 0.0625 μg/mL) against S. aureus compared to platensimycin.

Properties and Mechanisms of Flavin-Dependent Monooxygenases and Their Applications in Natural Product Synthesis

Natural products are usually highly complicated organic molecules with special scaffolds, and they are an important resource in medicine. Natural products with complicated structures are produced by enzymes, and this is still a challenging research field, its mechanisms requiring detailed methods for elucidation. Flavin adenine dinucleotide (FAD)-dependent monooxygenases (FMOs) catalyze many oxidation reactions with chemo-, regio-, and stereo-selectivity, and they are involved in the synthesis of many natural products. In this review, we introduce the mechanisms for different FMOs, with the classical FAD (C4a)-hydroperoxide as the major oxidant. We also summarize the difference between FMOs and cytochrome P450 (CYP450) monooxygenases emphasizing the advantages of FMOs and their specificity for substrates. Finally, we present examples of FMO-catalyzed synthesis of natural products. Based on these explanations, this review will expand our knowledge of FMOs as powerful enzymes, as well as implementation of the FMOs as effective tools for biosynthesis.

Asymmetric Catalytic Vinylogous Addition Reactions Initiated by Meinwald Rearrangement of Vinyl Epoxides

The first catalytic asymmetric multiple vinylogous addition reactions initiated by Meinwald rearrangement of vinyl epoxides were realized by employing chiral N,N'-dioxide/Sc^III complex catalysts. The vinyl epoxides, as masked β,γ-unsaturated aldehydes, via direct vinylogous additions with isatins, 2-alkenoylpyridines or methyleneindolinones, provided a facile and efficient way for the synthesis of chiral 3-hydroxy-3-substituted oxindoles, α,β-unsaturated aldehydes and spiro-cyclohexene indolinones, respectively with high efficiency and stereoselectivity. The control experiments and kinetic studies revealed that the Lewis acid acted as dual-tasking catalyst, controlling the initial rearrangement to match subsequent enantioselective vinylogous addition reactions. A catalytic cycle with a possible transition model was proposed to illustrate the reaction mechanism.

Microwave-assisted organic acid–base-co-catalyzed tandem Meinwald rearrangement and annulation of styrylepoxides

A microwave-assisted acid and base co-catalyzed strategy shows very high efficiency in the tandem reaction for the conversion of styrylepoxides into [1,1′-biaryl]-3-carbaldehydes.

Tris(pentafluorophenyl)borane-Catalyzed Reactions Using Silanes

The utility of an electron-deficient, air stable, and commercially available Lewis acid tris(pentafluorophenyl)borane has recently been comprehensively explored. While being as reactive as its distant cousin boron trichloride, it has been shown to be much more stable and capable of catalyzing a variety of powerful transformations, even in the presence of water. The focus of this review will be to highlight those catalytic reactions that utilize a silane as a stoichiometric reductant in conjunction with tris(pentafluorophenyl) borane in the reduction of alcohols, carbonyls, or carbonyl-like derivatives.

Short Enantioselective Formal Synthesis of (-)-Platencin

A short enantioselective formal synthesis of the antibiotic natural product platencin is reported. Key steps in the synthesis include enantioselective decarboxylation alkylation, aldehyde/olefin radical cyclization, and regioselective aldol cyclization.

[2.2.2]- to [3.2.1]-Bicycle Skeletal Rearrangement Approach to the Gibberellin Family of Natural Products

Synthetic studies toward the gibberellin family of natural products are reported. An oxidative dearomatization/Diels-Alder cascade assembles the carbon skeleton as a [2.2.2]-bicycle, which is then transformed to the [3.2.1]-bicyclic gibberellin core via a novel Lewis acid catalyzed rearrangement. Strategic synthetic handles allow for late-stage modification of the gibberellin skeleton and provides efficient access to this important family of natural compounds.

Versatile, mild, and selective reduction of various carbonyl groups using an electron-deficient boron catalyst

A mild and selective new method was discovered to reduce acetanilides and other carbonyl compounds. Unlike sodium borohydride, which is selective in reducing aldehydes and ketones, this new protocol is uniquely selective in reducing acetanilides and nitriles over other carbonyl containing functional groups. Additionally, β-ketoamides were shown to be reduced at the ketone preferentially over the amide.

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.

Dearomatisation approaches to spirocyclic dienones via the electrophilic activation of alkynes

Two simple metal-mediated dearomatising spirocyclisation protocols to generate spirocyclic dienones from anisole and phenol-tethered ynones in high yields are described.

Syntheses of Denudatine Diterpenoid Alkaloids: Cochlearenine, N-Ethyl-1α-hydroxy-17-veratroyldictyzine, and Paniculamine

The denudatine-type diterpenoid alkaloids cochlearenine, N-ethyl-1α-hydroxy-17-veratroyldictyzine, and paniculamine have been synthesized for the first time (25, 26, and 26 steps from 16, respectively). These syntheses take advantage of a common intermediate (8) that we have previously employed in preparing aconitine-type natural products. The syntheses reported herein complete the realization of a unified strategy for the preparation of C20, C19, and C18 diterpenoid alkaloids.

Review of Platensimycin and Platencin: Inhibitors of β-Ketoacyl-acyl Carrier Protein (ACP) Synthase III (FabH)

Platensimycin and platencin were successively discovered from the strain Streptomyces platensis through systematic screening. These natural products have been defined as promising agents for fighting multidrug resistance in bacteria by targeting type II fatty acid synthesis with slightly different mechanisms. Bioactivity studies have shown that platensimycin and platencin offer great potential to inhibit many resistant bacteria with no cross-resistance or toxicity observed in vivo. This review summarizes the general information on platensimycin and platencin, including antibacterial and self-resistant mechanisms. Furthermore, the total synthesis pathways of platensimycin and platencin and their analogues from recent studies are presented.

Tandem C-H oxidation/cyclization/rearrangement and its application to asymmetric syntheses of (-)-brussonol and (-)-przewalskine E

Natural products are a vital source of lead compounds in drug discovery. Development of efficient tandem reactions to build useful compounds and apply them to the synthesis of natural products is not only a significant challenge but also an important goal for chemists. Here we describe a tandem C–H oxidation/cyclization/rearrangement of isochroman-derived allylic silylethers, promoted by DDQ and InCl₃. This method allows the efficient construction of tricyclic benzoxa[3.2.1]octanes with a wide substrate scope. We employ this tandem reaction to achieve the asymmetric total syntheses of (−)-brussonol and (−)-przewalskine E.

The ability to form complex molecular architectures from simpler precursors is important for drug discovery and medicinal chemistry. Here, the authors report a cascade reaction giving access to tricyclic products and apply it in the total synthesis of two natural products.

A unified survey of Si-H and H-H bond activation catalysed by electron-deficient boranes

The bond activation chemistry of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si-H bonds through η(1) coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond reductions and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.

Formation of Fused Aromatic Architectures via an Oxidative Dearomatization - Radical Cyclization Rearomatization Approach

A new mild C-C bond forming cyclization approach of catechol derivatives is reported. This approach relies on an initial dearomatization step using lead (IV) acetate followed by a carefully controlled radical cyclization step, which under the reaction conditions also facilitates rearomatization. Triethylborane is the key to the success of this reaction as it enables the reaction to proceed at low temperatures and is also believed to aid rearomatization. The amount and ratio of triethylborane and reducing agent (tributyltinhydride) that is employed as well as the concentration the reaction is run at are all essential to the success of this new approach.

Total synthesis of natural products using hypervalent iodine reagents

We present a review of natural product syntheses accomplished in our laboratory during the last 5 years. Each synthetic route features a phenol dearomatization promoted by an environmentally benign hypervalent iodine reagent. The dearomatizations demonstrate the "aromatic ring umpolung" concept, and involve stereoselective remodeling of the inert unsaturations of a phenol into a highly functionalized key intermediate that may contain a quaternary carbon center and a prochiral dienone system. Several new oxidative strategies were employed, including transpositions (1,3-alkyl shift and Prins-pinacol), a polycyclization, an ipso rearrangement, and direct nucleophilic additions at the phenol para position. Several alkaloids, heterocyclic compounds, and a polycyclic core have been achieved, including sceletenone (a serotonin reuptake inhibitor), acetylaspidoalbidine (an antitumor agent), fortucine (antiviral and antitumor), erysotramidine (curare-like effect), platensimycin (an antibiotic), and the main core of a kaurane diterpene (immunosuppressive agent and stimulator of apoptosis). These concise and in some cases enantioselective syntheses effectively demonstrate the importance of hypervalent iodine reagents in the total synthesis of bioactive natural products.

Formal synthesis of (−)-platensimycin

An efficient formal synthesis of (−)-platensimycin was completed by using a tandem C–H oxidation/C–C coupling (cyclization)/rearrangement as the key step.

Evolution of an oxidative dearomatization enabled total synthesis of vinigrol

The evolution of the synthetic strategy resulting in a total synthesis of vinigrol is presented. Oxidative dearomatization/intramolecular Diels-Alder cycloaddition has served as the successful cornerstone for all of the approaches. Extensive radical cyclization efforts to form the tetracyclic core resulted in interesting and surprising reaction outcomes, none of which could be advanced to vinigrol. These cyclization obstacles were successfully overcome by using Heck instead of radical cyclizations. The total synthesis features a trifluoroethyl ether protecting group being used for the first time in organic synthesis. The logic of its selection and the group's importance beyond protecting the C8a hydroxyl group is presented along with a discussion of strategies for its removal. Because of the compact tetracyclic cage the route is built around many unusual reaction observations and solutions have emerged. For example, a first of its kind Grob fragmentation reaction featuring a trifluoroethyl leaving group has been uncovered, interesting interrupted selenium dioxide allylic oxidations have been observed as well as intriguing catalyst and counterion dependent directed hydrogenations.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Copper-catalyzed intramolecular alkene carboetherification: synthesis of fused-ring and bridged-ring tetrahydrofurans

Fused-ring and bridged-ring tetrahydrofuran scaffolds are found in a number of natural products and biologically active compounds. A new copper-catalyzed intramolecular carboetherification of alkenes for the synthesis of bicyclic tetrahydrofurans is reported herein. The reaction involves Cu-catalyzed intramolecular addition of alcohols to unactivated alkenes and subsequent aryl C-H functionalization provides the C-C bond. Mechanistic studies indicate a primary carbon radical intermediate is involved and radical addition to the aryl ring is the likely C-C bond-forming mechanism. Preliminary catalytic enantioselective reactions are promising (up to 75% ee) and provide evidence that copper is involved in the alkene addition step, likely through a cis-oxycupration mechanism. Catalytic enantioselective alkene carboetherification reactions are rare and future development of this new method into a highly enantioselective process is promising. During the course of the mechanistic studies a protocol for alkene hydroetherification was also developed.

Synthesis of allylic and homoallylic alcohols from unsaturated cyclic ethers using a mild and selective C-O reduction approach

Unsaturated cyclic ethers can be mildly and selectively reduced with catalytic amounts of B(C(6)F(5))(3) in the presence of an alkylsilane. The allylic position is preferentially reduced with minimal or no scrambling of olefin geometry. For electronically equivalent substrates, steric factors guide the reducing agent to the least substituted site.

Platensimycin and platencin: promising antibiotics for future application in human medicine

Platensimycin and platencin are novel antibiotics produced by Streptomyces platensis. They are potent and non-toxic natural products active against Gram-positive pathogens, including antibiotic-resistant strains and Mycobacterium tuberculosis. They were isolated using an intriguing target-based whole-cell antisense differential sensitivity assay as inhibitors of fatty acid biosynthesis of type II. This type of biosynthesis is not present in humans. Platensimycin inhibits the elongation-condensing enzyme FabF, whereas platencin inhibits both FabF and FabH. For these antibiotics to become successful drugs, their pharmacokinetics must be improved. They have too high a rate of clearance in the body, yielding a low degree of systematic exposure. They work well when administered by continuous infusion, but this is not a useful method of delivery to patients. The two antibiotics and many analogs have been prepared by chemical synthesis. Natural congeners have also been obtained from the producing actinomycete. However, none of these molecules are as active as platensimycin and platencin. Using tools of rational metabolic engineering, superior strains have been produced making hundreds of times more antibiotic than the natural strains.

Dearomatization strategies in the synthesis of complex natural products

Evolution in the field of the total synthesis of natural products has led to exciting developments over the last decade. Numerous chemoselective and enantioselective methodologies have emerged from total syntheses, resulting in efficient access to many important natural product targets. This Review highlights recent developments concerning dearomatization, a powerful strategy for the total synthesis of architecturally complex natural products wherein planar, aromatic scaffolds are converted to three-dimensional molecular architectures.