Metallacycle-Mediated Cross-Coupling in Natural Product Synthesis

Pathway-divergent coupling of 1,3-enynes with acrylates through cascade cobalt catalysis

Catalytic cascade transformations of simple starting materials into highly functionalized molecules bearing a stereochemically defined multisubstituted alkene, which are important in medicinal chemistry, natural product synthesis, and material science, are in high demand for organic synthesis. The development of multiple reaction pathways accurately controlled by catalysts derived from different ligands is a critical goal in the field of catalysis. Here we report a cobalt-catalyzed strategy for the direct coupling of inexpensive 1,3-enynes with two molecules of acrylates to construct a high diversity of functionalized 1,3-dienes containing a trisubstituted or tetrasubstituted olefin. Such cascade reactions can proceed through three different pathways initiated by oxidative cyclization to achieve multiple bond formation in high chemo-, regio- and stereoselectivity precisely controlled by ligands, providing a platform for the development of tandem carbon-carbon bond-forming reactions.

Cp2Ti(II) Mediated Rearrangement of Cyclopropyl Imines

Ti-catalyzed oxidative alkyne carboamination with alkenes and azo compounds can yield either α,β-unsaturated imines or cyclopropyl imines through a common azatitanacyclohexene intermediate. Herein, we report the synthesis of a model azatitanacyclohexene complex (3) through the ring-opening of a cyclopropyl imine with Cp2Ti(BTMSA) (BTMSA = bis(trimethylsilyl)acetylene). 3 readily undergoes thermal or reductant-catalyzed ring contraction to an azatitanacyclopentene (4), analogous to the proposed mechanism for forming α,β-unsaturated imines in the catalytic reaction. A cyclopropyl imine or an α,β-unsaturated imine could be liberated via the oxidation of 3 or 4 with azobenzene, respectively, further implicating the role of these metallacycles in the Ti-catalyzed carboamination reaction.

Acyclic Remote 1,6-Stereoselection

Stereochemical communication in homopropargylation and homoallylation of aldehydes was achieved by the Ti-O temporary linker strategy. Propargylic and allylic alcohol derivatives were employed as convenient pronucleophiles, obviating prefabrication of propargylation/allylation reagents. It was surprising that 1,6-diastereoselectivity was affected by not only the Grignard reagent but also the reaction solvent.

General Enantioselective and Stereochemically Divergent Four-Stage Approach to Fused Tetracyclic Terpenoid Systems

Tetracyclic terpenoid-derived natural products are a broad class of medically relevant agents that include well-known steroid hormones and related structures, as well as more synthetically challenging congeners such as limonoids, cardenolides, lanostanes, and cucurbitanes, among others. These structurally related compound classes present synthetically disparate challenges based, in part, on the position and stereochemistry of the numerous quaternary carbon centers that are common to their tetracyclic skeletons. While de novo syntheses of such targets have been a topic of great interest for over 50 years, semisynthesis is often how synthetic variants of these natural products are explored as biologically relevant materials and how such agents are further matured as therapeutics. Here, focus was directed at establishing an efficient, stereoselective, and molecularly flexible de novo synthetic approach that could offer what semisynthetic approaches do not. In short, a unified strategy to access common molecular features of these natural product families is described that proceeds in four stages: (1) conversion of epichlorohydrin to stereodefined enynes, (2) metallacycle-mediated annulative cross-coupling to generate highly substituted hydrindanes, (3) tetracycle formation by stereoselective forging of the C9-C10 bond, and (4) group-selective oxidative rearrangement that repositions a quaternary center from C9 to C10. These studies have defined the structural features required for highly stereoselective C9-C10 bond formation and document the generality of this four-stage synthetic strategy to access a range of unique stereodefined systems, many of which bear stereochemistry/substitution/functionality not readily accessible from semisynthesis.

Total synthesis of nahuoic acid A via a putative biogenetic intramolecular Diels-Alder (IMDA) reaction

Inspired by the biogenetic proposal of an intramolecular Diels-Alder (IMDA) cycloaddition, the total synthesis of natural product nahuoic acid A, a cofactor-competitive inhibitor of the epigenetic enzyme lysine methyl transferase SETD8, has been carried out. A non-conjugated pentaenal precursor was synthesized with high levels of stereoselectivity at seven stereogenic centers and with the appropriate control of double bond geometries. Although the IMDA reaction of the non-conjugated pentaenal using Me2AlCl for catalysis at -40 °C selectively afforded the trans-fused diastereomer corresponding to the Re-endo mode of cycloaddition, under thermal reaction conditions it gave rise to a mixture of diastereomers, that preferentially formed through the exo mode, including the cis-fused angularly-methylated octahydronaphthalene diastereomer precursor of nahuoic acid A. The natural product could be obtained upon oxidation and overall deprotection of the hydroxyl groups present in the Si-exo IMDA diastereomer.

Nickel-Catalyzed Intramolecular Hydroalkenylation of Imines

A ligand-enabled nickel-catalyzed intramolecular hydroalkenylation of imines with unactivated alkenes has been developed. A variety of five- and six-membered cyclic allylic amines were synthesized in high yields. The use of both wide-bite-angle diphosphine ligand and Brønsted acid is crucial for realizing the reaction. Preliminary investigation of the asymmetric intramolecular hydroalkenylation of imines shows promising potential for the application of the method in the synthesis of enantio-enriched cyclic allylic amines.

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4-8 transition metal catalysts

In this Perspective, we discuss recent syntheses of 5- and 6-membered aromatic heterocycles via multicomponent reactions (MCRs) that are catalyzed by group 4-8 transition metals. These MCRs can be categorized based on the substrate components used to generate the cyclized product, as well as on common mechanistic features between the catalyst systems. These particular groupings are intended to highlight mechanistic and strategic similarities between otherwise disparate transition metals and to encourage future work exploring related systems with otherwise-overlooked elements. Importantly, in many cases these early- to mid-transition metal catalysts have been shown to be as effective for heterocycle syntheses as the later (and more commonly implemented) group 9-11 metals.

NHC-Ni catalyzed 1,3- and 1,4-diastereodivergent heterocycle synthesis from hetero-substituted enyne

Diastereodivergent heterocycle synthesis has been recognized as an important tool for drug discovery in recent years, yet strategies based on nickelacycle formation have not been established. Here, we report a NHC-Ni catalyzed highly 1,3- and 1,4-diastereodivergent heterocycle synthesis from enyne, which is achieved by manipulating the enyne N-substituent (allowing switching of selectivity from up to 2:98 to 98:2). The key to success is the efficient diastereodivergent formation of a nickelacyclopentene, with broad enyne scope at mild conditions, which subsequently provides reductive hydroalkenylation, acylation and silylation products on demand. Diastereoisomers which are sterically hard to distinguish or difficult to access by conventional routes are now accessible easily, including those with very similar 4°, contiguous and skipped stereocenters.

Evolution of Anion Relay Chemistry: Construction of Architecturally Complex Natural Products

Multicomponent union tactics in which three or more fragments are rapidly connected are highly prized in the construction of architecturally complex natural products. Anion Relay Chemistry (ARC), a multicomponent union tactic, has just such potential to elaborate structurally diverse scaffolds in a single operation with excellent stereochemical control. Conceptually, the ARC tactic can be divided into two main classes: "Through-Bond," by the relay of negative charge through the bonding system of a molecule; and "Through-Space," by the migration of negative charge across space by a transfer agent. "Through-Space" Anion Relay Chemistry, the focus of this Account, can be further subdivided into two types: Type I ARC, originated from the Tietze-Schaumann-Smith coupling reaction, which for the first time permits controllable Brook rearrangements to construct unsymmetrical adducts, and as such has been successfully employed in the total syntheses of diverse natural products, including the mycoticins, bryostatin 1, spongistatins, rimocidin, indolizidine alkaloids, and enigmazole A; and Type II ARC, central to which is the design of novel bifunctional linchpins that enable rapid assembly of linear and cyclic fragments with diverse architectural features, ranging from polyols, spiroketals, and polyenes to polypropionate scaffolds. Recently, the Type II ARC tactic has been exploited as the key construction tactic in the total syntheses of the spirastrellolides, the cryptocarya acetates, secu'amamine A, mandelalide A, and nahuoic acid Ci (Bii). This Account will present the evolution of both the Type I and Type II Anion Relay tactics, in conjunction with some prominent applications.

A Complementary Process to Pauson-Khand-Type Annulation Reactions for the Construction of Fully Substituted Cyclopentenones

A complementary process to the Pauson-Khand annulation is described that is well suited to forging densely substituted/oxygenated cyclopentenone products (including fully substituted variants). The reaction is thought to proceed through a sequence of metallacycle-mediated bond-forming events that engages an internal alkyne and a β-keto ester in an annulation process that forges two C-C bonds. A variant of this annulation process has also been established that delivers deoxygenated cyclopentenones that lack the allylic tertiary alcohol.

A Highly Chemo-, Regio-, and Stereoselective Metallacycle-Mediated Annulation Between a Conjugated Enyne and an Ene-Diyne

Alkoxide-directed metal-centered intermolecular [2+2+2] annulation is shown to chemo-, regio-, and stereoselectively engage two polyunsaturated substrate in productive cyclization chemistry. This annulation process is unique in the field, revealing that it is possible to selectively engage three of five π-systems residing in the coupling partners in initial [2+2+2] reaction, and demonstrating that one of the two remaining π-systems (the TMS-alkyne) can ultimately serve to simply generate a new metallacyclopentene of great potential value in additional metallacycle-mediated coupling chemistry.

Total Synthesis of (-)-Nahuoic Acid Ci (Bii)

A convergent total synthesis of (-)-nahuoic acid Ci(Bii) (3), a novel cis-decalin polyketide, has been achieved. Key synthetic transformations include Type II Anion Relay Chemistry (ARC) to construct the polyol chain, a Ti-catalyzed asymmetric Diels-Alder reaction to generate the cis-decalin skeleton, and a late-stage large fragment union exploiting a Micalizio alkoxide-directed alkyne-alkene coupling tactic.

Synthetic nat- or ent-steroids in as few as five chemical steps from epichlorohydrin

Today, more than 100 Food and Drug Administration-approved steroidal agents are prescribed daily for indications including heart failure, inflammation, pain and cancer. While triumphs in organic chemistry have enabled the establishment and sustained growth of the steroid pharmaceutical industry, the production of highly functionalized synthetic steroids of varying substitution and stereochemistry remains challenging, despite the numerous reports of elegant strategies for their de novo synthesis. Here, we describe an advance in chemical synthesis that has established an enantiospecific means to access novel steroids with unprecedented facility and flexibility through the sequential use of two powerful ring-forming reactions: a modern metallacycle-mediated annulative cross-coupling and a new acid-catalysed vinylcyclopropane rearrangement cascade. In addition to accessing synthetic steroids of either enantiomeric series, these steroidal products have been selectively functionalized within each of the four carbocyclic rings, a synthetic ent-steroid has been prepared on a multigram scale, the enantiomer of a selective oestrogen has been synthesized, and a novel ent-steroid with growth inhibitory properties in three cancer cell lines has been discovered.

Synthesis of Highly Oxygenated Carbocycles by Stereoselective Coupling of Alkynes to 1,3- and 1,4-Dicarbonyl Systems

Densely substituted and highly oxygenated carbocycles are challenging targets for synthesis. In particular, those possessing numerous contiguous, fully substituted carbon atoms (i.e., tertiary alcohols and quaternary centers) are often not accessible in a direct fashion, necessitating the strategic decoupling of ring-formation from the establishment of functionality about the system. Here, we describe an approach to the construction of highly oxygenated mono-, di-, and polycyclic carbocycles from the reaction of disubstituted alkynes with β- or γ-dicarbonyl systems. These processes embrace a variant of metallacycle-mediated annulation chemistry where initial alkyne-carbonyl coupling is followed by a second, now intramolecular, stereoselective C-C bond-forming event. In addition to revealing the basic reactivity pattern in intermolecular settings, we demonstrate that this class of reactivity is quite powerful in a fully intramolecular context and, when terminated by a stereoselective oxidation process, can be used to generate polycyclic systems containing a fully substituted and highly oxygenated five-membered ring.

Some aspects of radical cascade and relay reactions

The ability to create carbon-carbon bonds is at the heart of organic synthesis. Radical processes are particularly apt at creating such bonds, especially in cascade or relay sequences where more than one bond is formed, allowing for a rapid assembly of complex structures. In the present brief overview, examples taken from the authors' laboratory will serve to illustrate the strategic impact of radical-based approaches on synthetic planning. Transformations involving nitrogen-centred radicals, electron transfer from metallic nickel and the reversible degenerative exchange of xanthates will be presented and discussed. The last method has proved to be a particularly powerful tool for the intermolecular creation of carbon-carbon bonds by radical additions even to unactivated alkenes. Various functional groups can be brought into the same molecule in a convergent manner and made to react together in order to further increase the structural complexity. One important benefit of this chemistry is the so-called RAFT/MADIX technology for the manufacture of block copolymers of almost any desired architecture.