Transannular reactions in asymmetric total synthesis

Enantioselective Synthesis of 2-Azanorbornanes: Chiral Trisimidazoline-Catalyzed Desymmetrizing Transannular Bromoaminocyclization

An enantioselective desymmetrizing transannular bromoaminocyclization of cyclopentenylamines has been achieved by using C3-symmetric trisimidazoline catalysis. This protocol could directly provide a range of chiral 2-azanorbornanes in excellent yields (up to 99%) and enantioselectivities (up to 97.5:2.5 enantiomeric ratio). The resulting enantioenriched 2-azanorbornanes are easily converted to synthetically useful building blocks. Further mechanistic investigations show that the presence of three imidazoline groups of the catalyst is crucial to the success of the enantioselective transformation.

Total Synthesis of Exiguolide Stereoisomers: Impact of Stereochemical Permutation on Reactivity, Conformation, and Biological Activity

(-)-Exiguolide is a marine macrolide natural product with potent anticancer activity. In this study, the total synthesis of exiguolide stereoisomers, (9R)-exiguolide, (9R,13S)-exiguolide, and (9R,13S,19R)-exiguolide, was achieved by capitalizing on our macrocyclization/transannular pyran cyclization strategy. The impact of the stereochemical permutation on the reactivity of advanced intermediates, the conformation of the macrocyclic skeleton, and the antiproliferative activity against human cancer cells were investigated in detail. The total synthesis of (9R,13S)-exiguolide and (9R,13S,19R)-exiguolide was completed in much the same way as that of the parent natural product using stereoisomeric building blocks. Nevertheless, the reactivity of the (9R,13S)- and (9R,13S,19R)-series of intermediates in macrocyclic ring-closing metathesis and transannular pyran-forming reactions was significantly different from that of naturally configured counterparts. The conformation of exiguolide stereoisomers, deduced by means of NMR spectroscopic analysis and DFT calculations, was clearly different from that of the parent natural product. Evaluation of the antiproliferative activity of exiguolide and its stereoisomers suggested the importance of the stereochemistry of the macrocyclic skeleton.

From Acyclic Intramolecular-[4 + 2]- to Transannular Bis-[4 + 2]-Cycloaddition of the Macrodiolide for the Stereoselective Synthesis of the Octahydronaphthalene Core of Polyenic Macrolactam Sagamilactam

The strategy for the synthesis of the octahydronaphthalene core of natural macrolide sagamilactam has unintentionally evolved from the acyclic intramolecular (IMDA) to the transannular (TADA) Diels-Alder reaction. Lewis acid-promoted IMDA of a protected 2Z,8E,10E-4,6,12-trihydroxy-2,8,10-decatrienal model with a diol of 4,6-anti relative configuration, as proposed by DP4+-based computational studies, afforded the cis-octahydronaphthalene diastereomer through the Re-endo approach. The 26-membered macrodiolide generated, under thermal reaction conditions, the trans-octahydronaphthalene by a double TADA reaction along the desired Si-exo orientation.

Enantioselective Total Synthesis of (-)-Artatrovirenol A

We report herein an enantioselective total synthesis of (-)-artatrovirenol A, a structurally unprecedented cage-like sesquiterpenoid. The synthesis features the following key steps: (a) cationic chiral oxazaborolidinium-catalyzed Diels-Alder reaction between isoprene and ethyl (E)-5-((tert-butyldimethylsilyl)oxy)-4-oxopent-2-enoate for the rapid synthesis of an enantioenriched 10-carbon bicyclic lactone; (b) union of two enantioenriched fragments by a diastereoselective Mukaiyama-Michael addition for the convergent assembly of an intermediate with all 15 carbons of the natural product; (c) intramolecular de Mayo [2 + 2] cycloaddition/retro-aldol sequence transforming a bicyclic compound to a tetracyclic one with concomitant generation of a five- and a seven-membered ring; (d) Lewis acid-triggered intramolecular ring opening of epoxide generating the norbornane substructure; and (e) Chugaev elimination converting the norbornane to the more strained norbornene.

Computational studies of Brønsted acid-catalyzed transannular cycloadditions of cycloalkenone hydrazones

The contribution to the energy barrier of a series of tethers in transannular cycloadditions of cycloalkenes with hydrazones has been computationally studied by using DFT. The Houk's distortion model has been employed to evaluate the influence of the tether in the cycloaddition reaction. That model has been extended to determine the contribution of each tether and, more importantly, the effect exerted between them. In addition to the distortion induced by the tethers, the entropy effects caused by them has also been studied. The analysis of the evolution of the electron localization function along the reaction revealed the highly concerted character of the reaction.

Brønsted Acid versus Phase-Transfer Catalysis in the Enantioselective Transannular Aminohalogenation of Enesultams

We have studied the enantioselective transannular aminohalogenation reaction of unsaturated medium-sized cyclic benzosulfonamides by using both chiral Brønsted acid and phase-transfer catalysis. Under optimized conditions, a variety of bicyclic adducts can be obtained with good yields and high enantioselectivities. The mechanism of the reaction was also studied by using computational tools; we observed that the reaction involves the participation of a conformer of the nine-membered cyclic substrate with planar chirality in which the stereochemical outcome is controlled by the relative reactivity of the two pseudorotational enantiomers when interacting with the chiral catalyst.

Bis(oxiranes) Containing Cyclooctane Core: Synthesis and Reactivity towards NaN3

Reactions of oxirane ring opening provide a powerful tool for regio- and stereoselective synthesis of polyfunctional and heterocyclic compounds, widely used in organic chemistry and drug design. Cyclooctane, alongside other medium-sized rings, is of interest as a novel molecular platform for the construction of target-oriented leads. Additionally, cyclooctane derivatives are well known to be prone to transannular reactions, which makes them a promising object in the search for novel approaches to polycyclic structures. In the present work, a series of cyclooctanediones was studied in Corey-Chaykovsky reactions, and novel spirocyclic bis(oxiranes) containing cyclooctane core, namely, 1,5-dioxadispiro[2.0.2.6]dodecane and 1,8-dioxadispiro[2.3.2.3]dodecane, were synthesized. Ring opening of the obtained bis(oxiranes) upon treatment with sodium azide was investigated, and it was found that the reaction path is determined by the reciprocal orientation of oxygen atoms in the oxirane moieties. Diastereomers of the bis(oxiranes) with cis-orientation underwent independent ring opening, supplying corresponding diazidodiols, while in the case of stereoisomers with trans-orientation, domino-like reactions occurred, including intramolecular nucleophilic attack and the formation of a novel three- or six-membered O-containing ring. Summarily, a straightforward approach to polyfunctional compounds containing cyclooctane or oxabicyclo[3.3.1]nonane cores, employing bis(oxiranes), was elaborated.

Diastereoselective Synthesis of Bicyclo[3.3.0]octenones by Copper-Catalyzed Transannular Ring-Closing Reaction

A novel and efficient copper-catalyzed transannular ring-closing reaction of eight-membered rings has been developed that provides a straightforward way to synthesize bicyclo[3.3.0]octane derivatives in good yields. Mechanistic studies revealed that the reaction pathway might involve chlorination followed by the Kornblum reaction. Readily accessible starting materials and good functional group tolerance make this procedure attractive.

Transannular Approach to 2,3-Dihydropyrrolo[1,2-b]isoquinolin-5(1H)-ones through Brønsted Acid-Catalyzed Amidohalogenation

A transannular approach has been developed for the construction of pyrrolo[1,2-b]isoquinolinones starting from benzo-fused nine-membered enelactams. This process takes place in the presence of a halogenating agent and under Brønsted acid catalysis and proceeds via a transannular amidohalogenation, followed by elimination. The reaction has been found to be wide in scope, enabling the formation of a variety of tricyclic products in good overall yield, regardless of the substitution pattern in the initial lactam substrate. The reaction has also been applied to the total synthesis of a reported topoisomerase I inhibitor and to the formal synthesis of rosettacin. Further extension of this methodology allows the preparation of 10-iodopyrrolo[1,2-b]isoquinolinones by using an excess of halogenating agent and these compounds can be further manipulated through standard Suzuki coupling chemistry into a variety of 10-aryl-substituted pyrrolo[1,2-b]isoquinolinones.

Tandem Macrolactone Synthesis: Total Synthesis of (-)-Exiguolide by a Macrocyclization/Transannular Pyran Cyclization Strategy

Tetrahydropyran-containing macrolactones were synthesized by integrating Meyer-Schuster rearrangement, macrocyclic ring-closing metathesis, and transannular oxa-Michael addition under gold and ruthenium catalysis. Single-step access to a variety of 14- to 20-membered macrolactones containing a tetrahydropyran ring was possible from readily available linear precursors in good yields and with moderate to excellent diastereoselectivity. A 13-step synthesis of (-)-exiguolide, an anticancer marine macrolide, showcased the feasibility of our tandem reaction sequence for macrolactone synthesis and also demonstrated the power of transannular reactions for rapid assembly of the tetrahydropyran rings of the target natural product.

Transannular Enantioselective (3 + 2) Cycloaddition of Cycloalkenone Hydrazones under Brønsted Acid Catalysis

Hydrazones derived from cycloalkenones undergo an enantioselective transannular formal (3 + 2) cycloaddition catalyzed by a chiral phosphoric acid. The reaction provides high yields and excellent stereocontrol in the formation of complex adducts with one or two α-tertiary amine moieties at the ring fusion, and these can be converted into very versatile stereodefined decalin- or octahydro-1H-indene-derived 1,3-diamines through simple reductive N-N cleavage.

Stereoselective Synthesis of Hydrindane and Hydroazulene Derivatives by Transannular Cyclization of Nine- and Ten-Membered Carbocycles

Treatment of cis-fused bicyclic diene dicarboxylates with Li/naphthalene triggers a tandem ring-opening and transannular cyclization process that stereoselectively yields hydroazulenes and hydrindanes derivatives. Cyclononadienyl diesters, which can be isolated after the ring-opening step by judicious choice of the reaction conditions, undergo a tandem conjugate addition/intramolecular Michael addition upon treatment with chiral lithium amides to give bicyclic β-amino esters in a process where 4 contiguous stereocenters are formed with high diastereocontrol. A concise route toward the highly enantioenriched AEF ring core of the aconitine-type alkaloids has been developed as an application of this methodology. The starting cis-fused bicyclic dicarboxylates are easily prepared in one step by reductive alkylation of diisopropyl phthalate (Na/THF, followed by the appropriate bis-electrophiles).

Five Step Total Synthesis of Lythranidine

A concise synthesis of the alkaloid lythranidine is reported. The strategy exploits the target's local C₂ symmetry by adopting a two directional synthetic approach, first in an acyclic environment, then in a cyclic system and finally in a bridged macrocyclic domain. The latter phase of the synthesis, which installs all four stereocenters, involves a thermodynamically controlled, twofold intermolecular/transannular aza-Michael addition and a twofold hydride reduction. The synthesis is one third of the length of the most step-economic previous approach, providing access to gram quantities of the natural product. The broad-spectrum nature of the synthesis is demonstrated through the preparation of three diastereomeric analogues of the natural product.

Role of Macrocyclic Conformational Steering in a Kinetic Route toward Bielschowskysin

Macrocyclic furanobutenolide-derived cembranoids (FBCs) are the biosynthetic precursors to a wide variety of highly congested and oxygenated polycyclic (nor)diterpenes (e.g. plumarellide, verrillin, and bielschowskysin). These architecturally complex metabolites are thought to originate from site-selective oxidation of the macrocycle backbone and a series of intricate transannular reactions. Yet the development of a common biomimetic route has been hampered by a lack of synthetic methods for the pivotal furan dearomatization in a regio- and stereoselective manner. To address these shortcomings, a concise strategy of epoxidation followed by a kinetically controlled furan dearomatization is reported. The surprising switch of facial α:β-discrimination observed in the epoxidation of the most strained E-acerosolide versus E-deoxypukalide and E-bipinnatin J derived macrocycles has been rationalized by the variation of the 3D conformational landscape between macrocyclic scaffolds. A careful conformational analysis of these macrocycles by VT-NMR and NOESY experiments at low temperature was supported by DFT calculations to characterize these equilibrating macrocyclic conformers. The shift in conformational topology associated with a swing of the butenolide ring in E-deoxypukalide is in general agreement with the reversal of β-selectivity observed in the epoxidation. We also describe the downstream functionalization of FBC-macrocycles and how the C-7 epoxide configuration is retentively translated to the C-3 stereogenicity in dearomatized products under kinetic control to secure the requisite 3S,7S,8S configurations for the bielschowskysin synthesis. Unlike previously speculated, our results suggest that the most strained FBC-macrocycles bearing a E-(Δ^7,8)-alkene moiety may stand as the true biosynthetic precursors to bielschowskysin and several other polycyclic natural products of this class.

Synergistic Pd(0)/Rh(II) Dual Catalytic [6 + 3] Dipolar Cycloaddition for the Synthesis of Monocyclic Nine-Membered N,O-Heterocycles and Their Alder-ene Rearrangement to Fused Bicyclic Compounds

The catalytic construction of a monocyclic medium-sized N,O-heterocyclic ring represents a formidable challenge in organic synthesis. Herein we report the synergistic palladium(0)/rhodium(II) dual catalytic cycloaddition of vinylpropylene carbonates with N-sulfonyl-1,2,3-triazoles to afford monocyclic nine-membered N,O-heterocycles. The catalytically generated 1,6-dipole-equivalent zwitterionic π-allyl palladium(II) complex and the 1,3-dipole-equivalent α-imino rhodium(II) carbenoid intermediate react with each other in a formal [6 + 3] dipolar cycloaddition to furnish nine-membered oxazonines, which can be transformed into cis-fused [4.3.0] bicyclic compounds via a transannular Alder-ene rearrangement. The tandem one-pot cycloaddition/Alder-ene rearrangement sequence is also possible.

Lessons from Natural Product Total Synthesis: Macrocyclization and Postcyclization Strategies

Macrocyclic natural products are plentiful in the bacteria, archaea, and eukaryote domains of life. For the significant advantages that they provide to the producing organisms, evolution has learned how to implement various types of macrocyclization reactions into the different biosynthetic pathways and how to effect them with remarkable ease. Mankind greatly benefits from nature’s pool, not least because naturally occurring macrocycles or derivatives thereof serve as important drugs for the treatment of many serious ailments.

In stark contrast, macrocyclization reactions are usually perceived as difficult to accomplish by purely chemical means. While it is true that ring closure necessarily entails an entropic loss and may result in the buildup of (considerable) ring strain that must be compensated for in one way or the other, it is also fair to note tremendous methodological advances during the last decades that greatly alleviated this traditional “macrocycle challenge”. It is therefore increasingly possible to explore the advantages provided by large as well as medium-size ring systems in a more systematic manner. This venture also holds the promise of increasing the “chemical space” amenable to drug development to a considerable extent.

In consideration of this and other important long-term perspectives, it is appropriate to revisit the current state of the art. To this end, a number of vignettes are presented, each of which summarizes a total synthesis project targeting macrocyclic natural products of greatly different chemotypes using a variety of transformations to reach these goals. Although we were occasionally facing “dead ends”, which are also delineated for the sake of a complete picture, these case studies illustrate the notion that the formation of a certain macrocyclic perimeter is (usually) no longer seriously limiting. In addition to substantial progress in the “classical” repertoire (macrolactonization and macrolactamization (pateamine A, spirastrellolide, and belizentrin)), various metal-catalyzed reactions have arguably led to the greatest leaps forward. Among them, palladium-catalyzed C–C bond formation (roseophilin and nominal xestocyclamine A) and, in particular, alkene and alkyne metathesis stand out (iejimalide, spirastrellolide, enigmazole, ingenamine, and sinulariadiolide). In some cases, different methods were pursued in parallel, thus allowing for a critical assessment and comparison.

To the extent that the macrocyclic challenge is vanishing, the opportunity arises to focus attention on the postmacrocyclization phase. One may stipulate that a well-designed cyclization precursor does not only ensure efficient ring closure but also fosters and streamlines the steps that come after the event. One way to do so is dual (multiple) use in that the functional groups serving the actual cyclization reaction also find productive applications downstream from it rather than being subject to simple defunctionalization. In this context, better insight into the conformational peculiarities of large rings and the growing confidence in their accessibility in a stereochemically well defined format rejuvenate the implementation of transannular reactions or reaction cascades that can lead to rapid and substantial increases in molecular complexity. The examples summarized herein showcase such possibilities, with special emphasis on tranannular gold catalysis and the emerging ruthenium-catalyzed trans-hydrometalation chemistry for the selective functionalization of alkynes.

The Formosalides: Structure Determination by Total Synthesis

Total synthesis allowed the constitution of the cytotoxic marine macrolides of the formosalide family to be confirmed and their previously unknown stereostructure to be assigned with confidence. The underlying blueprint was inherently modular to ensure that each conceivable isomer could be reached. This flexibility derived from the use of strictly catalyst controlled transformations to set the stereocenters, except for the anomeric position, which is under thermodynamic control; as an extra safety measure, all stereogenic centers were set prior to ring closure to preclude any interference of the conformation adopted by the macrolactone rings of the different diastereomers. Late-stage macrocyclization by ring-closing alkyne metathesis was followed by a platinum-catalyzed transannular 6-exo-dig hydroalkoxylation/ketalization to craft the polycyclic frame. The side chain featuring a very labile unsaturation pattern was finally attached to the core by Stille coupling.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Regiodivergent construction of medium-sized heterocycles from vinylethylene carbonates and allylidenemalononitriles

Medium-sized heterocycles exist in a broad spectrum of biologically active natural products and medicinally important synthetic compounds. The construction of medium-sized rings remains challenging, particularly the assembly of different ring sizes from the same type of substrate. Here we report palladium-catalyzed, regiodivergent [5 + 4] and [5 + 2] annulations of vinylethylene carbonates and allylidenemalononitriles. We describe the production of over 50 examples of nine- and seven-membered heterocycles in high isolated yields and excellent regioselectivities. We demonstrate the synthetic utility of this approach by converting a nine-membered ring product to an interesting polycyclic caged molecule via a [2 + 2] transannulation. Mechanistic studies suggest that the [5 + 2] annulation proceeds through palladium-catalyzed ring-opening/re-cyclization from the [5 + 4] adducts.

Here we report palladium-catalyzed, regiodivergent [5 + 4] and [5 + 2] annulations of vinylethylene carbonates and allylidenemalononitriles affording over 50 medium-sized heterocycles in high isolated yields with excellent regioselectivities.

Catalytic Stereoselective Borylative Transannular Reactions

Medium-sized carbocycles containing an α,β-unsaturated ketone moiety as Michael acceptor site and a ketone moiety as internal electrophilic site are ideal substrates to conduct Cu(I)-catalyzed conjugated borylation followed by electrophilic intramolecular trapping that results into a pioneer transannular borylative ring closing reaction. The relative configuration of three adjacent stereocenters is controlled, giving access to a single diastereoisomer for a wide range of substrates tested. Moreover, when a chiral ligand is incorporated, the reaction provides enantioenriched polycyclic products with up to 99 % ee.

Catalytic enantioselective domino Michael/transannular aldol reaction under bifunctional catalysis

Chiral bifunctional tertiary amine/squaramides catalyze the enantioselective Michael/transannular aldol reaction on medium-sized cyclic ketoenones leading to bicycle[5.4.0]undecanes.

A transannular approach toward lycopodine synthesis

A transannular reaction was proposed to access the Lycopodium alkaloid lycopodine. A key bicyclic precursor was synthesized via a ring-closing metathesis reaction. Initial evaluations of the transannular aza-Prins reaction to synthesize lycopodine were reported and discussed.

Cycloaddition Reactions of Cobalt-Complexed Macrocyclic Alkynes: The Transannular Pauson-Khand Reaction

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentenones through the efficient [2 + 2 + 1] cycloaddition of dicobalt alkyne complexes with alkenes. While intermolecular and intramolecular variants are widely known, transannular versions of this reaction are unknown and the basis of this study. Macrocyclic enyne and dienyne complexes were readily synthesized by palladium(II)-catalyzed oxidative macrocyclizations of bis(vinyl boronate esters) or ring-closing metathesis reactions followed by complexation with dicobalt octacarbonyl. Several reaction modalities of these macrocyclic complexes were uncovered. In addition to the first successful transannular Pauson-Khand reactions, other intermolecular and transannular cycloaddition reactions included intermolecular Pauson-Khand reactions, transannular [4 + 2] cycloaddition reactions, intermolecular [2 + 2 + 2] cycloaddition reactions, and intermolecular [2 + 2 + 1 + 1] cycloaddition reactions. The structural and reaction requirements for each process are presented.

Exploring sesquiterpene alkaloid-like scaffolds via Beckmann-transannular remodelling of beta-caryophyllene

Cascaded Beckmann-transannular protocol transformed macrocyclic beta-caryophyllene into poly-heterocyclic unnatural skeletal types.

Remote Stereoinductive Intramolecular Nitrile Oxide Cycloaddition: Asymmetric Total Synthesis and Structure Revision of (-)-11β-Hydroxycurvularin

The first total synthesis and structure revision of (-)-11β-hydroxycurvularin (1b), a macrolide possessing a β-hydroxyketone moiety, were accomplished. The β-hydroxyketone moiety in this natural product was introduced by cleavage of the N-O bond in an isoxazoline ring that was formed diastereoselectively in a 1,5-remote stereocontrolled fashion by employing intramolecular nitrile oxide cycloaddition.

Selective Halogen-Lithium Exchange of 1,2-Dihaloarenes for Successive [2+4] Cycloadditions of Arynes and Isobenzofurans

Successive [2+4] cycloadditions of arynes and isobenzofurans by site-selective halogen-lithium exchange of 1,2-dihaloarenes were developed, allowing the rapid construction of polycyclic compounds which serve as a useful synthetic intermediates for the preparation of various polyacene derivatives.