Recent applications of intramolecular Diels-Alder reactions to natural product synthesis

Simulation of a Diels-Alder reaction on a quantum computer

The simulation of chemical reactions is an anticipated application of quantum computers. Using a Diels-Alder reaction as a test case, in this study we explore the potential applications of quantum algorithms and hardware in investigating chemical reactions. Our specific goal is to calculate the activation barrier of a reaction between ethylene and cyclopentadiene forming a transition state. To achieve this goal, we use quantum algorithms for near-term quantum hardware (entanglement forging and quantum subspace expansion) and classical post-processing (many-body perturbation theory) in concert. We conduct simulations on IBM quantum hardware using up to 8 qubits, and compute accurate activation barrier in the reaction between cyclopentadiene and ethylene by accounting for both static and dynamic electronic correlation. This work illustrates a hybrid quantum-classical computational workflow to study chemical reactions on near-term quantum devices, showcasing the potential for performing quantum chemistry simulations on quantum hardware to predict activation barriers in agreement with those predicted by CASCI.

Late-Stage Glycosylation of Peptides by Methionine-Directed β-C(sp3)-H Functionalization with 1-Iodoglycals

Using l-methionine (Met) as the endogenous directing group, we developed Pd-catalyzed β-C(sp3)-H glycosylation of peptides with 1-iodoglycals. A wide range of tri- to hexapeptides containing the Ala-Met motifs underwent Ala C-H glycosylation under the standard conditions to give the glycopeptides smoothly. 15 proteinogenic amino acids (with easily removable protecting groups) were well tolerated. Control experiments indicated that Met acted as a N,S-bidentate directing group and exhibited an effect superior to other amino acid residues such as l-aspartic acid (Asp), l-asparagine (Asn), and S-protected l-cysteine (Cys). In addition, further transformation by HFIP-promoted 1,4-elimination furnished another type of glycopeptide with the 1,3-diene motif, which provides a handle for further derivatization.

Catalytic asymmetric construction of bridged bicyclo[m.3.1] rings using an intramolecular Diels-Alder reaction

Herein, we presented an enantioselective intramolecular Diels-Alder (IMDA) reaction with vinyl branched vinylidene ortho-quinone methide (VQM). The control of site selectivity in the IMDA reaction led to both chiral bridged bicyclo[4.3.1] and [5.3.1] architectures with high isolated yields (up to 85%) and excellent enantioselectivities (up to 97% ee).

Sorbremnoids A and B: NLRP3 Inflammasome Inhibitors Discovered from Spatially Restricted Crosstalk of Biosynthetic Pathways

Crosstalk-oriented chemical evolution of natural products (NPs) is an efficacious strategy for generating novel skeletons through coupling reactions between NP fragments. In this study, two NOD-like receptor protein 3 (NLRP3) inflammasome inhibitors, sorbremnoids A and B (1 and 2), with unprecedented chemical architectures were identified from a fungus Penicillium citrinum. Compounds 1 and 2 exemplify rare instances of hybrid NPs formed via a major facilitator superfamily (MFS)-like enzyme by coupling reactive intermediates from two separate biosynthetic gene clusters (BGCs), pcisor and pci56. Both sorbremnoids A and B are NLRP3 inflammasome inhibitors. Sorbremnoid A demonstrated strong inhibition of IL-1β by directly binding to the NLRP3 protein, inhibiting the assembly and activation of the NLRP3 inflammasome in vitro, with potential application in diabetic refractory wound healing through the suppression of excessive inflammatory responses. This research will inspire the development of anti-NLRP3 inflammasome agents as lead treatments and enhance knowledge pertaining to NPs derived from biosynthetic crosstalk.

Progress in Lewis-Acid-Templated Diels-Alder Reactions

The synthesis of natural products with complicated architectures often requires the use of segments with functional groups that can be structurally transformed with the desired stereogenic centers. Bicyclic 𝛾-lactones have great potential as a suitable segment for natural product synthesis. However, the stereoselective construction of such functionalized bicyclic 𝛾-lactones is not as straightforward as one might expect. The template-mediated Diels-Alder reaction is one of the most powerful and versatile methods for providing bicyclic 𝛾-lactones with high regioselectivity and stereoselectivity. In this reaction, the diene is linked to the dienophile by a temporary tether, allowing the reaction to proceed efficiently, yielding a product that can be used for natural product synthesis. This review describes some important instances of the template-mediated Diels-Alder reaction and its application to the synthesis of biologically active compounds.

Enantioselective Total Synthesis of (-)-Vinigrol: The Evolution of a Transannular Diels-Alder Strategy

Vinigrol is a structurally and stereochemically complex diterpenoid that displays various potent pharmacological activities. Two generations of synthetic routes were designed and pursued based on a transannular Diels-Alder (TADA) cycloaddition strategy. An intramolecular [2 + 2]photocycloaddition in the presence of the chelating Lewis acid (MgBr2·Et2O) was first discovered to enable the reaction of sterically challenging substrates, which was followed by [2 + 2]cycloreversion to provide α-pyrones fused with a 10-membered ring. Eventually, a new and scalable synthetic route toward (-)-vinigrol was developed and provided over 600 mg materials, manifesting the power of macrocyclic stereocontrol and TADA reaction.

Palladium-Catalyzed Aminoalkylative Cyclization Enables Modular Synthesis of Exocyclic 1,3-Dienes

A novel and efficient palladium-catalyzed regioselective and stereodivergent ring-closing reaction of aminoenynes with aldehydes and boronic acids or hydrosilane is developed. This three-component reaction allows for the modular synthesis of a series of exocyclic 1,3-dienes bearing 5- to 8-membered saturated N-heterocycles. The reactions utilize a simple Pd-catalyst and work with broad range of aminoenynes, aldehydes and organometallic reagents under mild reaction conditions. The products represent useful intermediates for chemical synthesis due to the versatility of the conjugated diene. Preliminary mechanistic details of the method are also revealed.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

Photochemical Isomerization of Cyclohept-1-ene-1-carbaldehyde: Strain-Release Cycloadditions and Ene Reactions

Cyclohept-1-ene-1-carbaldehyde undergoes photoinduced E → Z isomerization at λ = 350 nm. The ring strain facilitates Diels-Alder cycloaddiions with 1,3-dienes, [3 + 2] cycloadditions with 1,3-dipoles, and ene reactions with olefins. Products are trans-fused at the cycloheptane core and were obtained in yields of up to 82%. Single crystal X-ray analyses corroborated the constitution and relative configuration of key products. With BF3 as a Lewis acid and 2,3-dimethylbuta-1,3-diene, cyclohept-1-ene-1-carbaldehyde reacted in the dark and rearranged stereoselectively to a tricyclic ketone (87%).

Construction of a Bicyclo[2.2.2]octene Skeleton via a Visible-Light-Mediated Radical Cascade Reaction of Amino Acid Derivatives with N-(2-Phenyl)benzoyl Groups

Bridged polycyclic ring systems constitute the core structures of numerous natural products and biologically active molecules. We found that simple biphenyl substrates derived from amino acids participate in a radical cascade reaction under visible light irradiation in the presence of [Ir{dF(CF₃)ppy}₂(dtbpy)]PF₆ to enable the direct construction of bicyclo[2.2.2]octene structures. Isotopic labeling experiments suggested that intramolecular hydrogen atom transfer is involved in the cascade processes.

In(OTf)3-catalyzed reorganization/cycloaddition of two imine units and subsequent modular assembly of acridinium photocatalysts

Herein, we disclose a novel reorganization/cycloaddition between two imine units catalyzed by In(OTf)3 Lewis acid that differs from the well-known [4 + 2] cycloaddition version via the Povarov reaction. By means of this unprecedented imine chemistry, a collection of synthetically useful dihydroacridines has been synthesized. Notably, the obtained products give rise to a series of structurally novel and fine-tuneable acridinium photocatalysts, offering a heuristic paradigm for synthesis and efficiently facilitating several encouraging dihydrogen coupling reactions.

Enantioselective Total Synthesis of (+)-Pedrolide

The first total synthesis of (+)-pedrolide, a tigliane-derived diterpenoid featuring an unprecedented 5-5-6-6-3 carbon skeleton, is reported. Key to the approach is the construction of the bicyclo[2.2.1]heptane core via an intramolecular cyclopentadiene-Diels-Alder cycloaddition. To this end, a norbornadiene serves as an effective surrogate for cyclopentadiene, which is unmasked under mild conditions involving a complex Diels-Alder reaction cascade. In addition, the synthesis provides a novel approach to a densely functionalized carane in an efficient and enantioselective manner.

Ring forming transformations of ynamides via cycloaddition

Ynamides are N-alkyne compounds bearing an electron withdrawing group at the nitrogen atom. They offer unique pathways for the construction of versatile building blocks owing to their exceptional balance between reactivity and stability. Recently several studies have been reported that explore and illustrate the synthetic potential of ynamides and ynamide-derived advanced intermediates in cycloadditions with different reaction partners to yield heterocyclic cycloadducts of synthetic and pharmaceutical value. Cycloaddition reactions of ynamides are the facile and preferable routes for the construction of structural motifs having striking importance in synthetic, medicinal chemistry, and advanced materials. In this systematic review, we highlighted the recently reported novel transformations and synthetic applications that involved the cycloaddition reaction of ynamides. The scope along with the limitations of the transformations are discussed in detail.

Gram-Scale Enantioselective Synthesis of (+)-Lucidumone

The first enantioselective total synthesis of (+)-lucidumone is described through a 13-step synthetic pathway (longest linear sequence). The key steps involve the formation of a bridged bicyclic lactone by an enantioselective inverse-electron-demand Diels-Alder cycloaddition, C-O bond formation to assemble two fragments, and a one-pot retro-[4 + 2]/[4 + 2] cycloaddition cascade. The synthesis is scalable, and more than one gram of natural product was synthesized in one batch.

Nonenzymatic Reactions in Natural Product Formation

Biosynthetic mechanisms of natural products primarily depend on systems of protein catalysts. However, within the field of biosynthesis, there are cases in which the inherent chemical reactivity of metabolic intermediates and substrates evades the involvement of enzymes. These reactions are difficult to characterize based on their reactivity and occlusion within the milieu of the cellular environment. As we continue to build a strong foundation for how microbes and higher organisms produce natural products, therein lies a need for understanding how protein independent or nonenzymatic biosynthetic steps can occur. We have classified such reactions into four categories: intramolecular, multicomponent, tailoring, and light-induced reactions. Intramolecular reactions is one of the most well studied in the context of biomimetic synthesis, consisting of cyclizations and cycloadditions due to the innate reactivity of the intermediates. There are two subclasses that make up multicomponent reactions, one being homologous multicomponent reactions which results in dimeric and pseudodimeric natural products, and the other being heterologous multicomponent reactions, where two or more precursors from independent biosynthetic pathways undergo a variety of reactions to produce the mature natural product. The third type of reaction discussed are tailoring reactions, where postmodifications occur on the natural products after the biosynthetic machinery is completed. The last category consists of light-induced reactions involving ecologically relevant UV light rather than high intensity UV irradiation that is traditionally used in synthetic chemistry. This review will cover recent nonenzymatic biosynthetic mechanisms and include sources for those reviewed previously.

Alcohol-Incorporating Diels-Alder Dimerization of In Situ Formed ortho-Quinamine via Co-Nitrenoid Insertion

We disclose herein a Cp*Co(III)(LX)-catalyzed dearomative Diels-Alder dimerization of 2,6-disubstituted phenyl azidoformates. Upon the postulated cobalt-nitrenoid insertion into the neighboring ortho-carbon, the key intermediate of ortho-quinamine was generated for the subsequent dimeric cycloaddition process. A series of experimental and computational studies suggested that the quinolinol ligand of the cobalt catalyst plays a crucial role in the alcoholic solvent incorporation into the o-quinamine moiety, thereby enabling the Diels-Alder dimerization to furnish the bridged tricyclic bisamidation products.

How Ionization Catalyzes Diels-Alder Reactions

The catalytic effect of ionization on the Diels-Alder reaction between 1,3-butadiene and acrylaldehyde has been studied using relativistic density functional theory (DFT). Removal of an electron from the dienophile, acrylaldehyde, significantly accelerates the Diels-Alder reaction and shifts the reaction mechanism from concerted asynchronous for the neutral Diels-Alder reaction to stepwise for the radical-cation Diels-Alder reaction. Our detailed activation strain and Kohn-Sham molecular orbital analyses reveal how ionization of the dienophile enhances the Diels-Alder reactivity via two mechanisms: (i) by amplifying the asymmetry in the dienophile's occupied π-orbitals to such an extent that the reaction goes from concerted asynchronous to stepwise and thus with substantially less steric (Pauli) repulsion per reaction step; (ii) by enhancing the stabilizing orbital interactions that result from the ability of the singly occupied molecular orbital of the radical-cation dienophile to engage in an additional three-electron bonding interaction with the highest occupied molecular orbital of the diene.

Origin of Catalysis and Selectivity in Lewis Acid-Promoted Diels-Alder Reactions Involving Vinylazaarenes as Dienophiles

The poorly understood factors controlling the catalysis and selectivity in Lewis acid-promoted Diels-Alder cycloaddition reactions involving vinylazaarenes as dienophiles have been quantitatively explored in detail by means of computational methods. With the help of the activation strain model and the energy decomposition analysis methods, it is found that the remarkable acceleration induced by the catalysis is mainly due to a significant reduction of the Pauli repulsion between the key occupied π-molecular orbitals of the reactants and not due to the proposed stabilization of the lowest unoccupied molecular orbital (LUMO) of the dienophile. This computational approach has also been helpful to understand the reasons behind the extraordinary regio- and diastereoselectivity observed experimentally. The insight gained in this work allows us to predict even more reactive vinylazaarene dienophiles, which may be useful in organic synthesis.

Fe-BPsalan Complex-Catalyzed Asymmetric [4 + 2] Cycloaddition of Cyclopentadiene with α,β-Unsaturated Heterocycles

An efficient iron-catalyzed asymmetric [4 + 2] cycloaddition of cyclopentadiene with α,β-unsaturated acyl imidazoles or 2-cinnamoylisoindoline-1,3-dione derivatives was developed to afford the addition products in high yield and selectivity. Interestingly, the absolute structures of the addition products were controlled by the auxiliaries via different coordination modes with the same type of catalyst.

Recent advances in cascade reactions and their mechanistic insights: a concise strategy to synthesize complex natural products and organic scaffolds

The beauty of cascade reactions to bestow us with cumbersome organic scaffolds has made them a cutting-edge area of research. Although the planning of cascades may require intuition, their results can be highly impactful. The development of cascades to provide specific targeted molecules of an appropriate structural and stereochemical framework poses a significant challenge but can serve as one of the most impressive tools in organic synthesis. This review shares a broad interest in compiling cascade transformations towards the construction of polycyclic frameworks, induction of chirality/asymmetry in the protocol, etc. to solve diverse challenges in organic synthesis pursuits, as cascades enable the rapid and efficient construction of complex architectures from simple molecules. The studies highlighted herein manifest the utilization of a range of cascade reactions under various classifications for generating natural product skeletons such as palau'amine, benzosimuline, arcutinine, and others from simple building blocks, with emphasis on breakthroughs and potential for asymmetric synthesis. The exquisite synthetic designs of recently completed total synthesis of natural products with a focus on strategic concerns are also highlighted in this review.

A mild tetradehydro-Diels-Alder reaction of aryldiyne compounds affords exclusively linear products

The thermal tetradehydro-Diels-Alder (TDDA) reaction for the synthesis of polysubstituted aromatic compounds remains underestimated probably due to the harsh conditions and multiproduct results. Herein, a mild intramolecular TDDA reaction of aryldiyne compounds is presented with linear naphthalenes only, exhibiting good functional group tolerance. The reaction is easy to operate and amenable to multigram-scale synthesis. From the preliminary work, it was found that the mild conditions may be the key to the completely linear product in the reactions.

Asymmetric Total Synthesis of Antibiotic Elansolid A

Elansolid A is a structurally complex polyketide macrolactone natural product that exhibits promising antibacterial properties. Its challenging asymmetric total synthesis was achieved by a convergent strategy, in which the tetrahydroindane core of the molecule and an eastern vinyl iodide moiety were combined as the main fragments. The central tetrahydroindane motif was constructed with high stereoselectivity by a bioinspired intramolecular Diels-Alder cycloaddition, generating four stereogenic centers in a single step. The stereocontrol of this key step could be achieved by virtue of a 1,3-allylic strain generated by the temporary introduction of a steric-directing iodine substituent on the substrate. The formation of the macrolactone motif that completes the synthesis was achieved via two different retrosynthetic disconnections, namely, a Suzuki-Miyaura cross-coupling or an alternative Mukaiyama esterification reaction.

BF3–Catalyzed Diels–Alder Reaction between Butadiene and Methyl Acrylate in Aqueous Solution—An URVA and Local Vibrational Mode Study

In this study we investigate the Diels–Alder reaction between methyl acrylate and butadiene, which is catalyzed by BF3 Lewis acid in explicit water solution, using URVA and Local Mode Analysis as major tools complemented with NBO, electron density and ring puckering analyses. We considered four different starting orientations of methyl acrylate and butadiene, which led to 16 DA reactions in total. In order to isolate the catalytic effects of the BF3 catalyst and those of the water environment and exploring how these effects are synchronized, we systematically compared the non-catalyzed reaction in gas phase and aqueous solution with the catalyzed reaction in gas phase and aqueous solution. Gas phase studies were performed at the B3LYP/6-311+G(2d,p) level of theory and studies in aqueous solution were performed utilizing a QM/MM approach at the B3LYP/6-311+G(2d,p)/AMBER level of theory. The URVA results revealed reaction path curvature profiles with an overall similar pattern for all 16 reactions showing the same sequence of CC single bond formation for all of them. In contrast to the parent DA reaction with symmetric substrates causing a synchronous bond formation process, here, first the new CC single bond on the CH2 side of methyl acrylate is formed followed by the CC bond at the ester side. As for the parent DA reaction, both bond formation events occur after the TS, i.e., they do not contribute to the energy barrier. What determines the barrier is the preparation process for CC bond formation, including the approach diene and dienophile, CC bond length changes and, in particular, rehybridization of the carbon atoms involved in the formation of the cyclohexene ring. This process is modified by both the BF3 catalyst and the water environment, where both work in a hand-in-hand fashion leading to the lowest energy barrier of 9.06 kcal/mol found for the catalyzed reaction R1 in aqueous solution compared to the highest energy barrier of 20.68 kcal/mol found for the non-catalyzed reaction R1 in the gas phase. The major effect of the BF3 catalyst is the increased mutual polarization and the increased charge transfer between methyl acrylate and butadiene, facilitating the approach of diene and dienophile and the pyramidalization of the CC atoms involved in the ring formation, which leads to a lowering of the activation energy. The catalytic effect of water solution is threefold. The polar environment leads also to increased polarization and charge transfer between the reacting species, similar as in the case of the BF3 catalyst, although to a smaller extend. More important is the formation of hydrogen bonds with the reaction complex, which are stronger for the TS than for the reactant, thus stabilizing the TS which leads to a further reduction of the activation energy. As shown by the ring puckering analysis, the third effect of water is space confinement of the reacting partners, conserving the boat form of the six-member ring from the entrance to the exit reaction channel. In summary, URVA combined with LMA has led to a clearer picture on how both BF3 catalyst and aqueous environment in a synchronized effort lower the reaction barrier. These new insights will serve to further fine-tune the DA reaction of methyl acrylate and butadiene and DA reactions in general.

Intramolecular Diels-Alder Cycloaddition of Furan-Derived β-Enamino Diketones: An Entry to Diastereoselective Synthesis of Polycyclic Pyrano[3,2-c]quinolin-5-one Derivatives

A series of 1,3-cyclodiketone- and tetrahydroepoxyisoindole-fused β-enamino dicarbonyl heterocycles were synthesized via a 1,4-diazabicyclo[2.2.2]octane-catalyzed, CH₃NO₂-mediated three-component reaction of 1,3-cyclodiketone, furfural, and allylamine in toluene. The target compounds were generated via the formation of β-enamino diketone as a key intermediate, followed by intramolecular Diels-Alder cycloaddition. The prepared molecules bearing a quinoline-2,4-dione moiety could be further brominated with N-bromosuccinimide and diastereoselectively reduced by NaBH₄ to afford pyrano[3,2-c]quinolin-5-one-derived heterocycles with six vicinal stereogenic centers.

Inverse Electron-Demand Aza-Diels-Alder Reaction of α,β-Unsaturated Thioesters with In Situ-Generated 1,2-Diaza-1,3-dienes for the Synthesis of 1,3,4-Thiadiazines

A highly regioselective inverse electron-demand aza-Diels-Alder reaction of α,β-unsaturated thioesters with 1,2-diaza-1,3-dienes generated in situ from α-halogeno hydrazones was developed. With α,β-unsaturated thioesters as C═S dienophiles, the developed protocol enables the formation of diverse 3,6-dihydro-2H-1,3,4-thiadiazine derivatives in excellent yields. In the presence of lithium aluminum hydride, 3,6-dihydro-2H-1,3,4-thiadiazine derivatives could be further transformed into 5,6-dihydro-4H-1,3,4-thiadiazines in good yields.

Me2(CH2CH)SiCN: a bifunctional ethylene equivalent for Diels–Alder reaction based controllable tandem synthesis

A bifunctional silyl reagent Me₂(CH₂CH)SiCN has been developed as a novel ethylene equivalent for the Diels–Alder (DA) reaction. The use of this reagent enables the controllable synthesis of value-added cyclohexenyl ketones or 2-acyl cyclohexancarbonitrile derivatives through a five- or six-step tandem sequence based on a Wittig/cyanosilylation/DA reaction/retro-cyanosilylation/isomerization sequence that involves a temporary silicon-tethered intramolecular DA reaction.

We report an unprecedented tandem Wittig/cyanosilylation/DA reaction/retro-cyanosilylation/isomerization sequence by using our designed bifunctional ethylene equivalent Me₂(CH₂CH)SiCN.

Hyperbaric reactions in organic synthesis. Progress from 2006 to 2020

This comprehensive review summarizes the published literature data concerning above 1 kbar reactions for the purposes of preparative organic synthesis (more then 50 mg of the initial substance) from 2006...

Synthesis of a Bicyclo[2.2.1]heptane Skeleton with Two Oxy-Functionalized Bridgehead Carbons via the Diels-Alder Reaction

We describe a synthetic method for a bicyclo[2.2.1]heptane skeleton with two oxy-functionalized bridgehead carbons. This method involves an intermolecular Diels-Alder reaction using 5,5-disubstituted 1,4-bis(silyloxy)-1,3-cyclopentadienes, the diene structure of which has never been synthesized. Furthermore, the intramolecular Diels-Alder reaction using a diene bearing a dienophile moiety at the C-5 position can provide a tricyclic carbon framework that includes the bicyclo[2.2.1]heptane skeleton. The novel bicyclo[2.2.1]heptane derivatives could be utilized as versatile building blocks for organic synthetic chemistry.

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.

Functional Porphyrinic Metal-Organic Framework as a New Class of Heterogeneous Halogen-Bond-Donor Catalyst

Biomimetic metal-organic frameworks have attracted great attention as they can be used as bio-inspired models, allowing us to gain important insights into how large biological molecules function as catalysts. In this work, we report the synthesis and utilization of such a metal-metalloporphyrin framework (MMPF) that is constructed from a custom-designed ligand as an efficient halogen bond donor catalyst for Diels-Alder reactions under ambient conditions. The implementation of fabricated halogen bonding capsule as binding pocket with high-density C-Br bonds enabled the use of halogen bonding to facilitate organic transformations in their three-dimensional cavities. Through combined experimental and computational studies, we showed that the substrate molecules diffuse through the pores of the MMPF, establishing a host-guest system via the C-Br⋅⋅⋅π interaction. The formation of halogen bonds is a plausible explanation for the observed boosted catalytic efficiency in Diels-Alder reactions. Moreover, the unique capability of MMPF highlights new opportunities in using artificial non-covalent binding pockets as highly tunable and selective catalytic materials.

Crystal structure and Hirshfeld surface analysis of (3aSR,6RS,6aSR,7RS,11bSR,11cRS)-2,2-dibenzyl-2,3,6a,11c-tetra-hydro-1H,6H,7H-3a,6:7,11b-di-epoxy-dibenzo[de,h]isoquinolin-2-ium tri-fluoro-methane-sulfonate

In the cation of the title salt, C30H28NO2 +·CF3O3S-, the four tetra-hydro-furan rings adopt envelope conformations. In the crystal, pairs of cations are linked by dimeric C-H⋯O hydrogen bonds, forming two R 2 2(6) ring motifs parallel to the (001) plane. The cations and anions are connected by further C-H⋯O hydrogen bonds, forming a three-dimensional network structure. Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H⋯H (47.6%), C⋯H/H⋯C (20.6%), O⋯H/H⋯O (18.0%) and F⋯H/H⋯F (9.9%) inter-actions.

On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects

Even at the end of the twentieth century, the view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism was widely accepted as the only possible one, regardless of the nature of the reaction components. Much has changed in the way these reactions are perceived since then. In particular, multi-step mechanisms with zwitterionic or diradical intermediates have been proposed for a number of processes. This review provided a critical analysis of such cases.

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).

Origin of asynchronicity in Diels-Alder reactions

Asynchronicity in Diels-Alder reactions plays a crucial role in determining the height of the reaction barrier. Currently, the origin of asynchronicity is ascribed to the stronger orbital interaction between the diene and the terminal carbon of an asymmetric dienophile, which shortens the corresponding newly formed C-C bond and hence induces asynchronicity in the reaction. Here, we show, using the activation strain model and Kohn-Sham molecular orbital theory at ZORA-BP86/TZ2P, that this rationale behind asynchronicity is incorrect. We, in fact, found that following a more asynchronous reaction mode costs favorable HOMO-LUMO orbital overlap and, therefore, weakens (not strengthens) these orbital interactions. Instead, it is the Pauli repulsion that induces asynchronicity in Diels-Alder reactions. An asynchronous reaction pathway also lowers repulsive occupied-occupied orbital overlap which, therefore, reduces the unfavorable Pauli repulsion. As soon as this mechanism of reducing Pauli repulsion dominates, the reaction begins to deviate from synchronicity and adopts an asynchronous mode. The eventual degree of asynchronicity, as observed in the transition state of a Diels-Alder reaction, is ultimately achieved when the gain in stability, as a response to the reduced Pauli repulsion, balances with the loss of favorable orbital interactions.

Facile generation of bridged medium-sized polycyclic systems by rhodium-catalysed intramolecular (3+2) dipolar cycloadditions

Bridged medium-sized bicyclo[m.n.2] ring systems are common in natural products and potent pharmaceuticals, and pose a great synthetic challenge. Chemistry for making bicyclo[m.n.2] ring systems remains underdeveloped. Currently, there are no general reactions available for the single-step synthesis of various bridged bicyclo[m.n.2] ring systems from acyclic precursors. Here, we report an unusual type II intramolecular (3+2) dipolar cycloaddition strategy for the syntheses of various bridged bicyclo[m.n.2] ring systems. This rhodium-catalysed cascade reaction provides a relatively general strategy for the direct and efficient regioselective and diastereoselective synthesis of highly functionalized and synthetically challenging bridged medium-sized polycyclic systems. Asymmetric total synthesis of nakafuran-8 was accomplished using this method as a key step. Quantum mechanical calculations demonstrate the mechanism of this transformation and the origins of its multiple selectivities. This reaction will inspire the design of the strategies to make complex bioactive molecules with bridged medium-sized polycyclic systems.

The bridged medium-sized ring bicyclo[m.n.2] family of natural products are commonly found but difficult to synthesize efficiently. Here the authors present a cascade reaction to form the carbon skeleton, via a [3+2] cycloaddition of a captured azavinyl carbene intermediate.

A Direct Organocatalytic Enantioselective Route to Functionalized trans-Diels-Alder Products Having the Norcarane Scaffold

An enantioselective methodology to construct trans-Diels-Alder scaffolds by organocatalysis with excellent selectivity, high yield and up to five contiguous stereocenters is presented. The reaction concept integrates the halogen effect and a novel discovered pseudo-halogen effect to direct an endo-selective, secondary-amine catalyzed Diels-Alder reaction allowing for the subsequent formation of trans-Diels-Alder cycloadducts featuring the norcarene scaffold. The methodology relies on the reaction between an in situ generated trienamine and an α-brominated or α-pseudo-halogenated enone to form a fleeting cis-Diels-Alder intermediate. The endo-transition state-enhanced by the (pseudo-)halogen effect-sets the stereochemistry that allows for a subsequent S_N 2-like reaction at a tertiary center to obtain the trans-Diels-Alder scaffold. The mechanism was investigated and supported by experimental results as well as computational studies.