Synthetic applications of type II intramolecular cycloadditions

Divergent Synthesis of Sulfur-Containing Bridged Cyclobutanes by Lewis Acid Catalyzed Formal Cycloadditions of Pyridinium 1,4-Zwitterionic Thiolates and Bicyclobutanes

Bridged cyclobutanes and sulfur heterocycles are currently under intense investigation as building blocks for pharmaceutical drug design. Two formal cycloaddition modes involving bicyclobutanes (BCBs) and pyridinium 1,4-zwitterionic thiolate derivatives were described to rapidly expand the chemical space of sulfur-containing bridged cyclobutanes. By using Ni(ClO4)2 as the catalyst, an uncommon higher-order (5+3) cycloaddition of BCBs with quinolinium 1,4-zwitterionic thiolate was achieved with broad substrate scope under mild reaction conditions. Furthermore, the first Lewis acid-catalyzed asymmetric polar (5+3) cycloaddition of BCB with pyridazinium 1,4-zwitterionic thiolate was accomplished. In contrast, pyridinium 1,4-zwitterionic thiolates undergo an Sc(OTf)3-catalyzed formal (3+3) reaction with BCBs to generate thia-norpinene products, which represent the initial instance of synthesizing 2-thiabicyclo[3.1.1]heptanes (thia-BCHeps) from BCBs. Moreover, we have successfully used this (3+3) protocol to rapidly prepare thia-BCHeps-substituted analogues of the bioactive molecule Pitofenone. Density functional theory (DFT) computations imply that kinetic factors govern the (5+3) cycloaddition reaction between BCB and quinolinium 1,4-zwitterionic thiolate, whereas the (3+3) reaction involving pyridinium 1,4-zwitterionic thiolates is under thermodynamic control.

Synthesis of Bridged Five-Membered Ring Systems by Type II [3 + 2] Annulation of Allenylsilane-ene

The first type II intramolecular [3 + 2] annulation of allenylsilane-ene has been achieved, enabling diastereoselective and efficient construction of synthetically challenging bridged five-membered ring systems such as bicyclo[3.2.1]. This mild and direct process shows a broad substrate scope and is highly stereospecific. Particularly, this work represents the first stereoselective method for the direct synthesis of bicyclo[3.2.1] ring systems from acyclic precursors. Additionally, the first asymmetric total syntheses of (+)- and (-)-strepsesquitriol, and the efficient formation of the synthetically challenging tetracyclic core of pierisjaponol D are achieved by this type II [3 + 2] annulation reaction.

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).

Gold-Catalyzed Oxidative Rearrangement Strategy to Yield 2-Hydroxycyclohepta-1,3-diene-1-carbonyl Compounds

A gold-catalyzed oxidative rearrangement of propargyl alcohols, derived from commercially available cyclohex-2-en-1-ones and alkynes, was successfully developed for the efficient synthesis of seven-membered rings. Thorough investigations were conducted to optimize the reaction conditions and evaluate its compatibility with various functional groups. Additionally, this methodology was applied to the formal total synthesis of guanacastepene A, demonstrating its practical utility in complex natural product synthesis. This versatile and efficient approach opens up new possibilities for the construction of diverse seven-membered ring systems, providing valuable building blocks for further exploration in drug discovery and the synthesis of intricate molecules.

Room Temperature Diels-Alder Reactions of 4-Vinylimidazoles

In the course of studying Diels-Alder reactions of 4-vinylimidazoles with N-phenylmaleimide, it was discovered that they engage in cycloaddition at room temperature to give high yields of the initial cycloadduct as a single stereoisomer. In certain cases, the product precipitated out of the reaction mixture and could be isolated by simple filtration, thereby avoiding issues with aromatization observed during chromatographic purification. Given these results, intramolecular variants using doubly activated dienophiles were also investigated at room temperature. Amides underwent cycloaddition at room temperature in modest yields, but the initial adducts were not isolable with Nimid-benzyl-protected systems. Attempts to extend these results to the corresponding esters and hydroxamate were less successful with these substrates only undergoing cycloaddition at elevated temperatures in lower yields. Density functional theory calculations were performed to evaluate the putative transition states for both the inter- and intramolecular variants to rationalize experimental observations.

Enantioselective and Regiodivergent Gold and Chiral Brønsted Acid Catalyzed Cycloisomerization/Diels-Alder Reaction of 1,10-Dien-4-yn-3-yl Acetates: Synthesis of Norbornene-Embedded Tricarbocycles

A synthetic method for the enantioselective and regiodivergent synthesis of hexahydro-2H-2,4a-methanonaphthalen-4-yl and octahydro-2,4-methanoazulen-1-yl esters that relies on the gold(I)- and chiral Brønsted acid-catalyzed cycloisomerization/Diels-Alder (CDA) reaction of (E)-1,10-dien-4-yn-3-yl acetates is described.

Total Synthesis of abeo-Steroids via Cycloaddition Strategy

ConspectusSteroids continue to play a significant role in organic chemistry, medicinal chemistry, and drug discovery due to their important biological activities and diverse intriguing structures. Although synthetic organic chemists have successfully constructed and elaborated the classical [6-6-6-5] tetracyclic steroid skeleton for nearly a century, synthesis of the unusual rearranged steroids, particularly abeo-steroids with a medium-sized ring, remains a challenge in the synthetic community. Furthermore, the structures of abeo-steroids are complex and diverse, containing a seven-membered ring embedded in the fused or bridged A/B ring system and possessing numerous stereogenic centers. Besides their structural complexity, various abeo-steroids have shown remarkable biological activities. However, the relative scarcity of abeo-steroids in natural sources has impeded the systematic evaluation of their biological activities. In addition, direct strategies to build the core structures of abeo-steroids are very rare, partially because of the high ring-strain energies of their rearranged A/B ring systems. Therefore, the development of direct and efficient synthetic approaches to these complex molecules is highly desired.Our long-standing interest in the total synthesis of abeo-steroids and the development of new cycloaddition reactions for streamlining complex molecule synthesis have led us to develop a series of unique and powerful intramolecular cycloaddition strategies to access a diverse array of highly strained abeo-steroids. These strategies include Ru-catalyzed [5 + 2] cycloaddition, acid-promoted type I [5 + 2] cycloaddition, Rh-catalyzed [2 + 2 + 1] cycloaddition, and type II [5 + 2] cycloaddition. Since 2018, we have accomplished the first total syntheses of five synthetically challenging abeo-steroids, i.e., bufogargarizins A and B, phomarol, bufospirostenin A, and cyclocitrinol, thus facilitating the evaluation of their pharmacological potentials. In this Account, we summarize our laboratory's systematic efforts in the total synthesis of these abeo-steroids via cycloaddition strategies. We highlight the efficiency and versatility of each cycloaddition strategy for constructing structurally complex abeo-steroid cores by forming the A/B ring system. The evolution of each strategy and key lessons learned from the synthetic journey are also discussed. We believe that our unique perspective in this field will promote advances in the total synthesis of abeo- and related steroids.

Photoswitchable Oxidopyrylium Ylide for Photoclick Reaction with High Spatiotemporal Precision: A Dynamic Switching Strategy to Compensate for Molecular Diffusion

We describe a novel type of photoclick reaction between 2,3-diaryl indenone epoxide (DIO) and ring-strained dipolarophiles, in which DIO serves as a P-type photoswitch to produce mesoionic oxidopyrylium ylide (PY) to initiate an ultra-fast [5+2] cycloaddition (k_2hν =1.9×10⁵  M^-1  s^-1 ). The photoisomerization between DIO and PY can be tightly controlled by either 365 or 520 nm photo-stimulation, which allows reversion or regeneration of the reactive PY dipole on demand. Thus, this reversible photoactivation was exploited to increase the chemoselectivity of the [5+2] cycloaddition in complex environments via temporal dual-λ stimulation sequences and to recycle the DIO reagent for batch-wise protein conjugation. A dynamic photoswitching strategy is also proposed to compensate for molecular diffusion of PY in aqueous solution, enhancing the spatial resolution of lithographic surface decoration and bioorthogonal labeling on living cells via a spatiotemporal dual-λ photo-modulation.

Selective [2σ + 2σ] Cycloaddition Enabled by Boronyl Radical Catalysis: Synthesis of Highly Substituted Bicyclo[3.1.1]heptanes

In contrast to the traditional and widely-used cycloaddition reactions involving at least a π bond component, a [2σ + 2σ] radical cycloaddition between bicyclo[1.1.0]butanes (BCBs) and cyclopropyl ketones has been developed to provide a modular, concise, and atom-economical synthetic route to substituted bicyclo[3.1.1]heptane (BCH) derivatives that are 3D bioisosteres of benzenes and core skeleton of a number of terpene natural products. The reaction was catalyzed by a combination of simple tetraalkoxydiboron(4) compound B2pin2 and 3-pentyl isonicotinate. The broad substrate scope has been demonstrated by synthesizing a series of new highly functionalized BCHs with up to six substituents on the core with up to 99% isolated yield. Computational mechanistic investigations supported a pyridine-assisted boronyl radical catalytic cycle.

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.

Catalytic asymmetric Michael/cyclization reaction of 3-isothiocyanato thiobutyrolactone: an approach to the construction of a library of bispiro[pyrazolone-thiobutyrolactone] skeletons

Here, we demonstrate the first example of 3-isothiocyanato thiobutyrolactone serving as a useful building block in the Michael/cyclization reaction with alkylidene pyrazolones for the enantioselective construction of optically active structural bispiro[pyrazolone-thiobutyrolactone] skeletons containing three contiguous stereocenters with two spiroquaternary stereocenters. These products were smoothly afforded in up to 90% yield, >20 : 1 dr and >99% ee with chiral squaramide as the catalyst under mild conditions. Notably, this is also the first example of the merger of a spirocyclic pyrazolone scaffold with a spirocyclic thiobutyrolactone scaffold, potentially useful in medicinal chemistry.

Recent advances in [5+2] cycloadditions

Abstract:

The existence of a seven-membered cyclic core in several natural products and biomolecules vitalized the research on its synthesis. [5+2] cycloaddition has become a promising strategy for the construction of seven-membered ring systems by the formation of carbon-carbon bonds in a single step, with strong regioselectivity and stereoselectivity. This review mainly focuses on recent developments in the area of [5+2] cycloaddition since 2019. Total synthesis of natural products involving [5+2] cycloaddition as key step leading to heptacyclic core is also discussed. Synthesis of fused and bridged ring systems via the reactions involving inter and intramolecular [5+2] cycloadditions like oxidopyrylium-mediated [5+2] cycloadditions, [5+2] cycloadditions of vinyl cyclopropanes (VCPs), vinyl phenols, etc is explained in the review with the latest examples. This review provides a useful guide for researchers exploring this powerful strategy to create more elegant heptacycles in their future research.

Construction of Bridged Aza- and Oxa-[n.2.1] Skeletons via an Intramolecular Formal [3+2] Cycloaddition of Aziridines and Epoxides with Electron-Deficient Alkenes

An intramolecular formal [3+2] cycloaddition of activated aziridines and epoxides with electron-deficient alkene has been developed for the general and efficient construction of bridged aza- and oxa-[n.2.1] (n = 3 or 4) skeletons. This strategy can be efficiently promoted by lithium iodide. To demonstrate its potential, the intramolecular formal [3+2] cycloaddition was used to access the important intermediate of homoepiboxidine.

Fused-Ring-Linked Covalent Organic Frameworks

The development of linkage chemistry in the research area of covalent organic frameworks (COFs) is fundamentally important for creating robust structures with high crystallinity and diversified functionality. We reach herein a new level of complexity and controllability in linkage chemistry by achieving the first synthesis of fused-ring-linked COFs. A series of bicyclic pyrano[4,3-b]pyridine COFs have been constructed via a cascade protocol involving Schiff-base condensation, intramolecular [4 + 2] cycloaddition, and dehydroaromatization. With a broad scope of Brønsted or Lewis acids as the catalyst, the designed monomers, that is, O-propargylic salicylaldehydes and multitopic anilines, were converted into the fused-ring-linked frameworks in a one-pot fashion. The obtained COFs exhibited excellence in terms of purity, stability, and crystallinity, as comprehensively characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, and so on. Specifically, the highly selective formation (>94%) of pyrano[4,3-b]pyridine linkage was verified by quantitative NMR measurements combined with ¹³C-labeling synthesis. Moreover, the fused-ring linkage possesses fully locked conformation, which benefits to the high crystallinity observed for these COFs. Advancing the linkage chemistry from the formation of solo bonds or single rings to that of fused rings, this study has opened up new possibilities for the concise construction of sophisticated COF structures with high controllability.

Synthesis of Various Bridged Ring Systems via Rhodium-Catalyzed Bridged (3+2) Cycloadditions

Here, we describe the rhodium-catalyzed bridged (3+2) cycloaddition cascade reactions of N-sulfonyl-1,2,3-triazoles, which allowed the efficient diastereoselective construction of various functionalized and synthetically challenging bridged ring systems. This simple, direct transformation had a broad substrate scope and excellent functional group tolerance. The highly strained polycyclic bicyclo[2.2.2]octa[b]indole core of fruticosine was synthesized efficiently using this methodology.

Direct Access to Bridged Polycyclic Skeletons by Merging Oxidative C-H Annulation and Cascade [4 + 2] Cycloaddition

We report a step-economic strategy for the direct synthesis of bridged polycyclic skeletons by merging oxidative C-H annulation and cascade cycloaddition. In the protocol, spiro[cyclopentane-1,3'-indoline]-2,4-dien-2'-ones were first synthesized by oxidative C-H annulation of ethylideneoxindoles with alkynes. Subsequent cascade [4 + 2] cycloaddition with dienophiles gave the bridged bicyclo[2.2.1]quinolin-2(1H)-ones and enabled the one-pot construction of two quaternary carbon centers and three C-C bonds. Mechanistic investigations of the latter suggest a cascade ring-opening, 1,5-sigmatropic rearrangement, and [4 + 2] cycloaddition process.

Vinylazaarenes as dienophiles in Lewis acid-promoted Diels-Alder reactions

Described are the first examples of Lewis acid-promoted Diels–Alder reactions of vinylpyridines and other vinylazaarenes with unactivated dienes. Cyclohexyl-appended azaarenes constitute a class of substructures of rising prominence in drug discovery. Despite this, thermal variants of the vinylazaarene Diels–Alder reaction are rare and have not been adopted for synthesis, and Lewis acid-promoted variants are virtually unexplored. The presented work addresses this gap and in the process furnishes increased scope, dramatically higher yields, improved regioselectivity, and high levels of diastereoselectivity compared to prior thermal examples. These reactions provide scalable access to druglike scaffolds not readily available through other methods. More broadly, these studies establish a useful new class of dienophiles that, based on preliminary mechanistic studies, should be amenable to conventional strategies for enantioselective catalysis.

Vinyl-substituted azaarenes are rare and challenging substrates as dienophiles in Diels–Alder reactions; by employing Lewis acid activation, high yielding and highly selective cycloadditions with unactivated dienes are enabled.

Asymmetric Synthesis of Bridged N-Heterocycles with Tertiary Carbon Center through Barbas Dienamine-Catalysis: Scope and Applications

A direct aza-Diels-Alder reaction between 2-aryl-3H-indolin-3-ones and cyclic-enones has been developed to access chiral indolin-3-one fused polycyclic bridged compounds. This method proceeds via proline-catalyzed Barbas-dienamine intermediate formation from various cyclic-enones such as 2-cyclopenten-1-one, 2-cyclohexene-1-one, and 2-cycloheptene-1-one, followed by a reaction with 2-aryl-3H-indol-3-ones. Several indolin-3-ones fusing [2.2.2], [2.2.1], and [3.2.1] skeletons decorated with a tertiary carbon chiral center have been prepared. Computational studies (DFT) supported the observed stereoselectivity in the method. The synthesized compounds have shown exciting photophysical activities and selective sensing of Pd²⁺ and Fe³⁺ ions through the fluorescence quenching "switch-off" mode.

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.

Synthesis of Indole-Fused Oxepines via C-H Activation Initiated Diastereoselective [5 + 2] Annulation of Indoles with 1,6-Enynes

A rhodium-catalyzed diastereoselective formal [5 + 2] annulation of indoles with cyclohexadienone-containing 1,6-enynes has been established via indole 2,3-difunctionalization. The reaction, probably proceeding through tandem indole C2-H alkenylation and intramolecular Friedel-Crafts alkylation relay, provides rapid construction of indole-fused oxepines in good to excellent yields with a broad substrate scope. This method also features concomitant construction of cis-hydrobenzo[b] oxepine scaffolds, a core unit found in numerous natural products of important biological activities.

Synthesis of the Bridged [5-7-7] Tricyclic Core of Eurifoloid A

We herein describe a new approach for the efficient and asymmetric construction of the tricyclic core of eurifoloid A, which possesses a unique and highly strained bicyclo[4.4.1] ring system. A rhodium-catalyzed intramolecular [3 + 2] dipolar cycloaddition was developed to install synthetically challenging bridged bicyclo[4.3.1] ring systems. The reported chemistry shows the feasibility of constructing the eurifoloid A framework using a diastereoselective intramolecular [3 + 2] cycloaddition and a ring enlargement.

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.

Construction of bridged polycycles through dearomatization strategies

Bridged polycycles are privileged molecular skeletons with wide occurrence in bioactive natural products and pharmaceuticals. Therefore, they have been the pursing target molecules of numerous chemists. The rapid and convenient generation of sp3-rich complex three-dimensional molecular skeletons from simple and easily available aromatics has made dearomatization a highly valuable synthetic tool for the construction of rigid and challenging bridged rings. This review summarizes the-state-of-the-art advances of dearomatization strategies in the application of bridged ring formation, discusses their advantages and limitations and the in-depth mechanism, and highlights their synthetic value in the total synthesis of natural products. We wish this review will provide an important reference for medicinal and synthetic chemists and will inspire further development in this intriguing research area.

Catalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones: stereoselective construction of dihydrobenzofuran-bridged polycyclic skeletons

An organocatalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones is developed for the construction of dihydrobenzofuran-bridged polycyclic skeletons with good results.

Organocatalytic domino annulation of in situ generated tert-butyl 2-hydroxybenzylidenecarbamates with 2-isothiocyanato-1-indanones for synthesis of bridged and fused ring heterocycles

An asymmetric domino annulation of 2-isothiocyanato-1-indanones with tert-butyl 2-hydroxybenzylidenecarbamates was developed for the enantioselective construction of bridged and fused ring hererocycles in high yields with excellent stereoselectivities.

Copper(i)-catalyzed [4 + 2] cycloaddition of aza-ortho-quinone methides with bicyclic alkenes

An efficient copper(i)-catalyzed [4 + 2] cycloaddition of aza-ortho-quinone methides (ao-QMs) and bicyclic alkenes to prepare tetrahydroquinoline-fused bicycles bearing multiple stereocenters in good yields is reported.

Synthesis of an isomer of lycoplanine A via cascade cyclization to construct the spiro-N,O-acetal moiety

A 6/10/5/5 tetracyclic isomer of lycoplanine A is synthesized using D–A reaction and cascade cyclization to respectively establish the [9.2.2] pentadecane skeleton and spirocenter, providing sufficient experience to synthesize lycoplanine A.

Synthetic Study toward the Total Synthesis of Lycoclavatumide

The synthetically challenging and highly functionalized azabicyclo[6.4.1] ring system, which is found in lycopodium alkaloid lycoclavatumide and some natural molecules, was synthesized for the first time. The key reaction was a diastereoselective type II [5+2] cycloaddition with excellent functional group compatibility. We tried to install the desired eight-membered ring in the final product by RCM reaction.