Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

Base-Promoted (5 + 2) Annulation between 2-(Alkynylaryl)acetonitriles and Arylalkynes for the Synthesis of Benzocycloheptene Derivatives

Herein, we describe a novel approach to the synthesis of benzocycloheptene derivatives via base-promoted (5 + 2) annulation between 2-(alkynylaryl)acetonitriles and arylalkynes. In this chemistry, 2-(alkynylaryl)acetonitriles are employed as a new C5 synthon to construct various benzocycloheptene(s) derivatives by building two C-C bonds in one single step. This method features excellent regioselectivity, the use of readily available starting materials, and good functional group tolerance. The practicality of the strategy was further demonstrated by gram-scale synthesis, late-stage functionalizations, and the post-modification of natural products such as probenecid and tetrahydrofurfuryl alcohol.

21-Carba-23-selenaporphyrinoid Dyads-An Azepine Unit as a Merging Motif

The oxidation of 10,15-diaryl-21-carba-23-selenaporphyrinoids resulted in the creation of dyads. The dimerization process follows a [5+2] cycloaddition path with the formation of an azepine unit. The arrays display two direct bonds between the peripheral carbocyclic carbon atoms of one carbaselenaporphyrinic subunit and the central carbon and nitrogen atoms of the second subunit. This results in a unique canted arrangement of two carbaporphyrinoid planes resembling an open seashell-like motif.

Construction of the A-B-C Ring of Simplicissin through an Oxidative Dearomatization/Iodination/[3+2] Annulation Cascade

As an attractive DMOA-derived spiromeroterpenoid, simplicissin shares a common A-B-C ring skeleton with other natural analogues. On the basis of the development of an oxidative dearomatization/iodination/[3+2] annulation cascade, a concise synthetic pathway to the A-B-C ring of simplicissin has been successfully established, and the substrate generality of the novel oxidative dearomatization/iodination/[3+2] annulation cascade has been checked.

Enantioselective Synthesis of Tropane Scaffolds by an Organocatalyzed 1,3-Dipolar Cycloaddition of 3-Oxidopyridinium Betaines and Dienamines

Tropane alkaloids constitute a compound-class which is structurally defined by a central 8-azabicyclo[3.2.1]octane core. A diverse bioactivity profile combined with an unusual aza-bridged bicyclic framework has made tropanes molecules-of-interest within organic chemistry. Enantioselective examples of (5+2) cycloadditions between 3-oxidopyridinium betaines and olefins remain unexplored, despite 3-oxidopyridinium betaines being useful reagents in organic synthesis. The first asymmetric (5+2) cycloaddition of 3-oxidopyridinium betaines is reported, affording tropane derivatives in up to quantitative yield and with excellent control of peri-, regio-, diastereo-, and enantioselectivity. The reactivity is enabled by dienamine-activation of α,β-unsaturated aldehydes combined with in situ formation of the pyridinium reaction-partner. A simple N-deprotection protocol allows for liberation of the tropane alkaloid motif, and synthetic elaborations of the cycloadducts demonstrate their synthetic utility to achieve highly diastereoselective modification around the bicyclic framework. DFT computations suggest a stepwise mechanism where regio- and stereoselectivity are defined during the first bond-forming step in which the pyridinium dipole exerts critical conformational control over its dienamine partner. In the second bond-forming step, a kinetic preference toward an initial (5+4) cycloadduct was identified; however, a lack of catalyst turn-over, reversibility, and thermodynamic bias favoring a (5+2) cycloadduct rendered the reaction fully periselective.

Strain-Release-Controlled [4 + 2 + 1] Reaction of Cyclopropyl-Capped Diene-ynes/Diene-enes and Carbon Monoxide Catalyzed by Rhodium

Achieving transition-metal-catalyzed reactions of diene-ynes/diene-enes and carbon monoxide (CO) to deliver [4 + 2 + 1] cycloadducts, rather than the kinetically favored [2 + 2 + 1] products, is challenging. Here, we report that this can be solved by adding a cyclopropyl (CP) cap to the diene moiety of the original substrates. The resulting CP-capped diene-ynes/diene-enes can react with CO under Rh catalysis to give [4 + 2 + 1] cycloadducts exclusively without forming [2 + 2 + 1] products. This reaction has a broad scope and can be used to synthesize useful 5/7 bicycles with a CP moiety. Of the same importance, the CP moiety in the [4 + 2 + 1] cycloadducts can act as an intermediate group for further transformations so that other challenging bicyclic 5/7 and tricyclic 5/7/5, 5/7/6, and 5/7/7 skeletons, some of which are widely found in natural products, can be accessed. The mechanism of this [4 + 2 + 1] reaction has been investigated by quantum chemical calculations, and the role of the CP group in avoiding the possible side [2 + 2 + 1] reaction has been identified, showing that the [4 + 2 + 1] is controlled by releasing the ring strain in the methylenecyclopropyl (MCP) group (about 7 kcal/mol) in the CP-capped dienes.

(4+3) Annulation of Donor-Acceptor Cyclopropanes and Azadienes: Highly Stereoselective Synthesis of Azepanones

Azepanes are important seven-membered heterocycles, which are present in numerous natural and synthetic compounds. However, the development of convergent synthetic methods to access them remains challenging. Herein, we report the Lewis acid catalyzed (4+3) annulative addition of aryl and amino donor-acceptor cyclopropanes with 2-aza-1,3-dienes. Densely substituted azepane derivatives were obtained in good to excellent yields and with high diastereoselectivity. The reaction occurred under mild conditions with ytterbium triflate as the catalyst. The use of copper triflate with a trisoxazoline (Tox) ligand led to an enantioselective transformation. The obtained cycloadducts were convenient substrates for a series of further modifications, showing the synthetic utility of these compounds.

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.

Recent Advances on the Synthesis of 2,3-Fused Quinazolinones

As one of the most important structural units in pharmaceuticals and medicinal chemistry, quinazolinone and its derivatives exhibit a wide range of biological and pharmacological activities, including anti-inflammatory, antitubercular, antiviral,...

Synthesis of 1,4-enynes via nickel-catalyzed cross-coupling of allylic alcohols with alkynylzinc reagents

Synthesis of 1,4-enynes was performed via nickel-catalyzed cross-coupling of allylic alcohols with alkynylzinc reagents. The reaction features high regio- and E/Z-selectivity when aryl-substituted allylic alcohols were employed. The method also exhibits a wide scope of substrates and good compatibility of functional groups.

Asymmetric [4 + 3] Annulations for Constructing Divergent Oxepane Frameworks via Cooperative Tertiary Amine/Transition Metal Catalysis

We report asymmetric [4 + 3] annulations between isatin-derived Morita-Baylis-Hillman carbonates and two types of vinyl carbonates synergistically catalyzed by tertiary amines and transition metals, through chemoselective assemblies of in situ formed allylic ylides and metal-containing 1,4-dipoles. A range of oxepane frameworks are generally constructed in moderate to good yields with high stereocontrol. Moreover, all four diastereomers for the products bearing vicinal stereocenters are accessible by tuning tertiary amine and metal catalysts.

Diastereoselective [2+2+1] Hydrative Annulation of Phenol-Linked 1,6-Enynes with Acetylenes Through Rhodium Catalysis: A Rapid Access to Cyclopenta[b]benzofuranols

An unprecedented [2+2+1] hydrative annulation of 1,6-enynes with terminal alkynes is achieved using catalytic cationic Rh(I). Thus, a modular assembly of cyclopenta[b]benzofuranols with two consecutive quarternary stereocenters is achieved from readily available alkynes. The reaction is proposed to go through a sequence of 5-membered rhoda-cycle formation, regioselective acetylene insertion, 1,5 H-shift, substrate controlled stereoselective addition of water molecule followed by 1,2-rhodium migration gave contracted rhoda-cycle D and reductive elimination. Necessary control/labelling experiments were conducted to gain insight in to the mechanism.

Intermolecular [5+2] Annulation between 1-Indanones and Internal Alkynes by Rhodium-Catalyzed C-C Activation

Herein, we report a [5+2] cycloaddition between readily accessible 1-indanones and internal alkynes through Rh-catalyzed activation of less strained C-C bonds. The reaction is enabled by a strongly σ-donating NHC ligand and a carefully modified temporary directing group. A wide range of functional groups is tolerated, and the method provides straightforward access to diverse benzocycloheptenones that are hard to access otherwise. DFT studies of the reaction mechanism imply the migration insertion as the turnover-limiting step and suggest beneficial π-π interactions in the transition states.

Electrochemical ODI-[5+2] Cascade for the Syntheses of Diversely Functionalized Bicyclo[3.2.1]octane Frameworks

A metal- and hypervalent iodine reagent-free electrochemical oxidative dearomatization-induced [5+2] cycloaddition/pinacol rearrangement cascade reaction was described. The electrosynthetic method showed strong tolerance for vinylphenols, ethynylphenols, and allenylphenols, which thus enabled the rapid assembly of diversely functionalized bicyclo[3.2.1]octanes in 41-95% yields and up to >20:1 dr. This protocol could be scaled up to gram amounts and should find wide application in complex natural product synthesis.

Direct α-Alkenylation of Cyclic Amines with Maleimides through Fe(III)-Catalyzed C(sp3)-H/C(sp2)-H Cross Dehydrogenative Coupling

Presented herein is a novel and efficient α-C(sp³)-H alkenylation of cyclic amines with maleimides. Mechanistically, this C(sp³)-H/C(sp²)-H cross dehydrogenative coupling (CDC) reaction involves a cascade procedure including oxidative α-amino radical formation from the cyclic amine substrate and nucleophilic addition of the in situ formed α-amino radical onto the electron-deficient carbon-carbon double bond of maleimide followed by oxidation and β-elimination. Notably, this direct α-functionalization provides an effective alternative to the conventional ionic reaction mode, in which an imine or iminium intermediate is formed to react with electron-rich coupling partners other than electron-deficient ones. In general, this method features readily available and structurally diverse substrates, a green and economical catalyst, a unique reaction pathway, mild reaction conditions, high efficiency, and excellent atom economy. This new reaction enriches the application of Fe(III)-catalyzed C(sp³)-H activation and functionalization.

Highly Diastereoselective Intramolecular Asymmetric Oxidopyrylium-olefin [5 + 2] Cycloaddition and Synthesis of 8-Oxabicyclo[3.2.1]oct-3-enone Containing Ring Systems

We have the investigated base mediated asymmetric intramolecular oxidopyrylium-alkene [5 + 2]-cycloaddition reaction which resulted in the synthesis of functionalized tricyclic ring systems containing an 8-oxabicyclo[3.2.1]octane core. Intramolecular cycloaddition constructed two new rings, three new stereogenic centers, and provided a tricyclic cycloadduct with high diastereoselectivity and isolated yield. We incorporated an α-chiral center and an alkoxy alkene tether on the substrates and examined the effect of the size of alkyl groups and alkene tether length on diastereoselectivity. The requisite substrates for the oxidopyrylium-alkene cycloaddition reaction were synthesized in a few steps involving alkylation of optically active α-hydroxy amide, furyllithium addition, reduction of resulting ketone, and Achmatowicz reaction followed by acylation of a lactol intermediate. We have proposed stereochemical models for the [5 + 2] cycloaddition reaction via the oxidopyrylium ylide. Interestingly, the alkoxy substituent on the stereocenter and the chain length are responsible for the degree of stereoselectivity of the cycloadduct.

Formal (3 + 4)-Annulation of Propargylic p-Quinone Methides with 2-Indolylmethanols: Synthesis of Polysubstituted Indole-Fused Oxepines

A novel Brønsted acid catalyzed 1,8-addition mediated (3 + 4)-annulation of in situ generated propargylic p-quinone methides with 2-indolylmethanols is described. This method provides a convenient and mild approach to structurally interesting and synthetically important polysubstituted indole-fused oxepines in high yields. Moreover, 2-indolylmethanols as four-atom synthons in the (3 + 4)-annulations under Brønsted acid conditions have been explored for the first time.

Enantioselective construction of the 8-azabicyclo[3.2.1]octane scaffold: application in the synthesis of tropane alkaloids

The 8-azabicyclo[3.2.1]octane scaffold is the central core of the family of tropane alkaloids, which display a wide array of interesting biological activities. As a consequence, research directed towards the preparation of this basic structure in a stereoselective manner has attracted attention from many research groups worldwide across the years. Despite this, most of the approaches rely on the enantioselective construction of an acyclic starting material that contains all the required stereochemical information to allow the stereocontrolled formation of the bicyclic scaffold. As an alternative, there are a number of important methodologies reported in which the stereochemical control is achieved directly in the same transformation that generates the 8-azabicyclo[3.2.1]octane architecture or in a desymmetrization process starting from achiral tropinone derivatives. This review compiles the most relevant achievements in these areas.

Ruthenium-Catalyzed Cycloadditions to Form Five-, Six-, and Seven-Membered Rings

Ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings are summarized, including applications in natural product total synthesis. Content is organized by ring size and reaction type. Coverage is limited to processes that involve formation of at least one C-C bond. Processes that are stoichiometric in ruthenium or exploit ruthenium as a Lewis acid (without intervention of organometallic intermediates), ring formations that occur through dehydrogenative condensation-reduction, σ-bond activation-initiated annulations that do not result in net reduction of bond multiplicity, and photochemically promoted ruthenium-catalyzed cycloadditions are not covered.

Asymmetric synthesis of dihydro-1,3-dioxepines by Rh(ii)/Sm(iii) relay catalytic three-component tandem [4 + 3]-cycloaddition

Heterocycles have been widely used in organic synthesis, agrochemical, pharmaceutical and materials science industries. Catalytic three-component ylide formation/cycloaddition enables the assembly of complex heterocycles from simple starting materials in a highly efficient manner. However, asymmetric versions remain a yet-unsolved task. Here, we present a new bimetallic catalytic system for tackling this challenge. A combined system of Rh(ii) salt and chiral N,N'-dioxide-Sm(iii) complex was established for promoting the unprecedented tandem carbonyl ylide formation/asymmetric [4 + 3]-cycloaddition of aldehydes and α-diazoacetates with β,γ-unsaturated α-ketoesters smoothly, affording various chiral 4,5-dihydro-1,3-dioxepines in up to 97% yield, with 99% ee. The utility of the current method was demonstrated by conversion of products to optically active multi-substituted tetrahydrofuran derivatives. A possible reaction mechanism was provided to elucidate the origin of chiral induction based on experimental studies and X-ray structures of catalysts and products.

Tetracyclic Diterpenoid Synthesis Facilitated by ODI-Cascade Approaches to Bicyclo[3.2.1]octane Skeletons

Tetracyclic diterpenoids (C₂₀) mainly refer to the plant terpenoids bearing biogenetically related carbon skeletons derived from copalyl diphosphates (ent-CPP and syn-CPP). This large family contains over 1600 known members that can be categorized into 11 major structural types. Among them, more than three-quarters share a bridged bicyclo[3.2.1]octane subunit, which is also an important branching point in biosynthesis en route to the other types of bicyclic scaffolds, such as bicyclo[2.2.2]-, bicyclo[3.3.0]-, and tricyclo[3.2.1.0]octanes. Combined with the significance of its stereochemical importance in biological activity, the assembly of the bicyclo[3.2.1]octane skeletons is critical to the success of the whole synthesis blueprint toward tetracyclic diterpenoids. Although a number of inspiring methodologies have been disclosed, general approaches by the incorporation of innovative cascade reactions permitting access to diverse structural types of tetracyclic diterpenoids remain limited and in urgent demand.Because of the long-standing interest in the synthesis of bridged diterpenoids, we have recently developed two complementary types of oxidative dearomatization induced (ODI) cascade approaches to the rapid and efficient construction of bicyclo[3.2.1]octane skeletons. In this Account, we summarize our original synthesis design, methodology development, and the application of these two strategies in tetracyclic diterpenoid synthesis during the past few years in our laboratory.First, we detail our preliminary investigation of the ODI-[5 + 2] cycloaddition/pinacol rearrangement cascade reaction, which showed a wide scope of vinylphenol substrates and led to cyclopentane and cyclohexane-fused bicyclo[3.2.1]octanes in good yields with excellent dr values. Next, we describe the utilization of this ODI-[5 + 2] cascade reaction which resulted in the asymmetric total syntheses of four highly oxygenated ent-kauranoids. The strategy concerning accurate stereochemical control in the ODI-[5 + 2] cycloaddition was then successfully transplanted to the total syntheses of three stemaranoids, thus providing a straightforward and diastereoselective route to C9-ethano-bridged tetracyclic diterpenoids. To access more complex diterpenoid rhodomollanol A, we exploited two additional biomimetic rearrangements, namely, the retro-Dieckmann fragmentation/vinylogous Dieckmann cyclization cascade and the photo-Nazarov cyclization/intramolecular cycloetherification cascade. Taken together with the ODI-[5 + 2] cascade, the asymmetric total synthesis of the target molecule was realized, which shed light on the biogenetic pathway of the unprecedented rhodomollane-type carbon framework. Finally, we describe an ODI-Diels-Alder/Beckwith-Dowd cascade approach as a valuable supplement to the ODI-[5 + 2] cascade for the fabrication of cycloheptane-fused bicyclo[3.2.1]octane skeletons. Its versatility was also demonstrated by the total syntheses of two challenging grayanane diterpenoids. In view of the high functional-group compatibility and scalability, we anticipate that the two novel cascade approaches will find further use in the field of complex natural product synthesis.

Copper-catalyzed [3 + 2]/[3 + 2] carboannulation of dienynes and arylsulfonyl chlorides enabled by Smiles rearrangement: access to cyclopenta[a]indene-fused quinolinones

A radical Smiles rearrangement strategy for allowing an unprecedented [3 + 2]/[3 + 2] carboannulation of dienynes with arylsulfonyl chlorides using cheap copper catalysis is described.

Asymmetric β,γ′-regioselective [4 + 3] and [4 + 2] annulations of α-vinylenals via cascade iminium ion-dienamine catalysis

α-Vinylenal substrates have been designed and applied in β,γ′-regioselective asymmetric [4 + 3] and [4 + 2] annulation reactions via cascade aminocatalysis.

Divergent functionalization of terminal alkynes enabled alkynylative [5+1] benzannulation of 3-acetoxy-1,4-enynes

We here describe an alkynylative [5+1] benzannulation of 3-acetoxy-1,4-enynes with terminal alkynes, which enables both the construction of a benzene ring skeleton and intermolecular incorporation of an alkynyl group in a single reaction using Pd and Cu cooperative catalysts. The method represents efficient access to internal aryl alkynes through divergent functionalization of two terminal alkyne components: one alkyne serves as the one-carbon unit to realize the [5+1] benzannulation and the other alkyne as a nucleophile terminates the reaction.

Synthesis of Polycyclic Ether-Benzopyrans and In Vitro Inhibitory Activity against Leishmania tarentolae

Construction of a focused library of polycyclic ether-benzopyrans was undertaken in order to discover new therapeutic compounds that affect Leishmania growth and infectivity. This is especially of interest since there are few drug therapies for leishmaniasis that do not have serious drawbacks such high cost, side effects, and emerging drug resistance. The construction of these polycyclic ether-benzopyrans utilized an acetoxypyranone-alkene [5+2] cycloaddition and the Suzuki-Miyaura cross-coupling. The multi-gram quantity of the requisite aryl bromide was obtained followed by effective Pd-catalyzed coupling with boronic acid derivatives. Compounds were tested in vitro using the parasitic protozoan, Leishmania tarentolae. Effects of concentration, time, and exposure to light were evaluated. In addition, the effects on secreted acid phosphatase activity and nitric oxide production were investigated, since both have been implicated in parasite infectivity. The data presented herein are indicative of disruption of the Leishmania tarentolae and thus provide impetus for the development and testing of a more extensive library.

Synthetic applications of type II intramolecular cycloadditions

Type II intramolecular cycloadditions ([4+2], [4+3], [4+4] and [5+2]) have emerged recently as an efficient and powerful strategy for the construction of bridged ring systems. In general, type II cycloadditions provide access to a wide range of bridged bicyclo[m.n.1] ring systems with high regio- and diastereoselectivity in an easy and straightforward manner. In each section of this review, an overview of the corresponding type II cycloadditions is presented, which is followed by highlights of method development and synthetic applications in natural product synthesis. The goal of this review is to provide a survey of recent advances in the field covering literature up to 2020. The review will serve as a useful reference for organic chemists engaged in the total synthesis of natural products containing bridged bicyclo[m.n.1] ring systems and provide strong stimulus for invention and further advances in this exciting research field.

Metal-free aza-Claisen type ring expansion of vinyl aziridines: an expeditious synthesis of seven membered N-heterocycles

A metal-free approach for the synthesis of 7-membered aza-heterocycles has been developed by the intermolecular [5 + 2] cycloaddition of non-activated vinylaziridines and alkynes. This method has a broad substrate scope under mild reaction conditions to afford structurally diverse 7-membered N-heterocycles in high yield up to 92%.