The (4+3)-cycloaddition reaction: heteroatom-substituted allylic cations as dienophiles

Total Synthesis and Cytotoxicity Evaluation of Pareitropone and Analogues

A concise synthesis of pareitropone by oxidative cyclization of a phenolic nitronate is delineated. The use of TMSOTf as an additive to promote the facile formation of a strained norcaradiene intermediate provides convenient access to highly condensed multicyclic tropones in high yields. This synthesis is modular, efficient, and scalable, highlighting the synthetic utility of radical anion coupling reactions in annulation reactions. This work is discussed in the context of total syntheses of the tropoloisoquinoline alkaloids. Also included are the preparation of several congeners and a brief description of their biological activities.

Tandem (4 + 3)-Annulation of Aziridines: Stereoselective Access to Fused Azepinoindoles

A stereoselective tandem (4 + 3)-coupling of aziridines with 4-alkylidene indole malonates has been disclosed under Cu-catalysis involving a base-promoted annulation. The methodology serves as a potential approach toward the facile construction of fused azepinoindoles with good yields and diastereoselectivities. Late-stage natural product and drug modification as well as preliminary investigations for the enantioselective (4 + 3)-annulation are important practical features.

Dithioallyl Cations in Stereoselective Dearomative (3 + 2) Cycloadditions of Benzofurans: Mechanism and Synthetic Applications

A stereoselective dearomative cyclopentannulation of benzofurans is reported. A previously reported dearomative (3 + 2) cycloaddition of indoles with 1,4-dithiane-fused allyl cations was found to lack stereoselectivity when more substituted cyclopentene rings are targeted. However, for benzofuran substrates, excellent levels of stereoselectivity were observed for the same allyl cation reagents under very similar reaction conditions. In this full account, we provide a mechanistic rationale and some design principles that govern the stereoselectivity of the intriguing dearomative transformations using dithioallyl cations and demonstrate how the stereoselectivity depends on electronic factors of the starting materials. The stereoselective methodology is also applied in a straightforward dearomative synthesis of the tricyclic sesquiterpenoid natural product aplysin and its analogues, starting from a simple benzofuran.

Dithioallyl cation (3 + 2) cycloadditions under aprotic reaction conditions: rapid access to spiro-fused cyclopentane scaffolds

We report a general method to effect all-carbon (3 + 2) cycloadditions that can elaborate cyclopentenes from a range of olefins. The required dithioallyl cation reagents can be generated in a newly developed mild protocol starting from 2-allyloxypyridine precursors, thus avoiding the use of strong Brønsted acids. The novel method significantly expands the substrate scope, which now also includes acid-sensitive olefins, and thus enables the preparation of previously inaccessible spiro-fused scaffold types from simple and readily available starting materials.

Asymmetric (4+n) Cycloadditions of Indolyldimethanols for the Synthesis of Enantioenriched Indole-Fused Rings

Catalytic asymmetric construction of chiral indole-fused rings has become an important issue in the chemical community because of the significance of such scaffolds. In this work, we have accomplished the first catalytic asymmetric (4+2) and (4+3) cycloadditions of 2,3-indolyldimethanols by using indoles and 2-naphthols as suitable reaction partners under the catalysis of chiral phosphoric acids, constructing enantioenriched indole-fused six-membered and seven-membered rings in high yields with excellent enantioselectivities. In addition, this approach is used to realize the first enantioselective construction of challenging tetrahydroindolocarbazole scaffolds, which are found to show promising anticancer activity. More importantly, theoretical calculations of the reaction pathways and activation mode offer an in-depth understanding of this class of indolylmethanols. This work not only settles the challenges in realizing catalytic asymmetric cycloadditions of indolyldimethanols but also provides a powerful strategy for the construction of enantioenriched indole-fused rings.

Oxygen-Promoted 6-endo-trig Cyclization of β,γ-Unsaturated Hydrazones/Ketoximes with Diazonium Tetrafluoroborates for Pyridazin-4(1H)-ones/Oxazin-4(1H)-ones

An oxidative 6-endo-trig cyclization and [2 + 2] cycloaddition of β,γ-unsaturated hydrazones/ketoximes and diazonium tetrafluoroborates for a synthetic strategy to pyridazin-4(1H)-ones/oxazin-4(1H)-ones under metal-free conditions is presented in a one-pot procedure. This protocol features excellent functional group tolerance and remarkable regioselectivity. A mechanistic study has been verified via 18O labeling and the H218O labeling method, in which O2 acts as both a reaction component and an oxidant.

Computational insights into the reactivity for the [2+5] cycloaddition reactions of norbornene-linked group 14 element/P-based and Si/group 15 element-based frustrated Lewis pairs with benzaldehyde

The element effects of Lewis acid (LA) and Lewis base (LB) on the potential energy surfaces of [2+5] cycloaddition reactions of norbornene-based G14/P-based (G14 = group 14 element) and Si/G15-based (G15 = group 14 element) frustrated Lewis pair (FLP)-type molecules with benzaldehyde were theoretically examined via density functional theory and several sophisticated methods. The theoretical findings indicated that among the above nine norbornene-linked G14/G15-based FLPs, only the Si/N-Rea, Si/P-Rea, and Si/As-Rea FLP-assisted compounds can readily undergo cycloaddition reactions with doubly bonded organic systems from kinetic and thermodynamic viewpoints. The energy decomposition analysis showed that the bonding interactions between the norbornene-based G14/G15-FLPs and benzaldehyde are better described in terms of the singlet-singlet model (donor-acceptor model) rather than the triplet-triplet model (electron-sharing model). In particular, natural orbitals for chemical valence findings revealed that the forward bonding is the lone pair (G15) → p-π*(C) interaction, which is a significantly strong FLP-to-benzaldehyde interaction. However, the back-bonding is the p-π*(G14) ← lone-pair orbital(O) interaction, which is a weak benzaldehyde-to-FLP interaction. The analyses based on the activation strain model showed that the larger the atomic radius of either the G14(LA) or the G15(LB) atom, the greater the G14⋯G15 separation distance in the norbornene-based G14/G15-FLP molecule, the smaller the orbital overlaps between G14/G15-FLP and Ph(H)CO, and the higher the activation barrier during its cycloaddition reaction with benzaldehyde.

Construction of cyclopenta[b]dihydronaphthofurans via TsOH-catalyzed consecutive biscyclization of dithioallylic alcohols and 1-styrylnaphthols

An efficient TsOH-catalyzed consecutive biscyclization cascade reaction of dithioallylic alcohols with 1-styrylnaphthols is demonstrated for the concise construction of pharmaceutically important cyclopenta[b]dihydrobenzofuran scaffolds. This process involved an acid-catalyzed (3+2) cycloaddition followed by an intramolecular nucleophilic addition, providing cyclopenta[b]dihydronaphthofurans bearing a tetra- or fully substituted cyclopentane core in good yields with exclusive diastereoselectivities (>20 : 1 d.r.).

Copper-Catalyzed Enantioselective C1,N-Dipolar (3+2) Cycloadditions of 2-Aminoallyl Cations with Indoles

2-Aminoallyl cations are versatile 1,3-dipoles that could potentially be used for diverse (3+n) cycloaddition reactions. Despite some preliminary studies, the asymmetric catalytic transformation of these species is still underdeveloped. We herein report a binuclear copper-catalyzed generation of 2-aminoallyl cations from ethynyl methylene cyclic carbamates and their enantioselective (3+2) cycloaddition reaction with indoles to construct chiral pyrroloindolines. This transformation features a novel C1,N-dipolar reactivity for 2-aminoallyl cations.

1,4-Dithianes: attractive C2-building blocks for the synthesis of complex molecular architectures

This review covers the synthetic applications of 1,4-dithianes, as well as derivatives thereof at various oxidation states. The selected examples show how the specific heterocyclic reactivity can be harnessed for the controlled synthesis of carbon-carbon bonds. The reactivity is compared to and put into context with more common synthetic building blocks, such as 1,3-dithianes and (hetero)aromatic building blocks. 1,4-Dithianes have as yet not been investigated to the same extent as their well-known 1,3-dithiane counterparts, but they do offer attractive transformations that can find good use in the assembly of a wide array of complex molecular architectures, ranging from lipids and carbohydrates to various carbocyclic scaffolds. This versatility arises from the possibility to chemoselectively cleave or reduce the sulfur-heterocycle to reveal a versatile C2-synthon.

Carbocycloaddition Strategies for Troponoid Synthesis

Tropone is the prototypical aromatic 7-membered ring, and can be found in virtually any undergraduate textbook as a key example of non-benzenoid aromaticity. Aside from this important historical role, tropone is also of high interest as a uniquely reactive synthon in complex chemical synthesis as well as a valuable chemotype in drug design. More recently, there has been growing interest in the utility of tropones for catalysis and material science. Thus, synthetic strategies capable of synthesizing functional tropones are key to fully exploiting the potential of this aromatic ring system. Cycloaddition reactions are particularly powerful methods for constructing carbocycles, and these strategies in turn have proven to be powerful for generating troponoids. The following review article provides an overview of strategies for troponoids wherein the 7-membered carbocycle is generated through a cycloaddition reaction. Representative examples of each strategy are also provided.

Dearomative (4 + 3) Cycloaddition Reactions of 3-Alkenylindoles and 3-Alkenylpyrroles to Afford Cyclohepta[b]indoles and Cyclohepta[b]pyrroles

The dearomative (4 + 3) cycloaddition reactions of 3-alkenylindoles with in situ-generated oxyallyl cations furnish cyclohepta[b]indoles, functionality-rich frameworks found in many bioactive compounds, including all pentacyclic ambiguine alkaloids. The analogous reactions between oxyallyl cations and 3-alkenylpyrroles afford cyclohepta[b]pyrroles. The cycloadducts are generally formed in good to high yields and diastereoselectivities and can be readily transformed into useful derivatives. Additionally, we report preliminary investigations into the enantioselective catalysis of the dearomative (4 + 3) cycloaddition using imidodiphosphorimidate catalysts.

Studies towards the Synthesis of (+)‐Dictyoxetane

The dolabellane-type diterpene dictyoxetane represents a significant challenge to synthetic organic chemistry. Methodology directed towards the total synthesis of naturally occurring (+)-dictyoxetane is reported. Catalytic asymmetric synthesis of the trans-hydrindane ring system is achieved through chemoselective deoxygenation of the Hajos-Parrish ketone. An alternative to the Garst-Spencer furan annulation is developed for the synthesis of a 2,5-dimethyl, tetrasubstituted furan, employing a tandem 5-exo-dig alcohol to alkyne cyclisation/aromatisation reaction as a key step. The (4+3) cycloaddition reaction of an oxyallyl cation with a tetrasubstituted furan is established on a cyclohexanone-derived model system, and a range of related (4+3) cycloadditions investigated on a homochiral, trans-hydrindane-fused furan, where regio- and diastereoselectivity is required for the natural product synthesis. In an alternative (4+2) Diels-Alder approach, a C₂ -symmetric vinyl sulfoxide-based chiral ketene equivalent is used to prepare oxanorbornenes with the same oxygen bridge stereochemistry found in the 2,7-dioxatricyclo[4.2.1.0^3,8 ]nonane ring system of the natural product.

Oxidative bridgehead functionalization of (4 + 3) cycloadducts obtained from oxidopyridinium ions

Treatment of selected (4 + 3) cycloadducts derived from oxidopyridinium ions with N-iodosuccinimide (NIS) in hexafluoroisopropanol (HFIP) resulted in the formation of bridgehead ethers via a net oxidative C-H activation.

Heck Reaction of 2-Oxyacrylates with Aryl Bromides: A Common Route to Monoaryl Pyruvates and Ortho Ester-Protected Monoaryl Pyruvates

A palladium-catalyzed Heck reaction between 2-oxyacrylates and aryl bromides was developed, where DavePhos was a unique ligand that efficiently promoted the reaction. The products, 2-oxycinnamates, served as excellent precursors, providing synthetically useful monoaryl pyruvates or ortho ester-protected monoaryl pyruvates depending on the nature of the 2-oxy group. The formation of such ortho esters via alkoxide addition is novel, and computational studies identified a plausible mechanism with an oxyallyl zwitterion as the key intermediate.

Intramolecular (4+3) Cycloadditions of Oxidopyridinium Ions: Towards Daphnicyclidin A

N-alkylation of 5-hydroxynicotinic acid esters with electrophiles containing diene functionality produces salts that undergo intramolecular (4+3) cycloaddition reactions upon heating in the presence of base. Initial studies used a three-carbon tether to join the pyridinium ion and diene, revealing some aspects of the inherent selectivity of the reaction with such substrates. Much more challenging was the synthesis of related species possessing only a two-carbon tether. Nevertheless, the cycloaddition of such compounds was successful, leading directly to the ABC ring system of the alkaloid daphnicyclidin A.

Dearomative (3 + 2) Cycloadditions of Unprotected Indoles

The (3 + 2) cycloaddition of various indoles with a dithioallyl cation affords dearomatized cyclopentannulated adducts, with complete control of regioselectivity and excellent chemo- and diastereoselectivity. The success of the reaction critically relies on the use of an excess of very strong Brønsted acid, which paradoxically prevents carbocationic side reactions. The reaction tolerates sensitive functionalities such as basic amines or free hydroxyls, and we demonstrate its use in late stage derivatization of highly functionalized, unprotected indoles.

Tropane Skeleta from the Intramolecular Photocycloaddition of (4+3) Cycloadducts of Oxidopyridinium Ions and Dienes

Easily prepared cycloadducts derived from the (4+3) cycloaddition of oxidopyridinium ions with dienes reacted intramolecularly in a [2+2] cycloaddition process to afford complex polycyclic species in which the tropane skeleton was embedded.

Catalytic Asymmetric (3 + 3) Cycloaddition of Oxyallyl Zwitterions with α-Diazomethylphosphonates

The unique structure of oxyallyls represents a significant challenge for their catalytic asymmetric applications. Herein, an unprecedented chiral imidodiphosphoric acid-catalytic enantioselective (3 + 3) cycloaddition between oxyallyl zwitterions generated in situ from α-haloketones and α-diazomethylphosphonates was developed. Pharmaceutically interesting chiral pyridazine-4(1H)-ones were obtained in up to 98% yields with excellent stereoselectivities (up to 99% ee, > 99:1 dr).

Generation of the 7-Azabicyclo[4.3.1]decane Ring System via (4 + 3) Cycloaddition of Oxidopyridinium Ions

Oxidopyridinium ions bearing an ester group at the 5-position undergo (4 + 3) cycloaddition reaction to afford congeners of 7-azabicyclo[4.3.1]decane. The reaction generally proceeds in high yield, although an excess of diene is often required to achieve such yields. The reaction is highly regioselective, but not endo/exo selective. It appears the cycloaddition process can be either kinetically or thermodynamically controlled, depending on the nature of the diene used and the reaction time. An intramolecular Heck reaction was used to demonstrate that some chemistry is possible with the cycloadducts.

Stereoselective Gold(I)-Catalyzed Vinylcyclopropanation via Generation of a Sulfur-Substituted Vinyl Carbene Equivalent

A stereoselective gold(I)-catalyzed vinylcyclopropanation of alkenes has been developed. A gold-coordinated cationic vinyl carbene species, readily generated via a rearrangement of the ethylenedithioacetal of propargyl aldehyde, reacts with a wide range of alkenes to afford thio-substituted vinylcyclopropanes. The gold-catalyzed vinyl cyclopropanation proceeds under mild conditions at room temperature and is generally selective for the formation of cis-substituted cyclopropanes. The reaction allows the formal introduction of a "naked" vinyl carbene, by subsequent chemoselective hydrodesulfurisation of the ethylenedithio-bridge. The synthetic utility of the new method is demonstrated by a short, racemic formal synthesis of the alkaloid cephalotaxin, hinging on a key vinyl cyclopropane-cyclopentene rearrangement.

Pd/LA-catalyzed decarboxylation enabled exclusive [5 + 2] annulation toward N-aryl azepanes and DFT insights

A practical approach towards the synthesis of non-fused N-aryl azepane derivatives with diversity is described. DFT calculations revealed the origins of this unusual exclusive [5 + 2] rather than empirical [3 + 2] annulation process.

Access to Vinyl Ethers and Ketones with Hypervalent Iodine Reagents as Oxy-Allyl Cation Synthetic Equivalents

We report an Umpolung strategy of enol ethers to generate oxy-allyl cation equivalents based on the use of hypervalent iodine reagents. Under mild basic conditions, the addition of nucleophiles to aryloxy-substituted vinylbenziodoxolone (VBX) reagents, easily available in two steps from silyl alkynes, resulted in the stereoselective formation of substituted aryl enol ethers. The reaction was most efficient with phenols as nucleophiles, but preliminary results were also achieved for C- and N- nucleophiles. In absence of external nucleophiles, the 2-iodobenzoate group of the reagent was transferred. The obtained aryl enol ethers could then be transformed into α-difunctionalized ketones by oxidation. The described "allyl cation"-like reactivity contrast with the well-established "vinyl-cation" behavior of alkenyl iodonium salts.

Air-Stable Oxyallyl Patterns and a Switchable N-Heterocyclic Carbene

Oxyallyl derivatives are typically elusive compounds. Even recently reported "stabilized" 1,3-diaminooxyallyl species are still highly reactive and have short lifetimes at room temperature. Herein, we report the synthesis and preliminary study of mesoionic pyrimidine derivatives that feature 1,3-bis(dimethylamino)oxyallyl patterns with an unprecedented level of stabilization. The latter are not only insensitive towards air and moisture, but they are also compatible with the formation of an ancillary stable N-heterocyclic carbene moiety. As the oxyallyl pattern is proton-responsive, it allows the reversible switching of the electronic properties of the carbene, as a ligand.

Cooperative Catalysis for the Highly Diastereo- and Enantioselective [4+3]-Cycloannulation of ortho-Quinone Methides and Carbonyl Ylides

We describe herein a highly diastereo- and enantioselective [4+3]-cycloannulation of ortho-quinone methides and carbonyl ylides to furnish functionalized oxa-bridged dibenzooxacines with excellent yields and stereoselectivity in a single synthetic step. The combination of rhodium and chiral phosphoric acid catalysis working in concert to generate both transient intermediates in situ provides direct access to complex bicyclic products with two quaternary and one tertiary stereogenic centers. The products may be further functionalized into valuable and enantiomerically highly enriched building blocks.

Synthesis of Cyclohepta[b]indoles by (4 + 3) Cycloaddition of 2-Vinylindoles or 4H-Furo[3,2-b]indoles with Oxyallyl Cations

The synthesis of cyclohepta[b]indole derivatives through the dearomative (4 + 3) cycloaddition reaction of 2-vinylindoles or 4H-furo[3,2-b]indoles with in situ generated oxyallyl cations is reported. Oxyallyl cations are generated from α-bromoketones in the presence of a base and a perfluorinated solvent. Cyclohepta[b]indole scaffolds are obtained under mild reaction conditions, in the absence of expensive catalysts, starting from simple reagents, in good to excellent yields and with complete diasteroselectivity. Preliminary expansion of the scope to 3-vinylindoles and to aza-oxyallyl cations is reported.

Rhodium-Catalyzed (5 + 2) and (5 + 1) Cycloadditions Using 1,4-Enynes as Five-Carbon Building Blocks

Cycloaddition reactions are a hallmark in organic synthesis because they provide an efficient way to construct highly substituted carbo- and heterocycles found in natural products and pharmaceutical agents. Most cycloadditions occur under thermal or photochemical conditions, but transition-metal complexes can promote reactions that occur beyond these circumstances. Transition-metal complexation with alkynes, alkenes, allenes, or dienes often alters the reactivity of those π-systems and facilitates access to diverse cycloaddition products. This Account describes our efforts toward the design of novel five-carbon synthons for use in rhodium-catalyzed (5 + n) cycloadditions, which include 3-acyloxy-1,4-enynes (ACEs) for (5 + 1) and (5 + 2) cycloadditions and 3-hydroxy-1,4-enynes (HYEs) for (5 + 1) cycloadditions. Furthermore, this Account includes relevant computational information, mechanistic insights, and applications of these cycloadditions in the synthesis of various highly substituted carbo- and heterocycles. The (5 + n) cycloaddition reactions presented herein share the following common mechanistic features: the 1,2-migration of an acyloxy group in propargyl esters or the ionization of a hydroxyl group in propargylic alcohols, oxidative cyclization to form a metallacycle, insertion of the one- or two-carbon component, and reductive elimination to yield the final product. In conjunction with a cationic rhodium catalyst, we used ACEs for the intramolecular (5 + 2) cycloaddition with tethered alkynes, alkenes, and allenes. In some cases, an electron-deficient phosphine ligand improved the reaction yields, especially when the ACE featured an internal alkyne. We also demonstrated that chirality could be efficiently transferred from a relatively simple starting material to a more complex bicyclic product. Products derived from ACEs with tethered alkenes and allenes contained one or more stereocenters, and high diastereoselectivity was achieved in most of these cases. For ACEs tethered to an allene, the reaction preferentially occurred at the internal alkene. We also switched the positions of the alkene and the alkyne in the 1,4-enyne of our original ACE to provide an inverted ACE variant, which produced products with complementary functionalities. After we successfully developed the Rh-catalyzed intramolecular (5 + 2) cycloaddition, we optimized conditions for the intermolecular version, which required a neutral rhodium catalyst and phosphine ligand. When a terminal alkyne was used as the two-carbon component, high regioselectivity was observed. While investigating the effect of esters on the rate of the intermolecular (5 + 2) cycloadditions, we determined that an electron-rich ester significantly accelerated the reaction. Subsequently, we demonstrated that (5 + 1) cycloadditions undergo this rate enhancement as well in the presence of an ester. Aside from ACEs, we synthesized HYEs in four steps from commercially available 2-aminobenzoic acid for use in the (5 + 1) cycloaddition. Mechanistically, HYEs were designed so that the aniline nitrogen could serve as the nucleophile and the -OH could serve as the leaving group. Using HYEs, we developed a novel method to make substituted carbazoles, dibenzofurans, and tricyclic compounds with a cyclohexadienone moiety. Although the occurrence of transition-metal-catalyzed acyloxy migrations has been known for decades, only recently has their synthetic value been realized. We hope our studies that employ readily available 1,4-enynes as the five-carbon components in (5 + n) cycloadditions can inspire the design of new two-component and multicomponent cycloadditions.

(4 + 3) cycloadditions of allenyl ether-derived oxygen-stabilized oxyallyls with furans

(4 + 3) Cycloadditions between allenyl ethers and furans are described. The reaction features anin situformation of oxygen-stabilized oxyallylsviaepoxidations of allenyl ethers in the presence of H₂PO₄⁻.

Divergent syntheses of spirooxindoles from oxindole-embedded four-membered synthon via cycloaddition reactions

The construction of five and six membered heterocycle fused spirooxindoles was achieved via the [4 + 1] and formal [4 + 2] cycloadditions between our rationally designed four-membered synthons and pyridinium methylides and α-bromoacetophenones, respectively.