Cycloaddition of cyclopropanes for the elaboration of medium-sized carbocycles

Visible-Light-Photocatalyzed Self-Cyclopropanation Reactions of Dibenzoylmethanes for the Synthesis of Cyclopropanes

A new self-cyclopropanation of 1,3-diphenylpropane-1,3-dione, leading to tetrasubstituted cyclopropane containing three contiguous stereogenic centers with high stereoselectivity, has been achieved through violet-light-emitting diode-irradiated photocatalysis, featuring both cycloaddition and a distinctive rearrangement. Diverging from conventional cyclopropanation pathways, this reaction yields a tetrasubstituted cyclopropane through unprecedented rearrangement and cascade reactions.

B(C6F5)3-Catalyzed Formal (n + 3) (n = 5 and 6) Cycloaddition of Bicyclo[1.1.0]butanes to Medium Bicyclo[n.1.1]alkanes

Herein, a B(C6F5)3-catalyzed formal (n + 3) (n = 5 and 6) cycloaddition of bicyclo[1.1.0]butanes (BCBs) with imidazolidines/hexahydropyrimidines is described. The reaction provides a modular, atom-economical, and efficient strategy to two libraries of synthetically challenging medium-bridged rings, 2,5-diazabicyclo[5.1.1]nonanes and 2,6-diazabicyclo[6.1.1]decanes, in moderate to excellent yields. This reaction also features simple operation, mild reaction conditions, and broad substrate scope. A scale-up experiment and various synthetic transformations of products further highlight the synthetic utility.

Ruthenium-Catalyzed Carbonylation of α-Aminoaryl-Tethered Alkylidenecyclopropanes: Synthesis of Eight-Membered Benzolactams

The synthesis of medium-sized lactams is a great challenge because of the unfavorable transannular interactions and entropic barriers in the transition state. We have developed a ruthenium-catalyzed carbonylation of α-aminoaryl-tethered alkylidenecyclopropanes (ACPs) that allows for the efficient preparation of valuable eight-membered benzolactams under ligand-free conditions. The amino group served a dual role of both directing group and nucleophile to facilitate the metallacycle formation and the carbonylation.

Synthesis of medium-sized heterocycles from oxetanes based on an allylic amination/ring-opening strategy

The construction of medium-sized ring compounds has been a prominent research area in synthetic chemistry. In this study, we developed a tandem strategy that combines allylic amination and ring-opening of oxetanes to synthesize medium-sized heterocycles. Specifically, N-aryl oxetan-3-amines undergo allylic amination with zwitterionic π-allylpalladium, followed by intramolecular ring-opening, resulting in the formation of medium-sized heterocycles. Notably, we are able to achieve the synthesis of 7-8 membered heterocycles with moderate to good yields by employing different types of zwitterionic π-allylpalladium species.

Molybdenum-Mediated Reductive Hydroamination of Vinylcyclopropanes with Nitroarenes: Synthesis of Homoallylamines

A molybdenum-mediated reductive hydroamination of vinylcyclopropanes with nitroarenes has been developed. A broad range of substituted homoallylamines were prepared in good to excellent yields from readily available starting materials. No noble metal catalysts were used in this reaction, and Mo(CO)₆ acted as both catalyst and reductant. This protocol provides an effective method for the selective synthesis of substituted homoallylamines from easily available nitroarenes.

Recent Progress in Heterocycle Synthesis: Cyclization Reaction with Pyridinium and Quinolinium 1,4-Zwitterions

Heteroarene 1, n-zwitterions are powerful and versatile building blocks in the construction of heterocycles and have received increasing attention in recent years. In particular, pyridinium and quinolinium 1,4-zwitterions have been widely studied and used in a variety of cyclization reactions due to their air stability, ease of use, and high efficiency. Sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions, types of emerging heteroatom-containing synthons, have attracted much attention from chemists. These 1,4-zwitterions, which contain multiple reaction sites, have been successfully used in the synthesis of three- to eight-membered cyclic compounds over the last decade. In this review, we present the exciting progress made in the field of cyclization reactions of sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions. Moreover, the mechanistic insights, the transition states, some synthetic applications, and the challenges and opportunities are also discussed. We hope to provide an overview for synthetic chemists who are interested in the heterocycle synthesis from cyclization reaction with pyridinium and quinolinium 1,4-zwitterions pyridinium and quinolinium 1,4-zwitterions.

Intramolecular Rhodium-Catalyzed [(3+2+2)] Carbocyclization Reactions with Dienylidenecyclopropanes: A Concise and Stereoselective Total Synthesis of the Sesquiterpene (+)-Zizaene

The development of an intramolecular rhodium-catalyzed [(3+2+2)] carbocyclization reaction of alkylidenecyclopropanes (ACPs) tethered to 1,4- and 1,5-skipped dienes is described. This transformation offers a new approach for the construction of bridged tricyclic compounds with up to three quaternary centers, which are suitable for the synthesis of challenging bioactive natural products. For instance, the synthetic utility of this transformation is illustrated through a concise asymmetric total synthesis of the sesquiterpene (+)-zizaene in ten steps from a commercially available starting material.

Merging Two Strained Carbocycles: Lewis Acid Catalyzed Remote Site-Selective Friedel-Crafts Alkylation of in Situ Generated β-Naphthol

Lewis acid catalyzed tandem activation of the two smallest carbocycles, 3-ethoxy cyclobutanones, and donor-acceptor cyclopropanes has been demonstrated. The diphenyl-substituted 3-ethoxy cyclobutanone rearranges itself by intramolecular cyclization for the in situ generation of 1-phenyl 2-naphthol, which further undergoes remote site-selective Friedel-Crafts alkylation with donor-acceptor cyclopropane to synthesize a series of γ-naphthyl butyric acid derivatives. Further control experiments for mechanistic investigations and synthetic applications have also been carried out.

Synthesis of Indole-Fused Dihydrothiopyrano Scaffolds via (3 + 3)-Annulations of Donor-Acceptor Cyclopropanes with Indoline-2-Thiones

A new methodology for the synthesis of N-haloindole-fused dihydrothiopyrano derivatives via (3 + 3)-annulation of donor-acceptor cyclopropanes (DACs) with indoline-2-thiones in the presence of Sc(OTf)₃ as a Lewis acid catalyst has been developed. This protocol provides a variety of indole-fused dihydrothiopyrano molecules in good to excellent yields, which architecturally resemble other indole-fused tricyclic molecules having potential medicinal value. In addition, we have described a detailed reaction mechanism and transformation of the furnished product into N-fused thiazino indole molecule.

Ring Expansion of Isatins via 1,2-Phospha-Brook Rearrangement: A Route to the Synthesis of 2-Quinolinone-Derived p-Quinone Methides

A Lewis acid-mediated one-carbon homologation approach to installing a 2-quinolinone core embedded with para-quinone methides, in a high yield of up to 92%, and with high regioselectivity has been developed. Also, post-synthetic modifications, including C-P, C-S, and C-C bond formations, have been demonstrated by the 1,6-addition of suitable nucleophiles. Further, cyclopropanation of 2-quinolinone-embedded p-QM is also demonstrated affording a contiguous quaternary spiro center.

(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.

Enantioselective Synthesis of Bispiro[indanedione-oxindole-cyclopropane]s through Organocatalytic [2+1] Cycloaddition

A series of compounds featuring a novel bispiro[indanedione-oxindole-cyclopropane] moiety have been synthesized through a squaramide-catalyzed [2+1] cycloaddition reaction. The tandem Michael-alkylation reaction of 2-arylidene-1,3-indanediones with 3-bromooxindoles furnished the cycloadducts in high yields with excellent diastereo- and enantioselectivities. The ammonium ylide in the catalytic process, as a key intermediate, was revealed by the high-resolution mass spectrometry study.

A metal-free BF3·OEt2 mediated chemoselective protocol for the synthesis of propargylic cyclic imines

A chemoselective and metal/additive-free protocol for the synthesis of propargylic cyclic imine derivatives via (3 + 2)-cycloaddition of donor-acceptor cyclopropanes and alkynylnitriles in the presence of BF₃·OEt₂ has been established. The newly developed methodology provided access to a variety of propargylic cyclic imines in good to excellent yields. In addition, the synthesis of propargylic amines and the corresponding very stable enol derivatives from the title compound is also explored.

Enantioselective [3 + 2] Cycloaddition of Vinylcyclopropanes with Alkenyl N-Heteroarenes Enabled by Palladium Catalysis

The first catalytic enantioselective [3 + 2] cycloaddition reaction between vinylcyclopropanes and alkenyl N-heteroarenes in the presence of LiBr and a Pd(0)/SEGPHOS complex was developed. LiBr plays a key role in improving the reactivity of alkenyl N-heteroarenes as a mild Lewis acid.

High-order dipolar annulations with metal-containing reactive dipoles

Medium-sized heterocycles are widespread among a spectrum of structurally intriguing and biologically significant natural products and synthetic pharmaceuticals. Metal-catalyzed high-order dipolar annulations resembling reactions of metal-containing reactive dipoles with dipolarophiles constitute a highly efficient and flexible strategy for constructing medium-sized heterocycles. Mechanistically, these annulation reactions usually proceeding through stepwise pathways are different from the classic high-order pericyclic reactions that follow the Woodward-Hoffman rules. More significantly, asymmetric high-order dipolar annulations using chiral organometallic catalysts have been proven successful for constructing chiral medium-sized heterocycles with high enantio- and diastereoselectivity. This review highlights the impressive advances in this area and is focused on the reactivity, scope, mechanisms and applications of high-order dipolar annulation reactions.

Gold/scandium bimetallic relay catalysis of formal [5+2]- and [4+2]-annulations: access to tetracyclic indole scaffolds

Regiodivergent formal [5+2]- and [4+2]-annulation reactions of indole derivatives with 2-(2-alkynyl)aryl cyclopropane-1,1-diesters (ACPs) have been developed. A series of tetracyclic indole derivatives were delivered in a 77% average yield with excellent regioselectivities enabled by Au(I)/Sc(III) bimetallic relay catalysis. A gram-scale reaction and further transformation of the resulting tetracyclic indoles demonstrated the practical utility of this protocol. Moreover, the photophysical properties of the obtained multicyclic compounds were also investigated.

Lewis acid-catalyzed [3 + 2] annulations of oxindole based spirocyclic donor-acceptor cyclopropanes with ynamides

[3 + 2] annulations of oxindole based spirocyclic donor-acceptor cyclopropanes and ynamides catalyzed by copper triflate have been developed for the synthesis of biologically important spirocyclopenteneoxindoles. These reactions tolerated a wide scope of substrates and provided the desired products in good to high yields (up to 90%) with up to >40 : 1 diastereoselectivities under mild conditions.

Radical 1,5-Chloropentafluorosulfanylation of Unactivated Vinylcyclopropanes and Transformation into α-SF5 Ketones

The radical 1,5-chloropentafluorosulfanylation of vinyl cyclopropanes (VCPs) initiated by Et₃B/O₂ affords allylic pentafluorosulfanyl/homoallylic chloride products through the ring-strain release of the cyclopropane. The VCP substitution pattern was investigated. The utility of this reaction was illustrated in post-transformation of the C═C bond by ozonolysis, giving access to valuable α-SF₅ carbonyl compounds.

Visible-Light-Induced Dearomatization of Indoles/Pyrroles with Vinylcyclopropanes: Expedient Synthesis of Structurally Diverse Polycyclic Indolines/Pyrrolines

Visible-light-induced cycloaddition reactions initiated via energy-transfer processes have recently evolved as powerful methods for the construction of strained cyclic molecules that are not easily accessed using known ground-state synthetic methods. Particularly, the reactions initiated by the excitation of aromatic rings provide an alternative solution to the direct transformations of aromatic feedstocks under the scheme of dearomatization. Vinylcyclopropanes (VCPs) are well-known reagents in radical clock experiments, working as a probe to detect transient radical intermediates. However, the synthetic applications in this regard still remain limited due to uncontrollable selectivities. Herein, we report visible-light-induced dearomatization of indole- or pyrrole-tethered VCPs, in which several competitive reaction pathways, including [5 + 2], [2 + 2], interrupted [5 + 2], and [5 + 4] cycloadditions, can be well regulated by engineering substrate structures and tuning reaction conditions. The reaction mechanism has been explored by combined experimental and computational investigations. These reactions provide a convenient method to synthesize structurally diverse polycyclic molecules with high efficiency and good selectivity.

Visible-light-induced, catalyst and additive-free cycloaddition of vinylcyclopropanes: access to sulfur-containing seven-membered heterocycles

The first visible-light-induced, catalyst and additive-free protocol for [5 + 2] cycloadditions of vinylcyclopropanes (VCPs) has been achieved.