Catalytic Asymmetric Tandem Cycloisomerization/[5+2] Cycloaddition Reaction of N-Aryl Nitrone Alkynes with Methyleneindolinones

Construction of Chiral Spiro-Bridged Rings with Four Consecutive Stereocenters via Dearomative Diels-Alder Reactions of Anthracenes

A highly diastereo- and enantioselective dearomative Diels-Alder reaction was accomplished by chiral N,N'-dioxide/Mg(II) complex catalyst. Various anthracene derivatives and methyleneindolinones efficiently transformed into the corresponding chiral spiro-bridged cyclic products with four consecutive stereocenters in good yields, excellent dr and er values under mild conditions (46 examples, up to 99% yield, >19:1 dr, >99:1 er). Gram-scale synthesis of chiral products and their further transformations were feasible. On the basis of theoretical calculation, possible working modes were provided to understand the origin of stereoselectivity of this transformation.

Asymmetric Catalytic Aziridination to Synthesize Spiro-aziridine Oxindoles

Asymmetric catalytic aza-Michael-initiated ring closure of methyleneindolinones with N-tosyloxycarbamates has been established. The reaction using a chiral nickel complex catalyst enabled the formation of a series of spiro-aziridine oxindoles in good yields (up to 99 %) with high stereoselectivity (up to 97 % ee, >19 : 1 dr) under mild reaction conditions. Ring-opening of spiro-aziridine oxindole leads to formation of glycinate-bearing oxindoles with retention of configuration.

Asymmetric Catalytic (3 + 2) Cyclization and Sequential Reaction to Construct Dihydrofuran- and Azepine-Based Spirooxindoles

The enantioselective formal (3 + 2) cyclization and sequential reaction of 2-malononitrile-substituted oxindoles with benzaldehydes and ortho-aminobenzaldehydes were achieved by chiral N,N'-dioxide/metal complex Lewis acid catalysts. This protocol supplies facile and efficient access to highly functionalized chiral dihydrofuran- and azepine-based spirooxindoles. Based on the control experiments and the deuterium labeling studies, the interconversion of (3 + 2) diastereomeric intermediates under the reaction conditions and reversible 1,5-H transfer step were disclosed.

Crystallization-induced diastereomer transformation assists the diastereoselective synthesis of 4-nitroisoxazolidine scaffolds

This communication describes a solution to the vexing problem of synthesis of 4-nitroisoxazolidine rings from cyclic nitrones and β-nitrostyrenes. The trans-endo adduct 2 is quantitatively synthesised from E-β-nitrostyrene under mild conditions avoiding purification, while the trans-exo adduct 4 is obtained at higher temperatures. Furthermore, a Crystallization-Induced Diastereomer Transformation (CIDT) process was used to epimerise the NO2 bond from 2 to the cis-exo adduct 3 with total conversion assisted by the para-halogen substitution of the aromatic ring without any additives. By combining these approaches, we can establish a simple synthetic methodology that allows the diastereoselective synthesis of three diastereoisomers, overcoming the previous problems described in the literature.

Rhodium(I)-Catalyzed Cascade Annulation of 1,n-Diynyl Nitrones to 3,4-Fused Fully Substituted Furans

A method for the assembly of fully substituted furans containing a 3,4-fused 5-8-membered carbocycle, heterocycle, or spirocycle from rhodium(I)-catalyzed and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)-assisted cascade annulation of 1,n-diynyl nitrones has been developed.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Recent synthetic strategies of spiro-azetidin-2-one, -pyrrolidine, -indol(one) and -pyran derivatives-a review

Spiro-heterocycles have received special attention in medicinal chemistry because of their promising biological activity. Over the years, many synthetic methodologies have been established for the construction of spirocyclic compounds. Spiro heterocycles such as spiro-azetidin-2-one, -pyrrolidine, -indol(one) and -pyran derivatives have been found to exhibit diversified biological and pharmacological activity in addition to their therapeutic properties. In view of these facts, we decided in this review to present representative synthetic approaches of the aforementioned spiro heterocycles, especially in the past 20 years.

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.

Asymmetric synthesis of complex tricyclo[3.2.2.0]nonenes from racemic norcaradienes: kinetic resolution via Diels-Alder reaction

Herein, the enantioselective synthesis of complex tricyclo[3.2.2.0]nonenes through the Diels-Alder reaction is reported. Utilizing racemic norcaradienes prepared from the visible-light-mediated dearomative cyclopropanation of m-xylene as dienes and enone derivatives as dienophiles, the overall process represents a kinetic asymmetric transformation in the presence of a chiral cobalt(ii) complex of chiral N,N'-dioxide. High diastereo- and enantioselectivity could be obtained in most cycloaddition processes and part racemization of norcaradiene is observed. The topographic steric maps of the catalysts were collected to rationalize the relationship between reactivity and enantioselectivity with the catalysts.

Intramolecular Copper-Catalyzed Asymmetric Propargylic [4 + 2]- Cycloaddition toward Optically Active Tetrahydroisoindolo[2,1-a]quinoxalines

An intramolecular Cu-catalyzed asymmetric propargylic [4 + 2] cycloaddition of bis-N-nucleophile-functionalized propargylic esters has been realized in the support of a chiral tridentate N-ligand, (S,S)-Pybox-diOAc, leading to chiral tetrahydroisoindolo[2,1-a]quinoxalines in high yields and with good to excellent enantioselectivities. The reaction features high efficiency, simplicity, and broad substrate scope, thus providing a powerful and concise strategy for stereoselective access to optically active polycyclic heterocycle frameworks that are otherwise difficult to synthesize.

Asymmetric Synthesis of Nortropanes via Rh-Catalyzed Allylic Arylation

Tropane derivatives are extensively used in medicine, but catalytic asymmetric methods for their synthesis are underexplored. Here, we report Rh-catalyzed asymmetric Suzuki–Miyaura-type cross-coupling reactions between a racemic N-Boc-nortropane-derived allylic chloride and (hetero)aryl boronic esters. The reaction proceeds via an unexpected kinetic resolution, and the resolved enantiopure allyl chloride can undergo highly enantiospecific reactions with N-, O-, and S-containing nucleophiles. The method was applied in a highly stereoselective formal synthesis of YZJ-1139(1), a potential insomnia treatment that recently completed Phase II clinical trials. Our report represents an asymmetric catalytic method for the synthesis of YZJ-1139(1) and related 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.

Catalytic asymmetric transformation of nitrones and allenes to dihydropyridoindoles via chiral N,N'-dioxide/cobalt(II) catalysis

A chiral N,N'-dioxide/cobalt(II) complex catalytic system is developed to promote the multistep cascade reaction of α,β-unsaturated-N-aryl nitrones with allenes, giving a variety of chiral dihydropyridoindoles in moderate to good yields with excellent dr and ee values. Mechanistic studies support a [3+2] cycloaddition/[3,3]-rearrangement/retro-Mannich process.

Stereoselective synthesis of fused-, spiro- and bridged heterocycles via cyclization of isoquinolinium salts: A recent update

Isoquinoline and its derivatives are ubiquitous in natural alkaloids, synthetic materials and pharmaceuticals with broad spectrum of biological activities. In particular, isoquinolinium salts are important in organic synthesis because they...

An update on the progress of cycloaddition reactions of 3-methyleneindolinones in the past decade: versatile approaches to spirooxindoles

Cycloaddition reactions are of great interest due to their potential and rapid construction of optically enriched spiro-cyclic products. 3-Methyleneindolinones have been proven to be a valuable precursor in cycloaddition reactions for the construction of diverse 3,3'-spirocyclic oxindoles. Their versatile reactivity has provided a new forum for the development of a variety of building blocks and synthetic compounds, including bioactive molecules. Herein, significant accomplishments in the cycloaddition reactions of 3-methyleneindolinones for the synthesis of spirooxindoles have been summarised and elaborated. The review is outlined according to the type of cycloaddition such as [2 + 1], [2 + 2], [3 + 2], [4 + 2] and [5 + 2] cycloaddition reactions.

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.

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.

Asymmetric epoxidation of α,β-unsaturated ketones catalyzed by rare-earth metal amides RE[N(SiMe3)2]3 with chiral TADDOL ligands

The first application of rare-earth metal catalyst combined with TADDOLs in the asymmetric epoxidation of α,β-unsaturated ketones is disclosed.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Diversified Transformations of Tetrahydroindolizines to Construct Chiral 3-Arylindolizines and Dicarbofunctionalized 1,5-Diketones

Enantioselective diverse synthesis of a small-molecule collection with structural and functional similarities or differences in an efficient manner is an appealing but formidable challenge. Asymmetric preparation and branching transformations of tetrahydroindolizines in succession present a useful approach to the construction of N-heterocycle-containing scaffolds with functional group, and stereochemical diversity. Herein, we report a breakthrough toward this end via an initial diastereo- and enantioselective [3 + 2] cycloaddition between pyridinium ylides and enones, following diversified sequential transformations. Chiral N,N'-dioxide-earth metal complexes enable the generation of optically active tetrahydroindolizines in situ, across the strong background reaction for racemate-formation. In connection with deliberate sequential transformations, involving convenient rearomatic oxidation, and light-active aza-Norrish II rearrangement, the tetrahydroindolizine intermediates were converted into the final library including 3-arylindolizine derivatives and dicarbofunctionalized 1,5-dicarbonyl compounds. More importantly, the stereochemistry of four-stereogenic centered tetrahydroindolizine intermediates could be efficiently transferred into axial chirality in 3-arylindolizines and vicinal pyridyl and aryl substituted 1,5-diketones. In addition, densely functionalized cyclopropanes and bridged cyclic compounds were also discovered depending on the nature of the pyridinium ylides. Mechanism studies were involved to explain the stereochemistry during the reaction processes.