Diastereodivergent Asymmetric 1,3-Dipolar Cycloaddition of Azomethine Ylides and β-Fluoroalkyl Vinylsulfones: Low Copper(II) Catalyst Loading and Theoretical Studies

Enantioselective Synthesis of Allylic Sulfones via Rhodium-Catalyzed Direct Hydrosulfonylation of Allenes and Alkynes

A highly regio- and enantioselective hydrosulfonylation using commercially available sodium sulfinates is reported, providing the first direct asymmetric rhodium-catalyzed hydrosulfonylation of allenes/alkynes to synthesize chiral allylic sulfones. Ligand screening studies demonstrated the indispensable role of the C1-symmetric P,N-ligand (Rax,S,S)-StackPhim for achieving both high regioselecitivity (>20:1) and enantioselectivity (up to 97% ee). Notably, the operationally simple method and mild conditions allow for the rapid preparation of chiral allylic sulfones with a wide scope of functional groups. Moreover, the use of sodium tert-butyldimethylsilyloxymethanesulfinate enables the collective synthesis of various chiral sulfone derivatives after simple transformations of the protected hydroxymethyl product.

Organocatalyzed Asymmetric Conjugate Addition of Alcohols to β-Fluoroalkyl Vinylsulfones by Bifunctional Phosphonium Salt Catalyst

Chiral secondary alcohols, serving as essential structural motifs, hold significant potential for diverse applications. The exploration of effective synthetic strategies toward these compounds is both attractive and challenging. Herein, we present an asymmetric oxa-Michael reaction involving aliphatic alcohols as nucleophiles and β-fluoroalkyl vinylsulfones catalyzed by bifunctional phosphonium salt (BPS), achieving high yields and excellent enantioselectivities (up to 98 % yield and 98 % ee). Additionally, a sequential process including asymmetric oxa-Michael and debenzylation, facilitated by BPS/Lewis acid cooperation, was revealed for synthesizing diverse chiral secondary alcohol compounds in high yields (81-88 %) with consistent stereoselectivities. Furthermore, mechanistic explorations and subsequent results unveiled that the enantioselectivity originates from hydrogen-bonding and ion-pair interactions between the BPS catalyst and the substrates.

Well-defined Cu(I) complexes based on [N,P]-pyrrole ligands catalyzed a highly endoselective 1,3-dipolar cycloaddition

We herein report the synthesis and catalytic application of a new family of dinuclear Cu(I) complexes based on [N,P]-pyrrole ligands. The Cu(I) complexes (4a-d) were obtained in good yields and their catalytic properties were evaluated in the1,3-dipolar cycloaddition of azomethine ylides and electron-deficient alkenes. The air-stable complexes 4a-d exhibited high endo-diasteroselectivity to obtain substituted pyrrolidines, and the catalytic system showed excellent reactivity and wide substitution tolerance.

Density Functional Theory Calculations: A Useful Tool to Investigate Mechanisms of 1,3-Dipolar Cycloaddition Reactions

The present review contains a representative sampling of mechanistic studies, which have appeared in the literature in the last 5 years, on 1,3-dipolar cycloaddition reactions, using DFT calculations. Attention is focused on the mechanistic insights into 1,3-dipoles of propargyl/allenyl type and allyl type such as aza-ylides, nitrile oxides and azomethyne ylides and nitrones, respectively. The important role played by various metal-chiral-ligand complexes and the use of chiral eductors in promoting the site-, regio-, diastereo- and enatioselectivity of the reaction are also outlined.

Dipolarophile-Steered Formal Stereodivergent Synthesis of 2,5-cis/trans-Pyrrolidines Based on Asymmetric 1,3-Dipolar Cycloaddition of Imino Lactones

The stereodivergent asymmetric synthesis of 2,5-trans/cis pyrrolidines by 1,3-dipolar cycloaddition using two different types of activated alkenes is described. When ylidene-isoxazolones were employed as dipolarophiles, the Ag/(S,Sp )-iPr-FcPHOX-catalyzed asymmetric [3+2] cycloaddition of imino lactones proceeded with 2,5-trans selectivity. Subsequent decarboxylation of the isoxazolone rings produced pyrrolidines with 2,5-trans stereoretention. In the reaction using acyclic enones as activated alkenes, the Ag/(R,Sp )-ThioClickFerrophos complex-catalyzed asymmetric [3+2] cycloaddition afforded 2,5-cis substituted pyrrolidines in high yields and enantioselectivities. Therefore, these methods can be considered as a formal stereodivergent synthesis of 2,5-cis/trans pyrrolidines.

Copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of 1,3-enynes and azomethine ylides

Herein, we report a copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides and 1,3-enynes, which provides a series of chiral poly-substituted pyrrolidines in high regio-, diastereo-, and enantioselectivities. Both 4-aryl-1,3-enynes and 4-silyl-1,3-enynes serve as suitable dipolarophiles while 4-alkyl-1,3-enynes are inert. Moreover, the method is successfully applied in the construction of both tetrasubstituted stereogenic carbon centers and chiral spiro pyrrolidines. The DFT calculations are also conducted, which imply a concerted mechanism rather than a stepwise mechanism. Finally, various transformations started from the pyrrolidine bearing a triethylsilylethynyl group and centered on the alkyne group are achieved, which compensates for the inertness of 4-alkyl-1,3-enynes in the present reaction.

Electron-rich benzofulvenes as effective dipolarophiles in copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides

A series of benzofulvenes without any electron-withdrawing substituents were employed as 2π-type dipolarophiles for the first time to participate in Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition (1,3-DC) reactions of azomethine ylides. An intrinsic non-benzenoid aromatic characteristic from benzofulvenes serves as a key driving force for activation of the electron-rich benzofulvenes. Utilizing the current methodology, a wide range of multi-substituted chiral spiro-pyrrolidine derivatives containing two contiguous all-carbon quaternary centers were formed in good yield with exclusive chemo-/regioselectivity and high to excellent stereoselectivity. Computational mechanistic studies elucidate the origin of the stereochemical outcome and the chemoselectivity, in which the thermostability of these cycloaddition products is the major factor.

11H-Benzo[4,5]imidazo[1,2-a]indol-11-one as a New Precursor of Azomethine Ylides: 1,3-Dipolar Cycloaddition Reactions with Cyclopropenes and Maleimides

The possibility of generating azomethine ylides from 11H-benzo[4,5]imidazo[1,2-a]indol-11-one and amino acids is shown for the first time. Based on the cycloaddition reactions of these azomethine ylides with cyclopropenes and maleimides, cyclopropa[a]pyrrolizines, 3-azabicyclo[3.1.0]hexanes, and pyrrolo[3,4-a]pyrrolizines spiro-fused with a benzo[4,5]imidazo[1,2-a]indole fragment were synthesized. Spirocyclic compounds were obtained in moderate to good yields, albeit with poor diastereoselectivity. Density functional theory calculations were performed to obtain an insight into the mechanism of the 1,3-dipolar cycloaddition of 11H-benzo[4,5]imidazo[1,2-a]indol-11-one-derived azomethine ylides to cyclopropenes. The cytotoxic activity of some of the obtained cycloadducts against the human erythroleukemia (K562) cell line was evaluated in vitro by MTS-assay.

Cu-Catalyzed Switchable Asymmetric Defluoroalkylation and [3 + 2] Cycloaddition of Trifluoropropene

Chiral fluorinated amino esters and pyrrolidines are privileged scaffolds in synthetic chemistry and exhibit unique biological properties. We report the facile preparation of these compounds through copper-catalyzed switchable defluoroalkylation and [3 + 2] cycloaddition of trifluoropropene in an asymmetric fashion. The choice of solvent and chiral ligand was crucial for the efficient transformation and exquisite chemoselectivity pattern from identical starting materials that rapidly and reliably incorporate gem-difluoroalkene and trifluoromethyl (CF₃) motifs to generate a diverse range of enantioenriched fluorinated building blocks in good to excellent yields with high asymmetric induction.

Synergistic Catalysis between a Dipeptide Phosphonium Salt and a Metal-Based Lewis Acid for Asymmetric Synthesis of N-Bridged [3.2.1] Ring Systems

We present an unprecedented synergic catalytic route for the asymmetric construction of fluorinated N-bridged [3.2.1] cyclic members of tropane family via a bifunctional phosphonium salt/silver co-catalyzed cyclization process. A broad variety of substrates bearing an assortment of functional groups are compatible with this method, providing targeted compounds bearing seven-membered ring and four contiguous stereocenters in high yields with excellent stereoselectivities. The gram-scale preparations, facile elaborations and preliminary biological activities of the products demonstrate the application potential. Moreover, both experimental and computational mechanistic studies revealed that the cyclization proceeded via a "sandwich" reaction model with multiple weak-bond cooperative activations. Insights gained from our studies are expected to advance general efforts towards the catalytic synthesis of challenging chiral heterocyclic molecules.

Catalytic enantioselective intramolecular 1,3-dipolar cycloaddition of azomethine ylides with fluorinated dipolarophiles

An enantioselective synthesis of polycyclic fluorinated pyrrolidines has been achieved by Cu-catalyzed intramolecular 1,3-dipolar cycloaddition of azomethine ylides with fluorinated dipolarophiles. The method displays a wide scope and afforded the desired cycloadducts in high yields with up to 99% ee. These results demonstrate that fluoroalkyl substituents are excellent activating groups in this transformation.

Stereodivergent synthesis of enantioenriched azepino[3,4,5-cd]-indoles via cooperative Cu/Ir-catalyzed asymmetric allylic alkylation and intramolecular Friedel-Crafts reaction

The development of enantioselective annulation reactions using readily available substrates for the construction of structurally and stereochemically diverse heterocycles is a compelling topic in diversity-oriented synthesis. Herein, we report efficient catalytic asymmetric formal 1,3-dipolar (3 + 4) cycloadditions of azomethine ylides with 4-indolyl allylic carbonates for the construction of azepino[3,4,5-cd]-indoles fused with a challenging seven-membered N-heterocycle, a frequently occurring tricyclic indole scaffold in bioactive compounds and pharmaceuticals. Through cooperative Cu/Ir-catalyzed asymmetric allylic alkylation followed by intramolecular Friedel-Crafts reaction, an array of azepino[3,4,5-cd]-indoles were obtained in good yields with excellent diastereo-/enantioselective control. More importantly, the full stereodivergence of this transformation was established via synergistic catalysis followed by acid-promoted epimerization, and up to eight stereoisomers of the cycloadducts bearing three stereogenic centers could be predictably achieved from the same set of starting materials for the first time. Quantum mechanical computations established a plausible mechanism for the synergistic Cu/Ir catalysis to stereodivergently introduce two vicinal stereocenters whose stereochemical information is remotely delivered across the fused azepine ring to control the third chiral center. Epimerization of the last center involves protonation-enabled reversal of the thermodynamically controlled relative configuration.

exo'-Selective Construction of Spirobipyrrolidines by the Silver-catalyzed Asymmetric [3+2] Cycloaddition of Imino Esters with 4-Benzylidene-2,3-dioxopyrrolidines

The unprecedented Ag-catalyzed exo ' -selective [3+2] cycloaddition of imino esters with 4-benzylidene-2,3-dioxopyrrolidines is described. The reaction was efficiently catalyzed by AgOAc/( R , S p )-ThioClickFerrophos (TCF) leading to the construction of the corresponding spirobipyrrolidine scaffolds in excellent enantio- and diastereoselectivities. This reaction is the first example of a silver-catalyzed exo ' -selective asymmetric [3+2] cycloaddition, as well as the first exo ' -selective spirobipyrrolidine construction via a [3+2] cycloaddition process using imino esters. The wide substrate scope of this reaction enabled the preparation of structurally diverse spirobipyrrolidine derivatives, which are attracting attention as targets for drug discovery. Mechanistic studies suggested that the unusual exo ' -selectivity of this reaction is not due to epimerization following the common exo - or endo -selective cycloaddition, but instead is due to a stepwise Michael addition/Mannich sequence with bond rotation.

Asymmetric synthesis of chiral imidazolidines by merging copper and visible light-induced photoredox catalysis

A visible light induced copper catalyzed synthesis of decarboxylative radical coupling/cyclization reaction for the synthesis of chiral imidazolidines in high yields and enantioselectivities was reported.

Diastereodivergent synthesis of fully disubstituted spiro[indoline-3,2′-pyrrolidin]-2-ones via tuneable Lewis base/Brønsted base-promoted (3 + 2) cycloadditions

We have developed unique base-promoted diastereoselective (3 + 2) cycloadditions. A diastereodivergent synthesis of fully disubstituted spiro[indoline-3,2′-pyrrolidin]-2-ones was realized by tuning a Lewis base/Brønsted base.

Diastereodivergent Synthesis of Chiral 4-Fluoropyrrolidines (exo and exo') Based on the Cu(II)-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition

1,3-Dipolar cycloaddition of azomethine ylides and electron deficient alkenes is widely studied for rapid installation of pyrrolidine frameworks. Despite significant advances, the major limitations of this process are creating chiral pyrrolidines bearing a quaternary stereogenic center and controlling the diastereoselectivity. Herein, we present an exo-selective asymmetric 1,3-dipolar cycloaddition to access chiral pyrrolidines with four contiguous stereogenic centers, including a fluorinated quaternary stereogenic center at C4, wherein a Cu(OAc)₂/(S)-tol-BINAP catalyst and α-fluoro-α,β-unsaturated arylketone dipolarophiles are used. Epimerization promoted by 5.0 equiv of DBU at 90 °C results in the formation of chiral 4-fluoropyrrolidines (exo') while maintaining the optical purity.

Cu(I)-Catalyzed asymmetric exo-selective synthesis of substituted pyrrolidines via a 1,3-dipolar cycloaddition reaction

An (R)-DM-BINAP/Cu(CH3CN)4BF4 complex catalyzed exo-selective asymmetric 1,3-dipolar cycloaddition (1,3-DCA) reaction of imino esters with α,β-unsaturated pyrazoleamides has been developed. A series of highly functionalized pyrrolidines with multiple stereogenic centers were obtained with good yields and diastereoselectivities and excellent enantioselectivities (up to 99% ee).

Silver-Mediated [3 + 2] Cycloaddition of Azomethine Ylides with Trifluoroacetimidoyl Chlorides for the Synthesis of 5-(Trifluoromethyl)imidazoles

A silver-mediated [3 + 2] cycloaddition of azomethine ylides with trifluoroacetimidoyl chlorides for the rapid assembly of 5-(trifluoromethyl)imidazoles has been developed. Notable features of the reaction include readily accessible reagents, a broad substrate scope, and high efficiency. The protocol can be successfully applied to construct the analogue of the specific allosteric modulator of GABA_A receptors. The silver species could be recycled by a simple operation.

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 3] Cycloaddition of 4-Indolyl Allylic Alcohols with Azomethine Ylides

An unprecedented iridium-catalyzed asymmetric [4 + 3] cycloaddition of racemic 4-indolyl allylic alcohols with azomethine ylides is reported. The ability of acid promoter zinc triflate to perform multiple roles is the key factor for the success of this strategy. This method provides scalable and efficient access to biologically important azepino[3,4,5-cd] indoles in good yields with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Mild reaction conditions, easily accessible substrates and chiral catalyst, and broad substrate scope highlight the practicality of this methodology.

Highly Chemo- and Enantioselective Rh-Catalyzed Hydrogenation of β-Sulfonyl-α,β-unsaturated Ketones: Access to Chiral γ-Ketosulfones

Rh-catalyzed highly chemo- and enantioselective hydrogenation of β-sulfonyl-α,β-unsaturated ketones was first successfully developed. Remarkably, a variety of enantioenriched γ-ketosulfones were generated in good to high yields with excellent chemo/enantioselectivities (82-99% yields, >99:1 chemoselectivity, 88 to >99% ee). Moreover, the gram-scale asymmetric hydrogenation was carried out smoothly in 97% yield and 97% ee. Preliminary DFT computations furnished a reasonable explanation for the high chemoselectivity and enantioselectivity.

Diastereodivergent synthesis of dispiroheterocyclic structures comprising pyrrolidinyloxindole and imidazothiazolotriazine moieties

Highly diastereoselective methods for the synthesis of two different diastereomers of polynuclear dispiroheterocyclic compounds with five chiral centers comprising pyrrolidinyloxindole and imidazothiazolotriazine moieties (dispiro[imidazo[4,5-e]thiazolo[2,3-c]-1,2,4-triazine-7,3'-pyrrolidine-2',3''-indoles]) have been developed on the basis of a dipolar cycloaddition of azomethine ylides to benzylidene derivatives of imidazothiazolotriazines and an alkali-induced rearrangement of the thiazolotriazine fragment. The different sequence of the cycloaddition and rearrangement stages allows us to perform the targeted synthesis of two diastereomerically pure products from the same starting compounds.

Copper(i)/Ganphos catalysis: enantioselective synthesis of diverse spirooxindoles using iminoesters and alkyl substituted methyleneindolinones

A copper-catalyzed asymmetric 1,3-dipolar cycloaddition of glycine iminoesters with alkyl substituted 3-methylene-2-oxindoles is described. By using de novo design of P-stereogenic phosphines as ligands, spiro[pyrrolidin-3,3'-oxindole]s are generated in good to excellent yields with high asymmetric induction. A further reduced catalyst loading of 0.1 mol% is sufficient to achieve a satisfactory enantioselectivity of 90% ee. The DFT calculations suggest the second Michael addition of the 1,3-dipole to be the rate- and enantio-determining step. A key feature of this 1,3-dipolar cycloaddition is the wide substrate applicability, even with alkyl aldehyde-derived azomethine ylide; thus it has streamlined a highly enantioselective access to a new class of antiproliferative agents, MDM2-p53.

Catalytic asymmetric synthesis of diazabicyclo[3.1.0]hexanes by 1,3-dipolar cycloaddition of azomethine ylides with azirines

Substituted 1,3-diazabicyclo[3.1.0]hexanes with two contiguous quaternary stereocentres are readily prepared by catalytic asymmetric [3+2] cycloaddition of α-substituted iminoesters with azirines. High diastereoselectivities and enantioselectivities (up to 98% ee) are achieved using Cu^I/(R)-Fesulphos as the catalytic system.

H-bond donor-directed switching of diastereoselectivity in the Michael addition of α-azido ketones to nitroolefins

The development of catalyst-controlled stereodivergent asymmetric catalysis is important for providing facile access to all stereoisomers of chiral products with multiple stereocenters from the same starting materials. Despite progress, new design strategies for diastereodivergent asymmetric catalysis are still highly desirable. Here we report the potency of H-bond donors as the governing factor to tune diastereoselectivity in a highly diastereoselective switchable enantioselective Michael addition of α-azido ketones to nitroolefins. While a newly developed bifunctional tertiary amine, phosphoramide, preferentially afforded syn-adducts, an analogous squaramide catalyst selectively gave anti-adducts. The resulting multifunctional tertiary azides can be converted to spiro-pyrrolidines with four continuous stereocenters in a one-pot operation. Mechanistic studies cast light on the control of diastereoselectivity by H-bond donors. While the squaramide-catalyzed reaction proceeded with a transition state with both squaramide N–H bonds binding to an enolate intermediate, an unprecedented model was proposed for the phosphoramide-mediated reaction wherein an amide N–H bond and an alkylammonium ion formed in situ interact with nitroolefins, with the enolate stabilized by nonclassical C–H⋯O hydrogen-bonding interactions.

We report the successful reversal of the diastereoselectivity in an unprecedented Michael addition of α-azido ketones to nitroolefins catalyzed by bifunctional tertiary amines, simply by varying the H-bond donor from phosphoramide to squaramide.

Catalytic Asymmetric Formal [3+2] Cycloaddition of Azoalkenes with 3-Vinylindoles: Synthesis of 2,3-Dihydropyrroles

Chiral phosphoric acid-catalyzed highly enantioselective formal [3 + 2] cycloaddition reaction of azoalkenes with 3-vinylindoles has been established. Under mild conditions, the projected cycloaddition proceeded smoothly, affording a variety of 2,3-dihydropyrroles in high yields and excellent enantioselectivities, and also in a diastereospecific manner. As opposed to the common 4-atom synthons in the previous literature reports, azoalkenes served as 3-atom synthons. Besides, the observed selectivity was supported by primary theoretical calculation. The unique chemistry of azoalkenes disclosed herein will empower asymmetric synthesis of nitrogen-containing ring structural motifs in a broader context.

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Chiral phosphoric acid catalyzed formal [3 + 2] cycloaddition reaction

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2,3-Dihydropyrroles were enantioselectively synthesized

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Azoalkenes served as 3-atom synthons

Kinetic resolution of 2H-azirines via Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides

An efficient kinetic resolution of racemic 2H-azirines via copper(i)-catalyzed asymmetric cycloaddition of azomethine ylides was developed and enantioenriched 2H-azirines and 1,3-diazabicyclo[3.1.0]hexane derivatives were obtained simultaneously.