Stereoselective Synthesis of Axially Chiral Allenes and Styrenes via Chiral Phosphoric Acid Catalysis: An Overview

Chiral allenes and styrenes are essential components in fields like medicinal chemistry, materials science, and organic synthesis. They serve a crucial role as chiral ligands and catalysts in asymmetric synthesis. Over the past decade, there has been a significant advancement in the development of practical methods utilizing organocatalytic strategies for the synthesis of chiral allenes and styrenes. It is noteworthy that despite extensive studies on the formation of allenes and styrenes, existing reviews on their construction confined to a specific organocatalysis, called chiral phosphoric acid catalysis, are less documented. This review aims to explore different conceptual approaches based on the electrophilic species involved in the reaction to produce stereoselective chiral allenes and styrenes, providing insights into recent advancements in the field. Emphasis is placed on works published since 2017, with detailed discussions on reaction mechanisms and examples from recent literature.

Asymmetric catalytic [1,3]- or [3,3]-sigmatropic rearrangement of 3-allyloxy-4H-chromenones and their analogues

A highly efficient asymmetric [1,3]- and [3,3]-O-to-C sigmatropic rearrangement of 3-allyloxy-4H-chromenones and their analogues was developed. Chiral N,N'-dioxide complexes of 3d late transition metal complexes enabled two mechanistically different processes, giving a series of optically active 2,2-disubstituted chromane-3,4-diones and 2-allyl-3-hydroxy-4H-chromen-4-ones as well as their related compounds in excellent yield and enantioselectivity. Systemic mechanistic studies and DFT calculation revealed the nature of the vinyl ether unit of the substrate, which biased regioselectivity via a stepwise tight ion pair pathway and a concerted pericyclic pathway, respectively. The enantioselectivity of the two processes is also disclosed.

Catalytic enantioselective reductive Eschenmoser-Claisen rearrangements

An important challenge in enantioselective catalysis is developing strategies for the precise synthesis of neighboring congested all-carbon quaternary stereocenters. The well-defined transition states of [3,3]-sigmatropic rearrangements and their underlying stereospecificity render them powerful tools for the synthesis of such arrays. However, this type of pericyclic reaction remains notoriously difficult to catalyze, especially in an enantioselective fashion. Herein, we describe an enantioselective reductive Eschenmoser-Claisen rearrangement catalyzed by chiral 1,3,2-diazaphospholene-hydrides. This developed transformation enables full control of the two newly formed acyclic stereogenic centers, leading to amides with vicinal all-carbon quaternary-tertiary or quaternary-quaternary carbon atoms.

Directing the Stereoselectivity of the Claisen Rearrangement to Form Cyclic Ketones with Full Substitution at the α-Positions

We report an enantioselective synthesis of cyclic ketones with full substitutions at the α-positions in a highly diastereoselective manner. Our method is achieved by subjecting substrate motifs in 2-allyloxyenones to chiral organomagnesium reagents, which trigger the Claisen rearrangement upon direct 1,2-carbonyl addition. The observed diastereoselectivity of the allyl migration is proposed to originate from the intramolecular chelation of the magnesium alkoxide to the allyloxy moiety.

Cobalt(II)-Catalyzed Enantioselective Propargyl Claisen Rearrangement: Access to Allenyl-Substituted Quaternary β-Ketoesters

Highly enantioselective propargyl Claisen rearrangement of O-propargyl β-ketoesters was achieved under 2.5 mol % of the chiral cobalt complex as the catalyst under mild reaction conditions. With Co(OTf)2 as the Lewis acid and C1-symmetric imidazoline-pyrroloimidazolone pyridine as the ligand, diverse chiral allenyl-substituted all-carbon quaternary β-ketoesters were obtained in good yields (up to 97% yield) and high enantioselectivities (up to 98% ee).

A bridged backbone strategy enables collective synthesis of strychnan alkaloids

Bridged frameworks are of high chemical and biological significance, being ubiquitous in pharmaceutical molecules and natural products. Specific structures are usually preformed to build these rigid segments at the middle or late stage in the synthesis of polycyclic molecules, resulting in decreased synthetic efficiency and target-specific syntheses. As a logically distinct synthetic strategy, we constructed an allene/ketone-equipped morphan core at the outset through an enantioselective α-allenylation of ketones. Experimental and theoretical results revealed that the high reactivity and enantioselectivity of this reaction are attributed to the cooperative effects of the organocatalyst and metal catalyst. The bridged backbone generated was employed as a structural platform to guide and facilitate the assembly of up to five fusing rings, and the allene and ketone groups thereon were used to precisely install various functionalities at C16 and C20 at the late stage, leading to a concise, collective total synthesis of nine strychnan alkaloids.

Enantioselective aromatic Claisen rearrangement of allyl 2-naphthyl ethers catalyzed by π-Cu(ii) complexes

The first catalytic enantioselective aromatic Claisen rearrangement of allyl 2-naphthyl ethers using 5-10 mol% of π-copper(ii) complexes is reported. A Cu(OTf)₂ complex with an l-α-homoalanine amide ligand gave (S)-products in up to 92% ee. Conversely, a Cu(OSO₂C₄F₉)₂ complex with an l-tert-leucine amide ligand gave (R)-products in up to 76% ee. Density-functional-theory (DFT) calculations suggest that these Claisen rearrangements proceed stepwise via tight-ion-pair intermediates, and that (S)- and (R)-products are enantioselectively obtained via the staggered transition states for the cleavage of the C-O bond, which is the rate-determining step.

[3,3]-Sigmatropic Rearrangements of Naphthyl 1-Propargyl Ethers: para-Propargylation and Catalytic Asymmetric Dearomatization

The para-Claisen rearrangement of aryl 1-propargyl ethers involves two-step [3,3]-sigmatropic rearrangements and dearomatization process, which has high activation barriers and is of challenge. Here we discovered thermal para-Claisen rearrangement of naphthyl 1-propargyl ethers, and it enabled the formation of formal para-C-H propargylation products upon rearomatization. Chirality transfer occurred if optically active propargyl ethers were employed, leading to the construction of aryl/propargyl-containing stereogenic centers. Moreover, catalytic asymmetric dearomatization of naphthyl 1-propargyl ethers with different substitution at para-position gave access to benzocyclohexenones bearing all-carbon quaternary stereocenters. The reaction was accelerated by a chiral N,N'-dioxide/Co(OTf)₂ complex catalyst to achieve high yields (up to 98 %) and high enantioselectivities (up to 93 % ee). The DFT calculations and experimental results provided important clues to clarify the para-Claisen rearrangement process as well as the chiral induction and remote delivery.

Recent advances in metal-catalysed asymmetric sigmatropic rearrangements

Asymmetric sigmatropic rearrangement is a powerful organic transformation via substrate-reorganization to efficiently increase molecular complexity from readily accessible starting materials. In particular, a high level of diastereo- and enantioselectivity can be readily accessed through well-defined and predictable transition states in [3,3], [2,3]-sigmatropic rearrangements, which have been widely applied in the synthesis of various chiral building blocks, natural products, and pharmaceuticals. In recent years, catalytic asymmetric sigmatropic rearrangements involving chiral metal complexes to induce stereocontrol have been intensively studied. This review presents an overview of metal-catalysed enantioselective versions of sigmatropic rearrangements in the past two decades, mainly focusing on [3,3], [2,3], and [1,3]-rearrangements, to show the development of substrate design, new catalyst exploitation, and novel cascade processes. In addition, their application in the asymmetric synthesis of complex natural products is also exemplified.

Enantioselective Nickel-Catalyzed Reductive Aryl/Alkenyl-Cyano Cyclization Coupling to All-Carbon Quaternary Stereocenters

An enantioselective nickel-catalyzed intramolecular reductive cross-coupling of C(sp²) electrophiles and cyano groups is reported. Enantioenriched CN-containing all-carbon quaternary stereocenters are assembled by desymmetrizing cyclization of aryl/alkenyl halide-tethered malononitriles. The use of an organic reductant, (EtO)₂MeSiH, is crucial to the enantioselectivity and reactivity. Applications of the method are demonstrated through the synthesis of bioactive molecules and their cyanated analogues and the total synthesis of the natural product diomuscinone. This study exhibits the potential of desymmetrizing reductive coupling strategies to access structurally rigid and synthetically versatile molecules from readily available starting materials.

Cooperative Palladium/Brønsted Acid Catalysis toward the Highly Enantioselective Allenylation of β-Keto Esters

We report the first enantioselective allenylation of Pd enolates enabled by cooperative Pd/Brønsted acid catalysis employing β-keto esters and propargyl alcohols. The enantioselectivity originates solely from an in-situ-generated chiral metal enolate in an open transition state with no additional binding of the propargyl component to the catalyst. Thus a broad substrate scope was established, furnishing hitherto inaccessible products in very good yields with excellent enantioselectivities (up to >99:1 e.r.).

Visible-Light-Induced 1,4-Hydroxysulfonylation of Vinyl Enynes with Sulfonyl Chlorides: The Bridge of Chloride Linking Water and Enynes

A novel visible-light-induced 1,4-hydroxysulfonylation of vinyl enynes with sulfonyl chlorides has been established, providing a highly efficient protocol to access multisubstituted sulfonyl allenic alcohols. Control experiments and mechanistic studies disclose that the target products result from sequential reactions of hydroxyl and tosyl radicals, among which chloride anion plays a key role to generate the requisite ^•OH, thus bridging water and enynes. Moreover, the vinyl pendant is believed to decisively affect the site-selectivity of hydroxyl radical.

Gold-Catalyzed Asymmetric Thioallylation of Propiolates via Charge-Induced Thio-Claisen Rearrangement

A gold(I)-catalyzed enantioselective thioallylation of propiolates with allyl sulfides is described. The key mechanistic element is a sulfonium-induced Claisen rearrangement which helps minimize the allyl dissociation and render higher enantioselectivity. This protocol features remarkable scope of the allyl moiety, allowing enantiocontrolled synthesis of all-carbon quaternary centers, and exhibits exceptional functional group compatibility with many Lewis bases and π-bonds. This intermolecular variant of Claisen rearrangement forges both C-S and C-C bonds concomitantly, providing efficient access to interesting optically active organosulfur compounds which can be transformed further through the vinyl sulfide as a functional handle. The rate of the reaction was zeroth order with respect to allyl sulfides, which suggested a reversible inhibition, providing a resting state for the catalyst. The Hammett plot displayed a correlation with σ_p values, suggesting a turnover-limiting sigmatropic rearrangement where decreased electron-density on sulfur accelerated the rearrangement.

Chiral phosphoric acid-catalyzed stereodivergent synthesis of trisubstituted allenes and computational mechanistic studies

Chiral molecules with multiple stereocenters are widely present in natural products and pharmaceuticals, whose absolute and relative configurations are both critically important for their physiological activities. In spite of the fact that a series of ingenious strategies have been developed for asymmetric diastereodivergent catalysis, most of these methods are limited to the divergent construction of point chirality. Here we report an enantioselective and diastereodivergent synthesis of trisubstituted allenes by asymmetric additions of oxazolones to activated 1,3-enynes enabled by chiral phosphoric acid (CPA) catalysis, where the divergence of the allenic axial stereogenicity is realized by modifications of CPA catalysts. Density functional theory (DFT) calculations are performed to elucidate the origin of diastereodivergence by the stacking- and stagger-form in the transition state (TS) of allene formation step, as well as to disclose a Münchnone-type activation mode of oxazolones under Brønsted acid catalysis.

Copper-Catalyzed Enantioselective Radical 1,4-Difunctionalization of 1,3-Enynes

Chiral allenes are important structural motifs frequently found in natural products, pharmaceuticals, and other organic compounds. Asymmetric 1,4-difunctionalization of 1,3-enynes is a promising strategy to construct axial chirality and produce substituted chiral allenes from achiral substrates. However, the previous state of the art in 1,4-difunctionalization of 1,3-enynes focused on the allenyl anion pathway. Because of this, only electrophiles can be introduced into the allene backbones in the second functionalization step, consequently limiting the reaction and allene product types. The development of asymmetric 1,4-difunctionalization of 1,3-enynes via a radical pathway would complement previous methods and support expansion of the toolbox for the synthesis of asymmetric allenes. Herein, we report the first radical enantioselective allene formation via a group transfer pathway in the context of copper-catalyzed radical 1,4-difunctionalization of 1,3-enynes. This method addresses a longstanding unsolved problem in asymmetric radical chemistry, provides an important strategy for stereocontrol with free allenyl radicals, and offers a novel approach to the valuable, but previously inaccessible, chiral allenes. This work should shed light on asymmetric radical reactions and may lead to other enantioselective group transfer reactions.

Chiral Fe(ii) complex catalyzed enantioselective [1,3] O-to-C rearrangement of alkyl vinyl ethers and synthesis of chromanols and beyond

A highly efficient enantioselective [1,3] O-to-C rearrangement of racemic vinyl ethers that operates under mild conditions was developed. This method with chiral ferrous complex catalyst provided an efficient access to a wide range of chromanols with high yields and excellent enantioselectivities. In addition, an important urological drug (R)-tolterodine and others were easily obtained after simple transformations.

Cu-Catalyzed SN2' Substitution of Propargylic Phosphates with Vinylarene-Derived Chiral Nucleophiles: Synthesis of Chiral Allenes

A new Cu-catalyzed enantioselective three-component (i.e., styrenes, B₂pin₂, and propargylic phosphates) allenylation via an S_N2' substitution of propargylic electrophiles with vinylarene-derived chiral nucleophiles is presented. This method provides an efficient and enantioselective approach to access a range of optically pure di-(1,1-), tri-, and tetra-substituted allenes with α-central chirality and axial chirality in excellent chemo-, regio-, diastereo-, and enantioselectivities.

Mechanism-Guided Scaffold Diversification: Perturbing and Trapping the Intermediates of Maltol-Type Cascade Claisen Rearrangement

A mechanism-guided study to harness different kinds of active species in the cascade Claisen rearrangement for scaffold diversification has been developed. These robust and practical processes furnished a series of architectures with a large chemical space, varying from planar to three-dimensional. In addition, several interesting reactions were observed, such as [3 + 3] dimerization, quinone-based vinylogous Nazarov-type cyclization, and a rare 12e [σ2a + π2s + π2a + π2s + (π2a + π2s)] Mobius aromatic transition state mediated rearrangement.

Asymmetric Synthesis of Allenyl α-Amino Amides by an Isothiourea Catalyzed Enantioselective [2,3]-Sigmatropic Rearrangement

Highly efficient catalytic asymmetric [2,3]-sigmatropic rearrangements of propargyl ammonium salts have been accomplished under mild reaction conditions. In the presence of the chiral isothiourea catalyst, a wide range of allenyl α-amino amide derivatives were obtained in generally good yields (up to 99%) with excellent enantioselectivities (up to 96% ee).

Stereodivergent synthesis of vicinal quaternary-quaternary stereocenters and bioactive hyperolactones

Although great success has been achieved in asymmetric Claisen rearrangement for the synthesis of chiral γ,δ-unsaturated carbonyl compounds bearing vicinal tertiary-quaternary stereocenters, the development of asymmetric versions for stereodivergent construction of adjacent quaternary-quaternary stereocenters remains a formidable challenge because of the high steric hindrance. Here we report a catalytic enantioselective dearomatization Claisen rearrangement of allyl furyl ethers catalyzed by chiral N,N'-dioxide-Ni^II complex catalysts. A variety of chiral γ,δ-unsaturated carbonyl compounds bearing vicinal quaternary-quaternary stereocenters were obtained with excellent outcomes under mild conditions. Furthermore, we disclosed that by matching the configuration of the catalysts and the alkene unit of the substrates, four stereoisomers of the products could be prepared in excellent yields and stereoselectivities. Finally, the fascination of this strategy was demonstrated by stereodivergent synthesis of bioactive natural products hyperolactones B, C, and their epimers. A possible catalytic model was proposed to explain the origin of the asymmetric induction.

Gold(I) Catalyzed Dearomative Claisen Rearrangement of Allyl, Allenyl Methyl, and Propargyl Aryl Ethers

Claisen rearrangements of allyl aryl ethers to generate enones bearing all carbon quaternary centers are accelerated by Ph₃PAuNTf₂ under mild conditions in good yields. Multiple C-C bond containing variants of the allyl fragment are viable, including alkylidenecyclopropanes, allenes, and alkynes, which generate all-carbon stereogenic centers substituted with vinyl cyclopropanes, 1,3-butadienyl, and allenyl substituents, respectively, for subsequent synthetic manipulation. With allyl phenyl ethers, the product of the [3,3] rearrangements can be trapped by a tandem [4 + 2] cycloaddition to generate complex molecular scaffolds from readily available, achiral starting materials.

Chiral ligands designed in China

Asymmetric catalysis has become an indispensable and productive field within the Chinese organic chemistry society. The design of chiral ligands is one of the most prominent research areas in this field. Since the late 1990s, Chinese organic chemists have developed numerous chiral ligands possessing novel chiral skeletons and design concepts. Some of these ligands have been widely adopted and can be regarded as ‘privileged ligand’, which have shown excellent performance in many asymmetric catalytic reactions. In this review, we provide an overview of the chiral ligands designed by Chinese scientists with the aim of promoting the development of this area in China and with the hope of encouraging more scientists across the world to use these ligands when designing asymmetric reactions.

Recent advances in catalytic asymmetric synthesis of allenes

This minireview summarizes the recent advances in the synthesis of allenes with catalytic asymmetric methods.

Enantioselective construction of branched 1,3-dienyl substituted quaternary carbon stereocenters by asymmetric allenyl Claisen rearrangement

The availability of enantiomerically enriched 1,3-dienyl substituted quaternary stereocenters is highly valuable for the synthesis of complex natural compounds. Despite great advances in the area of construction of alkenyl-substituted types, a general, practical catalytic system that works for enantioselective formation of 1,3-diene derivatives still remains to be developed. Herein, we disclose the first asymmetric Claisen rearrangement of allenyl vinyl ethers to access optically active β-ketoesters, containing branched 1,3-butadienyl substituted stereocenters. A variety of substrates bearing a range of useful functional groups were well tolerated, thus affording the corresponding products with excellent enantioselectivities (up to 99% ee) and high yields (up to 91%).

Asymmetric [3 + 2] cycloaddition of donor-acceptor aziridines with aldehydes via carbon-carbon bond cleavage

An enantioselective [3 + 2] annulation of donor-acceptor aziridines with aldehydes has been realized using a Nd(OTf)3/N,N'-dioxide/LiNTf2 catalyst system, providing various chiral cis-1,3-oxazolidines in moderate to good yields with high levels of stereocontrol. A relay catalytic process is proposed where LiNTf2 promotes the formation of azomethine ylide intermediates, and a chiral Nd(iii)-N,N'-dioxide complex accelerates the asymmetric cycloaddition.

Efficient Synthesis of Chiral Trisubstituted 1,2-Allenyl Ketones by Catalytic Asymmetric Conjugate Addition of Malonic Esters to Enynes

An N,N'-dioxide/scandium(III) complex catalyzed, highly efficient conjugate addition of malonic esters to enynes is described. A range of trisubstituted 1,2-allenyl ketones were obtained in high yields (up to 99 %) with good d.r. (up to 95/5) and excellent ee values (97 %-99 %). Moreover, the products could be easily transformed into chiral furan and 5-hydroxypyrazoline derivatives, both of which are important skeletons of many biologically active compounds and pharmacologicals.