Enantioselective Reactions of 2‐Sulfonylalkyl Phenols with Allenic Esters: Dynamic Kinetic Resolution and [4+2] Cycloaddition Involving ortho ‐Quinone Methide Intermediates

Diastereo-divergent synthesis of chiral hindered ethers via a synergistic calcium(II)/gold(I) catalyzed cascade hydration/1,4-addition reaction

Hindered ethers are ubiquitous in natural products and bioactive molecules. However, developing an efficient method for the stereocontrolled synthesis of all stereoisomers of chiral hindered ethers is highly desirable but challenging. Here we show a strategy that utilizes in situ-generated water as a nucleophile in an asymmetric cascade reaction involving two highly reactive intermediates, 3-furyl methyl cations and ortho-quinone methides (o-QMs), to synthesize chiral hindered ethers. The Ca(II)/Au(I) synergistic catalytic system enables the control of diastereoselectivity and enantioselectivity by selecting suitable chiral phosphine ligands in this cascade hydration/1,4-addition reaction, affording all four stereoisomers of a diverse range of chiral tetra-aryl substituted ethers with high diastereoselectivities (up to >20/1) and enantioselectivities (up to 95% ee). This work provides an example of chiral Ca(II)/Au(I) bimetallic catalytic system controlling two stereogenic centers via a cascade reaction in a single operation.

Photocatalytic Enantioselective Hydrosulfonylation of α,β-Unsaturated Carbonyls with Sulfonyl Chlorides

This research explores the enantioselective hydrosulfonylation of various α,β-unsaturated carbonyl compounds via the use of visible light and redox-active chiral Ni-catalysis, facilitating the synthesis of enantioenriched α-chiral sulfones with remarkable enantioselectivity (exceeding 99 % ee). A significant challenge entails enhancing the reactivity between chiral metal-coordinated carbonyl compounds and moderate electrophilic sulfonyl radicals, aiming to minimize the background reactions. The success of our approach stems from two distinctive attributes: 1) the Cl-atom abstraction employed for sulfonyl radical generation from sulfonyl chlorides, and 2) the single-electron reduction to produce a key enolate radical Ni-complex. The latter process appears to enhance the feasibility of the sulfonyl radical's addition to the electron-rich enolate radical. An in-depth investigation into the reaction mechanism, supported by both experimental observations and theoretical analysis, offers insight into the intricate reaction process. Moreover, the versatility of our methodology is highlighted through its successful application in the late-stage functionalization of complex bioactive molecules, demonstrating its practicality as a strategy for producing α-chiral sulfones.

Asymmetric (4+1) Annulations by Cascade Allylation and Transient σ-Alkyl-Pd(II) Initiated Allylic Csp3 -H Activation

A unique Pd-catalyzed approach for asymmetric (4+1) annulations via cascade allylation and transient σ-alkyl-Pd(II) initiated methylene Csp3 -H activation is reported. The enolate fragment derived from the decarboxylation of vinyl methylene carbonate is crucial to stabilize the key intermediate. These reactions enable the synthesis of various useful dihydrobenzofurans with excellent enantioselectivity, typically >95 : 5 er, and exclusive (Z)-stereoselectivity. Compared with the well-established annulations via Heck-type C-H activations, this protocol showcases a conceptually new way to generate σ-alkyl-Pd(II) species that could initiate challenging asymmetric Csp3 -H activations.

Brønsted acid-mediated tandem cyclization of triarylphosphines and in situ generated ortho-alkynyl quinone methides: access to heterocyclic quaternary phosphonium salts

Heterocyclic Quaternary Phosphonium Salts (HQPS) have emerged as promising chemicals for organic synthesis and medicinal chemistry. However, the present synthetic methodology of this type of compound is still limited. Here, we report a deconstructive reorganization strategy based on Brønsted acid-mediated tandem 1,4 addition/intramolecular cyclization of triphenylphosphine derivatives and in situ generated o-AQMs for the first time. This protocol provides a novel approach to heterocyclic quaternary phosphonium salts. The method also features a non-metal catalyst, mild reaction conditions, high efficiency and wide substrate scope. Moreover, a series of obtained heterocyclic phosphonium salts can be converted to isotopically labelled 2-benzofuran compounds directly by simple deuteration reactions.

Functionalized Chromans from ortho-Quinone Methides and Arylallenes

ortho-Quinone methides (o-QMs) underwent formal [4 + 2]-cycloaddition reactions with arylallenes regioselectively at the styrenyl olefin to furnish the corresponding 3-methylene-2-arylchromans in moderate to good yields (up to 88%). When R ≠ H, the reactions also proceeded with moderate stereoselectivity (up to 5:1) which was governed by the nature of the R group. The 3-methylene-2-arylchromans could serve as common intermediates for further functionalization including epoxidation, oxidative cleavage/Baeyer-Villiger oxidation, Riley oxidation, acid-catalyzed rearrangement, and Pd-catalyzed cross-coupling reactions to furnish the corresponding derivatives in moderate to good yields.

Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.

Carbene-Catalyzed Enantioselective Sulfonylation of Enone Aryl Aldehydes: A New Mode of Breslow Intermediate Oxidation

A carbene-catalyzed sulfonylation reaction between enone aryl aldehydes and sulfonyl chlorides is disclosed. The reaction effectively installs sulfone moieties in a highly enantioselective manner to afford sulfone-containing bicyclic lactones. The sulfonyl chloride behaves both as an oxidant and a nucleophilic substrate (via its reduced form) in this N-heterocyclic carbene (NHC)-catalyzed process. The NHC catalyst provides both activation and stereoselectivity control on a very remote site of enone aryl aldehyde substrates. Water plays an important role in modulating catalyst deactivation and reactivation routes that involve reactions between NHC and sulfonyl chloride. Experimental studies and DFT calculations suggest that an unprecedented intermediate and a new oxidation mode of the NHC-derived Breslow intermediate are involved in the new asymmetric sulfonylation reaction.

Acid-promoted formal [3 + 2] cyclization/N,O-ketalization of in situ generated ortho-alkynyl quinone methides: access to bridged 2,3-cyclopentanoindoline skeletons

An acid-catalyzed formal [3 + 2] cyclization/N,O-ketalization of in situ formed ortho-alkynyl quinone methides (o-AQMs) and tryptophol derivatives was developed.

Asymmetric [4 + 2] cycloaddition synthesis of 4H-chromene derivatives facilitated by group-assisted-purification (GAP) chemistry

In this work, we present a strategy for the preparation of functionalized 4H-chromene derivatives via a Cs₂CO₃-catalyzed [4 + 2] cycloaddition of enantiopure chiral salicyl N-phosphonyl imines with allenoates. Fifteen examples were achieved in excellent yields and diastereoselectivity. The products were purified simply by washing the crude mixture with hexanes following the Group-Assisted Purification (GAP) chemistry/technology to bypass traditional separation methods. The absolute configuration was unambiguously determined by X-ray structure analysis.

A new asymmetric method for the synthesis of highly functionalized 4H-chromenes was developed via Group-Assisted Purification (GAP) chemistry and shown in good to high yield and excellent diastereoselectivity.

Organocatalytic Asymmetric [2 + 4] Cycloadditions of 3-Vinylindoles with ortho-Quinone Methides

Catalytic asymmetric [2 + 4] cycloadditions of 3-vinylindoles with ortho-quinone methides and their precursors were carried out in the presence of chiral phosphoric acid to afford a series of indole-containing chroman derivatives with structural diversity in overall high yields (up to 98%), good diastereoselectivities (up to 93:7 dr) and moderate to excellent enantioselectivities (up to 98% ee). This approach not only enriches the chemistry of catalytic asymmetric cycloadditions involving 3-vinylindoles but is also useful for synthesizing chiral chroman derivatives.

Organocatalytic Asymmetric Addition of Aromatic α-Cyanoketones to o-Quinone Methides: Synthesis of 3,4-Dihydrocoumarins and Tetrasubstituted Chromans

The first organocatalytic asymmetric addition of aromatic α-cyanoketones to in situ-generated o-quinone methides has been developed. The products 3,4-dihydrocoumarin and tetrasubstituted chroman were obtained via addition of aromatic α-cyanoketones to in situ-generated o-quinone methides followed by treatment with 0.7 N HCl. With 10 mol % catalyst, the desired products were obtained in high enantio- and diastereoselectivities.

Biomimetic approach to the catalytic enantioselective synthesis of tetracyclic isochroman

Polyketide oligomers containing the structure of tetracyclic isochroman comprise a large class of natural products with diverse activity. However, a general and stereoselective method towards the rapid construction of this structure remains challenging due to the inherent instability and complex stereochemistry of polyketide. By mimicking the biosynthetic pathway of this structurally diverse set of natural products, we herein develop an asymmetric hetero-Diels–Alder reaction of in-situ generated isochromene and ortho-quinonemethide. A broad range of tetracyclic isochroman frameworks are prepared in good yields and excellent stereoinduction (up to 95% ee) from readily available α-propargyl benzyl alcohols and 2-(hydroxylmethyl) phenols under mild conditions. This direct enantioselective cascade reaction is achieved by a Au(I)/chiral Sc(III) bimetallic catalytic system. Experimental studies indicate that the key hetero-Diels-Alder reaction involves a stepwise pathway, and the steric hindrance between in-situ generated isochromene and t-Bu group of Sc(III)/N,N’-dioxide complex is responsible for the enantioselectivity in the hetero-Diels–Alder reaction step.

General and stereoselective synthesis of tetracyclic isochroman-containing polyketide oligomers is challenging. Here, the authors report on an Au(I)/chiral Sc(III) bimetallic catalyst for a biomimetic asymmetric hetero-Diels–Alder reaction for in-situ generation of isochromene and ortho-quinonemethide.

A formal intermolecular [4 + 1] cycloaddition reaction of 3-chlorooxindole and o-quinone methides: a facile synthesis of spirocyclic oxindole scaffolds

Herein, we developed an efficient and straightforward method for the rapid synthesis of spirocyclic oxindole scaffolds via the [4 + 1] cyclization reaction of 3-chlorooxindole with o-quinone methides (o-QMs), which were generated under mild conditions. The products could be obtained in excellent yields with numerous types of 3-chlorooxindole. This methodology features mild reaction conditions, high atom-economy and broad substrate scope.

Herein, we developed an efficient and straightforward method for the rapid synthesis of spirocyclic oxindole scaffolds via the [4 + 1] cyclization reaction of 3-chlorooxindole with o-quinone methides (o-QMs), which were generated under mild conditions.

Catalytic Asymmetric Conjugate Addition/Hydroalkoxylation Sequence: Expeditious Access to Enantioenriched Eight-Membered Cyclic Ether Derivatives

A sequential enantioselective conjugate addition/hydroalkoxylation between in situ generated ortho-quinomethanes and ynones by combining bifunctional squaramide and DBU catalysis has been developed. A variety of eight-membered cyclic ethers with two contiguous tertiary stereocenters were obtained in high yields with excellent stereoselectivities. This reaction not only provides a new strategy for constructing enantioenriched eight-membered cyclic ethers but also demonstrates the practicability of ynones as C4-syntons for the synthesis of chiral medium-membered rings.

(4 + 2) cyclization of aza-o-quinone methides with azlactones: construction of biologically important dihydroquinolinone frameworks

A base-promoted (4 + 2) cyclization of aza-o-quinone methides (aza-o-QMs) in situ generated from N-(o-chloromethyl)aryl amides was established. In this approach, azlactones were utilized as competent two-atom reaction partners to undergo (4 + 2) cyclization with aza-o-QMs, which afforded a series of dihydroquinolinone derivatives in overall good yields (up to 98%). This protocol has not only advanced the development of aza-o-QM-involved reactions, but also offered a useful method for constructing biologically important dihydroquinolinone frameworks.

Divergent Asymmetric Reactions of ortho-Quinone Methides with α-Thiocyanato Indanones for the Synthesis of Spiro- and Fused-Indanones

Reported in this work is a water triggered chemo-divergent enantioselective spiro-annulation and cascade reaction of ortho-quinone methides (o-QMs) with α-thiocyanato indanones catalyzed by a chiral organic base. In the case of spiro-annulation, the use of trace amount of water as additive is critical to achieve high enantioselectivity (up to 96 % ee). We found that a cascade reaction was enabled by just tuning the ratio of water in solvent. Accordingly, two new highly efficient asymmetric reactions for the divergent synthesis of spiro- and fused-indanone scaffolds with excellent enantioselectivities (up to 99 % ee) were developed. Mechanistic investigations suggest that interfacial hydrogen bonding may play an important role in achieving the switchable reaction pathways.

Recent advances for the synthesis of chiral sulfones with the sulfone moiety directly connected to the chiral center

The recent development of strategies for the asymmetric synthesis of chiral sulfones with sulfone moieties directly connected to the stereocenters.

Hydroquinine-catalyzed asymmetric 1,4-hydrophosphination of in situ generated aza-o-quinone methides with H-phosphine oxides

An organocatalytic enantioselective 1,4-addition of H-phosphine oxides to in situ generated aza-o-quinone methides has been successfully established using hydroquinine.

Organocatalytic C3-functionalization of indolizines: synthesis of biologically important indolizine derivatives

A Brønsted acid-catalyzed C3-alkylation of indolizines has been established with different electrophiles such as ortho-hydroxybenzyl alcohols, other precursors of para-quinone methides and ortho-quinone methides, and 2-indolylmethanol as well. By using this approach, a series of C3-functionalized indolizines were synthesized in overall good yields (up to 89%). These examples demonstrate that the Brønsted acid-catalyzed C3-alkylation of indolizines has a wide applicability, which can serve as a useful method for synthesizing C3-functionalized indolizine derivatives with structural diversity. This reaction has fulfilled the task of developing organocatalytic C3-functionalization of indolizines for the synthesis of biologically important indolizine derivatives.

Organocatalytic asymmetric addition of thioglycolates to o-quinone methides: a route to 5-substituted-5H-benzoxathiepine-2(3H)-ones

Herein we have developed the first enantioselective synthesis of 5-substituted-5H-benzoxathiepine-2(3H)-ones. 2-Sulfonylmethyl phenols and thioglycolates were employed as reaction partners in this method. The desired thia Michael products were obtained via bifunctional squaramide catalyzed conjugate addition reaction to in situ generated o-quinone methides and then basic hydrolysis followed by cyclization led to the formation of 5-substituted-5H-benzoxathiepine-2(3H)-ones. Broad scope and moderate to high enantioselectivities were observed for both products.

Diastereo- and Enantioselective Construction of Biologically Important Chiral 1,3-Dioxolochroman Frameworks via Catalytic Asymmetric [4+2] Cycloaddition

A diastereo- and enantioselective construction of biologically important chiral 1,3-dioxolochroman frameworks has been established via chiral phosphoric acid (CPA)-catalyzed asymmetric [4+2] cycloaddition of ortho-quinone methides with 3-methyl-2-vinylindoles. By using this approach, a series of indole-based chiral 1,3-dioxolochromans were synthesized with structural diversity in generally good yields, excellent diastereoselectivities and high enantioselectivities (up to 98% yields, >95:5 dr, 97% ee). The evaluation on the cytotoxic activity of some selected products indicated that this class of chiral 1,3-dioxolochroman derivatives had some extent of anti-cancer activity. This reaction not only provides an efficient synthetic method for accessing chiral 1,3-dioxolochroman derivatives with structural diversity and optical purity but also will enrich the research contents of catalytic asymmetric [4+2] cycloadditions involving ortho-quinone methides. In addition, the bioassay of these compounds will cast a light on discovering useful bioactivities of chiral 1,3-dioxolochroman derivatives, which will be helpful for finding lead compounds.

Construction of chiral chroman scaffolds via catalytic asymmetric (4 + 2) cyclizations of para-quinone methide derivatives with 3-vinylindoles

The title reaction has been established in the presence of chiral phosphoric acid, affording chiral chroman derivatives bearing an indole moiety in high yields and with moderate to good stereoselectivities.

Migratory ability of quinone methide-generating acridine conjugates in DNA

Conversion of a bisquinone methide–acridine conjugate to its monofunctional analogue releases the constraints that limit migration of its reversible adducts within DNA.

Asymmetric synthesis of benzothiazolopyrimidines with high catalytic efficiency and stereoselectivity under bifunctional phosphonium salt systems

Bifunctional phosphonium salt-mediated formal [4+2] annulation towards chiral benzothiazolopyrimidine compounds with excellent yields and stereoselectivities.

Enantioselective Catalytic [4+1]-Cyclization of ortho-Hydroxy-para-Quinone Methides with Allenoates

The first highly asymmetric catalytic synthesis of densely functionalized dihydrobenzofurans is reported, which starts from ortho-hydroxy-containing para-quinone methides. The reaction relies on an unprecedented formal [4+1]-annulation of these quinone methides with allenoates in the presence of a commercially available chiral phosphine catalyst. The chiral dihydrobenzofurans were obtained as single diastereomers in yields up to 90 % and with enantiomeric ratios up to 95:5.

A Strategy To Obtain o-Naphthoquinone Methides: Ag(I)-Catalyzed Cyclization of Enynones for the Synthesis of Benzo[ h]chromanes and Naphthopyryliums

A new strategy to obtain o-NQM intermediates through a ring-formation strategy by Ag(I)-catalyzed cyclization of 2-alkenylphenyl alkynyl ketones and its [4 + 2] annulations with styrenes has been developed. This reaction features high efficiency, mild reaction conditions, as well as flexible substitutions and atom economy. The obtained benzo[ h]chromane products were further oxidized to naphthopyryliums, which displayed tunable photophysical properties.

FeCl3-catalyzed dimerization/elimination of 1,1-diarylalkenes: efficient synthesis of functionalized 4H-chromenes

An efficient synthesis of valuable poly-substituted 4H-chromenes was developed via a [4 + 2] cycloaddition followed by the elimination of 1,1-diarylalkenes. The inexpensive FeCl₃ salt has proven to be an efficient catalyst for this transformation. The commercial availability of the catalyst and reagents, together with a convenient procedure, makes this an attractive method for 4H-chromene synthesis.

Organocatalytic asymmetric [4 + 1] annulation of in situ generated ortho-quinomethanes with 4-halo pyrazolones: straightforward access to chiral spiro-benzofuran pyrazolones

Asymmetric [4 + 1] annulation of ortho-quinomethanes with 4-halo pyrazolones was developed to construct spiro-benzofuran pyrazolones.

Phosphine-catalyzed [4 + 2] cyclization of para-quinone methide derivatives with allenes

The first [4 + 2] cyclization of para-quinone methide derivatives with allenes has been established via phosphine catalysis, which afforded a series of chroman derivatives in high yields (up to 97%) and excellent (E/Z)-selectivities (all >95 : 5 E/Z).

Asymmetric organocatalytic conjugated addition of pyrazolin-5-ones to ortho-quinomethanes: construction of vicinal tertiary and all-carbon quaternary stereocenters

An organocatalytic conjugated addition of pyrazolin-5-ones to ortho-quinomethanes was developed to construct the vicinal tertiary and all-carbon quaternary stereocenters.

A visible light photoredox catalyzed carbon radical-mediated generation of ortho-quinone methides for 2,3-dihydrobenzofuran synthesis

A visible light photoredox-catalyzed carbon radical-mediated strategy for in situ formation of ortho-quinone methides from 2-vinyl phenols towards 2,3-dihydrobenzofuran synthesis is described.