Enantioselective Synthesis of Functionalized 4-Aryl Hydrocoumarins and 4-Aryl Hydroquinolin-2-ones via Intramolecular Vinylogous Rauhut-Currier Reaction of para-Quinone Methides

Design, synthesis and biological evaluation of diarylmethyl amine derivatives with anti-ulcerative colitis activity via inhibiting inflammation and oxidative stress

Two series of diarylmethylamine derivatives were synthesized by 1,6-addition reaction between para-quinone methides and 1-methylpiperazine or 2-oxazolidinone, and their structures were identified by 1H NMR,13C NMR and HRMS. In the lipopolysaccharide-induced inflammatory Raw264.7 cells model, 3-CF3 modified active derivative 1l was screened out by inhibiting the excessive production of NO (IC50 = 5.82 μM), and can inhibit the excessive production of ROS. Western blot analyses indicated that 1l can also inhibit the excessive production of pro-inflammatory cytokines (IL-6 and TNF-α) and the nuclear transfer of NF-κB in inflammatory cells. In the dextran sulfate sodium (DSS)-induced ulcerative colitis (UC) mice model, 1l can effectively inhibit the colonic shortening and suppress inflammatory symptoms of the colonic tissue (HE). Western blot analyses and biochemical indicators demonstrated that 1l can protect the colon of UC mice by regulating the inflammation-related TLR4/NF-κB signaling pathway and the oxidative stress-related Nrf2/HO-1 signaling pathway. Besides, the safety evaluation results of the UC mouse model (serum biochemical indicators, pathological tissue analysis and organ indexes) and the oral acute toxicity test revealed that 1l had certain safety in mice and can resist other tissues damage caused by DSS. In summary, 1l is an effective anti-inflammatory agent that can be developed as a potential drug for treating UC.

Chemoselective Intramolecular Morita-Baylis-Hillman Reaction; Acrylamide and Ketone as Sluggish Reacting Partners on a Labile Framework

Chemoselectivity is an important issue frequently encountered while working over labile precursors. Carbonyl compounds with a heteroatom at the β carbon are sensitive precursors because they are prone to elimination under different conditions. Morita-Baylis-Hillman (MBH) reaction, although a widespread method for C-C bond formation, has its own limitations. Acrylamide and ketone are such limitations of the MBH reaction. Using them together for an intramolecular MBH (IMBH) reaction on a labile framework prone to elimination is a significant 2-fold synthetic challenge. A highly chemoselective IMBH reaction on such precursors has been established using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a promoter. The protocol leads to quick access to a diversely substituted and functionalized piperidone framework in high yields. Various substitution patterns in the form of 34 successful examples have been studied. A diastereoselective version and tolerance to various functional and protecting groups are the added advantages of the developed methodology. A tertiary carbon at the β position of ketone, however, led to complete reversal of selectivity and gave only the elimination product. Control experiments toward a better understanding of the substitution pattern, role of catalyst, and mechanistic study have been carried out. As an application of the IMBH adduct, a one-step allylic rearrangement for the dihydropyridone framework has also been demonstrated.

Brønsted Acid-Mediated Multicomponent One-Pot Approach to Direct Construction of 4-Aryl-hydrocoumarin Derivatives

The facile and efficient synthesis of a unique class of 4-aryl-hydrocoumarins having enormous applications in medicinal chemistry and natural products is presented. We have for the first time developed a Brønsted acid-catalyzed, multicomponent, one-pot approach for producing various 4-aryl-coumarin derivatives. The feedstock availability of these precursors allowed access to a wide range of 2-chromanone derivatives in good to excellent yields under mild conditions. The practicality of this protocol was justified by the synthesis of bioactive compounds, late-stage functionalization of natural products, and gram-scale synthesis.

Phosphine-catalysed transformations of ortho- and para-quinone methides

Organocatalytic methodologies for the derivatization of o-QM, p-QM and the analogous aza-QM have been recently developed and involve different catalytic systems such as phosphoric acids, thioureas, squaramides, NHC carbenes or chiral ammonium salts. Besides, phosphines, commonly used as ligands in metal-catalysed reactions, can be also used as organocatalysts. In this case, they are mainly involved as nucleophilic catalysts in reactions such as the Rauhut-Currier (RC) reaction. In this review, an analysis of the recent developments in racemic and enantioselective phosphine-catalysed transformations of o-QM, p-QM and aza-o-QM has been carried out.

Synthesis of 4-aryl-3,4-dihydrocoumarins and 4-aryl-4H-chromenes via Er(OTf)3-catalyzed cascade reactions of p-quinone methides with 1,3-dicarbonyl compounds

The Er(OTf)₃-catalyzed cascade cyclization reaction of para-quinone methides (p-QMs) with various 1,3-dicarbonyl compounds has been developed, which efficiently constructed a series of versatile 4-aryl-3,4-dihydrocoumarins and 4-aryl-4H-chromenes. Herein, we not only propose a novel cyclization strategy of p-QMs, but also provide an easy access to structurally diverse coumarins and chromenes.

The crystal structure of methyl 4-((3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)methyl) benzoate, C23H28O3

C23H28O3, monoclinic, P21/n (no. 14), a = 7.3985 (2) Å, b = 16.6978 (5) Å, c = 16.5137 (5) Å, β = 96.699(3)°, V = 2026.15 (10) Å3, Z = 4, Rgt (F) = 0.0428, wRref (F 2) = 0.1116, T = 90 K.

The crystal structure of 2,6-di-tert-butyl-4-(4-(methylthio)benzylidene)cyclohexa-2,5-dien-1-one, C22H28OS

C22H28OS, monoclinic, P21/n (no. 14), a = 6.7729(3) Å, b = 17.3973(6) Å, c = 16.6277(6) Å, β = 92.380(3)°, V = 1957.55(13) Å3, Z = 4, R gt(F) = 0.041, wR ref(F 2) = 0.100, T = 90 K.

Chiral Aziridine Phosphines as Highly Effective Promoters of Asymmetric Rauhut–Currier Reaction

A series of chiral enantiomerically pure aziridines containing a phosphine moiety were synthesized and successfully applied as organocatalysts in asymmetric intramolecular Rauhut–Currier reactions of p-quinone derivatives. The desired chiral phenols were achieved in high chemical yields and with satisfactory values of enantiomeric excess (up to 98% ee, in some cases). The stereochemical course of the title reaction may be controlled by the use of an appropriate enantiomer of the catalyst. The individual enantiomers of the organocatalyst led to the formation of specific enantiomers of the chiral product.

Iron(III)-catalyzed tandem annulation of indolyl-substituted p-quinone methides with ynamides for the synthesis of cyclopenta[b]indoles

The unique reactivity of indolyl-substituted p-QMs as a new type of two-carbon synthon has been explored for the first time in a novel iron(III)-catalyzed tandem annulation. This (2+2) annulation/retro-4π electrocyclization/imino-Nazarov cyclization cascade reaction is characterized by an unusual structural reconstruction of indolyl-substituted p-QMs, leading to an expeditious assembly of synthetically important functionalized cyclopenta[b]indoles.

Effects of Anchimeric Assistance in Phosphonium Enolates Chemistry

Abstract

Phosphonium enolates are key intermediates of phosphine-catalyzed reactions extensively used in current organic synthesis for construction of carbon–carbon and carbon–heteroatom bonds. Herein, general methods for increasing the rates, chemo- and stereoselectivity on the basis of effects of anchimeric assistance for the reactions involving formation of phosphonium enolates are discussed.

Palladium Catalyzed Enantioselective Hayashi-Miyaura Reaction for Pharmaceutically Important 4-Aryl-3,4-dihydrocoumarins

The first palladium-catalyzed asymmetric addition of arylboronic acids to coumarins was successfully established, providing a straightforward asymmetric approach to achieving pharmaceutically important 4-aryl-3,4-dihydrocoumarins. This methodology features easily accessible and bench-stable ligands, a wide substrate scope, mild conditions, and accommodation of electron-withdrawing arylboronic acids.

Asymmetric cross Rauhut–Currier reactions of vinyl ketones with carbonyl para-quinone methides via phosphine catalysis

Novel cross Rauhut–Currier reactions, involving chiral phosphine catalysis, between vinyl ketones and terminal-carbonyl-substituted para-quinone methides are presented.

Diastereoselective synthesis of chroman bearing spirobenzofuranone scaffolds via oxa-Michael/1,6-conjugated addition of para-quinone methides with benzofuranone-type olefins

A simple and convenient cyclization of ortho-hydroxyphenyl-substituted para-quinone methides with benzofuran-2-one type active olefins via oxa-Michael/1,6-conjugated addition has been developed, which afforded an easy access to enriched functionalized chroman-spirobenzofuran-2-one scaffolds with good to excellent yields (up to 90%) and diastereoselectivities (up to >19 : 1 dr). This reaction provided an efficient method for constructing desired spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Highly diastereoselective synthesis of spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Synthesis of functionalized 3,2′-pyrrolidinyl spirooxindoles via domino 1,6-addition/annulation reactions of para-quinone methides and 3-chlorooxindoles

A highly efficient diastereoselective [4 + 1] cycloaddition of ortho-tosylaminophenyl-substituted p-QMs with 3-chlorooxindoles has been developed to afford 3,2′-pyrrolidinyl spirooxindoles.

Synthesis of Spiroindolenine Bearing Chroman Scaffolds Based on Cylization Reaction to para-Methylated Methides

An efficient diastereoselective cyclization reaction of ortho-hydroxyphenyl-substituted para-quinone methides with α,β-unsaturated imines generated in situ from aryl sulfonyl indoles was developed, which afforded spiroindolenine bearing chroman scaffolds in good to...

In situ phosphonium-containing Lewis base-catalyzed 1,6-cyanation reaction: a facile way to obtain α-diaryl and α-triaryl acetonitriles

A in situ phosphonium-containing catalyst and its successful application in promoting remote 1,6-cyanation reaction of p-QMs and fuchsones have been disclosed, providing a facile and direct access to both α-diaryl and α-triaryl acetonitriles.

Enantioselective Rauhut-Currier Reaction with β-Substituted Acrylamides Catalyzed by N-Heterocyclic Carbenes

β-Substituted acrylamides have low electrophilicity and are yet to be exploited in the enantioselective Rauhut-Currier reaction. By exploiting electron-withdrawing protection of the amide and moderate nucleophilicity N-heterocyclic carbenes, such substrates have been converted to enantioenriched quinolones. The reaction proceeds with complete diastereoselectivity, good yield, and modest enantioselectivity. Derivatizations are reported, as are computational studies, supporting decreased amide bond character with electron-withdrawing protection of the nitrogen.

Dearomatization of 2,3-Disubstituted Indoles via 1,8-Addition of Propargylic (Aza)-para-Quinone Methides

Dearomatization of indole is a useful strategy to access indolimines: a motif widely exists in biologically active molecules and natural products. Herein, an efficient method for the dearomatization of 2,3-disubstituted indoles to generate diverse indolimines with tetrasubstituted allenes is described. This work accomplishes dearomatization of 2,3-disubstituted indoles through 1,8-addition of (aza)-para-quinone methides, which are generated in situ from propargylic alcohols. A series of synthetically useful indolimines containing quaternary carbon centers and tetrasubstituted allenes can be accessed in good yields (up to 99%). Additionally, the separability of product isomers, diversified product transformations, and easy scale-up of the reaction demonstrate the potential application of this method.

Crystal structure of 2,6-di-tert-butyl-4-(4-methoxybenzylidene)cyclohexa-2,5-dien-1-one, C22H28O2

C22H28O2, orthorhombic, Pna21 (No. 33), a = 12.0841(9) Å, b = 16.6946(12) Å, c = 9.3837(8) Å, β = 90°, V = 1893.1(3) Å3, Z = 4, R gt (F) = 0.0497, wR ref (F 2) = 0.1057, T = 220 K.

Stereoselective Synthesis of Dihydrocoumarins via [1,2]-Phospha-Brook Rearrangement in Three-Component Coupling Reaction of α-Ketoesters, o-Quinone Methides, and Dialkyl Phosphites

A highly regio- and diastereoselective approach for the synthesis of phosphate substituted dihydrocoumarins via Brønsted base catalyzed [1,2]-phospha-Brook rearrangement is reported. The two-step, one-pot Michael addition of α-phosphonyloxy enolates proceeds by coupling of dialkyl phosphite and α-ketoesters to o-quinone methides, followed by an intramolecular cyclization, providing 3,4-dihydrocoumarin frameworks.

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.

Construction of Oxygen- and Nitrogen-based Heterocycles from p-Quinone Methides

In the last few years, there has been an explosive growth in the area of para-quinone methide (p-QM) chemistry. This boom is actually due to the unique reactivity pattern of p-QMs, and also their remarkable synthetic applications. In fact, p-QMs serve as synthons for unsymmetrical diaryl- and triarylmethanes, and also for the construction of diverse range of carbocycles and heterocycles. In the last few years, a wide range of structurally complex heterocyclic frameworks could be accessed through the synthetic transformations of structurally modified stable p-QMs. Therefore, the main focus of this review article is to cover the recent advancements in the transition-metal, Lewis acid and base-catalyzed/mediated synthetic transformations of the stable p-quinone methides (p-QMs) to oxygen- and nitrogen-containing heterocycles.

Formal (3 + 4)-Annulation of Propargylic p-Quinone Methides with 2-Indolylmethanols: Synthesis of Polysubstituted Indole-Fused Oxepines

A novel Brønsted acid catalyzed 1,8-addition mediated (3 + 4)-annulation of in situ generated propargylic p-quinone methides with 2-indolylmethanols is described. This method provides a convenient and mild approach to structurally interesting and synthetically important polysubstituted indole-fused oxepines in high yields. Moreover, 2-indolylmethanols as four-atom synthons in the (3 + 4)-annulations under Brønsted acid conditions have been explored for the first time.

Catalytic Asymmetric Hydrogenation of 3-Ethoxycarbonyl Quinolin-2-ones and Coumarins

A protocol of iridium catalyzed asymmetric hydrogenation of 4-alkyl substituted 3-ethoxycarbonyl quinolin-2-ones and coumarins has been reported, providing a wide range of chiral dihydroquinolin-2-ones and dihydrocoumarins in high yields with excellent enantioselectivities (up to 99% ee) and high turnover numbers (up to 28 000). This efficient protocol was successfully applied for the synthesis of MPR3160 and the key chiral intermediate of R-106578.

Brønsted Acid-Catalyzed Formal (3+3)-Annulation of Propargylic (Aza)-para-Quinone Methides with 4-Hydroxycoumarins and 1,3-Dicarbonyl Compounds

Herein, we describe a highly effective 1,8-conjugate-addition-mediated formal (3+3)-annulation of (aza)-para-quinone methides in situ generated from propargylic alcohols with 4-hydroxycoumarins and 1,3-dicarbonyl compounds under the catalysis of a Brønsted acid. This methodology affords efficient and practical access to synthetically important and highly functionalized pyranocoumarins and pyrans in excellent yields under mild conditions. Importantly, these products exhibit impressive inhibitory activity toward α-glucosidase.

Asymmetric Construction of α-Substituted β-Hydroxy Lactones via Ni Catalyzed Decarboxylative Addition Reaction

We described a Ni-bidentate oxazoline catalyzed highly enantio- and diastereoselective decarboxylative aldol reaction of 2-oxotetrahydrofuran-3-carboxylic acid/2-oxochromane-3-carboxylic acid derivatives with different kinds of carbonyls. Under optimal reaction conditions, α-substituted β-hydroxy butyrolactones and dihydrocoumarins with an all-carbon quaternary stereocenter have been generated with high levels of functional-group compatibility. Furthermore, proficient transformations of products were also described, in which an aliphatic tertiary alcohol and a multi-substituted 1,4-diol were smoothly constructed through hydrogenation and ring-opening reaction, respectively.

Chiral Tertiary Amine Catalyzed Asymmetric [4 + 2] Cyclization of 3-Aroylcoumarines with 2,3-Butadienoate

Coumarins and 2H-pyran derivatives are among the most commonly found structural units in natural products. Therefore, the introduction of 2H-pyran moiety into the coumarin structural unit, i.e., dihydrocoumarin-fused dihydropyranones, is a potentially successful route for the identification of novel bioactive structures, and the synthesis of these structures has attracted continuing research interest. Herein, a chiral tertiary amine catalyzed [4 + 2] cyclization of 3-aroylcoumarines with benzyl 2,3-butadienoate was reported. In the presence of Kumar’s 6’-(4-biphenyl)-β-iso-cinchonine, the desired dihydrocoumarin-fused dihydropyranone products could be obtained in up to 97% yield and 90% ee values.

Synthesis of heterocyclic compounds through nucleophilic phosphine catalysis

Nucleophilic phosphine catalysis is a practical and powerful tool for the synthesis of various heterocyclic compounds with the advantages of environmentally friendly, metal-free, and mild reaction conditions. The present report summarizes the construction of four to eight-membered heterocyclic compounds containing nitrogen, oxygen and sulphur atoms through phosphine-catalyzed intramolecular annulations and intermolecular [2+2], [3+2], [4+1], [3+1+1], [5+1], [4+2], [2+2+2], [3+3], [4+3] and [3+2+3] annulations of electron-deficient alkenes, allenes, alkynes and Morita-Baylis-Hillman carbonates.

Enantioselective Construction of Chiral Cyclopropa[c]coumarins via Lewis Base-Catalyzed Cyclopropanation

The first highly enantioselective construction of chiral cyclopropa[c]coumarins was described. Using commercially available (bis)cinchona alkaloid (DHQ)₂PYR as the chiral Lewis base catalyst, together with Cs₂CO₃ as the achiral base, the reaction of a series of coumarin-3-carboxylate and 3-benzoyl coumarins with tert-butyl 2-bromoacetate could give rise to the corresponding cyclopropa[c]coumarins bearing three continuous chiral stereocenters in 83-93% ee and 90-97% ee, respectively. The reaction is proposed to proceed via an in situ generated ammonium ylide intermediate.

Enantio- and diastereoselective diarylmethylation of 1,3-dicarbonyl compounds

A Cu-catalyzed enantio- and diastereoselective diarylmethylation of 1,3-dicarbonyl compounds is described. It is a successful example of constructing all-carbon quaternary stereocenters from 3 °C–H nucleophiles, a challenging topic in synthetic chemistry. In the present work, two contiguous stereocenters are constructed with high levels of stereoselectivity and atom economy. The broad scope of 1,3-dicarbonyl nucleophiles and the tolerance of a wide range of functional groups make this protocol of great importance in the synthesis of chiral diarylmethane compounds.

A Cu-catalyzed enantio- and diastereoselective diarylmethylation of 1,3-dicarbonyl compounds is described. Two contiguous stereocenters including one all-carbon quaternary stereocenter are constructed with good stereoselectivity and atom economy.

Asymmetric synthesis of dihydrocoumarins via catalytic sequential 1,6-addition/transesterification of α-isocyanoacetates with para-quinone methides

Dihydroquinidine-derived aminophosphine/AgNO₃ catalyzed asymmetric tandem 1,6-addition/transesterification of α-isocyanoacetates with o-hydroxyphenyl-substituted p-QMs was developed.

Metal free highly efficient C–N bond formation through 1,6-addition: synthesis and photophysical studies of diaryl methyl amino acid esters (DMAAEs)

A transition-metal free, proficient strategy for the one-pot synthesis of diverse diaryl methyl amino acid esters (DMAAEs) has been established from the easily accessible chiral amino acid esters and para-quinone methides (QMs) in very good to excellent yields.

Asymmetric organocatalytic double 1,6-addition: rapid access to chiral chromans with molecular complexity

We have developed a highly δ-selective organocatalytic asymmetric double 1,6-addition protocol by tethering ortho-hydroxyphenyl substituted p-QMs onto the linear unbiased 2,4-dienals towards the formation of synthetically enriched chiral chromans.

1,6-Conjugate addition initiated formal [4+2] annulation of p-quinone methides with sulfonyl allenols: a unique access to spiro[5.5]undeca-1,4-dien-3-one scaffolds

An expedient one-pot synthesis of carbocyclic spiro[5.5]undeca-1,4-dien-3-ones via 1,6-conjugate addition initiated formal [4+2] annulation sequences by employing p-quinone methides and sulfonyl allenols.

Regioselective catalytic asymmetric N-alkylation of isoxazol-5-ones with para-quinone methides

A highly regioselective and enantioselective N-alkylation of isoxazol-5-ones with para-quinone methides has been developed to obtain enantioenriched N-diarylmethane substituted isoxazolinones with high yields and enantioselectivities.

Organocatalytic Remote Stereocontrolled 1,8-Additions of Thiazolones to Propargylic Aza-p-quinone Methides

A remote stereocontrolled 1,8-conjugate addition of thiazolones to propargylic aza-p-quinone methides formed from propargylic alcohols has been developed with the aid of a chiral phosphoric acid, and this represents the first report on organocatalytic stereocontrolled 1,8-addition of propargylic aza-p-quinone methides. Notably, the remote stereocontrolled activation protocol enables the construction of vicinal sulfur-containing quaternary carbon stereocenters and axially chiral tetrasubstituted allenes and promotes the chemistry of chiral phosphoric acids.