Enantioselective Synthesis of Tetrahydropyrano[3,4‐b]indoles: Palladium(II)‐Catalyzed Aminopalladation/1,4‐Addition Sequence

A novel palladium(II)‐catalyzed cyclization of aniline‐tethered alkynyl cyclohexadienones is reported. This reaction offers an atom‐economical and redox‐neutral access to various cyclohexenone‐fused tetrahydropyrano[3,4‐b ]indoles with high yield and excellent enantioselectivity. Remarkably, this work represents the first example on a transition‐metal‐catalyzed asymmetric intramolecular aminopalladation/1,4 addition sequence.     

Enantioselective Redox-Divergent Chiral Phosphoric Acid Catalyzed Quinone Diels–Alder Reactions

An efficient enantioselective construction of tetrahydronaphthalene-1,4-diones as well as dihydronaphthalene-1,4-diols by a chiral phosphoric acid catalyzed quinone Diels–Alder reaction with dienecarbamates is reported. The nature of the protecting group on the diene is key to the success of achieving high enantioselectivity. The divergent “redox” selectivity is controlled by using an adequate amount of quinones. Reversible redox switching without erosion of enantioselectivity was possible from individual redox isomers.     

An Asymmetric Redox Arylation: Chirality Transfer from Sulfur to Carbon through a Sulfonium [3,3]‐Sigmatropic Rearrangement

A general, asymmetric redox arylation of ynamides and thioalkynes with chiral sulfoxides is reported. This is the first example of a general 1,4‐chirality transfer from sulfur to a carbon stereocenter through a sulfonium [3,3]‐sigmatropic rearrangement. This reaction delivers α‐arylated thioesters and amides under mild conditions in an atom‐economical manner. The products are formed in high yields with enantiomeric ratios up to 99.5:0.5. Quantum chemical calculations suggest a mechanism for the chirality transfer from sulfur to carbon and explain the experimentally observed correlation of the enantioselectivity with both the catalyst and the substrate.     

Enantioselective 1,4‐Addition Reaction of α,β‐Unsaturated Carboxylic Acids with Cycloalkanones Using Cooperative Chiral Amine–Boronic Acid Catalysts

An enantioselective 1,4‐addition of α,β‐unsaturated carboxylic acids with cycloalkanones has been developed by using chiral amine–boronic acid cooperative catalysts. In the presence of a chiral amine and boronic acid, cycloalkanones and carboxylic acids are activated as chiral enamines and mixed anhydrides, respectively. The corresponding 1,4‐adducts are obtained in high yield with high enantioselectivity. Furthermore, subsequent oxylactonization of the 1,4‐adducts gives spirolactones with high diastereoselectivity.     

Synthesis of Chiral 1,4‐Benzodioxanes and Chromans by Enantioselective Palladium‐Catalyzed Alkene Aryloxyarylation Reactions

A highly enantioselective alkene aryloxyarylation led to the high‐yielding formation of a series of 1,4‐benzodioxanes, 1,4‐benzooxazines, and chromans containing quaternary stereocenters with excellent enantioselectivity. The sterically bulky and conformationally well defined chiral monophosphorus ligand L4 or L5 was responsible for the high reactivity and enantioselectivity of these transformations. The application of this method to the synthesis of the chiral chroman backbone of α‐tocopherol was demonstrated.     

CuII/TEMPO‐Catalyzed Enantioselective C(sp3)–H Alkynylation of Tertiary Cyclic Amines through Shono‐Type Oxidation

A novel strategy for asymmetric Shono‐type oxidative cross‐coupling has been developed by merging copper catalysis and electrochemistry, affording C1‐alkynylated tetrahydroisoquinolines with good to excellent enantioselectivity. The use of TEMPO as a co‐catalytic redox mediator is crucial not only for oxidizing a tetrahydroisoquinoline to an iminium ion species but also for decreasing the oxidation potential of the reaction. A novel bisoxazoline ligand is also reported.     

Palladium‐Catalyzed Asymmetric Aminohydroxylation of 1,3‐Dienes

A Pd^II‐catalyzed asymmetric aminohydroxylation of 1,3‐dienes with N‐tosyl‐2‐aminophenols was developed by making use of a chiral pyridinebis(oxazoline) ligand. The highly regioselective reaction provides direct and efficient access to chiral 3,4‐dihydro‐2H‐1,4‐benzoxazines in high yield and enantioselectivity (up to 96:4 e.r.). The reaction employs readily available N‐tosyl‐2‐aminophenols as a unique aminohydroxylation reagent and is complementary to known asymmetric aminohydroxylation methods.     

Asymmetric Decarboxylative 1,4‐Addition of Malonic Acid Half Thioesters to Vinyl Sulfones: Highly Enantioselective Synthesis of 3‐Monofluoromethyl‐3‐Arylpropanoic Esters

An asymmetric decarboxylative 1,4‐addition of malonic acid half thioesters (MAHTs) to 2‐aryl‐substituted vinyl sulfones has been developed, yielding adducts with excellent enantioselectivity (up to 97 % ee). In view of tuning pK_a values, a quinine‐based benzyl‐substituted thiourea was designed and demonstrated as the most efficient catalyst. The enantioselective synthesis of 3‐monofluorinated analogues of 3‐methyl indanone and (+)‐turmerone has been accomplished from decarboxylative 1,4‐addition adducts with satisfactory results.     

Access to N‐Substituted 2‐Pyridones by Catalytic Intermolecular Dearomatization and 1,4‐Acyl Transfer

A novel rhodium‐catalyzed dearomatization of O‐substituted pyridines to access N‐substituted 2‐pyridones has been developed. A computational study suggests a mechanism involving the formation of a pyridinium ylide followed by an unprecedented 1,4‐acyl migratory rearrangement from O to C. Furthermore, the chiral dirhodium complexes serve as the catalyst for the asymmetric transformation with excellent enantioselective control. DFT calculations indicate the chirality is transferred from axial chirality to the central stereogenic centre. The stronger π–π interaction and CH–π interaction account for the high enantioselectivity.     

Organocatalytic 1,4‐Addition Reaction of 2‐Formyl(thio)esters to Vinylketones: An Efficient Access to Acyclic Chiral Building Blocks with a Quaternary Carbon Stereocenter

2‐Formyl(thio)esters were utilized as pronucleophiles to obtain less‐accessible acyclic chiral building blocks bearing versatile functional groups on a quaternary carbon atom for enantioselective 1,4‐addition to vinylketones. To achieve high enantioselectivity in the present 1,4‐addition reaction, thiourea‐tertiary amines containing a bulky chiral backbone were developed as catalysts, and several derivatizations of the products were performed to demonstrate the synthetic utility of the products.     

Enantioselective Organocatalytic Construction of Spiroindane Derivatives by Intramolecular Friedel–Crafts‐Type 1,4‐Addition

The highly enantioselective organocatalytic construction of spiroindanes containing an all‐carbon quaternary stereocenter by intramolecular Friedel–Crafts‐type 1,4‐addition is described. The reaction was catalyzed by a cinchonidine‐based primary amine and accelerated by water and p‐bromophenol. A variety of spiro compounds containing quaternary stereocenters were obtained with excellent enantioselectivity (up to 95 % ee). The reaction was applied to the asymmetric formal synthesis of the spirocyclic natural products (?)‐cannabispirenones A and B.     

Palladium‐Catalyzed Dearomative syn‐1,4‐Oxyamination

A palladium‐catalyzed dearomative syn‐1,4‐oxyamination protocol using non‐activated arenes has been developed. This one‐pot procedure utilizes arenophile chemistry, and the corresponding para‐cycloadducts are treated with oxygen nucleophiles via formal allylic substitution, providing direct access to syn‐1,4‐oxyaminated products. The reaction conditions permit a range of arenes, as well as different O‐nucleophiles, such as oximes and benzyl alcohols. Moreover, this process was established in an asymmetric fashion, delivering products with high enantioselectivity. The dearomatized products are amenable to a multitude of further derivatizations ranging from olefin chemistry to C?H activation, giving rise to a diverse set of new functionalities. Overall, this dearomative functionalization offers rapid and controlled formation of molecular complexity, enabling straightforward access to functionalized small molecules from simple and readily available arenes.     

Organocatalytic Enantioselective Synthesis of 1,4‐Dioxanes and Other Oxa‐Heterocycles by Oxetane Desymmetrization

A new asymmetric synthesis of chiral 1,4‐dioxanes and other oxa‐heterocycles has been developed by means of organocatalytic enantioselective desymmetrization of oxetanes. This mild process proceeds with exceedingly high efficiency and enantioselectivity to establish the quaternary stereocenters. This method complements the existing, yet limited, strategies for the synthesis of these oxa‐heterocycles.     

Enantioselective NiH/Pmrox‐Catalyzed 1,2‐Reduction of α,β‐Unsaturated Ketones

The enantioselective 1,2‐reduction of α,β‐unsaturated ketones was achieved using a NiH catalyst in the presence of pinacolborane. This mild process represents a general method to access a wide variety of structurally diverse α‐chiral allylic alcohols in excellent yields and enantioselectivity, as well as very high levels of ambidoselectivity for 1,2‐ over 1,4‐reduction. Furthermore, for reactions on a 10 mmol scale, catalyst loadings as low as 0.5 mol % could be employed to deliver product without any detrimental effect on the yield, enantio‐, or ambidoselectivity.     

Asymmetric Synthesis of Chiral 1,4‐Enynes through Organocatalytic Alkenylation of Propargyl Alcohols with Trialkenylboroxines

A highly enantioselective synthesis of 1,4‐enynes is described that proceeds through an organocatalytic reaction between propargyl alcohols and trialkenylboroxines. Our strategy relies on acid‐mediated generation of the carbocationic intermediate from propargyl alcohols followed by enantioselective alkenylation with trialkenylboroxines. A range of chiral 1,4‐enynes were obtained in moderate to good yields with high levels of enantioselectivity. Use of a highly acidic chiral N‐triflyl phosphoramide catalyst, which has two distant Lewis basic oxygen atoms, was found to be crucial for both high reactivity and selectivity in the present reaction.     

Theoretical study of enantiomeric and geometric control in chiral guanidine‐catalyzed asymmetric 1,4‐addition of 5H‐oxazol‐4‐ones

Density functional theory calculations are used to study the reaction mechanism and origins of high stereoselectivity in chiral guanidine‐catalyzed asymmetric 1,4‐addition of 5H‐oxazol‐4‐ones. The reaction involves proton abstraction of 5H‐oxazol‐4‐one, C—C bond formation, and proton transfer. N1 atom of chiral guanidine exchanges its character as base and acid to activate 5H‐oxazol‐4‐one and to facilitate the product formation. The role of N2—H2 is not only H‐bond donor for 5H‐oxazol‐4‐one but also electron accepter for N1. The enantioselectivity related with rate‐limiting step 1 and Z/E selectivity determined in step 2 are primarily influenced by a five to six‐membered ring link in the backbone of chiral guanidine. The reaction proceeds along the favorable path with smaller rotations of the linked bonds. The enantioselectivity is improved with guanidine involving an electron‐deficient and bulky substituent. With methyl ether‐protected hydroxy in structure, the catalytic ability and enantioselective control of guanidine are extraordinarily low, affording the opposite enantiomer as major product. Z‐isomers are preferred in all cases. © 2013 Wiley Periodicals, Inc.     

Intermolecular 1,4‐Carboamination of Conjugated Dienes Enabled by Cp*RhIII‐Catalyzed C−H Activation

A protocol for the three‐component 1,4‐carboamination of dienes is described. Synthetically versatile Weinreb amides were coupled with 1,3‐dienes and readily available dioxazolones as the nitrogen source using [Cp*RhCl₂]₂‐catalyzed C?H activation to deliver the 1,4‐carboaminated products. This transformation proceeds under mild reaction conditions and affords the products with high levels of regio‐ and E‐selectivity. Mechanistic investigations suggest an intermediate Rh^III–allyl species is trapped by an electrophilic amidation reagent in a redox‐neutral fashion.     

Sumanenemonoone Imines Bridged by Redox‐Active π‐Conjugated Unit: Synthesis,Stepwise Coordination to Palladium(II), and Laser‐Induced Formation of Nitrogen‐Doped Graphitic Carbon

Sumanenemonoone imine compounds bridged by a redox‐active π‐conjugated unit on the basis of the conversion between 1,4‐phenylenediamine and 1,4‐benzoquinonediimine were synthesized and characterized. The stepwise coordination of the imino groups to Pd^II in the sumanenemonoone imine compound bridged by 1,4‐benzoquinonediimine was indicated by the titration experiment. Laser irradiation of a film of the metal‐free quionediimine gave nitrogen‐doped graphitic carbon, which was supported by an increase in conductivity and by Raman spectroscopy. The obtained graphitic carbon corresponds to carbonous compounds thermally treated at approximately 700–1000 °C. The ratio of nitrogen and carbon relative to that in the starting compound was nearly completely retained (5.4 % decrease).     

Novel Cs‐Symmetric 1,4‐Diphosphine Ligands in the Copolymerization of Propene and Carbon Monoxide: High Regio‐ and Stereocontrol in the Catalytic Performance

New C_s‐symmetric aryl 1,4‐diphosphine ligands were synthesized and tested in the copolymerization of carbon monoxide and propene. The electronic properties of the two different P‐atoms did not affect the high enantioselectivity of the catalyst precursors, thus resulting in high ‘regio’‐ and ‘stereoregular’ copolymers.     

Regioselective Catalytic Asymmetric C‐Alkylation of Isoxazolinones by a Base‐Free Palladacycle‐Catalyzed Direct 1,4‐Addition

Isoxazolinones constitute a class of heterocycles utilized for the development of novel drug candidates. The cyclic oxime ester motif is also synthetically useful as it contains functional handles which have previously been used to provide access to an assortment of valuable compound classes not easily accessible by alternative approaches. However, asymmetric methods towards isoxazolinones are notoriously scarce. Herein we report the first catalytic asymmetric alkylations of isoxazolinones forming all‐C‐substituted quaternary stereocenters. The present studies were driven by the question of how to control the regioselectivity in the competition of different nucleophilic positions. The investigation of a direct 1,4‐addition uncovered that a sterically demanding palladacycle catalyst directs the reactivity in the absence of a base nearly exclusively to the nucleophilic C atom, while at the same time it allows for high enantioselectivity and TONs up to 1900.