Palladium-Catalyzed Enantioselective Intermolecular Carboetherification of Dihydrofurans

Phosphonium-Catalyzed Monoreduction of Bisphosphine Dioxides: Origin of Selectivity and Synthetic Applications

Controlling product selectivity in successive reactions of the same type is challenging owing to the comparable thermodynamic and kinetic properties of the reactions involved. Here, the synergistic interaction of the two phosphoryl groups in bisphosphine dioxides (BPDOs) with a bromo-phosphonium cation was studied experimentally to provide a practical tool for substrate-catalyst recognition. As the eventual result, we have developed a phosphonium-catalyzed monoreduction of chiral BPDOs to access an array of synthetically useful bisphosphine monoxides (BPMOs) with axial, spiro, and planar chirality, which are otherwise challenging to synthesize before. The reaction features excellent selectivity and impressive reactivity. It proceeds under mild conditions, avoiding the use of superstoichiometric amounts of additives and metal catalysts to simplify the synthetic procedure. The accessibility and scalability of the reaction allowed for the rapid construction of a ligand library for optimization of asymmetric Heck-type cyclization, laying the foundation for a broad range of applications of chiral BPMOs in catalysis.

Diastereodivergent and Regioselective Synthesis of Tetrahydrofuro[2,3-b]furans with Four Consecutive Stereocenters

Base-catalyzed diastereodivergent and regioselective domino processes of triketone enones with arylacetaldehydes for the synthesis of tetrahydrofuro[2,3-b]furans with four consecutive stereocenters are reported. Good yields and diastereoselectivities are obtained when DBU is employed as a catalyst; in contrast, Et3N delivers a different diastereomer in excellent diastereoselectivity. This work offers many advantages, including switchable diastereoselectivity, cheap base catalysts, and a simple operation.

Sadphos as Adaptive Ligands in Asymmetric Palladium Catalysis

ConspectusPalladium catalysis, as one of the most important strategies in asymmetric synthesis, has continuously attracted the attention of organic chemists. With the development of chiral ligands, increasingly challenging reactions and substantial progress in asymmetric catalysis are being realized.Since 2014, we have focused on exploiting a series of sulfinamide phosphine ligands called "Sadphos," including Ming-Phos, Xu-Phos, Xiao-Phos, Xiang-Phos, TY-Phos, PC-Phos, GF-Phos, and WJ-Phos. These ligands can be easily prepared in two to four steps using commercial materials. These new types of ligands have shown remarkable performance in transition-metal-catalyzed reactions, especially in Pd-catalyzed transformations. X-ray diffraction analysis, mechanistic studies, and density functional theory calculations have revealed that Sadphos ligands can coordinate with the Pd0 and PdII species in the Pd0/P, Pd0/P,S, or PdII/P,O modes.This Account summarizes our recent efforts toward palladium-catalyzed enantioselective reactions using Sadphos ligands. These ligands were found to be privileged and very crucial to promote the reactions by increasing the reactivity and enantioselectivity. Ming-Phos is an effective ligand in Pd-catalyzed asymmetric coupling and intramolecular Heck reactions, providing highly enantioselective trisubstituted allenes, axially chiral anilides, gem-diarylmethine silanes, and disubstituted dihydroisoquinolinones. Incorporation of an electron-rich cyclohexyl group in the phosphine moiety afforded Xu-Phos, which showed a unique effect in a series of asymmetric transformations, including reductive Heck, dearomative Mizoroki-Heck, tandem Heck/Suzuki coupling, carboiodination, carboamination, and cross-coupling reactions. Using a similar strategy, our group synthesized more electron-rich TY-Phos and Xiang-Phos ligands bearing t-butyl and 1-adamantyl group at P atoms, respectively. Regarding stereoelectronic features, these two characteristic ligands were the best choice to satisfy the requirements of the palladium-catalyzed fluoroarylation of gem-difluoroalkenes, intermolecular α-arylation of aldehydes, carboetherification of alkenyl oximes, and carboheterofunctionalization of 2,3-dihydrofurans. Compared with the aforementioned Sadphos ligands, the attractive features of Xiao-Phos, including high nucleophilicity originating from the CH2PPh2 group and the ortho-substituent effect at the side of the aryl ring, are presumably responsible for its efficiency. The Pd/Xiao-Phos catalyst system shows good performance in a series of cross-coupling reactions of secondary phosphine oxides, affording P-stereogenic products bearing multiple types of molecular skeletons. The modification of the basic Sadphos backbone by introducing a xanthene skeleton motivated us to design and synthesize monophosphines, named PC-Phos and GF-Phos. PC-Phos is effective in various reactions, including arylation of sulfenate anions, denitrogenative cyclization of benzotriazoles, and dearomatization of indoles. The practicability of GF-Phos was validated in the Pd-catalyzed asymmetric three-component coupling of N-tosylhydrazones, aryl halides, and terminal alkynes, as well as in the cross-coupling of N-tosylhydrazones and vinyl iodides with pendent amines. In addition, ferrocene-derived WJ-Phos was employed in the palladium-catalyzed Suzuki-Miyaura cross-coupling reaction, affording axially chiral biaryl monophosphine oxides in excellent enantiomeric excesses.

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Regio- and Stereospecific Hydrative Cloke-Wilson Rearrangement

The Cloke-Wilson rearrangement of unsymmetrical β-diketone-derived cyclopropanes inevitably yields a mixture of two 4-acylated 2,3-dihydrofuran regiomers. By using alkynes as masked acyls, Tf2NH-promoted Cloke-Wilson rearrangement of polysubstituted 1-(1-alkynyl)cyclopropyl ketones followed by alkyne hydration is described, regioselectively affording 2,3-dihydrofurans bearing 4-acyls nonequivalent to that involved in the Cloke-Wilson rearrangement. The 2,3-dihydrofuran rings with cis 2,3-diaryls are unexpectedly more stable than their trans diastereomers under the reaction conditions, guaranteeing the regiospecificity of this hydrative Cloke-Wilson rearrangement with high fidelity.

Nickel Chain-Walking Catalysis: A Journey to Migratory Carboboration of Alkenes

ConspectusChain-walking offers extensive opportunities for innovating synthetic methods that involve constructing chemical bonds at unconventional sites. This approach provides previously inaccessible retrosynthetic disconnections in organic synthesis. Through chain-walking, transition metal-catalyzed alkene difunctionalization reactions can take place in a 1,n-addition (n ≠ 2) mode. Unlike classical 1,2-regioselective difunctionalization reactions, there remains a scarcity of reports regarding migratory patterns. Moreover, the range of olefins utilized in these studies is quite limited.About five years ago, our research group embarked on a project aimed at developing valuable migratory difunctionalization reactions of alkenes through chain-walking. Our focus was on carboboration of alkenes utilizing nickel catalysis. The reaction commences with the migratory insertion of an olefin into a Ni-Bpin species. Subsequently, a thermodynamically stable alkyl nickel complex is generated through a chain-walking process. This complex then couples with a carbon-based electrophile, leading to the formation of an alkylboron compound. It is worth highlighting that the success of these transformations relies significantly on the utilization of a bisnitrogen-based ligand and LiOMe as a B2pin2 activator. Synthetically, these migratory carboboration reactions establish a robust platform for the rapid and efficient synthesis of a wide range of structurally diverse organoboron compounds, which are not facially accessed by conventional methods. The incorporation of a versatile boron group introduces a wealth of possibilities for subsequent diversifications, significantly enhancing the value of the resulting products and allowing for the creation of a broader range of valuable derivatives and applications.This Account provides a comprehensive overview of our research efforts and advancements in the field of migratory carboboration of unactivated alkenes using nickel catalysis. We begin by outlining the development of a series of 1,1-regioselective carboboration reactions of terminal alkenes. A significant focus is placed on the initial integration of boronate, which not only triggers the formation of thermodynamically stable metal species but also exerts control over remote stereochemistry in reactions involving substituted methylenecyclohexenes. Continuing our exploration, remarkable success is achieved in 1,3-regio- and cis-stereoselectivity when dealing with cyclic alkenes. Remarkably, nickel chain-walking catalysis enables heterocyclic alkenes to be viable coupling partners within our transformations. Moreover, it grants us the ability to achieve regioselectivity for cyclohexenes that was previously unattainable, thus expanding the horizons of regiochemical control in these reactions. Lastly, we present the evolution of ligand-modulated regiodivergent carboboration of allylarenes. By gaining insights into the underlying mechanisms driving regiodivergence, we lay a strong foundation for tackling challenges related to selecting specific sites in chain-walking reactions, especially when dealing with multiple stable factors. We anticipate that our findings, coupled with the mechanistic insights we've gained, will not only advance the realm of nickel chain-walking catalysis but also contribute to the broader understanding of selectivity control in reactions of this nature. This advancement will also catalyze the synthesis of intricate functional molecules, contributing to the creation of complex and valuable compounds in the realm of organic chemistry.

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Enantioselective Synthesis of 2,3,3a,8a-Tetrahydrofuro[2,3-b]benzofuran Scaffolds Enabled by Cu(II)/SPDO-Catalyzed [3+2] Cycloaddition of 2,3-Dihydrofuran and Quinone Esters

An asymmetric [3+2] cycloaddition of quinone esters with 2,3-dihydrofuran has been realized via a newly developed Cu(II)/SPDO complex. It provides straightforward access to 2,3,3a,8a-tetrahydrofuro[2,3-b]benzofurans (TFB) with high enantioselectivity (up to 97.5:2.5 er) and diastereoselectivity (all >20:1 dr). The resulting adducts contain two adjacent stereocenters and a continuously functionalized benzene ring. Additionally, this transformation could be easily performed on a gram scale, allowing for expedient synthesis of natural dihydroaflatoxin D2 and aflatoxin B2.

Ligand-Enabled NiII -Catalyzed Hydroxylarylation of Alkenes with Molecular Oxygen

The use of molecular oxygen as the terminal oxidant in transition metal catalyzed oxidative process is an appealing and challenging task in organic synthetic chemistry. Here, we report a Ni-catalyzed hydroxylarylation of unactivated alkenes enabled by a β-diketone ligand with high efficiency and excellent regioselectivity employing molecular oxygen as the oxidant and hydroxyl source. This reaction features mild conditions, broad substrate scope and incredible heterocycle compatibility, providing a variety of β-hydroxylamides, γ-hydroxylamides, β-aminoalcohols, γ-aminoalcohols, and 1,3-diols in high yields. The synthetic value of this methodology was demonstrated by the efficient synthesis of two bioactive compounds, (±)-3'-methoxyl citreochlorol and tea catechin metabolites M4.

Catalytic σ-Bond Annulation with Ambiphilic Organohalides Enabled by β-X Elimination

We describe a catalytic cascade sequence involving directed C(sp3 )-H activation followed by β-heteroatom elimination to generate a PdII (π-alkene) intermediate that then undergoes redox-neutral annulation with an ambiphilic aryl halide to access 5- and 6-membered (hetero)cycles. Various alkyl C(sp3 )-oxygen, nitrogen, and sulfur bonds can be selectively activated, and the annulation proceeds with high diastereoselectivity. The method enables modification of amino acids with good retention of enantiomeric excess, as well as σ-bond ring-opening/ring-closing transfiguration of low-strain heterocycles. Despite its mechanistic complexity, the method employs simple conditions and is operationally straightforward to perform.

Boosting the Enantioselectivity of Conjugate Borylation of α,β-Disubstituted Cyclobutenones with Monooxides of Chiral C2 -Symmetric Bis(phosphine) Ligands

Chiral bis(phosphine) monooxides (BPMOs) derived from C₂ -symmetric bis(phosphines) have been found to induce superior levels of enantioselection in copper-catalyzed conjugate borylation of α,β-disubstituted cyclobutenones. More precisely, enantiomeric excesses as well as chemical yields are exceedingly high with (R,R)-Bozphos as the chiral ligand while these values are low with parent (R,R)-Me-Duphos. A similar yet less pronounced effect was seen in the corresponding 1,6-addition to para-quinone methides.

Route to Chiral Tetrahydrofuran Acetals via Pd-Catalyzed Asymmetric Allylic Cycloaddition of Vinyl Epoxides with β-Keto Enol Ethers

An efficient method for the synthesis of functionalized chiral tetrahydrofuran (THF) acetals via Pd-catalyzed asymmetric allylic cycloaddition has been developed. With a palladium catalyst coordinated by a chiral phosphine ligand, the protocol is enabled to combine readily available vinyl epoxides and β-keto enol ethers to produce THF acetals bearing three stereocenters in a broad substrate scope with uniformly high levels of enantio- and diastereoselectivity.

Convergent Synthesis of Dihydrobenzofurans via Urea Ligand-Enabled Heteroannulation of 2-Bromophenols with 1,3-Dienes

We disclose a palladium and urea ligand-mediated heteroannulation of 2-bromophenols and 1,3-dienes. This method addresses synthetic challenges present in the palladium-catalyzed heteroannulation of bifunctional reagents and olefins by engaging a diverse scope of coupling partners under a unified set of reaction conditions. Our recently developed urea ligand platform outperforms phosphine ligands to generate the dihydrobenzofuran motif in a convergent manner.

Accessing Complex Tetrahydrofurobenzo-Pyran/Furan Scaffolds via Lewis-Acid Catalyzed Bicyclization of Cyclopropane Carbaldehydes with Quinone Methides/Esters

Herein, we report a straightforward one-pot synthesis of tetrahydrofurobenzopyran and tetrahydrofurobenzofuran systems via an in situ ring-expansion of the cyclopropane carbaldehydes followed by a [2 + n] cycloaddition with the quinone derivatives. The transformation not only unveils a new reaction mode of cyclopropane carbaldehydes with quinone methides/esters, but also promotes a step-efficient diastereoselective route to the sophisticatedly fused oxygen tricycles that can be further dehydrogenated to access the valued dihydro-2H-furo[2,3-b]chromene frameworks.

Copper(I)-Catalyzed Direct Oxidative Annulation of 1,3-Dicarbonyl Compounds with Maleimides: Access to Polysubstituted Dihydrofuran Derivatives

An efficient annulation method for the synthesis of polysubstituted dihydrofurans from 1,3-dicarbonyl compounds and maleimides is described. The reactions can afford furo[2,3-c]pyrrole derivatives with satisfactory yields. The developed strategy realizes the direct oxidative double C(sp³)-H functionalization in the presence of copper(I) salts and 2-(tert-butylperoxy)-2-methylpropane. Meanwhile, this protocol features a mild reaction condition and simple catalytic system. A reaction mechanism involving a single electron oxidation is also proposed.

Phosphine-catalyzed Divergent Domino Processes between γ-Substituted Allenoates and Carbonyl-Activated Alkenes

Highly enantioselective and chemodivergent domino reactions between γ-substituted allenoates and activated alkenes have been developed. In the presence of NUSIOC-Phos, triketone enone substrates smoothly reacted with γ-substituted allenoates to form bicyclic furofurans in good yields with high stereoselectivities. Alternatively, the reaction between diester-activated enone substrates and γ-substituted allenoates formed chiral conjugated 1,3-dienes in good yields with excellent enantioselectivities. Notably, by employing substrates with subtle structural difference, under virtually identical reaction conditions, we were able to access two types of chiral products, which are of biological relevance and synthetic importance.

Highly enantioselective and chemodivergent domino reactions between γ-substituted allenoates and activated alkenes have been developed.

Three-component synthesis of α-indole-β-sulfonyl tetrahydrofurans under metal-free conditions

An efficient multi-component reaction has been developed for the synthesis of 2,3-disubstituted tetrahydrofurans in a “one pot” manner, starting from readily available 2-arylindoles, arylsulfonyl azides, and tetrahydrofuran under simple and easily operated reaction conditions.

Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis

Whereas there is a significant interest in the rapid construction of diversely substituted saturated heterocycles, direct and modular access is currently limited to the mono-, 2,3-, or 3,4-substitution pattern. This Communication describes the straightforward and modular construction of 2,4-substituted saturated heterocycles from readily available materials in a highly stereo- and regioselective manner, which sets the stage for numerous readily accessible drug motifs. The strategy relies on chain walking catalysis.

[3 + 2] Coupling of Quinone Monoacetals with Vinyl Ethers Effected by Tetrabutylammonium Triflate: Regiocontrolled Synthesis of 2-Oxygenated Dihydrobenzofurans

The synthesis of 2-oxygenated dihydrobenzofurans involving the [3 + 2] coupling of quinone monoacetals with vinyl ethers has been realized by tetrabutylammonium triflate catalysis. The reaction involves a new activation method of the acetal moiety in quinone monoacetals under acid-free conditions affording the highly oxygenated dihydrobenzofurans. This new activation mode was achieved by using the triflate anion catalyst for stabilization of the highly reactive cationic intermediate.

Asymmetric Sequential Corey-Chaykovsky Cyclopropanation/Cloke-Wilson Rearrangement for the Synthesis of 2,3-Dihydrofurans

The first sequential Corey-Chaykovsky cyclopropanation/Cloke-Wilson rearrangement between propargyl sulfonium salts and acrylonitrile derivatives has been developed, affording the tetra-substituted 2,3-dihydrofurans in generally excellent yields (57-98%) with good diastereoselectivities (7:1-18:1). In addition, chiral propargyl sulfonium salt is also suitable for this strategy, giving the optically active 2,3-dihydrofurans with good enantioselectivities. This reaction sequence was designed upon in situ generated 10π-conjugated structures from the dearomatization of indole fragments and subsequent intramolecular 1,6-addition.

Recent advances in palladium-catalyzed (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides

Palladium has proven to be effective in catalyzing the (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides. Through the employment of appropriate ambiphilic coupling partners, efficient annulation of a variety of allenes, 1,3-dienes, strained alkenes, styrenes, and other C[double bond, length as m-dash]C bond variants can be achieved to provide direct access to numerous useful hetero- and carbocyclic scaffolds. In this Feature Article, we summarize palladium-catalyzed (hetero)annulation methods reported since 2005 (spanning just over 15 years) and discuss outstanding challenges in this area of study.

Anti-selective [3+2] (Hetero)annulation of non-conjugated alkenes via directed nucleopalladation

2,3-Dihydrobenzofurans and indolines are common substructures in medicines and natural products. Herein, we describe a method that enables direct access to these core structures from non-conjugated alkenyl amides and ortho-iodoanilines/phenols. Under palladium(II) catalysis this [3 + 2] heteroannulation proceeds in an anti-selective fashion and tolerates a wide variety of functional groups. N-Acetyl, -tosyl, and -alkyl substituted ortho-iodoanilines, as well as free -NH2 variants, are all effective. Preliminary results with carbon-based coupling partners also demonstrate the viability of forming indane core structures using this approach. Experimental and computational studies on reactions with phenols support a mechanism involving turnover-limiting, endergonic directed oxypalladation, followed by intramolecular oxidative addition and reductive elimination.

Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization under mild conditions

A mild and practical Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization reaction of 2,3-dihydrofurans is developed, leading to various optically active fused furoindolines and tetrahydrofurobenzofurans. The key to this transformation is employing two newly modified N-Me-Xiang-Phos ligands ((S, R S)- N-Me-X4/X5) as chiral ligands under mild conditions. Moreover, this synthetic methodology can be efficiently applied to a variety of complex polysubstituted heterocycles with high chemo-, regio-, and enantio-selectivities via introducing diverse substituents on furan rings, which were hard to access by other routes.

Highly Enantioselective Construction of Dihydrooxazines via Pd-Catalyzed Asymmetric Carboetherification

A straightforward synthesis of highly enantioenriched 5,6-dihydro-4H-1,2-oxazines is realized by Pd-catalyzed asymmetric carboetherification of γ,δ-alkenyl oximes with (hetero)aryl and alkenyl halides in the presence of a commercially available bisphosphine ligand. The enantioenriched products can be facilely converted to functionalized alcohols with high fidelity of chiral transfer.

Stereoselective synthesis of trifluoromethyl-substituted 2H-furan-amines from enaminones

A straightforward strategy for synthesis of highly functionalized trifluoromethyl 2H-furans is described.

Construction of a CF3-Containing Benzofurofuranone Skeleton from Coumarins via Reductive Coupling and Acid-Mediated Ring Contraction

Magnesium-promoted reductive introduction of a trifluoroacetyl group to coumarin in the presence of ethyl trifluoroacetate and the subsequent treatment with trifluoroacetic acid led to simple access to a trifluoromethylated benzofurofuranone at 8a-position with a high regio- and stereoselectivity. Trifluoromethylated or difluoromethylated benzofurofuranone derivatives were also prepared from coumarins including naturally occurring ones only in two successive steps, which might have potential bioactivity in medicinal chemistry.

Enantioselective, Organocatalytic, Dissymmetric 1,4- and 1,2-Addition of Malononitrile to a Keto-bisenone Followed by an Oxa-Michael Addition Cascade

An unprecedented enantioselective dissymmetric 1,4- and 1,2-addition of malononitrile to a keto-bisenone followed by an oxa-Michael addition cascade to trap the in situ generated unstable tertiary alcohol have been developed. The quinine-derived amino-squaramide bifunctional organocatalyst worked efficiently and provides the oxa-spiro-[4,4]-nonanes in good yields and excellent diastereo- and enantioselectivities (up to 99:1 dr and 99% ee). Notably, a complete chemoselective addition of a methylene unit to an aliphatic-tethered enone over the aromatic-tethered enone was observed.

Utilization of BozPhos as an Effective Ligand in Enantioselective C-H Functionalization of Cyclopropanes: Synthesis of Dihydroisoquinolones and Dihydroquinolones

The bisphosphine monoxide ( R, R)-BozPhos enables enantioselective C-H functionalization of cyclopropanes in a palladium-catalyzed cyclization. The synthesis of a broad spectrum of dihydroisoquinolones and dihydroquinolones in good yields and high enantiomeric excess was achieved through the use of this hemilabile ligand. Furthermore, the isolation of an intermediary palladium(II)-BozPhos complex after oxidative addition was successful and a second complex provided further insight into bond length and angles through a crystal structure.

Pd-Catalyzed carboannulation of γ,δ-alkenyl oximes: efficient access to 5-membered cyclic nitrones and dihydroazines

The Pd⁰-catalyzed carboannulation of γ,δ-alkenyl oximes with organohalides offers catalyst-controlled access to 5-membered cyclic nitrones or 6-membered cyclic oxime ethers.

Organocatalytic Asymmetric Synthesis of Bridged Acetals with Spirooxindole Skeleton

The first highly diastereo- and enantioselective synthesis of bridged O, O-acetals embedded with spirooxindoles has been developed. Dioxindoles and 2-hydroxy cinnamaldehydes were employed as the reaction partners in this method. The desired products were obtained via diaryl prolinol TBS ether catalyzed Michael reaction followed by acetal formation with TFA.

Recent Advances in Phthalan and Coumaran Chemistry

Oxygen-containing heterocycles are common in biologically active compounds. In particular, phthalan and coumaran cores are found in pharmaceuticals, organic electronics, and other useful medical and technological applications. Recent research has expanded the methods available for their synthesis. This Minireview presents recent advances in the chemistry of phthalans and coumarans, with the goal of overcoming synthetic challenges and facilitating the applications of phthalans and coumarans.

Synthesis of Spirocyclic Ethers by Enantioselective Copper-Catalyzed Carboetherification of Alkenols

Spirocyclic ethers can be found in bioactive compounds. This copper-catalyzed enantioselective alkene carboetherification provides 5,5-, 5,6- and 6,6-spirocyclic products containing fully substituted chiral carbon centers with up to 99 % enantiomeric excess. This reaction features the formation of two rings from acyclic substrates, 1,1-disubstituted alkenols functionalized with either arenes, alkenes, or alkynes, and clearly constitutes a powerful way to synthesize chiral spirocyclic ethers.

Intramolecular Pd-Catalyzed Formal anti-Carboalkoxylation of Alkynes: Access to Tetrasubstituted Enol Ethers

An intramolecular Pd-catalyzed formal anti-carboalkoxylation reaction is presented that provides access to tetrasubstituted enol ethers. The key to success is a cascade consisting of a formal anti-carbopalladation of a carbon-carbon triple bond followed by a nucleophilic attack of a hydroxy group at the emerging vinyl organopalladium species. The desired transformation proceeded smoothly with primary, secondary, and tertiary alcohols, and even with phenols. Depending on the substitution pattern of the enol ethers, a further Tsuji-Trost-type step may occur resulting in oligocyclic ketals.