Silyl Enol Ethers Research Articles

Dual Lewis Acid Promoted/Visible Light Driven Preparation of Aryl Diazenyl Oxazoles from Isocyanoacetamides and Arylazo Sulfones

While arylazo sulfones have been widely applied as arylating agents, their exploitation for diazenylation reactions has been previously limited to electron‐rich alkenes such as styrenes and silyl enol ethers. Herein we optimized a synthetic one‐pot protocol to access oxazolyl azo compounds via a selective domino ring‐closing/aryldiazenylation sequence by starting from arylazo sulfones and isocyanoacetamides in the presence of Sm(OTf)3 as the Lewis acid. The satisfactory substrate scope and functional group tolerance, along with scale‐up reaction both in batch and under continuous flow conditions provide a valuable approach to such a class of heteroaryl azo compounds.

Access to Carbonyl Azides via Iodine(III)-Mediated Cross-Coupling.

Herein, we present a prominent metal-free C-N cross-coupling platform that enables access to carbamoyl- and ketoazides from isocyanides or silyl enol ethers and trimethylsilyl azide (TMSN3) with an aid of iodine(III) promoter. This offers a rapid route to a diverse set of synthetically valuable azide decorated fragments with excellent substrate scope and good to excellent yields. The disclosed platform exemplifies the use of TMSN3 for incorporation of the azide fragment without the loss of N2.

Organocatalytic Applications of Sulfonyl Squaramides in Anion Recognition Strategies

A modular, 3‐steps protocol for the synthesis of N‐sulfonyl squaramides has been developed. The strategic installation of a tetrahedral, electron‐withdrawing sulfonyl group into the squaramido core allowed the prevention of undesired self‐aggregations, therefore upgrading the solubility in common organic solvents, and moreover, enhancing their H‐bond donor abilities for molecular recognition. These unique features have been efficiently exploited in two different ion‐pairing reactions: (i) the challenging C4‐selective dearomatization of 2‐picoline with silyl ketene acetals and (ii) the tritylation of Nmethylindole. Furthermore, their catalytic activities have been directly compared with other common and well‐established (thio)urea analogues and related H‐bond donors, revealing that highly acidic designs are essential to reach optimal catalytic performances.

Stereoselective Total Synthesis of Rhodocoranes I and J.

We report the stereoselective total synthesis of rhodocoranes I and J in 10 steps and 16.4% overall yield from (S)-limonene. The synthesis was accomplished through the convergent assembly of a highly substituted chiral cyclopentanone and a lithiated furanyl silyl ketene acetal. The requisite cyclopentanone framework was strategically constructed from the chiral pool, (S)-limonene, through a sequence of steps that included a hydroboration/oxidation, ozonolysis, aldol condensation, reduction, and palladium-catalyzed diastereoselective allylic transposition. This study provides a general approach to the synthesis of the rhodocorane family, known for their antibacterial, antifungal, and cytotoxic properties.

Photocatalytic Synthesis of α-Ketonyl Glycosyl Compounds from Glycosyl Thiols and Silyl Enol Ethers.

The synthesis of C1-ketonyl glycosyl compounds featuring α-selectivity has seldom been reported. We herein devise a glycosyl radical-based approach to facilely access stereoenriched ketonyl glycosyl compounds via an Ir photoredox-catalyzed desulfurative addition to silyl enol ethers, using in situ-generated tetrafluoropyridinyl thioglycosides from glycosyl 1-thiols as radical precursors. This protocol features readily prepared starting materials, mild conditions, excellent functional group tolerance, satisfactory scale-up, and notable amenability to late-stage modification of pharmaceutically relevant complex molecules.

Cu-Catalyzed [3+1+1] Cascade Cyclization of O-Acyl Oximes with Sulfur and Silyl Enol Ethers: Rapid Access to Naphthothiazoles.

A three-component cyclization reaction of O-acyl oximes, silyl enol ethers and elemental sulfur has been developed, in which silyl enol ether acts as a C1 synthon to participate in cyclization reaction and build series of 2-aroylnaphthothiazoles and 2-aroylbenzothienothiazoles. The preliminary exploration of the reaction mechanism indicated that this transformation probably proceeded through a radical process, involving S3•- as a key intermediate, enabling subsequent nucleophilic substitution with O-acyl oximes to afford iminosulfur radical, which undergoes 1,3-H shift to yield sulfur-centered radical intermediate. And then this intermediate undergoes radical addition with silyl enol ether, leading to the formation of the titled products through intramolecular cyclization and oxidation. Moreover, the products obtained exhibit favorable fluorescence properties, which indicates their potential application as functional materials.

Streamlining the Synthesis of Pyridones through Oxidative Amination of Cyclopentenones

AbstractHerein we report the development of an oxidative amination process for the streamlined synthesis of pyridones from cyclopentenones. Cyclopentenone building blocks can undergo in situ silyl enol ether formation, followed by the introduction of a nitrogen atom into the carbon skeleton with successive aromatisation to yield pyridones. The reaction sequence is operationally simple, rapid, and carried out in one pot. The reaction proceeds under mild conditions, exhibits broad functional group tolerance, complete regioselectivity, and is well scalable. The developed method provides facile access to the synthesis of 15N‐labelled targets, industrially relevant pyridone products and their derivatives in a fast and efficient way.

Enantioselective Rh‐Catalyzed Hydroboration of Dialkyl Ketone‐Derived Silyl Enol Ethers

AbstractA Rh‐catalyzed asymmetric hydroboration of dialkyl ketone‐derived silyl enol ethers with HBpin has been developed using a commercially available catalyst and ligand ([RhCl (cod)]2 and MeO‐BIBOP). A variety of silyl enol ethers undergo this asymmetric transformation, providing the corresponding products with high enantioselectivity (up to 97 : 3 % er). In addition, the utility of this protocol is demonstrated by the versatile transformation of chiral borylether products to a range of valuable derivatives.

Intermolecular Formal [2π + 2σ] Cycloaddition of Enol Silyl Ethers with Bicyclo[1.1.0]butanes Promoted by Lewis Acids.

Silyl enol ethers react with bicyclo[1.1.0]butanes (BCBs) through Yb(OTf)3-promoted formal [2π + 2σ] cycloaddition reactions to furnish bicyclo[2.1.1]hexanes (BCHs). This new reaction tolerated a wide range of enol silyl ethers and BCBs. Furthermore, the amplification experiments and synthetic transformations of the cycloaddition compounds further highlighted their practicality.

Exploration and Development of Nitrone Chemistry.

Nitrones are widely used as 1,3-dipoles in organic synthesis, but control of their reactions is not always easy. This review outlines our efforts to make the reactions of nitrones more predictable and easier to use. These efforts can be categorized into (1) 1,3-nucleophilic addition reaction of ketene silyl acetals to nitrones, (2) geometry-controlled cycloaddition of C-alkoxycarbonyl nitrones, (3) stereo-controlled cycloaddition using double asymmetric induction, and (4) generation of nitrones by N-selective modification of oximes.

Facile access to bicyclo[2.1.1]hexanes by Lewis acid-catalyzed formal cycloaddition between silyl enol ethers and bicyclo[1.1.0]butanes

Saturated three-dimensional carbocycles have gained increasing prominence in synthetic and medicinal chemistry. In particular, bicyclo[2.1.1]hexanes (BCHs) have been identified as the molecular replacement for benzenes. Here, we present facile access to a variety of BCHs via a stepwise two-electron formal (3 + 2) cycloaddition between silyl enol ethers and bicyclo[1.1.0]butanes (BCBs) under Lewis acid catalysis. The reaction features wide functional group tolerance for silyl enol ethers, allowing the efficient construction of two vicinal quaternary carbon centers and a silyl-protected tertiary alcohol unit in a streamlined fashion. Interestingly, the reaction with conjugated silyl dienol ethers can provide access to bicyclo[4.1.1]octanes (BCOs) equipped with silyl enol ethers that facilitate further transformation. The utilities of this methodology are demonstrated by the late-stage modification of natural products, transformations of tertiary alcohol units on bicyclo[2.1.1]hexane frameworks, and derivatization of silyl enol ethers on bicyclo[4.1.1]octanes, delivering functionalized bicycles that are traditionally inaccessible.

Oxidative Nitrogen Insertion into Silyl Enol Ether C═C Bonds.

Here, we demonstrate a fundamentally new reactivity of the silyl enol ether functionality utilizing an in situ-generated iodonitrene-like species. The present transformation inserts a nitrogen atom between the silyl enol ether olefinic carbons with the concomitant cleavage of the C═C bond. Overall, this facile transformation converts a C-nucleophilic silyl enol ether to the corresponding C-electrophilic N-acyl-N,O-acetal. This unprecedented access to α-amido alkylating agents enables modular derivatization with carbon and heteroatom nucleophiles and the unique late-stage editing of carbon frameworks. The reaction efficiency of this transformation is well correlated with enol ether nucleophilicity as described by the Mayr N scale. Applications presented herein include late-stage nitrogen insertion into carbon skeletons of natural products with previously unattainable regioselectivity as well as modified conditions for 15N labeling of amides and lactams.

Diastereoselective Synthesis of Pyridone ribo‐C‐Nucleosides via Heck Reaction and Oxidation

AbstractNucleosides find applications in medicinal chemistry, chemical biology and diagnostics. Among the nucleosides, C‐nucleosides have attracted particular attention because their carbon‐carbon bond between nucleobase and ribose provides stability against degradation. While several well‐established methods exist for the synthesis of 2′‐deoxy‐C‐nucleosides, the preparation of their ribofuranosyl counterparts is more challenging. Established methods for their synthesis give mixtures of anomers and require harsh reagents or conditions. Here we report a diastereoselective glycosylation method involving a Heck reaction between a glycal and an aryl iodide, followed by oxidation of the silyl enol ether with methyl(trifluoromethyl)dioxirane (TFDO). The resulting ketone is then diastereoselectively reduced to the ribonucleoside. The new route has been applied to pyridones and is expected to also yield other ribo‐C‐nucleosides in diastereoselective fashion.

Stereoselective Synthesis of Silyl Enol Ethers with Acylsilanes and α,β-Unsaturated Ketones.

Acylsilanes are emerging bench-stable reagents for the generation of electron-rich oxycarbenes that are difficult to access with unstable diazo compounds. Herein, we report a siloxycarbene-mediated stereoselective synthesis of silyl enol ethers through visible-light-induced intermolecular reactions between acylsilanes and α,β-unsaturated ketones. Both the solvent and low temperature are important for the success of the reaction. This approach features atomic economics, exclusive stereocontrol, and broad substrate scope. The synthetic potential of this methodology is demonstrated by gram-scale reaction and various downstream transformations including that requiring configuration purity of the silyl enol ethers.

Silver-Catalyzed Radical Umpolung Cross-Coupling of Silyl Enol Ethers with Activated Methylene Compounds: Access to Diverse Tricarbonyl Derivatives.

A silver-catalyzed protocol for the intermolecular radical umpolung cross-coupling protocol of silyl enol ethers with activated methylene compounds is disclosed. The protocol exhibits excellent functional group tolerance, enabling the expedient preparation of a variety of tricarbonyl compounds. Preliminary mechanistic investigations suggest that the reaction proceeds through a process involving free radicals in which silver oxide has a dual role, acting as both a catalyst and a base.

Streamlining the Synthesis of Pyridones through Oxidative Amination of Cyclopentenones.

Herein we report the development of an oxidative amination process for the streamlined synthesis of pyridones from cyclopentenones. Cyclopentenone building blocks can undergo in situ silyl enol ether formation, followed by the introduction of a nitrogen atom into the carbon skeleton with successive aromatisation to yield pyridones. The reaction sequence is operationally simple, rapid, and carried out in one pot. The reaction proceeds under mild conditions, exhibits broad functional group tolerance, complete regioselectivity, and is well scalable. The developed method provides facile access to the synthesis of 15N-labelled targets, industrially relevant pyridone products and their derivatives in a fast and efficient way.

Group transfer polymerization of polar conjugated monomers using pinacolborane and thiourea anions

AbstractGroup transfer polymerization (GTP) plays a crucial role in polymerizing (methyl) acrylate. The utilization of cheap and easily synthesized thiourea anion (TUA) combined with pinacolborane (HBpin) undergoes controlled GTPs of methyl methacrylate (MMA), methacrylate (MA), and α‐methylenebutyrolactone (MBL) at ambient temperature. The mechanistic study reveals that the TUA′‐1 having ‐NEt2 moiety works out as a cocatalyst to assist HBpin undergoes the 1,4‐hydroboration reaction with α, β‐unsaturated acrylates to in situ generate a boron enolate (boryl ketene acetal [BKA]). Different from typical GTP, in which silyl ketene acetal (SKA) is used as an initiator, this work shows that BKA could act as an initiator with the assistance of the cocatalyst TUA′‐1 to generate the polymer. The chain end groups of the obtained polymer are characterized by MALDI‐TOF‐MS, indicating one of the chain ends features a hydride from the HBpin and another chain end has a linear or back‐biting cyclic structure. Moreover, the polymerization process is strongly supported by density functional theory (DFT) calculations. The application of BKA expands the research area of GTP reactions. The tunable and structural properties of TUA/hydroboranes significantly broaden the types of initiators in the GTP and pave a new methodology to achieve functional polymers.

Stereoselective Entry into α,α'-C-Oxepane Scaffolds through a Chalcogen Bonding Catalyzed Strain-Release C-Septanosylation Strategy.

The utility of unconventional noncovalent interactions (NCIs) such as chalcogen bonding has lately emerged as a robust platform to access synthetically difficult glycosides stereoselectively. Herein, we disclose the versatility of a phosphonochalcogenide (PCH) catalyst to facilitate access into the challenging, but biologically interesting 7-membered ring α,α'-C-disubstituted oxepane core through an α-selective strain-release C-glycosylation. Methodically, this strategy represents a switch from more common but entropically less desired macrocyclizations to a thermodynamically favored ring-expansion approach. In light of the general lack of stereoselective methods to access C-septanosides, a remarkable palette of silyl-based nucleophiles can be reliably employed in our method. This include a broad variety of useful synthons, such as easily available silyl-allyl, silyl-enol ether, silyl-ketene acetal, vinylogous silyl-ketene acetal, silyl-alkyne and silylazide reagents. Mechanistic investigations suggest that a mechanistic shift towards an intramolecular aglycone transposition involving a pentacoordinate silicon intermediate is likely responsible in steering the stereoselectivity.

A Photoinduced Radical Cascade Cyclization for the Synthesis of Angularly Fused Tricyclic Compounds.

A photoinduced electron transfer (PET)-triggered cascade reaction has been devised for the conversion of second-generation enol silyl ethers into angularly fused tricyclic scaffolds. Utilizing readily available and cost-effective DCA and phenanthrene as the catalytic systems, this cascade transformation is achieved with high efficiency. The reaction demonstrates a good substrate scope and excellent stereoselectivity, thereby enriching the realm of PET-induced cascade reactions. Additionally, the radical adducts generated through this process can serve as valuable subunits for the synthesis of complex molecules.

Syntheses of α-monofluoroalkyl acetophenones from fluoroalkyl carboxylic NHPI esters and silyl enol ethers by photocatalytic decarboxylative reaction

A photoredox-catalyzed decarboxylative α-monofluoroalkylation of silyl enol ethers with fluoroalkyl carboxylic N-(hydroxy)phthalimide esters has been successfully developed. Diverse functionalized α-monofluoroalkyl acetophenones were produced in modest to good yields under mild and operationally simple conditions. The developed method was further applied to synthesize antalgic and ataractic activity compound containing α-monofluoroalkyl acetophenone skeleton.