Reaction of acylsilanes with α-sulfinyl carbanions: regioselective synthesis of silyl enol ethers

The reaction of acylsilanes with α-sulfinyl carbanions such as α-lithioalkyl sulfoxide is described. The reaction proceeds to give silyl enol ethers preferentially through the initial formation of the α-silyl alkoxide intermediates. In particular, the products derived from enolizable acylsilanes were the regio-defined silyl enol ethers that cannot be obtained by usual enolization of the corresponding unsymmetrical ketones with base.     

OsO4-catalyzed amination of silyl enol ethers: enantioselective synthesis of α-amino ketones

Osmium tetroxide catalyzed asymmetric aminohydroxylation of silyl enol ethers using cinchona alkaloids as chiral ligands and chloramine-T as the nitrogen source affords enantiomerically pure α-amino ketones.     

Zn-mediated rhodium-catalyzed α-trifluoromethylation of ketones via silyl enol ethers

The treatment of silyl enol ethers of ketones with CF₃-I and Et₂Zn in the presence of RhCl(PPh₃)₃ in DME gave α-trifluoromethyl ketones in good yields. The reaction can be widely applicable to silyl enol ethers derived from aliphatic or aromatic ketones. In the absence of the rhodium catalyst, the reaction was very slow and the yields were quite poor.     

Rhodium-catalyzed α-fluoroalkylation reaction of ketones using silyl enol ethers

The treatment of silyl enol ethers with fluoroalkyl halides (R_f-X) in the presence of RhCl(PPh₃)₃ gave α-fluoroalkylated ketones. It seems that a rhodium complex derived from the silyl enol ether and RhCl(PPh₃)₃ played an important role for the oxidative addition of fluoroalkyl halides and the reductive elimination of the product.     

PMA-SiO2 catalyzed synthesis of β-keto enol ethers

An efficient conversion of β-diketones into correspondingβ-keto enol ethers with catalytic amount of PMA-SiO_2 has been achieved.     

Sunlight-driven synthesis of γ-diketones via oxidative coupling of enamines with silyl enol ethers catalyzed by [Ru(bpy)3]2+

A photosensitizer [Ru(bpy)(3)](2+) catalyzes oxidative coupling reaction of enamines with silyl enol ethers under visible light irradiation by a Xe lamp or sunlight to produce γ-diketones. A 2e-oxidation process involved in this reaction is achieved by a combination of the photoexcited [Ru(bpy)(3)](2+) species and duroquinone, a 2e-acceptor.     

Asymmetric Michael addition of silyl nitronates to cyclic α,β-unsaturated ketones catalyzed by chiral quaternary ammonium bifluorides: isolation and selective functionalization of enol silyl ethers of optically active γ-nitro ketones

Highly enantioselective Michael addition of silyl nitronates to cyclic α,β-unsaturated ketones has been accomplished by the utilization of N-spiro C₂-symmetric chiral quaternary ammonium bifluoride 1 as an efficient catalyst, offering a new route to the enol silyl ethers of optically active γ-nitro ketones. The synthetic utility of this transformation has been demonstrated by the diastereoselective derivatizations of the optically active enol silyl ethers to the corresponding α-substituted cyclic ketones having three consecutive stereochemically defined stereocenters.     

A new practical synthesis of silyl enol ethers : II. From α,β-unsaturated aldehydes and ketones

In acetonitrile as the solvent and in the presence of a tertiary amine the trimethylchlorosilane/sodium iodide (trimethyliodosilane “in situ”) reagent provides the corresponding siloxy dienes when reacted with α,β-unsaturated series. These reactions occurs via the formation of 1,2 or -1,4 onium salt intermediates.     

Preparation of thermodynamically stable enol silyl ethers of γ,δ-unsaturated ketones by palladium-catalyzed decarboxylation-allylation of allyl 2,3-disubstituted 3-trimethylsiloxyacrylates

Allyl 2,3-disubstituted 3-trimethylsiloxyacrylates underwent palladium-catalyzed intramolecular decarboxylation-allylation to afford enol silyl ethers of γ,δ-unsaturated ketones     

Oxidative conversion of silyl enol ethers to α,β-unsaturated ketones employing oxoammonium salts

The oxidative conversion of silyl enol ethers to α,β-unsaturated ketones using a less-hindered class of oxoammonium salts (AZADO(+)BF(4)(-)) is described. The reaction proceeds via the ene-like addition of oxoammonium salts to silyl enol ethers.     

Aminomethylation of chiral silyl enol ethers: access to β-homotryptophane and β-homolysine derivatives

We describe here the aminomethylation of chiral silyl enol ether derivatives using iminium ion. The choice of judicious precursors with adequate protecting groups for the silylation/aminomethylation step was required to achieve the synthesis of β²-homotryptophane in few steps. The same methodology was used to prepare a β²-homolysine derivative.     

Preparation of proximal β-hydroxy silyl enol ethers from α,β-epoxyketones using silyllithium reagents

A new method for the stereoselective preparation of proximal β-hydroxy silyl enol ethers from α,β-epoxyketones using silyllithium reagents has been developed. The reaction is believed to proceed via Brook rearrangement assisted by opening of the adjacent epoxide. A number of α,β-epoxyketones were reacted with methyldiphenylsilyllithium to form the corresponding proximal β-hydroxy silyl enol ethers in good to excellent yield and excellent stereoselectivity.     

Synthesis of optically active α-hydroxy ketones by enantioselective oxidation of silyl enol ethers with a fructose-derived dioxirane

Optically active α-hydroxy ketones 3 have been prepared in moderate to high enantioselectivities by the asymmetric oxidation of the silyl enol ethers 2 with in situ generated dioxirane from the fructose-derived ketone 1. Best results (ee values up to 82%) for this novel non-transition metal mediated asymmetric α-hydroxylation may be obtained, when an excess of the fructose-derived ketone 1 is employed at pH ca. 8 and short reaction times. Valuable mechanistic information on the spiro versus planar transition states for the oxygen-transfer process has been aquired through the absolute configuration of the resulting α-hydroxy ketone 3 products.     

Microwave synthesis of α-cyano chalcones

A novel methodology for facile production of α-cyano chalcones under microwave irradiation is described. Utilizing a Knoevenagel condensation between benzoylacetonitriles and aromatic aldehydes, substituted chalcones are generated via a 15-min, one-pot synthesis. Diversification of aromatic groups, including electron-withdrawing, electron-donating, and heterocyclic substitutions, has led to the isolation of over twenty colored, solid chalcone products. Furthermore the methodology can be extended to the synthesis of benzylidenemalononitriles as well as methyl and ethyl α-cyano cinnamates.     

β-Nitroacrylates and silyl enol ethers as key starting materials for the synthesis of polyfunctionalized β-nitro esters and 1,2-oxazine-2-oxides

The reaction of silyl enol ethers with β-nitroacrylates, in the presence of tetrabutyl ammonium fluoride as catalyst, allows the formation of polyfunctionalized β-nitro esters, or hexahydro-4H-benzoxazine-2-oxides, depending on the nature of the starting silyl enol ethers.     

Asymmetric synthesis of γ-nitroesters by an organocatalytic one-pot strategy

An enantioselective synthesis of γ-nitroesters by a one-pot asymmetric Michael addition/oxidative esterification of α,β-unsaturated aldehydes is presented. The procedure is based on merging the enantioselective organocatalytic nitroalkane addition with an N-bromosuccinimide-based oxidation. The γ-nitroesters are obtained in good yields and enantioselectivities, and the method provides an attractive entry to optically active γ-aminoesters, 2-piperidones, and 2-pyrrolidones.     

Asymmetric synthesis of 2-alkyl-substituted 2-hydroxyglutaric acid γ-lactones

3-Alkyl-1,2-cyclopentanediones 1 are transformed into 2-alkyl-2-hydroxyglutaric acid γ-lactones 3 in up to 83% isolated yields and up to 96% ee, affording a simple access to many bioactive compounds, including diacylglycerol lactones (DAG-lactones).     

Suzuki-Miyaura cross-coupling of α-phosphoryloxy enol ethers with arylboronic acids

The Suzuki-Miyaura cross-coupling reaction of cyclic ketene acetal phosphates with arylboronic acids was found to be a convenient and highly efficient method for the construction of aryl vinyl ethers. A wide variety of differentially substituted electron-poor and electron-rich arylboronic acids smoothly underwent the coupling process to provide the desired dihydropyrans in moderate to excellent yields.     

New synthetic methods for α,β -unsaturated ketones,aldehydes, esters and lactones by the palladium-catalyzed reactions of silyl enol ethers,ketene silyl acetals,and enol acetates with allyl carbonates

Silyl enol ethers and ketene silyl acetals derived from ketones, aldehydes, esters and lactones are converted into α,β-unsaturated ketones, aldehydes, esters and lactones by treatment with allyl carbonates in high yields using the palladium—bis(diphenylphosphino)ethane (dppe) complex as catalyst. Phosphine-free palladium catalyst instead of the palladium—phosphine complex gives a higher selectivity for the preparation of cyclopentenone, cyclooctenone, dienones, α,β-unsaturated esters and lactones. As a solvent, the use of nitriles such as acetonitrile is essential. In other solvents, allylation takes place. Enol acetates derived from ketones are converted into α,β-unsaturated ketones by reaction with allyl carbonate in acetonitrile using the palladium complex and tributyltin methoxide as bimetallic catalysts.