Phenylthiomethylstannylation of silyl enol ethers and silyl dienol ethers

Silyl enol ethers (2) react with tributyl [(phenylthio) chloromethyl] stannane (1) in the presence of Zinc bromide to give β-phenylthiomethylstannyl ketones (3) in good yields. Silyl dienol ethers (4) mainly give products (5) due to γ-attack, under these conditions.     

Deracemization of silyl enol ethers

The deracemization by enantioselective protonation of silyl enol ethers was tested using 2,2-dimethyl 5-phenyl 1,3-dioxolan 4-one 1. The results obtained, especially with pantolactone as a chiral proton donor, are better than when the deracemization is carried out with the lithium enolate of 1.     

A radical cyclization between enol silyl ethers and aryl or alkenyl bromide moieties

Enol silyl ethers bearing aryl or alkenyl bromide moieties at their appropriate positions undergo radical cyclization in the presence of Bu₃SnH and AIBN to yield the corresponding 5- and 6-membered cycloalkanols in good yields.     

Regiospecific ureidoalkylation of silyl enol ethers

Site-specific ureidoalkylation of silyl enol ethers can be achieved by their reactions with chloromethylcarbamates at ?78° under the influence of titanium tetrachloride.     

Synthesis of mono- and difluoroacetyltrialkylsilanes and the corresponding enol silyl ethers

A variety of mono- and difluoroacetylsilanes and the corresponding silyl enol ethers were prepared from trifluoroethanol and chlorotrialkylsilanes in the presence of LDA through retro-Brook rearrangement. Sterically demanding silyl groups, especially those bound to oxygen, resulted in higher yields of difluoroacetylsilanes. The yields of difluoroacetylsilanes were also dramatically affected by the method of the termination of the reaction. Difluorohaloacetylsilanes were prepared from the corresponding difluoroethenyl silyl ethers with electrophilic halogenating reagents in good yields. A gamma-fluorinated beta-diketone 9a was prepared from monofluoroacetyltriisopropylsilane by nucleophilic acylation with methyl trifluoroacetate.     

Lewis acid-catalyzed nucleophilic substitutions of propargylic and allylic silyl ethers with enol silyl ethers

Nucleophilic reactions of various enol silyl ethers with carbocation species generated from propargyl silyl ethers by the action of a Lewis acid have been developed. The present method is highly useful for the introduction of allenic or enyne functionalities into the alpha-position of substituted ketones. [reaction--see text]     

Synthesis of 2-alkylidenepyrrolidines, pyrroles, and indoles by condensation of silyl enol ethers and 1,3-bis-silyl enol ethers with 1-azido-2,2-dimethoxyethane and subsequent reductive cyclization

The condensation of 1,3-bis-silyl enol ethers with 1-azido-2,2-dimethoxyethane and subsequent reductive cyclization allowed an efficient regio- and diastereoselective synthesis of a variety of 2-alkylidene-4-methoxypyrrolidines. The thermal elimination of methanol resulted in the formation of functionalized pyrroles. Similarly, 2,3,3a,4,5,6-hexahydro-2,3-benzopyrroles were prepared and transformed into 4,5,6,7-tetrahydro-2,3-benzopyrroles. In contrast, treatment of 2-alkylidenepyrrolidines with trifluoroacetic acid resulted in formation of indoles by [4 + 2] cycloaddition and subsequent extrusion of a nitrogen atom.     

A silver-catalyzed spirocyclization of alkynyl silyl enol ethers

Ring out the old: The cycloisomerization of alkynyl silyl enol ethers proceeds at ambient temperature under the mild conditions of silver catalysis. Mono- or bicyclic spiro compounds can be obtained by 5-exo-dig reactions. Trapping the vinyl silver species with an iodide source, such as N-iodosuccinimide (NIS), afforded the alkenyl iodide derivatives.     

Reaction of silyl enol ethers with cyanogen chloride

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1447–1448, June, 1988.     

Chemoselective oxygen-centered radical cyclizations onto silyl enol ethers

A new oxygen-centered radical cyclization onto silyl enol ethers has been developed and utilized for the synthesis of versatile siloxy-substituted tetrahydrofurans. The reactions display excellent chemoselectivity for cyclization onto the electron-rich silyl enol ether when competing terminal alkene cyclization, 1,5-hydrogen abstraction, and beta-fragmentation pathways are present. The increased chemoselectivity also allows for the synthesis of tetrahydropyrans, a challenging substrate class to access using oxygen-centered radical alkene cyclizations due to competing 1,5-hydrogen abstractions.     

Novel peparation of silyl enol ethers from allyl alcohols

A novel synthetic method for silyl enol ethers from allyl alcohols was developed using ruthenium hydride catalyst.     

Condensation of enol silyl ethers with 2-acetoxytetrahydrofuran and -tetrahydropyrans

Trimethylsilyl trifluoromethanesulfonate catalyzes stereoselective condensation of enol silyl ethers and 2-acetoxytetrahydrofuran or -tetrahydropyran derivatives.     

A novel rearrangement leading to methoxycarbonylmethylated silyl enol ethers

Methyl 2-siloxycyclopropanecarboxylates rearrange smoothly and quantitatively to the corresponding silyl enol ethers (3) by addition of a catalytical amount of iodo trimethylsilane. Scope and limitation of this novel process as well as the synthesis of the electron rich diene (10a) are described.     

Direct transformation of silyl enol ethers into functionalized allenes

The first elimination reactions of silyl enol ethers to lithiated allenes are reported. These reactions allow a direct transformation of readily available silyl enol ethers into functionalized allenes. The action of three to four equivalents of lithium diisopropylamide (LDA) on silyl enol ethers results in the formation of lithiated allenes by initial allylic lithiation, subsequent elimination of a lithium silanolate, and finally, lithiation of the allene thus formed. Starting with amide-derived silyl imino ethers, lithiated ketenimines are obtained. A variety of reactions of the lithiated allenes with electrophiles (chlorosilanes, trimethylchlorostannane, dimethyl sulfate and ethanol) were carried out. Elimination of silanolate is observed only for substrates that contain the hindered SiMe2tBu or Si(iPr)3 moiety, but not for the SiMe3 group. The reaction of 1,1-dilithio-3,3-diphenylallene with ketones provides a convenient access to novel 1,1-di(hydroxymethyl)allenes which undergo a domino Nazarov-Friedel-Crafts reaction upon treatment with p-toluenesulfonic acid.     

Highly diastereoselective construction of fused carbocycles from cyclopropane-1,1-dicarboxylates and cyclic enol silyl ethers: scope, mechanism, and origin of diastereoselectivity

Cyclopropane rings true: By selecting the appropriate substituents on the ester and silyl groups, fused cyclopentane derivatives with multiple contiguous stereocenters can be synthesized with excellent diastereoselectivity through Cu(II)/bisoxazoline-catalyzed intermolecular [3+2] cycloaddition reactions of cyclopropane-1,1-dicarboxylates and cyclic enol silyl ethers (see scheme).     

Tris(pentafluorophenyl)silyl enol ethers: synthesis and aldol reactions

Silyl enol ethers bearing three pentafluorophenyl groups at the silicon atom are described. These compounds undergo uncatalyzed aldol reactions with aliphatic, α,β-unsaturated, and aromatic aldehydes. The observed reactivity is analyzed in terms of the Lewis acidity of the silyl fragment.     

BF3-promoted cyclization reaction of imines and salicylaldehyde with silyl enol ethers: unexpected formation of dioxaspiro compounds

Unexpected spiro cyclic products were formed from the reaction of imines and salicylaldehyde with silyl enol ethers in the presence of BF₃·OEt₂. Different kinds of dioxaspiro products were afforded depending on the nature of starting materials. Furthermore, salicylaldehyde could also react directly with several silyl enol ethers, giving three products with different spiro cyclic structure under the same reaction conditions.