Cyclization Of Allylsilanes Research Articles

Glycosylation reactions mediated by hypervalent iodine: application to the synthesis of nucleosides and carbohydrates.

To synthesize nucleoside and oligosaccharide derivatives, we often use a glycosylation reaction to form a glycoside bond. Coupling reactions between a nucleobase and a sugar donor in the former case, and the reaction between an acceptor and a sugar donor of in the latter are carried out in the presence of an appropriate activator. As an activator of the glycosylation, a combination of a Lewis acid catalyst and a hypervalent iodine was developed for synthesizing 4’-thionucleosides, which could be applied for the synthesis of 4’-selenonucleosides as well. The extension of hypervalent iodine-mediated glycosylation allowed us to couple a nucleobase with cyclic allylsilanes and glycal derivatives to yield carbocyclic nucleosides and 2’,3’-unsaturated nucleosides, respectively. In addition, the combination of hypervalent iodine and Lewis acid could be used for the glycosylation of glycals and thioglycosides to produce disaccharides. In this paper, we review the use of hypervalent iodine-mediated glycosylation reactions for the synthesis of nucleosides and oligosaccharide derivatives.

Ir-Catalyzed Asymmetric and Regioselective Hydrogenation of Cyclic Allylsilanes and Generation of Quaternary Stereocenters via the Hosomi-Sakurai Allylation.

A number of cyclic dienes containing the allylsilane moiety were prepared by a Birch reduction and subjected to iridium-catalyzed regioselective and asymmetric hydrogenation, which provided chiral allylsilanes in high conversion and enantiomeric excess (up to 99 % ee). The compounds were successively used in the Hosomi-Sakurai allylation with various aldehydes employing TiCl4 as Lewis acid, providing adducts with two additional stereogenic centers in excellent diastereoselectivity.

Synthesis of O‐ and N‐Heterocycles by Silyl‐Prins Cyclization of Allylsilanes

Prins cyclization has emerged as one of the most valuable methods for the construction of heterocycles. It proceeds through intramolecular addition of a π‐nucleophile to an oxocarbenium or related ion. The use of electron‐rich alkenes, such as allylsilanes (so‐called silyl‐Prins cyclization) offers the benefit of allowing more selective reactions. The regio‐ and stereochemical outcomes of these transformations are explained by well‐stablished transition states. This review presents the most important contributions involving silyl‐Prins cyclization to the stereoselective preparation of oxygen and nitrogen heterocycles. The influence of different structural variants in the selectivity of the cyclization is discussed. These reactions have frequently been used as key steps in the synthesis of a wide variety of natural products.

ChemInform Abstract: Development of Novel Synthetic Reactions Using Allylsilane

Reported herein is the formation of both heterocycles and carbocycles by means of the nudeophilic attack of the hetero-atom onto the β-silyl carbocation intermediate generated from allylsilane. Thus, on treatment of β-keto ester with allylsilane bearing bulky silyl group in the presence of Lewis acid, [3+2] or [2+2] cycloaddition took place smoothly to afford silyl-substituted tetrahydrofuran or oxetane in good yield.ZrCl4 mediated [3+2] cycloannulation of allyldiisopropylphenylsilane with α, β-unsaturated diesters proceeded smoothly to afford silyl-substituted cyclopentanes highly stereoselectively in good yields. Subsequent oxidative cleavage of the carbon-silicon furnised cyclopentanols. Allyldiisopropylphenylsilane was found to work effectively as 2-hydroxy-1, 3-dipole equivalent in the [3+2] cycloaddition reactions.Bronsted acid catalyzed silicon-directed cyclization of allylsilane bearing hydroxy or sulfonamide moiety furnished silyl-substituted tetrahydrofuran or pyrrolidine derivatives stereoselectively in high yields. Chiral synthesis of pyrrolidine was achieved starting from enantiomerically pure aziridines.

On the [3 + 2] Annulation of Cyclic Allylsilanes with N-Phenyltriazolinedione: An Enantio- and Diastereoselective Synthesis of cis-1,3-Diaminocyclitols

Improved conditions were found to trigger [3 + 2] annulation of cyclic allylsilanes with N-phenyltriazolinedione (PTAD); the products from this reaction were readily tailored into cis-1,3-diaminocyclitols in highly enantioenriched form with full stereochemical control of up to four contiguous stereogenic centers.

A formal [3+2] cycloaddition for the synthesis of bicyclo[3.2.1]octanes

A new and a direct synthetic method for the construction of the bicyclo[3.2.1]octane system is envisioned by assembling cyclic allylsilanes with electron-deficient double bonds in a single step via a formal diastereoselective Lewis acid-promoted [3+2] cycloaddition.

Palladium‐Catalyzed Cyclization of Allylsilanes with Nucleophilic Displacement of the Silyl Group

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enantio- and diastereoselective synthesis of cyclic beta-hydroxy allylsilanes via sequential aldehyde gamma-silylallylboration and ring-closing metathesis reactions.

[reaction: see text] Highly enantioenriched cyclic allylsilanes are prepared via stereoselective gamma-silylallylboration reactions of beta- or gamma-unsaturated aldehydes followed by ring-closing metathesis.

Palladium‐Catalyzed Cyclization of Allylsilanes with Nucleophilic Displacement of the Silyl Group

AbstractFor Abstract see ChemInform Abstract in Full Text.

ChemInform Abstract: Synthesis of Stereodefined Vinyl‐tetrahydropyran and Vinyl‐octahydrochromene Derivatives via Acetalization—Cyclization of Allylsilanes with Aldehydes. Origin of the High Stereoselectivity.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Evidence for formation of silenes strongly influenced by reversed Si[double bond]C bond polarity.

[reaction: see text] Transient silenes that are strongly influenced by reversed Si[double bond]C bond polarization are formed upon heating of tris(trimethylsilyl)silylamides. The silenes are trapped with 2,3-dimethyl-1,3-butadiene to quantitatively yield only one of the possible diastereomers of the functionalized cyclic allylsilanes.

Synthesis of stereodefined vinyl-tetrahydropyran and vinyl-octahydrochromene derivatives via acetalization–cyclization of allylsilanes with aldehydes. Origin of the high stereoselectivity

Functionalized allylsilanes 1– 5 and aldehydes 6– 8 undergo Lewis-acid mediated ring closure to afford 2,3,5- or 6-substituted tetrahydropyrans ( 8– 15 ) and 2,3-substituted octahydrochromenes ( 16a– b ) with excellent stereoselectivity. According to DFT calculations the high stereoselectivity arises from electronically induced steric effects occurring in the key-intermediate of the cyclization.

Palladium-Catalyzed Cyclization of Allylsilanes with Nucleophilic Displacement of the Silyl Group

Allylsilanes containing hydroxy or tosylamide groups undergo palladium(II)catalyzed cyclization to afford derivatives of tetrahydrofuran, piperidine, and pyrrolidine. This catalytic reaction proceeds through an (eta3-allyl)palladium intermediate that is generated by allylic displacement of the silyl group of the allylsilane precursors. The internal nucleophilic attack on the (eta3-allyl)palladium intermediates proceeds with high chemo- and regioselectivity. Benzoquinone and copper(II) chloride can be used for regeneration of the palladium(II) catalyst precursor. Mechanistic studies revealed that the copper(II) chloride reoxidant also activates the (eta3-allyl)palladium intermediate towards nucleophilic attack. Kinetic studies on the formation of the (eta3-allyl)palladium intermediates showed that the reaction rate is highly dependent on the concentration of chloride ligand and the solvent. The structure and reactivity of the key intermediates of the palladadesilylation process were studied by density functional theory (DFT) calculations, which showed that coordination of the electrophilic palladium(II) catalyst precursor to allylsilanes leads to a relatively weak beta-silicon effect. The DFT studies also indicate that the cleavage of the carbon-silicon bond takes place by coordination of a chloride ion to the silicon atom.

アリルシランを用いた新規合成反応の開発

Reported herein is the formation of both heterocycles and carbocycles by means of the nudeophilic attack of the hetero-atom onto the β-silyl carbocation intermediate generated from allylsilane. Thus, on treatment of β-keto ester with allylsilane bearing bulky silyl group in the presence of Lewis acid, [3+2] or [2+2] cycloaddition took place smoothly to afford silyl-substituted tetrahydrofuran or oxetane in good yield.ZrCl4 mediated [3+2] cycloannulation of allyldiisopropylphenylsilane with α, β-unsaturated diesters proceeded smoothly to afford silyl-substituted cyclopentanes highly stereoselectively in good yields. Subsequent oxidative cleavage of the carbon-silicon furnised cyclopentanols. Allyldiisopropylphenylsilane was found to work effectively as 2-hydroxy-1, 3-dipole equivalent in the [3+2] cycloaddition reactions.Bronsted acid catalyzed silicon-directed cyclization of allylsilane bearing hydroxy or sulfonamide moiety furnished silyl-substituted tetrahydrofuran or pyrrolidine derivatives stereoselectively in high yields. Chiral synthesis of pyrrolidine was achieved starting from enantiomerically pure aziridines.

ChemInform Abstract: Chiral Syntheses of 2,3,5‐Trisubstituted Pyrrolidines by Silicon‐Directed Cyclization of Allylsilanes Bearing a Sulfonamide Moiety.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chiral syntheses of 2,3,5-trisubstituted pyrrolidines by silicon-directed cyclization of allylsilanes bearing a sulfonamide moiety

Brønsted acid-catalyzed cyclization of chiral N-tosyl-3-silyl-4-pentenamine proceeded smoothly by way of a β-silyl carbocation intermediate to furnish 2,3,5-trisubstituted pyrrolidines in enantiomerically pure form.

ChemInform Abstract: Stereoselective Synthesis of 2,3,5‐Substituted Tetrahydrofurans by Silicon‐Directed Cyclization of Allylsilanes Bearing a Hydroxy Moiety.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Stereoselective Synthesis of 2,3,5-Substituted Tetrahydrofurans by Silicon-Directed Cyclization of Allylsilanes Bearing a Hydroxy Moiety

Stereoselective Synthesis of 2,3,5-Substituted Tetrahydrofurans by Silicon-Directed Cyclization of Allylsilanes Bearing a Hydroxy Moiety

Intramolecular Cyclization of 4-[3-(Trimethylsilyl)prop-1-en-1-yl]cyclohexanecarbaldehyde with Loss of Formyl Carbon

Abstract Treatment of cis- and trans-4-[3-(trimethylsilyl)prop-1-en-1-yl]cyclohexanecarbaldehyde (2a and 2b) with tetrabutylammonium fluoride in dilute tetrahydrofuran afforded 7-vinylbicyclo[2.2.1]heptan-1-ol (3), the same product obtained from 4-[3-(trimethylsilyl)prop-1-en-1-yl]cyclohexanone (1). It was found that oxidative deformylation of 2a, b to 1 takes place under basic conditions before the cyclization to 3. It was also shown that cyclization of allylsilane with carbonyl does not proceed via an eclipsed conformation.

Stereochemistry in Reactions of Chiral Cyclic Allylsilanes with (Ethoxycarbonyl)nitrene. A New Route to Chiral N-substituted Allylamines.

The SE. reactions of chiral cyclic allylsilanes with (ethoxycarbonyl)nitrene generated by α- elimination of NsONHCO2Et with Et3N permit us to obtain chiral N-substituted allylic amines in good diastereomeric excess when stereoelectronic and conformational factors allow the anti attack.