Allylic Silanes Research Articles

ChemInform Abstract: Novel “Reverse Kahne-Type Glycosylation”: Access to O-, N-, and C-Linked Epipodophyllotoxin Conjugates.

Exposure of epipodophyllotoxin C4-sulfoxides to triflic anhydride, followed by a silyl glycoside, provides a glycoconjugate of the etoposide variety via formal “reverse Kahne glycosylation.” To our knowledge, this is the first example of this variant of the Kahne activation method wherein the activating functionality is positioned on the aglycon, rather than on the sugar. Phenols, anilines, or allyl silanes are also efficiently captured at C4, producing the corresponding O-, N-, and C-linked lignan conjugates.

ChemInform Abstract: Benzhydryldimethylsilyl Allylic Silanes: Syntheses and Applications to [3 + 2] Annulation Reactions.

A new silyl group, the benzhydryldimethylsilyl group, has been developed that is easily synthesized and that undergoes facile oxidation. The [3 + 2] annulation reactions of allylic silanes with this silyl group provide a variety of highly substituted five-membered carbocycles and heterocycles with high stereoselectivities. The silyl groups of these cyclic compounds have been oxidized to hydroxyl groups to demonstrate the general synthetic utility of the method.

ChemInform Abstract: Dramatic Enhancement of Reactivity of Organosilicon Compounds Induced by Complexation of Bis(allyl)silanes with Fluoride Ion.

New type of fluoride ion catalyzed allylation agent (1a, 1b), allenylation agent (9, 10), and alkynylation agent (17) can be successfully utilized for various carbonyl substrates. The rate acceleration is ascribable to the shift of equilibrium to the chelate complexes with Bu4NF by the favorable chelation of bis(silyl) compounds toward the fluoride ion. The 19F NMR spectrum (ethyl trifluoroacetate as external standard) of a mixture of 1a and Bu4NF in CDCl3 showed two peaks at δ −81.75 and −77.67 (integration ratio∼9:1). The large signal corresponds to the original peak of Bu4NF and the small one might be ascribed to the chelate complex [B] between 1a and Bu4NF. The fluoride ion mediated reaction of bis(crotyldimethylsilyl)methane (7) with benzaldehyde was examined and the only γ-adduct 8 was obtained regioselectively.

Fused Silacarbacycles Containing a Silole Unit: 1,2‐Hydroboration and 1,1‐Organoboration of Alkynyl(vinyl)silanes

AbstractThe reaction of various alkynyl(vinyl)silanes containing two alkynyl groups with 9‐borabicyclo[3.3.1]nonane (9‐BBN) proceeds by regioselective 1,2‐hydroboration of the vinyl group in the first step, followed by two intramolecular 1,1‐organoboration reactions to afford 1,6‐disilapentalene derivatives, fused silacarbacycles with a silole unit. Similarly, the analogous reaction of an alkynyl(allyl)silane gives a 1,7‐disilaindene derivative. The products were characterized by multinuclear magnetic resonance (1H, 11B, 13C and 29Si NMR) in solution and in two cases by X‐ray structural analysis in the solid state.

ChemInform Abstract: A Convergent Stereoselective Synthesis of Quinolizidines and Indolizidines: Chemoselective Coupling of 2‐Hydroxymethyl‐Substituted Allylic Silanes with Imines.

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Synthesis of 3‐Methylenecyclohexan‐1‐ols by Lewis Acid Catalyzed Cyclization of (Epoxy–allyl)silanes

AbstractA new route for the synthesis of (epoxy–allyl)silanes bearing the PhMe2Si group has been developed and their acid‐catalyzed cyclization studied. The so‐called normal products derived from 5‐exo or 6‐endo attack were never obtained. On the contrary, an interesting tandem rearrangement/cyclization process was observed, which selectively led to 3‐methylenecyclohexan‐1‐ols. A mechanism is proposed to explain this tandem reaction. The stereoselectivity of the cyclization process depends on the nature of the catalyst.

Catalytic coupling of N-benzylic sulfonamides with silylated nucleophiles at room temperature

In the presence of 2-10 mol% of Tf(2)NH, a range of N-benzylic sulfonamides smoothly react with allylic, propargylic, benzylic, or hydrido silanes at room temperature via sp(3) carbon-nitrogen bond cleavage to afford structurally diverse products in moderate to excellent yields and with high chemo- and regioselectivity.

Expedient access to branched allylic silanes by copper-catalysed allylic substitution of linear allylic halides

An unprecedented copper-catalysed allylic transposition enables the regioselective synthesis of branched allylic silanes from linear allylic halides through direct C-Si bond formation.

Pd-catalyzed synthesis of allylic silanes from allylic ethers.

Allylic phenyl ethers serve as electrophiles toward Pd(0) en route to a variety of allylic silanes. The reactions can be run at room temperature in water as the only medium using micellar catalysis.

Titanium-Induced Ring Opening of a Strained Bicyclic Acetal

Substituted tetrahydrofurans are a common structural motif found in numerous natural and medicinal compounds. The diastereoselective synthesis of trans-2,5-disubstituted tetra­hydrofurans has been studied by the groups of Reißig (Eur. J. Org. Chem. 2000, 3893) and Woerpel (J. Am. Chem. Soc. 2005, 127, 10879). In both cases, the addition of allyl silanes to five-membered cyclic oxacarbenium ions gave poor diastereoselectivities. In the present paper, the use of bicyclic 1 and TiCl4 promotes the ring opening by various C-nucleophiles to provide functionalized trans-substituted products in high selectivities.

A convergent stereoselective synthesis of quinolizidines and indolizidines: chemoselective coupling of 2-hydroxymethyl-substituted allylic silanes with imines.

A convergent synthesis of stereodefined indolizidines and quinolizidines through chemoselective allyl transfer between 2-hydroxymethyl-substituted allylic silanes and imines is described. Overall, highly substituted heterocycles that contain three stereogenic centers and up to four fused rings can be accessed in two steps from relatively simple coupling partners.

O-Benzenedisulfonimide as a Reusable Brønsted Acid Catalyst for Hosomi-Sakurai Reactions

Various acetals or alcohols react with allyl(trimethyl)silane or 1-phenyl-2-(trimethylsilyl)acetylene in the presence of a catalytic amount of the Brønsted acid o-benzenedisulfonimide under mild conditions to give good yields of the corresponding products. The catalyst can be easily recovered and purified for use in further reactions, which has economic and ecological advantages.

Siloles Bearing Si-Vinyl and Si-Allyl Functions. 1,1-Organoboration and Protodeborylation

1,1-Organoboration of dialkyn-1-yl(divinyl)silanes, dialkyn-1-yl(organo)(vinyl)silanes and dialkyn- 1-yl(allyl)(methyl)silane using triethylborane, BEt3, or 9-ethyl-9-borabicyclo[3.3.1]nonane, Et-9- BBN, afforded selectively silole derivatives with Si-vinyl and Si-allyl functions, respectively, bearing the dialkylboryl group in 3-position. The siloles are formed via intermolecular 1,1-alkylboration, followed by intramolecular 1,1-vinylboration. In the cases of several 3-diethylboryl-substituted siloles, smooth and essentially quantitative protodeborylation could be achieved by the reaction of the siloles with an excess of acetic acid at ambient temperature. All new siloles were characterized in solution by multinuclear magnetic resonance spectroscopy (1H, 13C, 11B and 29Si NMR).

Insertions of Silylenes into Vinyl Epoxides: Diastereoselective Synthesis of Functionalized, Optically Active trans-Dioxasilacyclooctenes

This communication describes a direct route to functionalized nonracemic trans-dioxasilacyclooctenes that involves stereoselective silylene insertions into vinyl epoxides to provide vinyl silaoxetane intermediates. These strained allylic silanes then undergo uncatalyzed allylation of aldehydes to afford trans-dioxasilacyclooctenes. Diastereoselective additions to these alkenes allow efficient transfer of planar chirality to chirality at stereogenic carbon atoms.

Electrophilic Fluorocyclization of Allyl Silanes

A refreshing cascade: General fluorocyclization reactions will breathe new life into the use of fluorinated hetero- and carbocycles as pharmaceuticals and agrochemicals. Allyl silanes have now been shown to undergo fluorination–cyclization with NF reagents to give cis- and trans-substituted fluorinated heterocycles selectively (see scheme). The correct choice of silyl group was critical to prevent competitive fluorodesilylation.

Silylene-Mediated Ring Contraction of Homoallylic Ethers To Form Allylic Silanes

(-)-Isopulegol derivatives undergo a ring contraction under silylene-mediated conditions to provide cyclopentane products. Silylene transfer to other homoallylic ethers did not provide the ring contraction products. Allylic silane products were elaborated to determine the stereochemical course of the ring contraction reaction. A mechanism for the transformation is proposed.

Formation of Medium-Sized Nitrogen Heterocycles from γ-Silyloxy-γ-Lactams

Nitrogen heterocycles can be prepared by performing ring-expansion reactions of gamma-silyloxy-gamma-lactams, which are available by the annulation reactions of allylic silanes. Nucleophilic substitution of the annulation products and subsequent translactamization of nitrogen-tethered gamma-lactams provide six-, seven-, and eight-membered ring lactams. An enantiomerically enriched delta-lactam formed from this method was elaborated to form the hydroxypiperidine core structure of the pseudodistomin alkaloids.

Synthesis of Enantiomerically Enriched Mono and Bis 2,2′-Bipyridine Alkyl Sulfoxides and Their First Application as Chiral Auxiliaries1

The direct asymmetric oxidation of 6,6′-bis(alkylsulfanyl)-2,2′-bipyridines 2a–e to non-racemic mono sulfoxides 3a–e and bis-sulfoxides 4a,c–e using either Davis oxaziridine or a modified Sharples reagent is reported. The use of isolated mono- and bis-sulfoxides as chiral auxiliaries was tested in either the asymmetric addition of the diethylzinc or allyl(trichloro)silane to benzaldehyde.

Silylene oxonium ylides: di- tert-butylsilylene insertion into C–O bonds

Allylic ethers undergo insertions of silylenes into C–O bonds to form allylic silanes. Silylene insertion into C–O acetal bonds was also observed. Formation of silylene ylide intermediates led to [1,2]-Stevens rearrangement products as well as [2,3]-sigmatropic products depending upon the steric environment of the starting allylic ether.

Allyl Silane Additions to Highly Electrophilic 5,6-Dihydro-2H-1,4- oxazinones

Allyl Silane Additions to Highly Electrophilic 5,6-Dihydro-2H-1,4- oxazinones