Silyl enol ethers rearrangements

A number of steric effects on the rate of rearrangement have been observed and can be accommodated by the chairlike transition state model. The f -silyl enol ethers rearrange somewhat more slowly than the corresponding Z-isomers. This is interpreted as resulting from the pseudoaxial placement of the methyl group in the transition state for rearrangement of the -isomer. 

This effect is the basis of the synthetic importance of ester enolate Claisen rearrangements in which enolates or silyl enol ethers of allylic esters are rearranged into 4-pentenoate esters. 

Geometrically defined a/ -epoxysilanes have been shown (6) to undergo a highly stereoselective rearrangement to silyl enol ethers (see also Chapter 15). This rearrangement is catalysed by boron trifluoride etherate, and seems to involved-opening of the epoxysilane, as shown ... 

The synthesis in Scheme 13.44 is also based on a carbohydrate-derived starting material. It controlled the stereochemistry at C(2) by means of the stereoselectivity of the Ireland-Claisen rearrangement in Step A (see Section 6.4.2.3). The ester enolate was formed under conditions in which the T -enolate is expected to predominate. Heating the resulting silyl enol ether gave a 9 1 preference for the expected stereoisomer. The... 

Oxidation of silyl enol ethers. Oxidation of silyl enol ethers to a-hydroxy aldehydes or ketones is usually effected with w-chloroperbenzoic acid (6, 112). This oxidation can also be effected by epoxidation with 2-(phenylsulfonyl)-3-( p-nitrophenyl) oxaziridine in CHC1, at 25-60° followed by rearrangement to a-silyloxy carbonyl compounds, which are hydrolyzed to the a-hydroxy carbonyl compound (BujNF or H,0 + ). Yields are moderate to high. Oxidation with a chiral 2-arene-sulfonyloxaziridine shows only modest enantioselectivity. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

As an extension of the above-mentioned method, a successful rearrangement starting from silyl enol ethers as an electron-rich iV-allyl moiety has been published. Allylamide 88 was synthesized in two steps starting from (-i-)-phen-ethylamine and acrolein via a condensation-acylation sequence. The auxiliary-... 

Silyl enol ethers have been prepared via a Brook rearrangement from the reaction of phenyldimethylsilyllithium with a-silyloxy ketones (see Scheme 68). The comparison of the rate of the base-catalysed Brook rearrangement in -substituted... 

Retro-Brook rearrangement of the [l,3]-variant will readily take place in sp and sp carbanion systems. Kuwajima and Takeda and Corey and Rticker have developed the [l,3]-retro-Brook rearrangement of silyl enol ether anions which provide a-silyl ketones (equation 100 and 101). 

Retro-Brook rearrangement is relatively common and enjoys widespread application in many facets of organic synthesis. For example, vinylsilane synthesis from aUyloxysUane (eqnation 103)"°, allylsilane synthesis (eqnation 104)"°, and lithium eno-late formation from silyl enol ether (equation 105)" were reported. 

C. Rearrangements of ester enolates and silyl enol ethers... 

Esters of allylic alcohols can be rearranged to y,<5-unsaturated carboxylic acids via the O-trimcthylsilyl ether of the ester enolate.161 This rearrangement takes place under much milder conditions than the ortho ester method. The reaction occurs at or slightly above room temperature. Entries 14 and 15 of Scheme 6.12 are examples. The example in entry 16 is a rearrangement of the enolate without intervention of the silyl enol ether. 

The stereochemistry of the silyl enol ether Claisen rearrangement is controlled not only by the stereochemistry of the double bond in the allyhc alcohol but also by the stereochemistry of the silyl enol ether. For the chair transition state, the configuration at the newly formed C—C bond is predicted to be determined by the E- or Z-configuration of the silyl enol ether. 

A number of steric effects on the rate of rearrangement have been observed and can be accommodated by the chairlike transition-state model.165 The A-silyl enol ethers... 

When the lactone silyl enol ether U is heated to 135°C, a mixture of four stereoisomers is obtained. Although the major one is that expected for a [3,3] sigmatropic rearrangement, lesser amounts of the other possible C-4a and C-5 epimers are also formed. When the reactant is heated to 100°C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic... 

Moreover, the trimethylsilyl ether 18 on sensitized photolysis rearranged to the silyl enol ether 19, which under acidic conditions gave l-methylbicyclo[3.2.0]heptan-3-one (20).78... 

THF if CuBr is present.1393 The reaction takes place either with or without allylic rearrangement.1394 Propargylic ethers give allenes.1395 Vinylic ethers can also be cleaved by Grignard reagents in the presence of a catalyst, in this case, a nickel complex.1396 Silyl enol ethers R2G=CROSiMe3 behave similarly.1397... 

Regiospecific route to silyl enol ethers.11 Silyl enol ethers arc formed regio-specifically by reaction of lithium dialkyl cuprates or diaryl cuprates with 2-trimcthylsiloxyallyl halides. An allylic rearrangement occurs in reactions with hindered halides. 

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