Enol silyl ethers formation

Silyl enol ether formation with RsSiCl-p EtsN gives thermodyanamic silyl enol ether... 

Corriu and coworkers have reported an alternative procedure for the conjugate addition of ketones to a.P-unsaturated acceptors which employs CsF-(RO)4Si (Scheme 56) 126 this procedure affords adducts with a,3-enones, oc.fj-unsaturated esters and a,3-unsaturated amides. Mechanistically, silyl enol ether formation occurs initially, followed by fluoride ion catalyzed enolate formation. 

Krapcho decarbomethoxylation of diester 216 provided monoester 217 (06SL1691). Chemoselective Swern oxidation of 3-(3-hydroxypropyl)-1,2,3,4,11, 1 lrt-hexahydro-6/T-pyrazino[l,2-fr]isoquinolin-4-ones 203 followed by silyl enol ether formation with TIPSOTf and Et3N in Et20 for 12 h at room temperature gave compounds 218 as a single isomer in excellent yields (08JA7148,09JOC2046). 

Silyl enol ether formation again results from silylation of carbonyl oxygen but this time no alcohol is added and a weak base, usually a tertiary amine, helps to remove the proton after silylation. 

Aldehydes can be converted to a,[3-unsaturated aldehydes in a one-pot transformation by in situ silyl enol ether formation followed by the Pd(II)-catalyzed dehydrosilylationA ... 

This reaction constitutes a method for the transformation of saturated cyclic or acyclic carbonyl compounds in three steps (silyl enol ether formation, halocarbene addition, rearrangement) to a,) -unsaturated carbonyl compounds with one-carbon ring enlargement or chain elongation, respectively. The rearrangement can be induced under acidic, basic, or thermolytic conditions, or with silver(I) salts. 

If the electrophile is a vinyl triflate, it is essential to add LiCl to the reaction so that the chloride may displace triflate from the palladium o-complex. Transmetallation takes place with chloride on palladium but not with triflate. This famous example illustrates the similar regioselectivity of enol triflate formation from ketones to that of silyl enol ether formation discussed in chapter 3. Kinetic conditions give the less 198 and thermodynamic conditions the more highly substituted 195 triflate. 

This approach can use the inherent regioselectivity of silyl enol ether formation (chapter 3) using kinetic or thermodynamic enolisation. Hence kinetic enolisation of enones (chapter 11) occurs on the a side leading to 2-Me3SiO-butadienes such as 222. Epoxidation of this silyl enol ether gives the unstable silyloxy ketone 223 which can be desilylated by fluoride ion and hence transformed into the hydroxyketone 225 or acetoxy ketone 224. These transformations are useful because the hydroxy ketones can be unstable34 (see below). 

A cleavage of the fe/7-butyldiphenylsilyl ether followed by thermodynamic silyl enol ether formation provided (269), which upon exposure to A-iodosuccinimide in tetrahydrofuran and subsequent treatment with tetra-n-butyl ammoniu m fluoride afforded (270). 

A stereoselective construction of 1,3-diol systems is based on the reaction of lactol acetates with allylsilanes or silyl enol ethers. Formation of the product is subject to 1,3-asymmetric induction by one or more substituents in the ring. Note that BFj OEt2 is not a suitable catalyst. 

The treatment of an ester (or lactone) with a base and a silyl halide or trillate gives rise to a particular type of sUyl enol ether normally referred to as a silyl ketene acetal. The extent of O- versus C-silylation depends on the structure of the ester and the reaction conditions. The less-bulky methyl or ethyl (or 5-tert-butyl) esters are normally good substrates for O-silylation using LDA as the base. Acyclic esters can give rise to two geometrical isomers of the silyl ketene acetal. Good control of the ratio of these isomers is often possible by careful choice of the conditions. The f-isomer is favoured with LDA in THF, whereas the Z-isomer is formed exclusively by using THF/HMPA (1.24). Methods to effect stereoselective silyl enol ether formation from acyclic ketones are less well documented. ... 

Deprotonation at the more substituted alpha carbon is slower since it is more crowded, but the resulting thermodynamic enolate is more stable since it has a more substituted double bond. The thermodynamic enolate is favored when the reaction is allowed to equilibrate using higher temperatures and either an excess of ketone or a weaker base allows the reverse reaction to occur. In another approach, an enolate is trapped with trimethyMyl chloride (TMSCl) to give the thermodynamic silyl enol ether. The reversible mechanism of the silyl enol ether formation, along with the warmer reaction conditions, promotes equilibration, and, therefore, favors the more stable product. 

Silyl enol ethers are very useful synthetic intermediates, but unfortunately, their synthesis, normally by conjugate addition-silyl enol ether formation, requires the use of air and moisture unstable organometaUic reagents with the consequent problems of functional group incompatibihties, and... 

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