Silylenol ethers enolates

Oxidation of silylenol ethers and enol carbonates to enones... 

The use of DAMgBr 39 in Et20/HMPA/TMSCPEt3N leads to the thermodynamic enolates or silyl enol ethers. This methodology is one of the best direct regiospecific preparations of thermodynamic silylenol ethers from unsymmetric cyclic ketones. 

Krafft and Holton have found that bromomagnesium diisopropylamide (BMDA) in an ethereal solution may be used in conjunction with the system TMSCl/EtsN/HMPA to prepare thermodynamic silylenol ethers. Reaction times of 8-12 h at 25 °C are required for the complete conversion to trimethylsilyl enolates (equation 67). 

The effectiveness of magnesium enolates as nucleophilic agents limits the interest of the reaction. With less substituted substrates (R = H), the aldol reaction is faster than the sily-lation. Moreover, due to solubility limitations, the authors are unable to determine whether the high thermodynamic kinetic ratio of silylenol ethers obtained accurately represents the magnesium enolate composition. Nonetheless, this method is an excellent procedure to selectively prepare the thermodynamic silylenol ether from an unsymmetrical ketone. ... 

An important reaction of silylenol ethers is their use as enolate equivalent in Mukaiyama aldol additions. An example of the synthetic utility of this reaction with a magnesium enolate as starting reagent is shown below. 

The iminium salts of 2,3-dihydropyridines are far more stable than the free bases and have been used extensively in the synthesis of alkaloids. N-Benzyl iminium salt 26, formed from the Polonovski-Potier reaction of V-oxide 25, was transformed into enol ether 27, which is a synthon for the unstable AT-benzvl-l, 2-dihvdropyridine 28 (Scheme 5) <2004LOC168>. The same transformation on a similar iminium salt has been used in the formation of macrocyclic marine alkaloids <1995TL2059>. Carbon nucleophiles, such as the silylenol ethers of esters, have been shown to undergo 1,2-addition rather than 1,4-addition to 2,3-dihydropyridinium salts <1999T14995>. 

Reaction of the benzylic bromide 1195 with silylenol ether 1196 in the presence of TBAI and Gingras salt ([//-If 111 ][I h SnF2]) affords the isochroman-4-one 1197 in poor yield. The product can be explained by the 1,2-addition of the enolate to the top side chain, followed by nucleophilic substitution of the benzylic bromide (Equation 459) <2000CEJ3887>. 

Actually, silylenol ethers can be cleaved off by many different nucleophiles (in particular, fluorides) to provide a large variety of enolates113. Hence, lithium amide in liquid ammonia114 or alcoholates115 transform efficiently the aldehyde silylenol ethers into the corresponding lithium enolates. To be extended to ketones silylenol ethers, this latter... 

Finally, RjSnLi (R = Me, Et, n-Bu) can also add 1,4 to unsaturated ketones, providing the expected /J-stannylated lithium enolates in good to very good yields185. Those can be simply hydrolyzed or transformed into the corresponding silylenol ether, or involved in subsequent aldolisation reactions. 

Kim and coworkers have evaluated the performance of a set of /S-hydroxyethers in the asymmetric protonation of the lithium enolate of 2-methyltetralone374. Their best results were obtained with a salt-free enolate (generated by adding methyllithium in ether to the corresponding silylenol ether in methylene chloride), and using a dichlorobenzylic alcohol as CPA, at any temperature between —25 and —98 °C (Scheme 80). 

Reactions with Enol Ethers, Silylenol Ethers and Enamines... 

Oxidative 1,3-coupling of dicarboxylic esters via treatment with lithium diisopropylamide in the presence of chlorotrimethylsilane gave bis-silylenol ethers, which on reaction with tita-nium(IV) chloride produced cyclopropanedicarboxylic esters 11 in moderate yield.It is assumed that initial conversion to an enoxy radical takes place via a titanium enolate, followed by a diradical 1,3-coupling. 

Lewis-acid-promoted alkylations of silylenol ethers and silyl ketene acetals [195] with Co-complexed acetylenic acetals [196] and acetylenic aldehydes [197,198] (Scheme 4-56) also proceed with fair to excellent syn diastereoselectivity, in contrast to the low selectivity reactions of the free acetylenic derivatives [199, 200]. Reactions of the complexed aldehydes with lithium enolates are stereospecific, with (Z)-enolates giving syn selectivity and ( )-enolates anti selectivity [201]. The complementary stereoselectivity of the crossed aldol reactions of free and cobalt-complexed propynals with silyl ketene 0,S-acetals has been elaborated by Hanoaka exclusive syn selectivity is exhibited by the complexes and high anti selectivity is found with pro-... 

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