Chlorotrimethylsilane, reaction with ester enolates

The scope of the reaction has been successfully extended to a,p-ethylenic aldehydes,5 esters,6 and amides7 as well as to a,p-acetylenic ketones8 (see Table IV). With esters, the reaction must be performed in the presence of chlorotrimethylsilane (MeaSiCI) to avoid the Claisen reaction by trapping the intermediate enolate. In most cases the organomanganese procedure is simple and more efficient than the organocopper procedure. 

The first formed enolate products from hydrodimerization of esters are trapped by reaction with added chlorotrimethylsilane. By starting with allyl esters, this hy-... 

The newest techniques employ silylation. Even the anions of saturated butenolides afford the enol ester on reaction with chlorotrimethylsilane91 applied to a butenolide the method readily supplies a 2-silyloxyfuran. Such ethers are even more easily procured by allowing a butenolide to react with chlorotrimethylsilane in the presence of triethylamine and zinc chloride but for reasons that remain unclear, acetonitrile is the only satisfactory solvent.92 A typical transformation of 2-trimethylsilyloxyfuran is shown in Scheme 19. 

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. 

As we saw in section 9.4.B,C, the reaction of an ester such as 623 with LDA, under kinetic control conditions, and subsequent reaction of the enolate anion (624) with chlorotrimethylsilane gives the silyl enol... 

Reaction of the lithium enolate of (1) followed by reaction with chlorotrimethylsilane provides ketene acetal (3). This enol reacts with aldehydes in the presence of a Lewis acid to provide unsaturated esters (eq 7). With conjugated enones, (3) undergoes conjugate addition when Titanium(IV) Chloride is used as catalyst (cf. eq 5). ... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

The combinations of chlorotrimethylsilane-hexamethylphosphoramide (HMPA) or chlorotrimethylsi-lane-4-(dimethylamino)pyridine (DMAP) are also powerful accelerants for copper(I)-catalyzed Grignard conjugate additions,33 and stoichiometric organocopper and homocuprate additions (Scheme 21 ).36 However, these reactions must be performed in tetrahydrofuran instead of ether.37 These procedures are noted for their high yields with stoichiometric quantities of Grignard reagents, excellent chemoselectivity and efficiency with a,3-unsaturated amides and esters and enals.58 Typically, additions to enals proceed via the S-trans conformers to afford stereo-defined silyl enol ethers for example, enals (122) and (124) give the ( )-silyl enol ether (123) and (Z)-silyl enol ether (125), respectively. 

The reaction of a-bromoketones with first lithium diisopropyl amide and then with chlorotrimethylsilane yields trimethylsilyl enol ethers quantitatively, and within a few minutes [295]. Pyrolysis of trimethylsilyl p keto esters affords high yields of silyl enol ethers [296], e.g. Eq. 3.130 ... 

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