F-butyldimethylsilyl enol ether

Recently Mukaiyama and coworkers introduced the use of trityl salts as efficient catalysts for the aldol reaction. Using a catalytic amount of trityl perchlorate (5 mol %) and f-butyldimethylsilyl enol ethers, the anti aldols were preferentially obtained (anti 73-84%) regardless of the double bond geometry.298 With trityl triflate (5 mol %) and dimethylphenylsilyl enol ethers, the syn isomers are produced predominantly (syn 63-79% Scheme l).29b Several variations of the catalyst system have been developed. Trityl... 

Transsilylation. Several reagents have been recommended for preparation of t-butyldimethylsilyl ethers by transsilylation. These include allyl-r-butyldimethyl-silane and r-butyldimethylsilyl enol ethers of pentane-2,4-dione and methyl aceto-acetate,2 both prepared with r-butyldimethylchlorosilane and imidazole. Unlike the reaction of f-butyldimethylchlorosilane with alcohols, which requires a base catalyst, these new reagents convert alcohols to silyl ethers under slightly acidic conditions (TsOH) in good yield. The trimethylsilyl ethers of pentane-2,4-dione and methyl acetoacetate convert alcohols to trimethylsilyl ethers at room temperature even with no catalyst. The former reagent is also useful for silylation of nucleotides.3... 

Excellent yields of silyl enol have also been obtained from enones using B(C6F5)3 as a catalyst.40 f-Butyldimethylsilyl, triethylsilyl, and other silyl enol ethers can also be made under these conditions. 

Oxidation of vinyllithiums.2 Vinyllithiums are oxidized to silyl enol ethers by bis(trimethylsilyl) peroxide (1) with retention of configuration. Yields can be increased by use of bis(f-butyldimethylsilyl) peroxide (2). 

On the other hand, unsaturated aldehydes and ketones were obtained using allylic alcohols as alkene components [68]. Similarly, allyl f-butyldimethylsilyl ether and N-allylamides gave silyl enol ethers [69] and enamides [70], respectively. The ruthenium-catalyzed alkene-alkyne coupling was successfully combined with the palladium-catalyzed intramolecular asymmetric allylic alkylation [71] to provide a novel one-pot heterocyclization method [72]. 

Although alkylation of 3-hydroxy ester dianions occurs with high diastereofacial selectivity, the aldol reaction of the dianion obtained from methyl 3-hydroxybutanoate with benzaldehyde gives all four dia-stereomeric aldols in a ratio of 43 34 14 9 (equation 117).155 On the other hand, dianions of 8-hydroxy esters show rather good diastereofacial preferences under the proper conditions. Deprotonation of t-butyl-5-hydroxyhexanoate with lithium diethylamide in the presence of lithium triflate gives an enolate that reacts with benzaldehyde to give aldols (196) and (197) in a ratio of 91 9 (equation 118).156 Use of the f-butyldimethylsilyl ether instead of the alcohol resulted in no facial preference. 

Scheme 8.80. A representation of the pathway from the f-butyldimethylsilyl ether of the enol of cyclohexanone to 2-methylcyclohexanone via the addition of methylene from the Simmons-Smith reagent and rearrangement of the cyclopropane formed (see Simmons, H. E. Smith, R. D. J. Am. Chem. Soc., 1959,81,4256).

Depending on the experimental conditions, reaction of (1) with f-butyldimethylsilyl trifluoromethanesulfonate leads either to the expected /3-trimethysilyl enol ether (11), or to its isomer (12) formed by a 1,3-migration of the trimethylsilyl group from the oxygen atom to the carbon atom (1 13). Reaction with f-butyldimethylchlorosilane always gives (12). Trimethylsilyl enol ether (11) is easily hydrolyzed to 2-(t-butyldimethylsilyl)-acetaldehyde (14) (eq 7). ... 

Enol silyl ethers Bromomagnesium diiso-propylamide. f-Butyldimethylchlorosilane. r-Butyldimethylsilyl trifluoromethanesul-fonate. Chlorotrimethylsily 1-Sodium iodide. Iron. Lithium f-octyl-t-butylamide. 

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