Allyl trimethylsilyl ketene acetal

Rearrangement of allyl trimethylsilyl ketene acetal, prepared by reaction of allylic ester enolates with trimethylsilyl chloride, to yield Y,5-unsaturated carboxylic a-cids. The Ireland-Claisen rearrangement seems to be advantageous to the other variants of the Claisen rearrangement in terms of E/Z geometry control and mild conditions. 

The interest generated by the Claisen rearrangement prompted the development of a considerable number of different versions of [3,3]-sigmatropic rearrangements. Thus, in 1972 Ireland reported the rearrangement of allyl trimethylsilyl ketene acetals to yield y,6-unsatura-ted carboxylic acids. ... 

In the Ireland-Claisen rearrangement (equation 11.17), an allylic ester is converted to an enolate that reacts with trimethylsilyl chloride to produce an allyl trimethylsilyl ketene acetal, which then imdergoes rearrangement. Depending on the reaction conditions, the product may be a y,S-unsaturated carboxylic acid or ester. ... 

Ireland-Claisen rearrangement of O-allyl-O -trimethylsilyl ketene acetals... 

This method was further improved when it was found that readily available allyl esters of the general formula 493 could also be involved in Claisen rearrangements via intermediate formation of ketene derivatives such as lithium enolates 494 or trimethylsilyl ketene acetals 495 (the Ireland-Claisen variant" ). Moreover, rearrangement of these substrates into unsaturated acids 496 occurred easily at room temperature or below. This was in striking contrast to all previous versions of the Claisen rearrangement, which required heating at elevated temperatures (140-160 °C). The Ireland (silyl ketene acetal) variant of... 

Metallation of 2,3,4,5-tetrahydrooxepin with Bu"Li or Bu Li leads to 7-lithio-2,3,4,5-tetra-hydrooxepin, i.e. vinylic deprotonation occurs <82JOC3094>. It is of interest that 2,3-dihydrooxepin, like 2,5-dihydrofuran, undergoes allylic deprotonation. The readily available trimethylsilyl ketene acetal of e-caprolactone is a convenient intermediate for the synthesis of 3-RO-substituted e-caprolactones (R is acyl or tosyl) using, as other reagents, lead(IV) carboxylates <83JOC4940> or [hydroxy(tosyloxy)iodo]benzene <89JOC1101>, respectively. 

Claisen Rearrangement of O-Allyl-O -trimethylsilyl Ketene Acetals OSiMe3 OSiMe,... 

Conversion of allyl alcohol esters into their corresponding trimethylsilyl or t-butyldimethylsilyl ketene acetals, followed by mild thermolysis, results in... 

In the rearrangement of allyl fluoroacetates, trialkylsilyl triflates have been introduced as a new reagent for the Z-selective generation of silyl ketene acetals485. Thus, when (T)-crotyl fluoroacetates are treated at ambient temperatures with a trialkylsilyl triflate in the presence of a tertiary amine, rearranged products with a svn relationship are preferentially obtained. The ketene acetal intermediates cannot be isolated and the geometry has been deduced from the stereochemistry of the products. The selectivity of this process improves in the order triisopropyl > ferf-butyldimethylsilyl > rerf-hexyldimethylsilyl > trimethylsilyl a triethylsilyl (see Table 11). 

LPDE mediated allylic substitutions have been investigated widely. Substitution reaction of allylic alcohols with silyl ketene acetals, allyl silanes, indoles, methoxymethyl vinyl ether, trimethylsilyl azide, trimethylsilyl cyanide, and propar-gyl silanes proceeds with LPDE (Scheme 3.9) [32, 33]. An experiment with allylic alcohol (5,6) proved that these reaction proceeds via generation of allylic carbocation (Scheme 3.10). Also with a combination of LPDE and AcOH (1 mol%), more efficient allylic substitutions proceed (Scheme 3.11) [34]. 

According to Mayr s nucleophilicity scale (N), silyl enol ethers derived from aldehydes (N > 3.5) and ketones (N > 5) and, in particular, silyl ketene acetals (N > 8) [70] represent powerful nucleophihc reagents. Indeed, the aldol-type addition of trichlorosilyl enol ethers 76a-d to aldehydes 1 proceeds readily at room temperature without a catalyst (Scheme 15.14), which is in contrast with the lack of reactivity of allyl silanes in the absence of a catalyst. As a result, the reaction exhibits simple first-order kinetics in each component [71, 72]. Nevertheless, the reaction is substantially accelerated by Lewis bases, which provides a sohd ground for the development of an asymmetric variant The required trichlorosilyl enol ethers 76 can be generated in various ways, for example (i) from the corresponding trimethylsilyl enol ethers on reaction with SiCLt, catalyzed by (AcO)2Hg,... 

Unfortunately, attempts to perform this substitution reaction on cyclohexenol and geraniol led to the exclusive formation of the corresponding silyl ethers. It thus would seem that one requirement for effective carbon-carbon bond formation is that allylic alcohols be secondary and have possess y,y-disubstitution. Pearson, however, discovered a method with less restriction on the natiue of the substrate he used allylic acetates with y-mono-substitution or primary alcohols [96]. Not only ketene silyl acetals but also a diverse set of nucleophiles including aUyl silane, indoles, MOM vinyl ether, trimethylsilyl azide, trimethylsilyl cyanide, and propargyl silane participate in the substitution of y-aryl allylic alcohol 90 to give allylated 91 (Sch. 45). Further experimental evidence suggests that these reactions proceed via ionization to allylic carboca-tions—alcohols 90 and 92 both afforded the identical product 93. 

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