Acetic trialkylsilyl -, esters

For a highly stereoselective formation of silylketene acetals from esters and trialkylsilyl perchlorates see C. 

The hydroxyl group was usually protected, because cyanohydrins have tendency to racemization or even decomposition. Vinyl ethers or acetal and acid catalysts furnish acetals [62]. Trialkylsilyl chlorides and imidazole are used to give silyl ethers [63]. Commonly used protective groups are silyl ether, ester, methoxy isopropyl (MIP) ether, and tetrahydro-pyranyl ether. ( -Protected cyanohydrins are tolerant to a wider range of cyanide/nitrile transformations and are utilized widely in the synthesis of compounds of synthetic relevance in organic chemistry. 

The role of the trialkylsilyl group is unclear. Changing the trimethylsilyl group not only increases the selectivity but also affects the product of the reaction—/3-hydroxy acetals are obtained instead of (5)-hydroxy esters. They investigated the course of the reaction with different terr-butyldimethylsilyl ketene acetals and aldehydes with catalyst 3f(Eq. 48). 

Trimethylsilyl chloride also gives the N -product alone in simple models, but this is probably the consequence of thermodynamic control in any event N -trialkylsilyl derivatives are much too labile to be useful as stepping stones to N"-alkylhistidines. The only thoroughly studied example of exclusive mono-N -derivatization is the reaction of acetic anhydride with acylhistidine esters, when thermodynamic control appears to operate. Thus, N -(benzyl-oxycarbonyl)histidine methyl ester gives only the M-acetyl derivative, providing a third kind of intermediate which is useful for N"-alkylation with reactive halides (see Section 2.6.2.3).1 1... 

Under the conditions of homogeneous catalysis, decomposition temperatures are normally significantly lower than with the heterogeneous catalysts mentioned above, and cyclopropane yields in general are higher. However, catalysts of type 2 must first be converted into the active form [presumably a copper(I) monochelate] by brief heating or by in situ reduction (see Table 10). Another soluble catalyst, copper(I) triflate, even decomposes diazoacetic esters and diazomalonic esters at temperatures below 0 °C and sterically more encumbered diazocarbonyl compounds (e.g. a-diazo-a-trialkylsilyl acetic esters " ) still at room temperature, and has shown its effectiveness in a number of cyclopropanation reactions. Since copper(I) triflate is... 

Based on the relative configuration of the products of Ireland-Claisen rearrangements, two groups have concluded that Z(0)-enolate formation predominates [65,66], On the other hand, two other groups quenched a-alkoxy ester enolates with trialkylsilyl chlorides and found mixtures of enol ether (ketene acetal) isomers [67,68],... 

Certain mixed organosilicon compounds can also be obtained in this way, e.g., (trialkylsilyl)acetic esters from trialkyliodosilanes and (iodomercuri)-acetic esters.273... 

A variety of substituents—including alkyl, alkenyl, cyano, acetyl, and alkoxy— is tolerated at the 2 and 5 positions of the oxazole ring for these cycloadditions. Acetylenic dienophiles with alkyl, trialkylsilyl, phenyl, ester, ketone, and acetal substituents, as well as terminal alkynes, are precedented. Ab initio calculations predict a slightly higher activation energy for the cycloaddition of oxazole with acetylene compared to the oxazole-ethylene reaction. ... 

Protection and Deprotection.—Carboxylic acids and amides can be silylated rapidly and in very high yield by exchange reactions with keten methyl trialkylsilyl acetals obtained from methyl propionate." Known procedures have been modified to allow the preparation of keten bis(phenylthio) acetals from adds or esters using aluminium thiophenoxide yields are generally very good."... 

A new and efficient combination for silylation is trimethylsilyl chloride-lithium sulphide. Even hindered hydroxy-groups are silylated at room temperature in neutral conditions, although the mechanism of the process is not yet clear. 4-Dimethylaminopyridine (DMAP) has been shown to be an efficient catalyst for the silylation of alcohols by t-butyldimethylsilyl chloride and to be more selective than the imidazole traditionally used. Two new methods for removal of the t-butyldimethylsilyl group are treatment with boron trifluoride etherate and reaction with aqueous HF in acetonitrile. The O-silyl derivatives of normal carboxylic esters, i.e. keten methyl trialkylsilyl acetals (59), have been investigated in silyl transfer to alcohols (Scheme 31) they have the advantage of... 

Very potent carbon nucleophiles formally equivalent to ester enolates are generated by the interaction of TASF(Me) with unhindered trialkylsilyl ketene acetals. In contrast to lithium enolates, these TAS enolates add 1,4 (nonstereoselectively) to a,fi-unsaturated ketones. These adducts can be alkylated in situ to form two new C-C bonds in one pot, or they can be hydrolyzed to give 1,5-dicarbonyl compounds (eq 5). ... 

Trimethylsilyl ketene acetals 92 react in a similar way (note that a bulky trialkylsilyl group works well) ketene acetals derived from t-Bu esters proved to exhibit slightly higher enantioselectivities than their Me, Et, or Ph counterparts (Scheme 15.18 and Table 15.5) [83]. Owing to the higher reactivity of ketene acetals, the catalyst loading could be reduced to 1-5 mol%. However, in contrast to the trichlo-rosilyl enolates, this reaction is believed to proceed via an open transition state, since both (E) and (Z) isomers 92b,c produce anti-aldol adducts 93 with high diastereo- and enantioselectivity (Scheme 15.18 and Table 15.6) [83]. 

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