Silylmethyl ketones

The anion from a-trimethylsilylacetate reacts with aldehydes to give a,0-unsaturated esters, and it has been found that by employing magnesium as the counter-ion pure E-isomers are obtained. Another application of silicon-stabilized anions involves the addition of lithiated trimethylsilyl methane [produced in situ from tributyl(trimethylsilylmethyl)tin and Bu"Li] to acid halides to give silylmethyl ketones, and the cyclization by Bu4N F" of (123) to (124) avoids the problems arising from the corresponding base-induced reaction in the absence of silicon. ... 

Af-(Silylmethyl)thioimidates (34) also undergo water-induced desilylation leading to the N-protonated azomethine ylides (38). These ylides react with a range of electron-deficient alkenes and alkynes, aldehydes, and ketones followed by elimination of methane thiol to give formal nitrile ylide adducts (e.g., 40) (23,24). The reactivity of these species is rather dependent on the nature of R (e.g., good for R = Ph but less so for R=Et or i-Pr), which may be due to competition from tautomerization to give the A -methylthioimidate (39). 

Recently, the chemoselective addition of the a-silylmethyl anion to aldehydes has been accomplished with titanium (equation 7). In studies by Kauffinann, the Grignard derivative was treated with TiCU to prepare the titanium species in situ As indicated in Table 2, this reagent added in good yield to aldehydes to produce the desired methylene compounds but was ineffective for the conversion of ketones to the corresponding methylene compound. ... 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

Further work from Vollhardt s group on cobalt-catalysed cycloadditions of olefins and acetylenes has led to a synthesis of cyclohexadiene derivatives (188 n = 0—2) by addition of trimethyl silylacetylenes the reactions can also be carried out in an intramolecular sense, leading to tricyclic compounds. The novel tricyclic compound (189) has been obtained from naphthoquinone by a [2 + 2]addition of l,l-bis(silylmethyl)ethylene followed by treatment with a Lewis acid. A key step in the first total synthesis of a decipiene diterpene is the formation of tricyclic ketone (190) by an intramolecular aldol condensation, which required the use of methanolic barium oxide. ... 

Chiral ( )-crotylsilanes give tetrahydrofurans, cyclohexanones lead to spiro[4,51decane through a-(silylmethyl)divinyl ketone substitution, P-(alkoxycatbonyl)allylsilanes with epoxides give a-methylene-y-lactones fused to carbocycles, benzyl cations add to give tetrahydronaphthalenes or dihydro(lf/)indenes while tricyclic -2-phenyl-4H-1,3,2-benzodioxaborins with allylSiMes give polycyclic chromans l. 

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