Acyl carbanions, masked

In dithioacetals the proton geminal to the sulfur atoms can be abstracted at low temperature with bases such as Bu"Li. Lithium ion complexing bases such as DABCO, HMPA and TMEDA enhance the process. The resulting anion is a masked acyl carbanion, which enables an assortment of synthetic sequences to be realized via reaction with electrophiles. Thus, a dithioacetal derived from an aldehyde can be further functionalized at the aldehyde carbon with an alkyl halide, followed by thioacetal cleavage to produce a ketone. Dithiane carbanions allow the assemblage of polyfunctional systems in ways complementary to traditional synthetic routes. For instance, the p-hydtoxy ketone systems, conventionally obtained by an aldol process, can now be constructed from different sets of carbon groups. ... 

The most systematically investigated acyl anion equivalents have been the IMS ethers of aromatic and heteroaromatic aldehyde cyanohydrins, TBDMS-protected cyanohydrins, benzoyl-protected cyanohydrins, alkoxycarbonyl-protected cyanohydrins, THP-protected cyanohydrins, ethoxyethyl-protected cyanohydrins, a-(dialkylamino)nitriles, cyanophosphates, diethyl l-(trimethylsiloxy)-phenylmethyl phosphonate and dithioacetals. Deprotonation of these masked acyl anions under the action of strong base, usually LDA, followed by treatment with a wide variety of electrophiles is of great synthetic value. If the electrophile is another aldehyde, a-hydroxy ketones or benzoins are formed. More recently, the acyl carbanion equivalents formed by electroreduction of oxazolium salts were found to be useful for the formation of ketones, aldehydes or a-hydroxy ketones (Scheme 4). a-Methoxyvinyl-lithium also can act as an acyl anion equivalent and can be used for the formation of a-hydroxy ketones, a-diketones, ketones, y-diketones and silyl ketones. - ... 

Cuprous iodide dimethyl sulfide Syntheses with a-alkoxyvinyllithium Masked acyl carbanions -Diketones and ) -ketocarboxylic acid esters from a,/ -ethyleneketones... 

Syntheses with masked acyl carbanions Metalation and alkylation of a-siloxynitriles Ketones from aldehydes via oc-siloxynitriles... 

The carbanion can usually be masked or trapped as a more metalloid derivative and be released in the presence of an electrophile. Trialkylsilane and trialkylstannane derivatives are most often used (81T4069, 88G211), for example in ipso substitution with acyl halides. 

Finally, ketone formation by acylation of carbanions can proceed in several ways, including masking of the ketonic product until isolation, the use of carbanion equivalent reagents which do not readily react with the product or the use of substrates which are more reactive than the ketone towards acylation. An... 

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