Dicarbonyl compounds dienolates

Dicarbonyl compounds,2 The alkylation of O-silyl dienolates is y-regioselec-tive and thus provide a useful route to protected 1,5-dicarbonyl compounds. Examples ... 

Dienolic forms of 1,4-dicarbonyl compounds are dehydrated by sulfuric acid, phosphorus pentoxide, and like catalysts to substituted furans. Diacylethylenes, RCOCH = CHCOR, undergo similar ring closure in reducing media. ... 

The a-alkylation of carbonyl compounds by their conversion into nucleophilic enoiates or enolate equivalents and subsequent reaction with electrophilic alkylating agents provides one of the main avenues for regio- and stereo-selective formation of carbon-carbon a-bonds. " Classical approaches to a-alkylation typically involve the deprotonation of compounds containing doubly activated methylene or methine groups and having p/iTa values of 13 or below by sodium or potassium alkoxides in protic solvents. Since these conditions lead to monoenolates derived from deprotonation only at the a-site of the substrate, the question of the regioselectivity of C-alkylation does not arise (however, there is competition between C- and 0-alkylation in certain cases). In more recent years, dienolates of p-dicarbonyl compounds have been utilized in -alkylations with excellent success. 

The alkylation of O-silylated dienolates with 1,3-dithienium tetrafluoroborate shows useful y-selectivity [equation (59)]. The y-alkylated products are selectively protected 1,5-dicarbonyl compounds. Unsaturated 1,5-diketones, precursors of various heterocycles, can be prepared by the reaction of the potassium enolates of methyl ketones with acyl keten dithioacetals [equation (60)]. ... 

Scheme 7.46) [77]. The reaction was applicable to the synthesis of a-ethenyl ke- tones possessing acidic a-protons, and isomerization to the thermodynamically stable conjugated enone was not observed. Equatorial preferences were observed in the ethenylation of cyclohexanone enolates for example, the ethenylation of a silyl enol ether derived from trans-3-decalone predominantly gave an equatorial isomer (Scheme 7.47) [78]. Silyl dienolates, synthesized by the a-ethenylation of thioesters followed by silylation, were ethenylated by this method at the a-position [79]. Ethenylation also occurred with silylated 1,3-dicarbonyl compounds, and ethenylmalonate possessing an acidic a-proton was obtained (Scheme 7.48) [80]. 

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