Dianions, active methylene alkylation

In the presence of a very strong base, such as amide ion or an organolithium reagent, it is possible to convert dicarbonyl compounds to their dianions. Subsequent alkylation of such dianions leads to alkylation at the more strongly basic enolate site, rather than at the carbon atom between the two carbonyl carbons. The more acidic methylene group activated by two carbonyl substituents is the preferred site in the monoanion, as discussed earlier. The ability to determine the site of monoalkylation by choice of the amount and nature of the basic catalyst has significantly expanded the synthetic utility of enolate alkylations. Scheme 1.7 gives some examples of formation and alkylation of dianions. 

The dianions of /S-keto-esters undergo Claisen ester condensation in the expected manner via the y-carbanion. In an extension of a method reported for the preparation of amines from alcohols loc. cit.), the same Japanese authors have described the direct alkylation of active methylene compounds with alcohols (Scheme 42) the method is limited to relatively acidic compounds, with pJ Tg < 11 diethyl malonate (pX 13.3) does not react. 

Alkylation of a 1,3-dicarbonyl compound at a flanking methyl or methylene group instead of at the doubly activated C-2 position does not usually take place to any significant extent. It can be accomplished selectively and in good yield, however, by way of the corresponding dianion, itself prepared from the dicarbonyl compound and two equivalents of a suitable strong base. For example, 2,4-pentanedione 2 is converted into 2,4-nonanedione by reaction at the more-reactive, less-resonance-stabilized carbanion (1.17). ... 

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