Diethyl oxalate, mixed Claisen

Mixed Claisen condensations (Section 21.3) Diethyl carbonate, diethyl oxalate, ethyl formate, and benzoate esters cannot form ester enolates but can act as acylating agents toward other ester enolates. 

Diethyl oxalate, (CCqEt, often gives high yields in mixed Claisen reactions. What product would you expect to obtain from a mixed Claisen reaction of ethyl acetate with diethyl oxalate ... 

Strategy A mixed Claisen reaction is effective when only one of the two partners has an acidic cy hydrogen atom. In the present case, ethyl acetate can be converted into its enolate ion, but cliethyl oxalate cannot. Thus, ethyl acetate acts as the donor and diethyl oxalate as the acceptor. 

The synthesis of the disubstituted malonic ester cannot be achieved by successive ethylation and phenylation procedures [cf. Section 5.11.6, p. 680] since for the latter reaction the aryl halide is not sufficiently reactive. The procedure described below (formulated in Expt 8.37) provides a suitable indirect alternative route. This involves a mixed Claisen ester condensation (cf. Section 5.14.3, p. 738) between ethyl phenylacetate and diethyl oxalate, followed by decarbony-lation of the a-keto diester on heating to 175 °C. 

Practice Problem 23.4 Diethyl oxalate, (C02Et)2, can give high yields in mixed Claisen reactions. 

Problem 23.12 What product would you expect from a mixed Claisen-Iike reaction of 2,2-dimethyI-cyclohexanone with diethyl oxalate (Practice Problem 23.4) ... 

Synthesis a is a Dieckmann condensation featuring attack of the a-carbon of the ketone enolate on the ester carbonyl at the other end of the molecule. S5mthesis fc is a mixed Claisen condensation in which cyclohexanone enolate attacks one of the two ester groups of diethyl ethanedioate (diethyl oxalate). 

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