Crossed Claisen ester condensations between two different esters
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص644-645
2025-07-01
487
To illustrate some Claisen reactions which are easy to do, we shall now give a few examples of crossed Claisen ester condensations between ordinary esters and the compounds we have just discussed. First, a reaction between a simple linear ester and diethyl oxalate performed under equilibrating conditions with ethoxide as the base. The weak base means a lower enolate concentration.
Only the simple ester can give an enolate, and the low concentration of this enolate reacts preferentially with the more electrophilic diethyl oxalate in a typical acylation at carbon. No self-condensation of the simple ester occurs as the oxalate is much more electrophilic.
The product has an acidic hydrogen atom so it is immediately converted into a stable enolate, which is protonated on work-up in aqueous acid to give the tricarbonyl compound back again.
Another important example leads to the preparation of diethyl phenylmalonate. This com pound cannot be made by ‘alkylation’ of diethyl malonate as aryl halides do not undergo nucleophilic substitution. A crossed Claisen ester condensation between very enolizable ethyl phenylacetate and un enolizable but electrophilic diethyl carbonate works very well indeed under equilibrating conditions.
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