Enolate anions, malonic acid, reaction with aldehydes

Malonic esters can be converted to the enolate anion and condensed with aldehydes, ketones, or acid derivatives. The reaction of malonic acid with an aldehyde using pyridine as a base is called the Knoevenagel condensation. 

The reaction of diethyl malonate (90) with sodium hydride generates enolate anion 91 as the conjugate base, and hydrogen gas is the conjugate acid. It has the three resonance contributors shown in the illustration, although 91A has the highest concentration of electron density, and 91 will react as a carbanion nucleophile. There is one extra resonance form in the malonate enolate anion relative to a simple ester due to the second carbonyl unit, and it means that 91 is more stable than the enolate derived from a monoester. In part, this accounts for the enhanced acidity and easier formation of the enolate anion using a weaker base. Once formed, 91 is a carbon nucleophile and it will react with both aldehydes and ketones, as well as with other esters. 

The intra-molecular Claisen condensation is called a Dieckmann condensation, and it generates a cyclic compound 58,99,101,118. Malonic esters can be converted to the enolate anion and condensed with aldehydes, ketones, or add derivatives. The reaction of malonic acid with an aldehyde using pyridine as a base is called the Knoevenagel condensation 59, 60, 61, 62, 69, 99,108,110,112, 113,119,124. 

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