Propanedioic acid, decarboxylation

The mechanisms of thermal decarboxylation probably are not the same for all cases, but when the acid has a double-bonded function such as 0=C, N=C, 0=N, or C=C attached to the a carbon then a cyclic elimination process appears to occur. For propanedioic acid the process is... 

Draw 2-benzyl-2-ethyl-l,3-propanedioic acid. Draw the transition state required for decarboxylation and the final product expected after heating to 200°C. 

Alkyl-substituted 3-oxobutanoic and propanedioic esters can be hydrolyzed under acidic conditions to the corresponding acids, and when these are heated they readily decarboxylate (Section 18-4). Alkyl 3-oxobutanoic esters thus yield methyl alkyl ketones, whereas alkylpropanedioic esters produce carboxylic acids ... 

In Section 23-2 two reactions of /3-tlicarbonyl compoiiiiJs arc presented. The (irst is alkylation of the readily deprotonated "carbon in the middle." The second pertains to /3-dicarbonyi compounds where at least one of the carbonyl groups is an ester. Ester hydrolysis leads to a carboxylic acid that readily loses COi (decarboxylation). When this sequence is carried out on a /3-ketocster, the result is a ketone. When carried out on a diester of propanedioic (malonic) acid, the result is a carboxylic acid. In each case, groups attached in the preliminary alkylation slep(s) wind up in the product. 

Malonic ester synthesis (Section 18.7) A reaction in which the a-hydrogen of diethyl propanedioate (diethyl malonate, also called malonic ester ) is removed, creating a resonance-stabilized anion which can serve as a nucleophile in an 5 2 reaction. The a-carbon can be substituted twice the ester functionalities can be converted into a carboxylic acid which, after decarboxylation, will yield a substituted ketone. 

Loss of CO2 occurs readily only from the neutral carboxylic acid. If the ester is hydrolyzed with base, acid must be added to protonate the resulting carboxylate salt in order for decarboxylation to occur. Decarboxylation of substituted propanedioic (malonic) acids follows the same mechanism. 

In Summary jS-Dicarbonyl compounds such as ethyl 3-oxobutanoate (acetoacetate) and diethyl propanedioate (malonate) are versatile synthetic building blocks for elaborating more complex molecules. Their unusual acidity makes it easy to form the corresponding anions, which can be used in nucleophilic displacement reactions with a wide variety of substrates. Their hydrolysis produces 3-ketoacids that are unstable and undeigo decarboxylation on heating. 

Recall that A-alkylation of 1,2-benzenedicarboxylic imide (phthalimide) anion followed by acid hydrolysis furnishes amines (Section 21-5). To prepare an amino acid instead, we can use diethyl 2-bromopropanedioate (diethyl 2-bromomalonate) in the first step of the reaction sequence. This alkylating agent is readily available from the bromination of diethyl propanedioate (malonate). Now the alkylation product can be hydrolyzed and decarboxylated (Section 23-2). Hydrolysis of the imide group then furnishes an amino acid. 

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