Unsaturated acids decarboxylation

A simple application of the reaction may bo mentioned. Refluxing of (I) with 48 per cent, hydrobromic aeid and glacial acetic acid leads to hydrolysis and decarboxylation and the production of a mixture of the yl tctone yr-di-phonylbutyrolaotone (II) and the isomeric unsaturated acid yY-dlphenyl-vinylacotic acid (III) reduction by the Clemmonsen method or catalytically... 

Here too there is an enol that tautomerizes to the product. The mechanism is illustrated for the case of P-keto acids, ° but it is likely that malonic acids, a-cyano acids, a-nitro acids, and P,y-unsaturated acids behave similarly, since similar six-membered transition states can be written for them. Some a,P-unsaturated acids are also decarboxylated by this mechanism by isomerizing to the p,y-isomers before... 

On the other hand, y, 8-unsaturated acids give lactones that involve cyclization without decarboxylation. 

Some evidence for this mode of decarboxylation of the free acid has been obtained by trapping the enol intermediate (48). (3y-Unsaturated acids (49) probably also decarboxylate by an analogous pathway ... 

With respect to the biosynthesis of the solvents it has been speculated on the basis of quantitative data and the identification of (3,y-unsaturated acids in primitive oxytelid beetles that pairs of 1-alkenes and y-lactones are synthesized from corresponding 3-alkenoic acids by either lactonization or by decarboxylation [118]. 

Migration of double and triple bonds 2-40 Decarboxylation of unsaturated acids... 

Although imidazolinones are usually resistant to hydrolysis, oxazolinone rings are often easily opened. In acid-catalyzed reactions of this type, water converts azlactones (234) into a-acylamino-a,(3-unsaturated acids (235) (77AHC(2i)l75). 1,3,4-Oxadiazolinones are readily opened by hot water to give hydrazine carboxylic acids which undergo decarboxylation. 

This reaction type leading to oc,/ -unsaturated acids and esters is exemplified in the Perkin reaction (Section 6.12.3, p. 1036) and the Knoevenagel reaction (Section 5.11.6, p. 681). The Doebner reaction, which is illustrated in this section, is the condensation of an aldehyde with malonic acid in pyridine solution, often in the presence of a trace of piperidine. The reaction mechanism involves the addition of a malonate anion to the aldehydic carbonyl carbon atom followed by the elimination of water accompanied by decarboxylation. 

If the product 63 is hydrolysed and heated in acid, decarboxylation occurs to give the unsaturated acid and re-esterification gives the ester that might have been the product from condensation of ethyl acetate and acetaldehyde. 

If the original reaction is carried out under more vigorous conditions with malonic acid 67, the decarboxylation occurs during the reaction to give the unsaturated acid in one step. This is a simple way to make11 substituted cinnamic acids 68. 

Acids of low molecular weight form readily with IOB [bis(acyloxy)iodo]benzenes, PhI(OOCR)2, in solution. However, a-ketoacids did not afford the corresponding compounds, because they were readily decarboxylated, in dioxane, at room temperature. Decarboxylation also occurred with a,/i-unsaturated acids, especially those having an aryl group at the /3-position. The reaction started with the acid and IOB the intermediate formed reacted further with A-chloro-, A-bromo-, or A-iodosuccinimide to afford the decarboxylated products, i.e. haloalkenes the stereochemistry of the double bond was largely retained and best results were obtained with A-bromosuccinimide (NBS). 

Highly conjugated diphenylpolyenes, CjHj (CH= CH) C sHs, are obtained directly from the condensation of olefinic aldehydes such as cin-namaldehyde, C Hj CH=CHCHO, with sodium salts of phenylacetic or succinic acids in the presence of lead oxide and acetic anhydride. ° The unsaturated acids first formed are decarboxylated under the conditions of the condensation. 

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