Carbon dioxide 3-keto acid formation

The carbon-carbon bond forming potential inherent m the Claisen and Dieckmann reac tions has been extensively exploited m organic synthesis Subsequent transformations of the p keto ester products permit the synthesis of other functional groups One of these transformations converts p keto esters to ketones it is based on the fact that p keto acids (not esters ) undergo decarboxylation readily (Section 19 17) Indeed p keto acids and their corresponding carboxylate anions as well lose carbon dioxide so easily that they tend to decarboxylate under the conditions of their formation... 

This thiamin pyrophosphate-dependent enzyme [EC 4.1.1.1] catalyzes the conversion of an a-keto acid (or, a 2-0X0 acid) to an aldehyde and carbon dioxide. This enzyme will also catalyze acyloin formation. 

A variation on the transamination approach that also starts with an a-keto acid substrate is to perform a reductive amination catalyzed by amino acid dehydrogenases (dHs) (Scheme 9.31) in combination with the formate dH cofactor recycling system, although other reducing systems can be used. " The generation of carbon dioxide from formate drives the coupled reactions to completion. 

As shown in Scheme 12.54, when ort/io-nitrotoluene is allowed to react with diethyl oxylate in the presence of sodium (or potassium) ethoxide, a condensation occurs and ethoxide is eUminated. Then, with zinc (Zn) in acetic acid, reduction of the nitro group and cyclization to the keto carbonyl occurs. Hydrolysis results in the formation of the indole-2-carboxyUc acid and pyrolysis leads to decarboxylation with the formation of the free base and carbon dioxide (CO2). 

Reactions.—A variety of interesting and useful syntheses have been published involving the reaction of dimsyl anion [MeS(0)CHa ] with esters and lactones,with disulphides, with chlorosilanes, with sulphinate esters, with organoboranes, and with stilbenes. Simple and functionalized a-sulphinyl carbanions can be condensed with carbonyl compounds or alkylated, often in a stereocontrolled manner, as in a nicely conceived synthesis of biotin. Considerable attention has been given to methods for the removal of the sulphoxide function following carbon-carbon bond formation. Among the methods used are reduction by aluminium amalgam (with j3-keto-sulphoxides), reduction with Raney nickel, pyrolytic elimination of sulphenic acid, elimination of sulphur dioxide from sultines, e.g. (64), and sulphoxide-... 

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