Acetoacetate synthesis

Biotin is a growth factor for many bacteria, protozoa, plants, and probably all higher animals. In the absence of biotin, oxalacetate decarboxylation, oxalosuccinate carboxylation, a-ketoglutarate decarboxylation, malate decarboxylation, acetoacetate synthesis, citrulline synthesis, and purine and pyrimidine syntheses, are greatly depressed or absent in cells (Mil, Tl). All of these reactions require either the removal or fixation of carbon dioxide. Together with coenzyme A, biotin participates in carboxylations such as those in fatty acid and sterol syntheses. Active C02 is thought to be a carbonic acid derivative of biotin involved in these carboxylations (L10, W10). Biotin has also been involved in... 

The Mechanism of the Ethyl Acetoacetate Synthesis—Before the tautomerism of ethyl acetoacetate is discussed we must consider the mechanism of its formation, which for decades has been the subject of lively discussion and was conclusively explained only in recent years (Scheibler). It has been found that even the C=0-group of the simple carboxylic esters, although in other respects inferior in activity to the true carbonyl group, can be enolised by alkali metals. Thus ethyl acetate is converted by potassium into the potassium salt of the tautomeric enol with evolution of hydrogen ... 

Now such enolates have an exceptionally reactive double bond which makes them capable of taking part in all kinds of addition reactions. The ethyl acetoacetate synthesis is the most important process of this type. The enolate is able to combine with a second molecule of the ester the latter is added as the two fragments R—jC=0 and OR... 

It is not only the esters of organic acids which combine, in the manner of the ethyl acetoacetate synthesis , with the enolates of ketones and of esters an analogous behaviour is shown by the esters of nitrous and nitric, acids. The process which leads to the formation of isonitroso-and atinitro-compounds yields products fundamentally similar to those already described just as with ethyl acetate the group CO.CHs enters, so here, the NO- and N02-groups are involved, and enolise " exactly as does >O=0 ... 

In order to prepare 5-fluoro-2-pentanone by the acetoacetate synthesis, the experimenter alkylated ethyl acetoacetate with l-chloro-2-fluoro-ethane in the presence of sodium ethoxide in ethanol. Instead of the expected ethyl 3-ethoxycarbonyl-5-fluoro-2-pentanone, he obtained a fluorine-free compound. What was this product, and how was it formed ... 

Their acute toxicity to mammals is low, but the two active substances may cause dermatitis. In rats fed daily on a diet containing quinomethionate a high cumulative toxicity was observed. A dietary level of 500 mg/kg for 90 days reduced body-weight, caused hypertrophy of the liver, and inhibited acetoacetate synthesis and the microsomal enzymes. It primarily inhibited the function of the HS-enzymes (pyruvate dehydrogenase, succinate dehydrogenase, malate dehydrogenase and a-ketoglutarate oxidase) (Carlson and DuBois, 1970). 

The synthetic procedures known as the acetoacetate synthesis and the malonic ester synthesis are very useftil. Acetoacetate can be used to make ketones and malonic esters can be used to make carboxyhc acids. 

Be sure to update your file cards for ketone syntheses with this sequence of reactions, which is called the (l-keto ester or acetoacetate synthesis. This reaction is not easy to see, as it requires a number of steps. In practice, you must dissect a potential target molecule into its parts to determine if it is available through this route. You must identify the R groups to be attached through alkylation, then piece together the original p-keto ester (we will learn how to make P keto esters in Section 19.8b). 

WORKED PROBLEM 19.16 Show how the following ketones can be made from suitable P-keto esters through the acetoacetate synthesis. Mechanisms are not required. 

A convenient way to determine when to apply the acetoacetic ester or malonic ester approach to a synthetic problem is to incorporate the synthon concept into retrosynthetic analysis. A synthon is a structural unit in a molecule that is related to a synthetic operation. A CH2CO2H group is a synthon that alerts us to the possibility of preparing a target by a malonic ester synthesis. Likewise, a CH2C(0)CH3 group is a synthon that suggests an acetoacetic synthesis. 

Before leaving the subject of Lipmann s laboratory I should mention the results of two other studies that were key to an understanding of the mechanism of fatty acid metabolism. These studies were carried out in collaboration with Michael Douderoff who spent a few weeks in Lipmann s laboratory while I was there. To determine whether acetoacetate synthesis involved the condensation of two molecules of acetyl CoA or one equivalent each of acetyl-CoA and acetate, Douderoff and I studied the formation of acetoacetate in a coupled enzyme system composed of phosphotransacetylase and acetoacetate synthetase from pigeon liver. We found that the acetoacetate produced from 1-P C] acetyl-P and unlabeled acetate was equally labeled in the carboxyl and carboxyl carbons, whereas acetoacetate was produced from unlabeled acetyl-P and P C] acetate contained no... 

Fio. 9. Synthesis of isoph3dol from citral. Numbers along arrows indicate 1, condensation with acetone , hydrogenation S, sodium acetylide in liquid ammonia 4, partial hydrogenation S, phosphorus tribromide and acetoacetate synthesis, or condensation with diketene followed by pyrolysis. 

Fig. 12. Synthesis of farnesylgeranyllinalool from farnesylacetone. 1, Sodium acetylide in liquid ammonia j , partial hydrogenation S, phosphorus tribromide followed by acetoacetate synthesis.

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