Claisen condensations retro

Q Nucleophilic addition of coenzyme A to the keto group occurs, followed by a retro-Claisen condensation reaction. The products are acetyl CoA and a chain-shortened fatty acyl CoA. 

It was unfortunate that we did not detect any product derived from a diketone in the reaction of w-dimethoxybenzene with tetrafluoroben-zyne. We therefore carried out a reaction of tetrafluorobenzyne with 1,3,5-trimethoxybenzene. The di-enol ether (80) could not be isolated, and after the removal of unreacted 1,3,5-trimethoxybenzene we isolated the phenolic acid (81) in good yield. This compound is undoubtedly formed by the hydrolysis of (80) followed by a retro-Claisen condensation, and aromatisation as shown below. 

Industry, in fact, has a major interest in these diesters as building blocks for nylon 6,6 and nylon 7,7 in the production of polyesters and polyamides. However, their present synthesis raises an environmental concern. For instance, the oxidation of cyclohexanone by nitric acid (for the preparation of adipic acid), accounts for more than 10% of the total yearly release of N2O, which is among the main gases responsible for the greenhouse effect. The reaction of Scheme 4.14 represents an eco-friendly alternative synthesis of a,(i)-diesters which uses green reagents and, relevantly, has a 100% atom economy. The overall process is mechanistically described as a retro-Claisen condensation. 

The conversion of the substituted 1,3-dicarbonyl compound into homophthalic acid is remarkably facile loss of the acetyl group by a retro-Claisen condensation and hydrolysis of the ester group are complete in a few minutes in aqueous sodium hydroxide. The overall synthesis of homophthalic acids from o-bromobenzoic acids occurs in high yield and provides an attractive route. 

Unexpected cleavage of acetylacetone occurred when slowly adding excess of Hacac to a solution of Y50(0iPr)13 (Eq. 39) [276]. The formation of an acetato moiety could be explained in terms of a retro-Claisen condensation. 

B as an ester- or lactone-substituted aldehyde enolate. Such enolates undergo condensations with all kinds of aldehydes, including paraformaldehyde. An adduct E is formed initially, acy-lating itself as soon as it is heated. The reaction could proceed intramolecularly via the tetrahedral intermediate D or intermolecularly as a retro-Claisen condensation. In both cases, the result is an acyloxy-substituted ester enolate. In the example given in Figure 13.50, this is the formyloxy-substituted lactone enolate C. As in the second step of an Elcb elimination, C eliminates the sodium salt of a carboxylic acid. The a,/)-unsaturated ester (in Figure 13.50 the 0J,/3-unsaturated lactone) remains as the aldol condensation product derived from the initial ester (here, a lactone) and the added aldehyde (here, paraformaldehyde). 

When o-phcnylenediamine reacts with aliphatic or aliphatic-aromatic ketones the initially formed benzimidazolines can be thermally decomposed, losing a hydrocarbon fragment to yield 2-substituted benzimidazoles [81, 131]. While this method does not appear to have major synthetic importance, the analogous reaction with a, 6-diketonc has some application [132]. Presumably it involves an acid-catalysed retro-Claisen condensation of the S-dicarbonyl compound. When acetylacetonc is used, acetone is formed. The process, then, offers an unambiguous approach to compounds such as 2-methyl-4-nitrobenzimidazoles (not available by direct nitration). Neither the numbers of substituents on the arylenediamine nor their natures appear to affect yields or reaction times significantly. 

The alkylation of p-keto ester enolates followed by decarboxylation affords substituted ketones (acetoacetic ester synthesis). The ester group acts as a temporary activating group. Retro-Claisen condensation can be a serious problem during hydrolysis of the ester, particularly in basic solution if the product has no protons between the carbonyl groups. In these cases, the hydrolysis should be carried out under acidic conditions or using one of the methods of decarbalkoxylation described in the next section. 

Other procedures for decarbalkoxylation include heating P-keto esters in the presence of boric acid or 4-(//,A -dimethylamino)pyridine (DMAP) in HjO-tolueneP This latter approach is also useful for the decarboxylation of a,a-disuhstituted P-keto esters, which are prone to undergo cleavage via retro-Claisen condensation. 

HCO2H, THF, H2O, 82% yield. In this case all fluoride-based methods failed. This may be do to the potential for this system to undergo a retro Claisen condensation with the often basic fluoride reagents. 

The biological oxidation of fatty acids starts with an a,P-dehydrogenation of fatty acid-CoA thioesters followed by a hydration and oxidation of the resulting P-alco-hol. The resulting p-ketoacylCoA looses acetylCoA (retro-Claisen condensation) and the cycles start again (Scheme 2.4.1). Fatty acids are thus split up in acetic acid units, which is one of the most time-consuming processes in the body s metabolism. 

Retro-Claisen condensation The reverse of Claisen conden.sation upon treal-meni of certain /3-dicarbonyl compounds with base. 

Treatment of the 4-acetyl-l,2-dioxan-3-ols with alkah resulted in the deacetylated l,2-dioxan-3-ols via the retro-Claisen condensation by hydroxide ion followed by recyclization [79]. 

In 1940, the reaction was unexpectedly discovered by M. F. Carroll while attempting the acetylation of various allylic alcohols. When cinnamyl alcohol (186) was treated with ethyl acetoacetate (185) in the presence of catalytic sodium acetate, the anticipated acetate 187 was not formed via a retro-Claisen condensation. However, the methyl ketone 189 was isolated. 

The effect of the angle change is clearly apparent in an intramolecular retro-Claisen condensation (Scheme 7.2). The free alkyne does not react, other than simple deprotonation, because the 180° bond angle cannot be accommodated within the 7-membered ring intermediate. The complex, however, does react giving the ring-expanded product." ... 

It has been shown that microwave irradiation speeds up the benzil-benzilic acid rearrangement and increases the yield of product. Spiro ketones of the type (223) have been found to be susceptible to nucleophilic-induced retro-Claisen condensations. These have led to molecular rearrangements that destroy spiro connectivity see Scheme 55. The rearrangements of radical anions derived from aliphatic ketones have been studied using electrochemical techniques. A model system has been unveiled in which nucleophilic catalysis of amide isomerization is characterized for... 

A C-C=0 bond in 1,3-dicarbonyl compounds is known to be cleaved by a simple alkoxide base via retro-Claisen condensation (Eq. (6.9)) [20]. A number of reactions have been reported in which catalytic transformation and base-promoted retro-Claisen condensation occur in tandem [21]. In addition to such examples, several other metal-catalyzed transformations of C-C=0 bonds in 1,3-dicarbonyls have been developed in recent years. 

Retro-Claisen condensation jS-Ketoesters lacking central hydrogens are cleaved by alkoxide... 

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