Methyl ketones Claisen condensation

The Claisen condensation of an aliphatic ester and a thiazolic ester gives after acidic hydrolysis a thiazolylketone (56). For example, the Claisen condensation of ethyl 4-methyl-5-thiazolecarboxylate with ethyl acetate followed by acid hydrolysis gives methyl 4-methyl-5-thiazolyl ketone in 16% yield. 

Synthesis of pyrazole 3 by the Medicinal Chemistry route was straightforward from N-Boc isonipecotic acid (45), so we utilized the route after some optimizations, as summarized in Table 2.4. The key 1,3-diketone intermediate 48 was prepared from 45 without issues. A minor problem in the original route was the exothermic nature of the Claisen condensation between methyl ketone 47 and methyl phenylacetate. Slow addition of l.lequiv of methyl phenylacetate to a mixture of 47, 0.2equiv of MeOH, and 2.5equiv of NaH in THF at room temperature solved this exothermic issue and reduced the amount of self-condensation of... 

Crossed aldol condensations, where both aldehydes (or other suitable carbonyl compounds) have a-H atoms, are not normally of any preparative value as a mixture of four different products can result. Crossed aldol reactions can be of synthetic utility, where one aldehyde has no a-H, however, and can thus act only as a carbanion acceptor. An example is the Claisen-Schmidt condensation of aromatic aldehydes (98) with simple aliphatic aldehydes or (usually methyl) ketones in the presence of 10% aqueous KOH (dehydration always takes place subsequent to the initial carbanion addition under these conditions) ... 

There is a pronounced preference for the formation of a trans double bond in the Claisen-Schmidt condensation of methyl ketones. This stereoselectivity arises in the dehydration step. In the transition state for elimination to a cis double bond, an unfavorable steric interaction between the ketone substituent (R) and the phenyl group occurs. This interaction is absent in the transition state for elimination to the trans double bond. 

Polycarboxylic acid synthases. Several enzymes, including citrate synthase, the key enzyme which catalyzes the first step of the citric acid cycle, promote condensations of acetyl-CoA with ketones (Eq. 13-38). An a-oxo acid is most often the second substrate, and a thioester intermediate (Eq. 13-38) undergoes hydrolysis to release coenzyme A.199 Because the substrate acetyl-CoA is a thioester, the reaction is often described as a Claisen condensation. The same enzyme that catalyzes the condensation of acetyl-CoA with a ketone also catalyzes the second step, the hydrolysis of the CoA thioester. These polycarboxylic acid synthases are important in biosynthesis. They carry out the initial steps in a general chain elongation process (Fig. 17-18). While one function of the thioester group in acetyl-CoA is to activate the methyl hydrogens toward the aldol condensation, the subsequent hydrolysis of the thioester linkage provides for overall irreversibility and "drives" the synthetic reaction. 

Diketones are synthesized by Claisen condensation of appropriate acetyl methyl ketone and ethyl perfluoroalkyl carboxylate. For example, 4,4/-bis(l",l,/,l//,2//,2",3//,3//-heptafluoro-4//,6//-hexanedion-6"-yl)-chlorosulfo-o-terphenyl (BHHCT) was synthesized from o-terphenyl by three step reactions (scheme 3 (Yuan et al., 1998a, 1998b)). The o-ter-phenyl are acetylated by acetyl chloride with anhydrous aluminum chloride as a Lewis acid and 4,4 -diacetyl-e>-terphcnyl is obtained. Then, the 4,4/-diacetyl-o-terphenyl is reacted with perfluoropropionic acid ethyl ester with sodium methoxide as a base. Finally, 4,4,-bis(l", 1",l//,2,2,3,/,3"-heptafluoro-4//,6"-hexanedion-6//-yl)-o-terphenyl is chloro-sulfonylatcd by chlorosulfonic acid to form BHHCT. 

Unfortunately neither reaction will work The black route requires a controlled condensation between two different enolizable esters—a recipe for a mixture of products. The simple alkylation route above removes the need for control. The green route requires a condensation between an unsymmetrical ketone and diethyl carbonate. This condensation will work all right, but not to give this product. As you saw on p. 730, Claisen condensations prefer to give the less substituted dicarbonyl compound, and condensation would occur at the methyl group of the ketone on the right to give the other unsymmetrical keto-ester. 

The keto-aldehyde can be made by a simple Claisen ester condensation (Chapter 28) using the enolate of the methyl ketone with ethyl formate (HCC Et) as the electrophile. It actually exists as a stable enol, like so many 1,3-dicarbonyl compounds (Chapter 21). 

Auxiliary cleavage with concomitant carbon-carbon bond formation is a particularly attractive option, which has been demonstrated in a bimolecular sense using the dianion of methyl sulfone (giving a methyl ketone), and in an intramolecular sense using a Claisen-type condensation of a p-acetoxy enolate (giving a 5-lactone). An interesting halolactonization procedure has... 

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. 

Self-metathesis describes the reachon of an unsaturated fatty acid with itself. For example, methyl oleate gives a mixture of starting material (50%), unsaturated hydrocarbon (25%), and long-chain unsaturated diester (25%), aU as a mixture of T-and trani-isomers. (Figure 18). The diester can be converted to the musk component civetone, but a more efficient route is through self metathesis of the ketone oleon derived from methyl oleate by Claisen condensation (104) (Figure 18). 

Methyl ethyl ketone can be made to undergo the Claisen condensation with a given ester to yield either of two products, depending upon experimental conditions, (a) What are these two products (b) How could you tell quickly and simply which product you had obtained Note Use ethyl benzoate as the ester.)... 

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