Claisen catalysed

Besides the obvious biological interest, chorismate mutase is important for being a rare example of an enzyme that catalyses a pericyclic reaction (the Claisen rearrangement), which also occurs in solution without the enzyme, providing a unique... 

The [3, 3] sigmatropic rearrangement of allyl vinyl ethers is known as Claisen rearrangement. Allylic alcohols can be converted to allyl vinyl ethers by mercuric acetate catalysed exchange with ethyl vinyl ether. The allyl vinyl ether need not be isolated because it undergoes rearrangement to 4-pentenal. 

In contrast to Mori s synthesis, Pawar and Chattapadhyay used enzymatically controlled enantiomeric separation as the final step [300]. Butanone H was converted into 3-methylpent-l-en-3-ol I. Reaction with trimethyl orthoacetate and subsequent Claisen-orthoester rearrangement yielded ethyl (E)-5-methyl-hept-4-enoate K. Transformation of K into the aldehyde L, followed by reaction with ethylmagnesium bromide furnished racemic ( )-7-methylnon-6-ene-3-ol M. Its enzyme-catalysed enantioselective transesterification using vinylacetate and lipase from Penicillium or Pseudomonas directly afforded 157, while its enantiomer was obtained from the separated alcohol by standard acetylation. 

The SnCU-catalysed Claisen and Cope rearrangements of A -allylanilines and N-allylenamines, and the effect of meffl-substituents in the aromatic ring on the Claisen aromatic amino rearrangement of a series of fluorinated anilines, have been investigated. 

The asymmetric aza-Claisen rearrangement of allyl imidates, (86) (87), has been shown to be catalysed by homochiral cationic palladium(II) complexes, and a series of enantiopure cyclopalladated ferrocenyl amines and imines have been... 

The Claisen reaction (sometimes Claisen condensation) is formally the base-catalysed reaction between two molecules of ester to give a P-ketoester. Thus, from two molecules of ethyl acetate the product is ethyl acetoacetate. 

The reverse Claisen reaction is common, especially with cyclic P-ketoesters, such as one gets from the Dieckmann reaction (see Section 10.8). If one only wants to hydrolyse the ester, it thus becomes necessary to use the rather less effective acid-catalysed hydrolysis method (see Section 7.9.2). 

Citrate synthase catalyses an aldol reaction rather than a Claisen reaction... 

Mechanistically, we can consider it as attack of an enolate anion equivalent from acetyl-CoA on to the ketone group of oxaloacetate. However, if we think carefully, we come to the conclusion that this is not what we would really predict. Of the two substrates, oxaloacetate is the more acidic reagent, in that two carbonyl groups flank a methylene. According to the enolate anion chemistry we studied in Chapter 10, we would predict that oxaloacetate should provide the enolate anion, and that this might then attack acetyl-CoA in a Claisen reaction (see Box 10.4). The product expected in a typical base-catalysed reaction would, therefore, be an acetyl derivative of oxaloacetate. 

Other interesting examples of asymmetric syntheses involving chiral monoterpenoids include the Claisen reaction between (—)-menthyl phenylacetate and benz-aldehyde (optical purity is confirmed by microcalorimetry), a highly enan-tioselective carbenoid cyclopropanation catalysed by (4), ° and the crossed aldol... 

Claisen rearrangement plays an important part in the biosynthesis of several natural products. For example, the chorismate ion is rearranged to the prephenate ion by the Claisen rearrangement, which is catalysed by the enzyme chorismate mutase. This prephenate ion is a key intermediate in the shikimic acid pathway for the biosynthesis of phenylalanine, tyrosine and many other biologically important natural products. 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

Nonpolymerizable cyclic imides like N-methylsuccinimide undergo condensation in the presence of basic catalysts (56). Similar condensations, remembering Claisen condensations of esters can occur also with imides which were added or which were formed by disproportionation during the base catalysed polymerization of caprolactam. The water which could be set free as a result of such a condensation consumes one equivalent of sodium caprolactam which is converted into the inactive sodium 6-amino-capronate. On the other hand the condensation products can take off directly another equivalent of the base because of their acidic nature. 

The attempted Pd(II)-catalysed aza-Claisen rearrangement of the trichloroacetimidate (145) yielded the diastereoisomerically pure cyclopropane derivative (146) (Scheme 55).275... 

Al)-Acetyl-(A6)-aroyl-2,5-dithio-3,4,7,8-tetramethylglycolurils (92) and related compounds undergo a t-butoxide-catalysed Claisen-type condensation.268 Rate constants have been measured for the template-directed process, Hammett plots have been constructed, and deuterium isotope effects measured at the acetyl group. The... 

The aldol and Claisen reactions both achieve carbon-carbon bond formation and in typical base-catalysed chemical reactions depend on the generation of a resonance-stabilized enolate anion from a suitable carbonyl system (Figure 2.7). Whether an aldol-type or Claisen-type product is formed depends on the nature of X and its potential as a leaving group. Thus, chemically, two molecules... 

Figure 4.11). This reaction, a Claisen rearrangement, transfers the PEP-derived side-chain so that it becomes directly bonded to the carbocycle, and so builds up the basic carbon skeleton of phenylalanine and tyrosine. The reaction is catalysed in nature by the enzyme chorismate mutase, and, although it can also occur thermally, the rate increases some 106-fold in the presence of the enzyme. The enzyme achieves this by binding the pseudoaxial conformer of chorismic acid, allowing a transition state with chairlike geometry to develop. 

The allyl chloride 353 is prepared from diprenyl ether (352), and its Pd-catalysed 1,4-elimination in the presence of AcONa affords the conjugated diene 354. Citral (355) is obtained by the consecutive Claisen and Cope rerrangements of 354 [168],... 

PdCl2(PhCN)2-catalysed Claisen rearrangement of the allyl vinyl ether 474 derived from cyclic ketone at room temperature affords the syn product 475 with high diastereoselectivity [203]. In contrast to thermal Claisen rearrangement, the Pd(II)-catalysed Claisen rearrangement is always stereoselective, irrespective of the geometry of allylic alkenes. The anti product is obtained by the thermal rearrangement in the presence of 2,6-dimethylphenol at 100 °C for lOh. 

The Pd(II)-catalysed reaction of an allylic alcohol with the ketene acetal 476 at room temperature generates the ortho ester 477. Its Claisen rearrangement via 478 in boiling xylene with a catalytic amount of PdCl2(PhCN)2 gives the y,S-unsaturated ester 479 [204]. 

Competitive experiments with 2H-, 13C- and 180-labelled chorismate derivatives and three different chorismate mutase enzymes have shown that in all catalysed and non-catalysed Claisen rearrangements a non-synchronous, concerted, pericyclic transition state is involved, with C-O bond cleavage considerably in advance of C-C bond formation. Some evidence has suggested that the ionic active site of the enzymes may polarize the transition state more than occurs in solution. Similar findings apply to the retro-ene fragmentation of chorismate to 4-hydroxybenzoate.17... 

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