Carbonyls enolisable

This reaction gives only a poor yield. Why Enolisation is involved if you re not sure about this, see frames 169 ff of the Carbonyl Programme. 

You may wonder why aldehyde A doesn t react, with itself but reacts instead with formaldehyde. This is just one aspect of control in carbonyl condensations, treated thoroughly in frames 217-315 of the Carbonyl Programme. In this case, only aldehyde A can enolise but formaldehyde is more electrophilic. Now try this problem How would you... 

AnsueP This diketo-ester has five sites for enolisation ( in 30a). Attack of these enols on the other carbonyl groups could produce ring sizes from ihree to nine but only one stable non-bridged cyclic... 

The second route gave 84% of TM(35). There is no problem with the cross condensation as only acetone can enolise and (36) has the more reactive carbonyl group. Synthe s-is ... 

Control will be needed for the Michael addition, and it proved necessary to protect one carbonyl of (14) as an acetal and add an activating group to the other to give (16), There is no ambiguity in either of these steps as protection of one carbonyl also deactivates the other (Chapter T5) and (15) can enolise on one side only. Removal of the acetal from (17), cyclisation, and decarboxylation can all be accomplished in one step. Synthesis ... 

Carbonyl compounds, simple, enolisation and related reactions of, 18, 1... 

Electronically excited states of organic molecules, acid-base properties of, 12,131 Energetic tritium and carbon atoms, reactions of, with organic compounds, 2, 201 Enolisation of simple carbonyl compounds and related reactions, 18,1 Entropies of activation and mechanisms of reactions in solution, 1,1 Enzymatic catalysis, physical organic model systems and the problem of, 11, 1 Enzyme action, catalysis of micelles, membranes and other aqueous aggregates as models of, 17. 435... 

One of the most widespread theories is the theory of enolisation mechanism put forward by Beckmans in the last century and explains beautifully racemisation in compounds containing a carbonyl group next to the chiral centre ... 

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 ... 

Once regioselective and stereoselective controls have been exerted, the cis-decalins must be isomerised to rrans-decalins, the configuration present in the target molecules. Since frans-decalins are thermodynamically more stable than the corresponding cis-decalins, it is possible to isomerise the latter through enolisation, a process that can be favored by the presence of a carbonyl group near to the centre to be inverted. 

We have an ingenious way of getting round this problem. There is one base, and one only, which we can use to enolise ethyl acetate so that even if it does attack the carbonyl group and displace Et0, we don t care. What base is that ... 

It must be clear to you by now that the carbonyl group not only allows certainly reactions to occur which would otherwise be impossible it also directs where they will occur by selecting exactly which hydrogen atom shall be removed in enolisation and therefore exactly where the new carbon-carbon bond is to be... 

A recent modification of the Peterson reaction involves the use of fluoride ion catalysts. Reaction of bis(trimethylsilyl)methyl derivatives 166 and carbonyl compounds gives the expected alkenes 167 (as shown in equation 137) in high yields, especially for non-enolisable carbonyl compounds, and in some cases with high stereoselectivity209. 

Bromination of the methyl group can be restricted to monosubstitution when the reaction is carried out in acidic media. The mechanism involves protonation of the carbonyl-oxygen, followed by proton loss to give the enol. After monobromination, protonation of the bromoketone is less favourable owing to the presence of the electron-withdrawing halogen atom. Further enolisation does not occur therefore and halogenation ceases (contrast the behaviour of methyl ketones on bromination under alkaline conditions, p. 667) the product is an aryl bromomethyl ketone or phenacyl bromide. 

Enolisation of Simple Carbonyl Compounds and Related Reactions... 

If a compound cannot enolise because it has no protons on the a-carbon atoms then it can take part in a carbonyl condensation only as the electrophile. This will be useful only if it is strongly electrophilic (to avoid self-condensation of the other compound, see point 3 above) so you will see mainly aldehydes and acid chlorides in this list. 

The triester 70 was needed to study pericyclic reactions with electron-rich (a) and electron poor (b) alkenes.12 The a, 3-unsaturated carbonyl disconnection reveals an enolisable ester 72 (X is some activating group such as CCbR) and a very electrophilic keto-diester 71. The synthesis of the allyl ester 72 is all right but the tricarbonyl compound 71 with two 1,2-diCO relationships, is a challenge. 

The dienophile 31 can be disconnected in two ways as there is a carbonyl group at each end of the alkene. Controlling disconnection 31a might be difficult as both components can enolise, but 31 is unambiguous except for the regiochemistry of enolisation of the ketone. 

Current understanding of the reaction suggests that an unprecedented mechanism is operating. Unlike in classical Lewis acid catalysed reactions [28], the metal complex does not activate the carbonyl moiety but is understood to enhance the degree of enolisation and thus create the necessary nucleophilic enol structure for reaction with the fluorinating agent [29]. 

Ketones may direct lateral lithiation even if the ketone itself is enolised enolates appear to have moderate lateral-directing ability. Mesityl ketone 433, for example, yields 434 after silylation - BuLi is successful here because of the extreme steric hindrance around the carbonyl group.396 The lithium enolate can equally well be made from less hindered ketones by starting with a silyl enol ether.396... 

Difluoroboron complexes are used to protect enolisable carbonyl functions against electrophilic bromination, thus allowing regiospecific radical bromination of other (e.g. benzylic) sites. This is demonstrated in the benzylic bromination of condensed tetralones948. 

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