Ketones magnesium enolates from

Colonge and Grenet have reported that this type of reaction may be utilized in the preparation of magnesium enolates from simple aliphatic a-bromo ketones. ... 

Different protocols have been tested to prepare enolates from /1-aryl-a-iodoketones. Reactions nsing EtsB/PhsSnH in benzene, EtsB in benzene or ether and w-BuLi in ether failed to provide the corresponding enolates. Alternatively, the use of EtMgBr succeeds in generating reactive magnesium enolates from a-iodo ketones. In most cases, the formation of the enolate in THF is cleaner than that in EtiO (equation 8). 

On the other hand, the predominant formation of the diastereomeric aldols 3 b results from the titanium enolate 1 b of (S )-5,5-dimethyl-4-tert-butyldimethylsilyloxy-3-hexanone. For this purpose, the ketone is first deprotonated with A-(bromomagnesio)-2,2,6,6-tetramethylpiperidine and the magnesium enolate, presumably (E) configurated, formed is thereby treated with hexamethylphosphoric triamide and triisopropyloxytitanium chloride. After sonification, the aldehyde is added to give predominantly aldol adducts 3b the diastereomeric ratio of 3b/2b surpasses 95 5 and the chemical yields range from 85 to 88%53b. 

The magnesium amides of choice for the preparation of magnesium enolates via met-allation are the Hauser bases, such as 39 and 40, or (bis)amidomagnesium reagents, such as 46 and 47. The reaction has been successfully applied to the preparation of enolates derived from cyclic, acyclic and a-siloxyketones, benzyUc ketones, aldehydes, carboxylic esters and amides, even with the less hindered Hauser bases. 

The effectiveness of magnesium enolates as nucleophilic agents limits the interest of the reaction. With less substituted substrates (R = H), the aldol reaction is faster than the sily-lation. Moreover, due to solubility limitations, the authors are unable to determine whether the high thermodynamic kinetic ratio of silylenol ethers obtained accurately represents the magnesium enolate composition. Nonetheless, this method is an excellent procedure to selectively prepare the thermodynamic silylenol ether from an unsymmetrical ketone. ... 

Magnesium enolates derived from hindered ketones are also possible Michael donors. For example, enolization of f-butyl alkylketones with (i-Pr)2Mg allows the 1,4-addition on the chalcone. A long reaction time (>3 h) limits the competing 1,2-addition and increases the proportion of the threo isomer (equation 77). 

Enantioselective protonation of ketone metal enolates constitutes an important method for the preparation of optically active ketones. Fuji and coworkers have shown interest in the magnesium countercation in the enantioselective protonation of such enolates. Pertinent results are obtained with protonation of Mg(II) enolates of 2-alkyltetralones and carbamates derived from l,l -binaphtalene-2,2 -diol as chiral proton sources, as indicated in equation 82 and Table 11. 

Magnesium enolates derived from hindered ketones are able to initiate polymerization. For example, addition of 2, 4, 6 4-trimethylacetophenone in toluene to a suspension of (DA)2Mg results in the isolation of (DA)Mg(OC(=CH2)-2,4,6-Me3C6H2), which is found to be an excellent initiator for the living syndioselective (a > 0.95) polymerization of methyl methacrylate (equation 86). 

Keto Sulfoxides. Cyclic p-keto sulfoxides are readily obtained from the magnesium enolate of the ketone and (—)-menthyl (S)-p-toluenesulfinate as a mixture of diastereomers in which the major epimer has the sulfoxide group in the equatorial orientation (eq 14). 

The disconnection for these three-component syntheses is to remove the trans substituents from the a and P positions. Thus the ketone 65 disconnects to the enone 66, a vinyl-Cu derivative and an electrophilic bromoester. In the event, a Cu2I2 catalysed Grignard addition followed by alkylation of the magnesium enolate gives pure trans-65 in 91% yield.27... 

Ketone (27) and reagents related to it have been used in synthesis. In equation (50) is shown an application of the magnesium enolate in Still s synthesis of monensin the facial selectivity in this case is 5 1 and the reaction proceeds in 85% yield. The lithium enolate of (27) has been employed in a synthesis of the C-l.C-7 segment of eiythronolide A (equation 51) the facial selectivity in this case is 6 1. Ketone (31) was used in a synthesis of the basic nucleus of crassin acetate (equation 52). The aldol reaction of (31) with (32), derived from geraniol, occurs in 58% yield to give only one isomer. Four further... 

SoUadid has introduced a-sulfinyl acetates as reagents for asymmetric aldol reactions.Compound (211) is prepared in good optical purity from the menthyl ester of p-tolylsulfinic acid. The magnesium enolate of (211), prepared by reaction of the sulfrnyl ester with r-butylmagnesium bromide, reacts with aldehydes and ketones to give diastereomeric mixtures of a-sulfinyl-3-hydroxy esters (Scheme 13). No... 

This new methodology is quite general and can be applied to the magnesium complexes of 1,2-dimethylenecyclopentane, 1,2-dimethylenecyclohep-tane, and 2-methyl-3-phenyl-l,3-butadiene. Likewise, other carboxylic esters, such as butyl and ethyl benzoates, can also be used to make various fused carbocyclic enols or p,y-unsaturated ketone products. Significantly, the overall synthetic process to form fused carbocyclic enols from the corresponding l,2-bis(methylene)cycloalkanes represents a formal [4+1] annulation. 

Magnesium enolates of ketones can be prepared from a-bromoketones by reaction with magnesium metal. 

At a glance, the descriptors Z and E might seem to be appropriate for O - metal-bound enolates like 6. Indeed, E/Z nomenclature causes no problems when the configuration of preformed enolates derived from aldehydes, ketones, and amides has to be assigned, because the O-metal residue at the enolate double bond has the higher priority. However, application of the E/Z descriptors to ester enolates leads to the dilemma that enolates with different metals but otherwise identical structures will be classified by opposite descriptors, as illustrated by lithium and magnesium enolates 9 and 10, respectively the former would have to be termed Z, and the latter E (Scheme 1.4). 

Figure 3.2 (a) Structure of dimeric THF-solvated lithium enolate of p-fluorophenyl benzyl ketone. Copied from Ref [7]. (b) Structure of a c/s-configured magnesium enolate of t-butyl ethyl ketone. Copied from Ref [8a]. 

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