Extended enolates Ketones

This principle can be extended to ketones whose enolates have less dramatic differences in stability. We said in Chapter 21 that, since enols and enolates are alkenes, the more substituents they carry the more stable they are. So, in principle, even additional alkyl groups can control enolate formation under thermodynamic control. Formation of the more stable enolate requires a mechanism for equilibration between the two enolates, and this must be proton transfer. If a proton source is available— and this can even be just excess ketone—an equilibrium mixture of the two enolates will form. The composition of this equilibium mixture depends very much on the ketone but, with 2-phenylcyclo-hexanone, conjugation ensures that only one enolate forms. The base is potassium hydride it s strong, but small, and can be used under conditions that permit enolate equilibration. 

Actually, silylenol ethers can be cleaved off by many different nucleophiles (in particular, fluorides) to provide a large variety of enolates113. Hence, lithium amide in liquid ammonia114 or alcoholates115 transform efficiently the aldehyde silylenol ethers into the corresponding lithium enolates. To be extended to ketones silylenol ethers, this latter... 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

Any equilibrium will produce the thermodynamically most stable enolate. The most stable enolate will have the greatest charge delocalization. In the above example, the thermodynamically favored enolate is conjugated the kinetically favored enolate is not. Common conditions for thermodynamic control are to use average bases (like sodium ethoxide or potassium tert-butoxide, p abH 16 to 19) in alcohol solvents. Proton transfer equilibria rapidly occur among base, solvent, ketone, and enolate. Sodium hydride or potassium hydride in an ether solvent are also thermodynamic reaction conditions that allow equilibration between the ketone and the enolate. Enones have two possible enolates weaker bases give the thermodynamically more stable extended enolate, whereas kinetic conditions produce the cross-conjugated enolate. 

Introduction The extended enolate problem Kinetic and thermodynamic control Wittig and Horner-Wadsworth-Emmons Reactions Extended Aza-Enolates Extended Lithium Enolates of Aldehydes Summary a-Alkylation of Extended Enolates Reaction in the y-Position Extended Enolates from Unsaturated Ketones Diels-Alder Reactions Extended Enolates from Birch Reductions The Baylis-Hillman Reaction The Synthesis of Mniopetal F... 

A device for getting extended enolates of esters to combine with aldehydes and ketones in the y position is to use a Wittig approach. The bromo-ester 28 is commercially available. Reaction with a trialkyl phosphite gives the phosphonate ester 29 that gives dienes such as 30 with aldehydes.11 Phosphonium salts can also be used.12... 

The true extended aldol reaction, the combination of an extended enolate in the y-position, with an aldehyde or ketone, can best be realised by combining a silyl enol ether 54 with an acetal 70 under Lewis acid catalysis.20 The Lewis acid, usually TiCl4, catalyses the formation of the oxonium ion 71 which adds in the y-position to the silyl enol ether [cf. 64] to give the adduct 72 from which the remaining OMe group can be removed with base to give the dienal 73, the extended aldol product. 

A recent synthesis of the natural tetronic acid vertinolide 175 illutrates several aspects of extended enolate chemistry. An enone disconnection suggests an aldol reaction between the methyl ketone 176 and crotonaldehyde 35. The ketone 167 might be made by conjugate addition of a reagent for the y-extended enolate 178 to butenone46 177. 

This is just a version of aldol condensation, where the nucleophile is the extended enolate ion at the y-carbon of an a, (3-unsaturated ketone, instead of the simple enolate at the a-carbon of a saturated ketone. 65. Work backward. 

The direct aldol reaction of carboxylic acid derivatives and aldehydes (or ketones) is very difficult. Recently, Kobayashi and co-workers (146) extended the ketones to amides as suitable candidates successfully for the direct aldol reaction. The screening of metal sources revealed that Ba(0-tBu)2 was the better catalyst than Sr(0-iPr)2, Ca(0-iPr)2, or Mg(0-iPr)2, whereas rare earth metals (eg, La(0-iPrls) are catalytically inactive in the presence of p-methoxyphenol. In this aldol reaction, barium enolate formed in situ from barium alkoxide and acylamide and... 

The Simmons-Smith reaction has been used as the basis of a method for the indirect a methylation of a ketone. The ketone (illustrated for cyclohexanone) is first converted to an enol ether, an enamine (16-12) or silyl enol ether (12-22) and cyclopropanation via the Simmons-Smith reaction is followed by hydrolysis to give a methylated ketone. A related procedure using diethylzinc and diiodomethane allows ketones to be chain extended by one carbon. In another variation, phenols can be ortho methylated in one laboratory step, by treatment with Et2Zn and... 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

Following their success with chiral ketone-mediated asymmetric epoxidation of unfunctionalized olefins, Zhu et al.113 further extended this chemistry to prochiral enol silyl ethers or prochiral enol esters. As the resultant compounds can easily be converted to the corresponding a-hydroxyl ketones, this method may also be regarded as a kind of a-hydroxylation method for carbonyl substrates. Thus, as shown in Scheme 4-58, the asymmetric epoxidation of enol silyl... 

LiAlH4 as this avoids protonation of the enolate and the production of any over-reduction products. Cholest-4-en-3-one may be reduced to cholestanone (5a 5/8,1 19) with alkali-metal carbonyl chromates. The studies on intramolecular hydride shifts on hydroxy-ketones and -aldehydes have been extended. " The hydride shifts were examined in a number of y- and 5-hydroxy-carbonyI compounds by heating the substrates with alkaline alumina containing D2O. Exchange of protons on the carbon a to both oxygen functions signals the intramolecular hydride shift typically, the hemiacetals (95) and (96) each incorporate up to six deuterium atoms. The general conclusion, in common with literature precedent, is that, whereas 1,5-shifts are common, 1,4-shifts are rare. 

Deprotonation of the A -acyl substituent of benzothiazines gives a nucleophile that reacts by deacylation with a second molecule of starting material (Equation 46) < 1980TL3001 >. Such anions also react with ketones in an erythro-selective aldol condensation (Equation 47) <1983TL3883>. The selectivity is due to the formation of a Z-enolate and the reaction was also extended to A -acylphenothiazines. 

Enantioselective deprotonation can also be successfully extended to 4,4-disubstituted cyclohexanones. 4-Methyl-4-phenylcyclohexanone (3) gives, upon reaction with various chiral lithium amides in THF under internal quenching with chlorotrimethylsilane, the silyl enol ether 4 having a quaternary stereogenic carbon atom. Not surprisingly, enantioselectivities are lower than in the case of 4-tm-butylcyclohexanone. Oxidation of 4 with palladium acetate furnishes the a./i-unsaturated ketone 5 whose ee value can be determined by HPLC using the chiral column Chiralcel OJ (Diacel Chemical Industries, Ltd.)59c... 

Enolates may be derived from a,/l-unsaturated ketones 16 by base-catalyzed proton abstraction. Under kinetic control the a -proton is abstracted and a cross-conjugated metal dienolate is formed, whereas under thermodynamic conditions the extended dienolate is the major product3,, l. Successful alkylations of dienolates derived from cyclic a,/l-unsaturated ketones have been performed (see Section 1.1.1.3.1.1.2.1.). The related a,/ -unsaturated ester systems have also been investigated22-24. Open-chain structures 16 pose a rather complicated... 

The treatment of a-bromo t-butyl alkyl ketone 5 with magnesium gives almost exclusively the (Z)-enolate 6. The process can be extended to cyclic systems, such as 2-bromo-2,5-trimethyl cyclopentanone 7 that leads to the euolate 8 (equatiou 3). ... 

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