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Metal enolates ester enolate reactions

The decarboxylation of allyl /3-keto carboxylates generates 7r-allylpalladium enolates. Aldol condensation and Michael addition are typical reactions for metal enolates. Actually Pd enolates undergo intramolecular aldol condensation and Michael addition. When an aldehyde group is present in the allyl fi-keto ester 738, intramolecular aldol condensation takes place yielding the cyclic aldol 739 as a main product[463]. At the same time, the diketone 740 is formed as a minor product by /3-eIimination. This is Pd-catalyzed aldol condensation under neutral conditions. The reaction proceeds even in the presence of water, showing that the Pd enolate is not decomposed with water. The spiro-aldol 742 is obtained from 741. Allyl acetates with other EWGs such as allyl malonate, cyanoacetate 743, and sulfonylacetate undergo similar aldol-type cycliza-tions[464]. [Pg.392]

Addition Reactions of Metal Enolates of Non-stabilized Esters, Amides, and Ketones to Epoxides... [Pg.295]

In spite of their intrinsic synthetic potential, addition reactions of metal enolates of non-stabilized esters, amides, and ketones to epoxides are not widely used in the synthesis of complex molecules. Following the seminal work of Danishefsky [64], who introduced the use of Et2AlCl as an efficient catalyst for the reaction, Taylor obtained valuable spiro lactones through the addition reaction of the lithium eno-late of tert-butyl acetate to spiro-epoxides, upon treatment of the corresponding y-... [Pg.295]

A direct application of the ring-opening reaction of an epoxide by a metal enolate amide for the synthesis of a complex molecule can be found in the synthesis of the trisubstituted cyclopentane core of brefeldin A (Scheme 8.35) [68a]. For this purpose, treatment of epoxy amide 137 with excess KH in THF gave a smooth cyclization to amide 138, which was subsequently converted into the natural product. No base/solvent combination that would effect cyclization of the corresponding aldehyde or ester could be found. [Pg.296]

Stereoselective aldol condensation. The stereoselectivity of the reaction of 1 with the ester 2 can be controlled by the choice of the metal enolate. The products are intermediates to 1-methylcarbapenems. [Pg.122]

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... [Pg.394]

Ketone and ester enolates have historically proven problematic as nucleophiles for the transition metal-catalyzed allylic alkylation reaction, which can be attributed, at least in part, to their less stabilized and more basic nature. In Hght of these limitations, Tsuji demonstrated the first rhodium-catalyzed allylic alkylation reaction using the trimethly-silyl enol ether derived from cyclohexanone, albeit in modest yield (Eq. 4) [9]. Matsuda and co-workers also examined rhodium-catalyzed allylic alkylation, using trimethylsilyl enol ethers with a wide range of aUyhc carbonates [22]. However, this study was problematic as exemplified by the poor regio- and diastereocontrol, which clearly delineates the limitations in terms of the synthetic utihty of this particular reaction. [Pg.197]

Lin, M.-H. RajaBabu, T. V. Metal-catalyzed acyl transfer reactions of enol esters role of Y5(OiPr)i30 and (thd)2Y(OiPr) as transesterification catalysts. Org. Lett. 2000, 2, 997-1000. [Pg.278]

The Claisen rearrangement, Cope rearrangement, and associated variants are powerful tools that can be used to create a number of new chiral centers in an expeditious manner, but the use of heavy metals, such as mercury, should be avoided. Of these reactions, the Ireland-Claisen ester enolate reaction provides the most versatile synthetic pathway with minimal scale up problems. [Pg.520]

Not much is currently known concerning diastereoselective addition of metal enolates to ketones 48,108), but selectivities are expected to be lower. In case of titanium enolates, several examples have been studied 77). The reaction shown in Equation 67 involves an ester-enolate21 and proceeds strictly in a 1,2 manner with 90% diastereoselectivity. The observation is significant because similar reactions with aldehydes are essentially stereo-random77). Also, the lithium analog of 203 affords a 1 1 mixture of diastereomers. Diastereoface-selectivity in Equation 67 is not an exception, because 203 adds to acetophenone and pinacolone to afford 85 15 and >76 24 diastereomer mixtures, respectively 77). Although stereochemical assignments have not been made in all cases, the acetophenone adduct was converted stereospecifically into the p-lactone which was decarboxylated to yield an 85 15 mixture of Z- and E-2-phenyi-2-butene 77). [Pg.38]

The best results are obtained with trisyl azide, which again leads to high yields of the azide transfer product 2, especially if the enolate 1 is added to trisyl azide (see entries 1 and 2). Interestingly, the best chemoselectivity and, in addition, identical yields of azide (73%) result from the reaction of the lithium enolate with trisyl azide (entry 3). The reaction of the ester enolate 1 with trisyl azide is less sensitive to the nature of the enolate metal than is the corresponding imide enolate reaction (see Section 7.1.1.). Acetic acid quench, on the other hand, again proved to be useful. Unfortunately, bis-azidation to 3 and diazo transfer to 4 are also observed. [Pg.642]

Three main types of redox reactions of keto compounds leading to the formation of metal enolates have been reported (i) two-electron reduction of diketones or a,(f-unsaturated ketones or esters (equation 1), (ii) oxidative addition reactions (equation 2) and (iii) threefold deprotonation of diketoamines followed by a two-electron oxidation of the trianion by the metal (equation 3). [Pg.256]

Michael addition of metal enolates to a,/3-unsaturated carbonyls has been intensively studied in recent years and provides an established method in organic synthesis for the preparation of a wide range of 1,5-dicarbonyl compounds (128) under neutral and mild conditions . Metal enolates derived from ketones or esters typically act as Michael donors, and a,-unsaturated carbonyls including enoates, enones and unsaturated amides are used as Michael acceptors. However, reaction between a ketone enolate (125) and an a,/3-unsaturated ester (126) to form an ester enolate (127, equation 37) is not the thermodynamically preferred one, because ester enolates are generally more labile than ketone enolates. Thus, this transformation does not proceed well under thermal or catalytic conditions more than equimolar amounts of additives (mainly Lewis acids, such as TiCU) are generally required to enable satisfactory conversion, as shown in Table 8. Various groups have developed synthons as unsaturated ester equivalents (ortho esters , thioesters ) and /3-lithiated enamines as ketone enolate equivalents to afford a conjugate addition with acceptable yields. [Pg.380]

Trichlorotitanium enolates are directly prepared from a ketone, TiCU, and a tertiary amine [122,123] and undergo aldol reactions with aldehydes [124-129], ketones [129], and imines [130,131], Intramolecular condensation with esters is also known [132-137], Although these reactions, based on a titanium enolate [16], which often results in high diastereoselectivity in aldol and related reactions [122], will not be discussed in detail in this article, the success of the alkylation of this titanium enolate with SNl-active electrophiles should be discussed owing to the high Lewis acidity of the metal center [123], Equation (37) shows stereoselective alkylation with an orthoacetate, which is usually inert to alkali metal enolates [138], Aminoalkylation of trichlorotitanium enolates with (a-chloroalkyl)amine has been performed analogously [139,140],... [Pg.673]

Enolate anions are an important class of carbanions that appear in a variety of important reactions, including alkylation a- to a carbonyl group and the aldol (reaction 16-34) and Claisen condensation (reaction 16-85) reactions. Metal enolate anions of aldehydes, ketones, esters, and other acid derivatives exist as aggregates in ether solvents, and there is evidence that the hthium enolate of... [Pg.263]

The direct oxidation of enolates with (lb) gives a-hydroxy carbonyl compounds in excellent yield <91TL715>. The reaction of (lb), isolated or in situ, with the enolates and/or enols of ketones, esters, /i-diketones, (i-oxo esters, and fi-oxo lactones affords the a-hydroxy carbonyl compounds in good yields <91CB2369>. In 1994, Adam et al. have shown that the diastereoselectivity of the dioxirane-enolate reaction is dependent on the metal partner in the enolate. Specifically, use of titanium enolates give much higher diastereoselectivities than sodium enolates <94JOC2358>. [Pg.447]


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Enol esters

Enol esters reaction

Enolates enol esters

Ester enolate

Esters enolates

Esters enolization

Esters metalation

Metal enolate

Metal enolates

Metal enolates 2- ester

Metalation ester enolates

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