Allyl ester enolates

Rearrangement of allyl trimethylsilyl ketene acetal, prepared by reaction of allylic ester enolates with trimethylsilyl chloride, to yield Y,5-unsaturated carboxylic a-cids. The Ireland-Claisen rearrangement seems to be advantageous to the other variants of the Claisen rearrangement in terms of E/Z geometry control and mild conditions. 

Scheme 6.14. Possible transition structures for the [2,3]-Wittig rearrangement of the R-allylic ester enolates shown in Scheme 6.13. For amide enolates, see Scheme 6.22.

A synthesis of a-substituted y,S-unsaii rangement of allyl ester enolates, which organocopper reagents to allyl 2-alkenoai Ketone synthesis. d-Keto-a-amino... 

A synthesis of a-substituted S-unsaturated acids exploits the facile Claisen rearrangement of allyl ester enolates, which can be generated by conjugate addition of organocopper reagents to ally 2-alkenoates. ... 

A modification of the [3,3] sigmatropic rearrangement of allyl ester enolates allows the elaboration of 4-ethoxybutyrolactones in ca. 70% overall yield (Scheme 11). 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

This effect is the basis of the synthetic importance of ester enolate Claisen rearrangements in which enolates or silyl enol ethers of allylic esters are rearranged into 4-pentenoate esters. 

When ethyl trifluoroacetylacetate is treated with an allylic alkoxide, tran-sesterification is followed by ester enolate Claisen rearrangement m a process that on decarboxylation yields stereospecifically the tnfluoromethyl ketone product [22] (equation 19)... 

Schemes 28 and 29 illustrate Curran s synthesis of ( )-hirsutene [( )-1]. Luche reduction58 of 2-methylcyclopentenone (137), followed by acetylation of the resulting allylic alcohol, furnishes allylic acetate 138. Although only one allylic acetate stereoisomer is illustrated in Scheme 28, compound 138 is, of course, produced in racemic form. By way of the powerful Ireland ester enolate Clai-sen rearrangement,59 compound 138 can be transformed to y,S-unsaturated tm-butyldimethylsilyl ester 140 via the silyl ketene acetal intermediate 139. In 140, the silyl ester function and the methyl-substituted ring double bond occupy neighboring regions of space, a circumstance that favors a phenylselenolactonization reac-...

It is possible to treat ketones with allyl alcohol and an acid catalyst to give y,5-unsaturated ketones directly, presumably by initial formation of the vinylic ethers, and then Claisen rearrangement.In an analogous procedure, the enolates (126) of allylic esters [formed by treatment of the esters with lithium isopropylcyclohex-... 

We might have recognised this as a Claisen-Cope product since it is a y > < -unsaturaLed ester. Disconnecting and inverting the allylic group from the ester enolate gives the start ing material 32), Ortho esters are usually used as the reagents. 

Entries 3 to 6 are examples of ester enolate alkylations. These reactions show stereoselectivity consistent with cyclic TSs in which the hydrogen is eclipsed with the enolate and the larger substituent is pseudoequatorial. Entries 4 and 5 involve SN2 substitutions of allylic halides. The formation of the six- and five-membered rings, respectively, is the result of ring size preferences with 5 > 7 and 6 > 8. In Entry 4, reaction occurs through a chairlike TS with the tertiary C(5) substituent controlling the conformation. The cyclic TS results in a trans relationship between the ester and vinylic substituents. 

The silyl ketene acetal rearrangement can also be carried out by reaction of the ester with a silyl triflate and tertiary amine, without formation of the ester enolate. Optimum results are obtained with bulky silyl triflates and amines, e.g., f-butyldimethylsilyl triflate and (V-methyl-Af, /V-dicyclohcxylaminc. Under these conditions the reaction is stereoselective for the Z-silyl ketene acetal and the stereochemistry of the allylic double bond determines the syn or anti configuration of the product.243... 

Enolates of allyl esters of a-amino acids are also subject to chelation-controlled Claisen rearrangement.249... 

Scheme 3.7. Diastereoselective formation of /S-silyl ( )- or (Z)-ester enolates by silylcuprate conjugate addition followed by alkylation with aldehydes [49]. Stereoselective synthesis of ( )-and (Z)-allyl silanes [50].

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

The first formed enolate products from hydrodimerization of esters are trapped by reaction with added chlorotrimethylsilane. By starting with allyl esters, this hy-... 

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. 

The method of preparation of 5-dodecen-2-one presented here is a version of the literature procedure published earlier. It offers several advantages over existing methodology (1) The ester enolate modification of the Carroll rearrangement provides the allylic acetoacetates via a mild, fast, and high yield synthesis. This procedure represents a significant Improvement over... 

Esters of allylic alcohols can be rearranged to y,<5-unsaturated carboxylic acids via the O-trimcthylsilyl ether of the ester enolate.161 This rearrangement takes place under much milder conditions than the ortho ester method. The reaction occurs at or slightly above room temperature. Entries 14 and 15 of Scheme 6.12 are examples. The example in entry 16 is a rearrangement of the enolate without intervention of the silyl enol ether. 

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