Enol ester, allylation

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

Reagent control This involves the addition of a chiral enolate or allyl metal reagent to an achiral aldehyde. Chiral enolates are most commonly formed through the incorporation of chiral auxiliaries in the form of esters, acyl amides (oxazolines), imides (oxazolidinones) or boron enolates. Chiral allyl metal reagents are also typically joined with chiral ligands. 

Anodic oxidation of cyclic enol esters with /1-hydrogens leads to allyl radicals, which then lose acyl radical to form a, /1-unsaturated ketones. When the electrolysis is performed in an undivided cell, these are converted by the cathode into enolate anion radicals, which then couple to form /1-dimers (Scheme 66)164. 

In contrast, some rr- allyl ruthenium complexes containing a chelating diphosphine ligand were the first metal complexes which favoured the anti-Markovnikov addition of carboxylic acids to terminal alkynes to form (Z)-enol and (E)-enol esters with high regioselectivity and stereoselectivity [17-19] according to Eq. (1). 

The success of the Claisen-Cope rearrangement need not be limited to the production of aldehydes via enol ethers. Allylic alcohol (58) is successively transposed into a mixture of allylic isomers (59 Scheme 4), and is subjected to an orthoester Claisen rearrangement at 150 "C to provide ester (61). The moderate temperature of the Claisen step permits the isolation of an intermediate (c/. Scheme 3) prior to the final Cope rearrangement (195 C) to. y-unsaturated esters (60). The esters (60) are a 55 45 mixture of ( )- and (Z)-double bond isomers owing to the near equal steric bulk of the methyl and acetic acid residues in the transition state for the Cope rearrangement. ... 

SnCLrinduced cyclizations between alkenes and enol acetates result in cycloalkanes or bicycloalkanes in high yield (Eq. 59). It is interesting to note that the MesSiOTf-catalyzed reaction can yield fused rather than bicyelo products. Alkenic carboxylic esters, allylic alcohols, sulfones, and sulfonate esters are also cyclized in the presence of SnCU alkenic oxiranes, however, often cyclize in poor yield [89a]. 

Reduction of enol ethers or enol esters of 1,3-diketones, followed by acid-catalyzed allylic rearrangement of the reduction product (see p. 85 in ref. 5) is a useful route to a,P-unsaturated ketones. Ali-phatic - and alicyclic enones have thus been prepared in good yields at low temperatures with NaAlH2(0CH2CH20Me)2.2 6... 

Scheme 9 demonstrates the further synthetic application of the thus obtained N,0-acetals. Substitution of the alkoxy or acyloxy group by nucleophiles like enol ethers, enol esters, enamines, other electron-rich olefins, CH-acidic compounds, electron-rich aromatics, isocyanides, trimethylsilyl cyanide, organometallics, vinyl and allyl silanes, hydroxy functions, or trialkylphosphites either catalyzed by Lewis acids or proton acids leads to the product of the amidoalkylation reaction (path a). In the presence of stereocenters as control elements, diasteroselective amidoalkylation reactions can be performed as shown in a large number of examples. On the other side, as Nyberg showed for the first time [196], elimination with formation of enecarbamates [208] and enamides [196,208,209] followed by reaction with electrophiles or nucleophiles (path b) also is possible. 

Keto stannylenolates can be prepared by the reaction of Sn-O or Sn-N bonded compounds with diketene, which can be regarded as a cyclic enol ester. The adducts formed from bis(tributyltin) oxide can undergo further reaction, with subsequent decarboxylation, to give the same products as those from the simple enolates. Alkylation with alkyl iodides or benzyl or allyl bromides is strongly catalysed by lithium bromide (e.g. Scheme 14-5). Double alkylation can be achieved with HMPA as solvent.120 The product of alkylation before the final hydrolysis is itself a tin enolate, which can be used in reactions with further carbon electrophiles. 

Claisen rearrangement of allyl esters. Ireland and Mueller6 report that lithium enolates of allyl esters rearrange rapidly at room temperature or slightly above to the corresponding y,<5-unsaturated acids. Thus the allyl ester (1) is converted into the lithium enolate (2) by treatment with lithium isopropylcyclohexylamide in THF at —78°. The solution of (2) is then allowed to warm to 25° for 10 min. The y,S-unsaturated acid (3)... 

In this reaction, the nucleophile (an enolate ion) comes from the enol ester part of the system, which is connected to the allyl group by a carbonate ester. Scheme 12.11 shows the steps in the catalytic process. 

The preparation of 1,1-difluorocyclopropanes from enol ethers, enol esters,esters of allylic alcohols,and esters of 2-phenylpropenoic acid, have also been reported. Selected examples are given in Table 4. 

Halocarbene adducts 1 of enol esters rearrange readily to form (a-halo)-a,/8-unsaturated ketones 2 under all reaction conditions that remove the acyl group via intermediate 2-halocyclo-propanols. Contrary to cationic cyclopropyl to allyl rearrangements, fluoride can be extruded in these rearrangements. 

Hydrogenolysis of esters to acids can occur provided the R —O bond is weakened, for instance when R is a vinyl, allyl, or aryl group [equation (d)]. Thus, hydrogenolysis of an enol ester is a synthetic method for facile removal of carbonyl oxygens that applies to enol trifluoromethanesulfonates [equation (e)] ... 

Corey and Lee [696] have recently proposed a variant of the Ireland-Claisen rearrangement that uses boron enolates of allylic esters derived from 2.62. The E-crotyi (Re = Me) or E-cinnamyl (Rz = Ph) derivatives could be selectively transformed into the Z- or E-boron enolates 10.46 at low temperature (Figure 10.16). The rearrangements take place at about 0°C, and the Z-enolates lead vay selectively to anti acids 10.47 with an excellent enantiomeric excess while the E-enolates lead to syn acids 10.48, with an interesting selectivity if R = Me or Et (Figure 10.16). In most cases, the enantiomeric excesses are excellent however, when the reaction is conducted with ally] esters (R = H), the ee s are a little bit lower (74 - 84%). These results are interpreted via a chair transition state that minimizes steric interactions [696],... 

In work [153-155] taken to support the irreversible formation of a perepoxide, the reactivity of 02(1Ag) towards a series of enol esters has been examined. The key finding was that in certain cases, along with allylic hydroperoxide and dioxetane, a product is formed in which the acyl function had undergone migration for instance the enol ester 37 reacts according to Eq. (56). 

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