Enolate anions rearrangement

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Enol formation provides the necessary 1,5-diene for the Cope-like rearrangement. If one provides an enolate anion, the ionic nature of the reaction provides the expected acceleration of rate. 

A novel base-catalyzed Fries-like rearrangement of a 3-acetoxythiophene has been reported (Scheme 149) (79JOC3292). This has been explained as the result of iodide-induced deacylation and subsequent C-acylation of the intermediate enolate anion. Some transformations of 4-hydroxy-2-methylthiophene-3-carboxylic ester are shown in Scheme 150 (69KGS567,75KGS914,71KGS759). 

Enolate anions 54, which can be generated from peri-acyl-oxy-substituted naphthyl benzyl ketones 52 or from their enolacylates 53, undergo rearrangement into 5-methoxynaphtholactone 48 or 2-benzyl-... 

Alkynolate anions.2 Treatment of the enolate anion 1 of a-chloroacetophenone with t-butyllithium (1.5-2.1 equiv.)3 in THF involves deprotonation to the dianion a followed by loss of Cl" and migration of the group on carbon to give the alkynolate anion b. The rearrangement was established by use of 3C. The anion b reacts with benzaldehyde to form the /Mactone c, which can be isolated as such or as the product 2 of hydrolysis. Quenching b with methanol affords the ester C6H5CH2COOCH3 (57% yield). 

Similar a,a -annulations were achieved from reaction of oc,/ -unsaturated acid chlorides with cyclic ketone enamines which afforded bicyclo[3.3.1]nonane-2,9-diones41,60 65 or bicyclo[4.3.1]decanones66. In this reaction, 7V-acylation of the enamine 109 occurs as the first step giving 110, followed by a [3,3] sigmatropic rearrangement to a ketene intermediate (111). Ketene 111 subsequently cyclized via 112 to a bicyclic immonium salt (113) which after hydrolysis gave the corresponding dione 11463. If there is an axially oriented electrophilic substituent at C-4 of the enamine (for example, R = COPh) the enolate anion 115 may cyclize to an adamantane derivative 116 (equation 20). 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

CLAISEN IRELAND Rearrangment Rearrangement ol allyl phenyl ethers to o (or p-)allylphenols or of allyl vinyl ethers to 7,5-unsaturated aldehydes or ketones (Claisen) Rearrangement of allyl esters as enolate anions to Y. unsaturated acids (Ireland)... 

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