Enones tert-butoxide

Reaction of that with potassium tert-butoxide affords the corresponding carbanion this is thought to first add to the enone in (5-3). The anion from the reaction with a second equivalent of base then adds to the enone function to form the spiw ring. The fact that the product from this reaction has the same relative stereochemistry as the natural product is attributed to the better overlap of the enolate with the triple bond in the transition state leading to that isomer. The product from the reaction is thus + griseo-fulvin (5-6) [5]. 

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. 

A mild base such as diisopropylethylamine or DBU can be used for the Homer-Wads worth-Emmons reaction in the presence of a complexing agent such as lithium chloride. These conditions are useful for sensitive substrates, such as chiral compounds with an enolizable stereocentre. For example, the enone 66 is formed in high optical purity under these conditions (2.81), whereas use of the base potassium tert-butoxide gives the product as a racemic mixture. 

Isomerizations of Unsaturated Compounds. r-BuOK is an effective base for bringing about migrations of double bonds in alkenes and alkynes via carbanion intermediates, but since the base promotes these reactions most effectively in DMSO, they will be described in more detail under Potassium tert-Butoxide-Dimethyl Sulfoxide. Important exanples of enone deconjugations with i-BuOK/f-BuOH which proceed via di- and trieno-late intermediates are shown in eqs 30 and 31. Potassium i-pentoxide is effective in promoting the latter reaction, but various lithium amide bases are not, apparently because they de-protonate the enone at the a -position regioselectively. The isomerization of Q ,/8-unsaturated imines to alkenyl imines (eq 32) is an important step in an alternative method for reduction-alkylation of a ,/8-unsaturated ketones. ... 

The first example of a catalytic asymmetric Michael addition of a Horner-Wadsworth-Emmons reagent to enones was developed by the Shibasaki group using (J )-ALB ent-18 as the catalyst (Scheme 19.28). When sodium tert-butoxide was used as a base instead of butyllithium, the competitive 1,2-addition could be suppressed and the yield of 1,4-addition product increased. 

Potassium tert-butoxide Cyclopent-3-enone ring from a-cyanoketones Johnson-Stobbe condensation... 

Miller disclosed the first enantioselective intramolecular Rauhut-Currier reaction (also called vinylogous MBH reaction). A stoichiometric amount of protected cysteine derivative 84 and potassium tert-butoxide (1.5 eq.) were employed, and various bis-enones 85 were converted into cyclohexene products 86 in good yields and with high enantioselectivity (Scheme 36.22). Using a lower loading of 84 leads to a reduced yield but nearly identical ee [28a]. 

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