Ketones enone formation

Enone Formation from Ketones, and Oxidation of Alcohols... 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

The oldest but still occasionally useful method for enone formation from saturated ketones involves bromination and elimination of HBr from the bromoketone 133. The problem here is that the proton to be removed (Hb in 133) in the elimination is not the most acidic proton in the molecule Ha is much more acidic. 

Preparation of enones from saturated ketones by Pd(II)-promoted dehydrosilyla-tion via silyl enol ethers was reported by Ito. Transmetallation of the silyl enol ether of cyclohexanone 519 with Pd(OAe)2 gives the oxo-TT-allylpalladium complex 520 (Pd enolate), which undergoes -H elimination to afford cyclohexenone. BQ is used as an oxidant of Pd(0) [211], However, the enone formation can be carried out using a catalytic amount of Pd(OAc)2 in DMSO under oxygen without other oxidants at room temperature. Also aldehyde 521 is converted to unsaturated aldehyde 522 via silyl ether in DMSO [4],... 

The enone formation has been applied to a number of natural product syntheses. The enone 524 was prepared from the complex molecule 523 and successfully applied to the total synthesis of pallescensin [212], Even the phenolic OH in 525 was converted to the conjugated ketone. The reaction was utilized as a key step in hypoxyxylerone synthesis [213]. In the total synthesis of galbulimima alkaloid GB 13, Mander converted a cyclohexanone in the complicated molecule 526 to the corresponding cyclohexenone via silyl enol ether in 82% yield [214]. 

H elimination occurs by the treatment of jr-allylpalladium enolates in boiling MeCN to give a.jS-unsaturated ketones and aldehydes. Shibasaki and co-workers used the enone formation in the total synthesis of strychnine [204], The silyl enol ether 555 was treated with diallyl carbonate 556 in MeCN and the cyclohexanone 557 was obtained in high yield. 

Enone Formation. The transformation of saturated carbonyl compounds into their ajS-unsaturated analogues by a process of homogeneous liquid-phase oxidative dehydrogenation has been described use of oxygen or air in the presence of a palladium(ii) catalyst and a co-catalyst, usually copper(ii) salts or quinone, gives good yields based on ketone consumed, though overall conversion is low. 

Protonation of the a-carbanion (50), which is formed both in the reduction of enones and ketol acetates, probably first affords the neutral enol and is followed by its ketonization. Zimmerman has discussed the stereochemistry of the ketonization of enols and has shown that in eertain cases steric factors may lead to kinetically controlled formation of the thermodynamically less stable ketone isomer. Steroidal unsaturated ketones and ketol acetates that could form epimeric products at the a-carbon atom appear to yield the thermodynamically stable isomers. In most of the cases reported, however, equilibration might have occurred during isolation of the products so that definitive conclusions are not possible. 

As first demonstrated by Stork,the metal enolate formed by metal-ammoni reduction of a conjugated enone or a ketol acetate can be alkylated in liquic ammonia. The reductive alkylation reaction is synthetically useful since ii permits alkylation of a ketone at the a-position other than the one at whicf thermodynamically controlled enolate salt formation occurs. Direct methyl-ation of 5a-androstan-17-ol-3-one occurs at C-2 whereas reductive methyl-... 

The formation of an epoxyketone (1) is generally favoured when the expected product of oxidation of an allylic alcohol is a cisoid enone. This type of reaction is promoted by acid conditions and may be prevented by using the chromium trioxide-pyridine reagent which gives only the unsaturated ketone (2) corresponding to the starting alcohol. ... 

Until recently, pyridine-type bases have been commonly used to produce conjugated enones from 2-halo ketones yields are usually poor °° and these reactions are frequently accompanied by rearrangement, reduction and salt formation. Thus, Warnhoff found that dehydrobromination of (28) with 2,4-lutidine gave a mixture of (29), (30) and (31) in the ratio 55 25 20. Collidine gave a ratio of 38 25 37, whereas pyridine gave mainly the salt (32). 

TMM cycloadditions to cyclic and conjugated ketones have also been reported (Scheme 2.22) [31]. The steric nature of the substrate does play a critical role in determining product formation. Thus the cyclic ketone (73) produced 55% yield of the tetrahydrofuran, but no cycloadduct could be obtained from the cyclic ketone (74). The enone (75) gave only carbonyl cycloaddition, whereas enone (76) yielded only olefin adduct. Interestingly, both modes of cycloaddition were observed with the enone (77). The ynone (78) also cycloadds exclusively at the carbonyl function [34]. 

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