Enol acetates Enolates

Coupling of two electron-rich components like alkenes, enol acetates, enol ethers, carbanions, or carboxylates ... 

Monodentate phosphoramidites, in particular (9) and its octahydro analogue, are found to be excellent ligands for the rhodium-catalysed asymmetric hydrogenation of aromatic enol acetates, enol carbamates, and 2-dienol carbamates with up to 98%... 

Several procedures have been developed for the preparation of a-arylselenocarbonyl compounds including phenylselenenylation of enol acetates, enol ethers and lithium and copper enolates. Oxi-... 

Enamines, Enol Acetates, Enol Ethers, and Vinyl Halides.—Diphenylcyclopropene-thione reacts with the morpholine enamines of cyclo-octanone and cyclododecanone to give adducts (337), which gave the strained cyclopropenes (338) when treated with dimethyl acetylenedicarboxylate. Other reactions of (337) have been reported." Whereas reactions of enamines with imidodialkylsulphonium chlorides usually give... 

Electrolysis of enol acetates, enol ethers, and silyl enol ethers in the presence of ammonium halides gives rises to high yields of a-halogenoketones. In alcoholic solvents, a-halogenoacetals are formed. One-half equivalent of... 

It is readily prepared by the action of metallic sodium on dry ethyl acetate. The reaction, which occurs only in the presence of a trace of ethanol, is complex, but may be considered (in effect) as a condensation of two molecules of ethyl acetate under the influence of sodium ethoxide, the sodium derivative of the enol form being thus obtained. Clearly, only a trace of ethanol is thus initially... 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

Lewis acid promoted condensation of silyl ketene acetals (ester enolate equiv.) with aldehydes proceeds via "open" transition state to give anti aldols starting from either E- or Z- enolates. 

Mukaiyarna-Johnson AJdoJ- Lewis acid promoted condensation of silyl enol ethers with acetals ... 

Low molecular mass enol esters (e.g. acetates H.O. House, 1965) or enol ethers (e.g. silyl ethers H.O. House, 1969) of ketones can be synthesized regioselectively and/or separated by distillation. Treatment with lithium alkyls converts them into the corresponding lithi-... 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

The oxidation of the cyclic enol ether 93 in MeOH affords the methyl ester 95 by hydrolysis of the ketene acetal 94 formed initially by regioselective attack of the methoxy group at the anomeric carbon, rather than the a-alkoxy ketone[35]. Similarly, the double bond of the furan part in khellin (96) is converted ino the ester 98 via the ketene acetal 97[l23],... 

The isoflavone 406 is prepared by the indirect a-phenylation of a ketone by reaction of phenylmercury(II) chloride with the enol acetate 405, prepared from 4-chromanone[371]. A simple synthesis of pterocarpin (409) has been achieved based on the oxypalladation of the oriho-mercurated phenol derivative 408 with the cyclic alkene 407[372,373]. 

Tin enolates of ketones can be generated by the reaction of the enol acetate 733 with tributyltin methoxide[60i] and they react with alkenyl halides via transmetallation to give 734. This reaction offers a useful method for the introduction of an aryl or alkenyl group at the o-carbon of ketones[602]. Tin enolates are also generated by the reaction of siiyl enol ethers with tributyltin fluoride and used for coupling with halides[603]. 

It was claimed that the Z-form of the allylic acetate 430 was retained in homoallylic ketone 431 obtained by reaction with the potassium enolate of 3-vinylcyclopentanone (429), after treatment with triethylborane[282]. Usually this is not possible. The reaction of a (Z)-allylic chloride with an alkenylaluminum reagent to give 1,4-dienes proceeds with retention of the stereochemistry to a considerable extent when it is carried out at -70 C[283]. 

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]... 

Preparation of o,/3-Unsaturated Carbonyl Compounds by the Reactions of Silyl Enol Ethers and Enol Acetates with Ally Carbonates... 

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. 

The decarboxylation of allyl /3-keto carboxylates generates 7r-allylpalladium enolates. Aldol condensation and Michael addition are typical reactions for metal enolates. Actually Pd enolates undergo intramolecular aldol condensation and Michael addition. When an aldehyde group is present in the allyl fi-keto ester 738, intramolecular aldol condensation takes place yielding the cyclic aldol 739 as a main product[463]. At the same time, the diketone 740 is formed as a minor product by /3-eIimination. This is Pd-catalyzed aldol condensation under neutral conditions. The reaction proceeds even in the presence of water, showing that the Pd enolate is not decomposed with water. The spiro-aldol 742 is obtained from 741. Allyl acetates with other EWGs such as allyl malonate, cyanoacetate 743, and sulfonylacetate undergo similar aldol-type cycliza-tions[464]. 

The unsaturated c.vo-enol lactone 17 is obtained by the coupling of propargylic acetate with 4-pentynoic acid in the presence of KBr using tri(2-furyl)-phosphine (TFP) as a ligand. The reaction is explained by the oxypalladation of the triple bond of 4-pentynoic acid with the ailenyipailadium and the carbox-ylate as shown by 16, followed by reductive elimination to afford the lactone 17. The ( -alkene bond is formed because the oxypalladation is tnins addition[8]. 

The cyclic enol ether 255 from the functionalized 3-alkynoI 254 was converted into the furans 256 by the reaction of allyl chloride, and 257 by elimination of MeOH[132], The alkynes 258 and 260, which have two hydroxy groups at suitable positions, are converted into the cyclic acetals 259 and 261. Carcogran and frontalin have been prepared by this reaction[124]. 

Owing to the instability of a-halogenoaldehydes it is occasionally preferable to use more stable derivatives, such as enol acetate prepared according to Bedoukian s method (204) and a-bromoacetals (4, 8, 10, 16, 22, 67, 101, 426). An advantage is said to be in the yield however, this appears to be slight. The derivatives react in the same sense as the aldehydes themselves, that is, the acetal group as the more polarized reacts first and enters the C-4 position. It is likely that the condensation and cyclization occur by direct displacement of alkoxide ions. Ethyl-a,/3-dihalogeno ethers (159, 164, 177, 248) have also been used in place of the free aldehydes in condensation with thioamides. 

The transition state involves the carbonyl oxygen of one carboxyl group—the one that stays behind—acting as a proton acceptor toward the hydroxyl group of the carboxyl that IS lost Carbon-carbon bond cleavage leads to the enol form of acetic acid along with a molecule of carbon dioxide... 

The enol intermediate subsequently tautomerizes to acetic acid... 

Step 1 Proton abstraction from the a carbon atom of ethyl acetate to give the corresponding enolate... 

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... 

---