Zinc enolates isolation

Because zinc enolates are intrinsically reactive species, especially towards carbonyl compounds, the attempted synthesis of such species often results in the formation of selfcondensation products which are often polymeric materials. Only in a few cases could pure organozinc enolates be isolated and structurally characterized. 

The reaction has been further extended into a tandem conjugate addition/ enolate trapping sequence, whereby the in situ generated zinc enolate was trapped with benzaldehyde. This resulted in an approximately 3 7 mixture of trans-erythro trans-threo aldol adducts, isolated in 88% yield. Subsequent oxidation of these products gave a single isomer of the corresponding diketone with 95% ee. 

The subsequent synthetic steps leading to glycinoeclepin A (50) are shown in Figure 2.39. Successful aldol reaction of A with B was achieved by employing the zinc enolate derived from B to give C, which was so unstable that the product could be isolated only after esterification to give D. Treatment of D with sodium hydride yielded E, whose functional group transformation afforded F, the pivotal intermediate for... 

NATURE OF THE REFORMATSKY REAGENT /. 8J2.I Isolation and Stability of Zinc Enolates... 

Metallation of the arylsulfinyl-N-methoxyacetimidate (49), which may be prepared in two steps from commercially available V-hydroxyacetimidate, followed by reaction with aldehydes provides adducts that after sequential desulfurization and hydrolysis may be converted into -hydroxy esters with 280% enantiomeric excess (Scheme 20). Thus, under kinetic conditions the reaction of the anion derived from (49) with aldehydes gives mixtures of the syn and anti products, (50) and (51) respectively, in nearly equal amounts. Under thermodynamic conditions, however, the more stable anti adducts (51) dominate, and after desulfurization and hydrolysis the P-hydroxy esters (53) are obtained in 75-94% enantiomeric excess. When the zinc enolate derived from (49) is condensed with aldehydes, the anti adducts (51) are again the major products and the p-hydroxy esters (53) can be isolated in 76-86% enantiomeric excess. On the other hand, the reaction of the zirconium enolate of (49), which is obtained by the addition of Cp2ZrCl2 to the corresponding lithium enolate, with aldehydes followed by desulfurization gives p-hy-... 

The 1,4-reduction of a,/3-unsaturated aldehydes is best carried out with diphenylsilane in the presence of zinc chloride and tetrakis(triphenylphosphine) palladium436 or a combination of triethylsilane and tris(triphenylphosphine) chlororhodium 437 Other practical approaches use phenylsilane with nickel (0) and triphenylphosphine438 and diphenylsilane with cesium fluoride.83 It is possible to isolate the initial silyl enol ether intermediate from the 1,4-hydrosilylation of o, /3-unsaturated aldehydes (Eq. 264).73,411 The silyl enol ethers are produced as a mixture of E and Z isomers. 

L = JV-acetylpyrazole) has been isolated.467 The ligand is coordinated to the metal through nitrogen and the acetyl oxygen atom, and the acetyl group is found to exist as the enol tautomer. A similar mode of bonding is adopted in the tris(A-carbamoylpyrazole)zinc(II) ion, which has also been isolated as its BF4 salt. 

Carboxylic acids (511) and their chlorides, anhydrides, amides, esters and nitriles, condense with o-aminothiophenols (507) or their zinc salts, affording 2-substituted benzothiazoles (513), the only exception being acetic acid. The intermediate o -acylaminothioph-enols (512) could be isolated and readily cyclized. The activation of carboxylic acids in... 

Halogen substituents in a "-haloketones are easily removed even by zinc dust, because of the greater tendency of the halogen to ionisation. The reduction may be represented by a mechanism like that for acetoxy ketones, the ease of removal of halogens following their usual order of reactivity F < Cl < Br < I. In one case the intermediacy of an enolate anion was revealed by direct isolation of an enol acetate aa-chloro-Sa-cholestan 3 One was dechlorinated with zinc in acetic anhydride to give 3-acetoxycholest-2 ene [ 112]. 

Tetranortriterpenoids.—The structure of sendanin (72), from the bark of a Japanese variety of Melia azedarach, has been confirmed by AT-ray analysis. The novel enol-ether (73) has been isolated from the heartwood of Khaya anthotheca along with 11/3- and lla-acetoxyazadirone (74) and (75). Zinc-copper couple is a very convenient reagent for reduction of epoxides, a,/S-epoxy-lactones, a/S-unsaturated ketones, and a-ketols. The full details of the X-ray analysis of prieurianin have appeared. " ... 

The (chloromethyl)furan (45) gives mainly the butenolide (46) on treatment with aqueous potassium cyanide.A detailed study of the reaction of furfural with primary aromatic amines has shown that 2,4-di(arylamino)cyclopent-2-enones (47) are usually formed intermediate 4,5-di(arylamino)cyclopent-2-en-ones (48) are occasionally isolated.The zinc-chloride-induced rearrangement of the 2-furyl alcohols (49 R = Bu, allyl, or Ph) to the cyclopentenolones (51) is thought to involve the cations (50) as key intermediates. The enolate that is formed by the action of butyl-lithium on 2-benzylfuran has been isolated... 

Takai reported a simple, general, and stereoselective method for the alkylidenation of ester to give Z-enol ethers The titanium carbene complex is easily prepared in situ by the reaction of RCHBr2 with a low-valent titanium species generated by treatment of TiCU with zinc and tetramethylenediamine (TMEDA) in THE. Without isolation, the complex is used for carbonyl alkenation. It was reported that the presence of a small amount of lead in the zinc was crucial to the reaction ... 

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