Zinc enolates structure

The method involves a regioselective, trans-diastereoselective, and enantioselective three-component coupling, as shown in Scheme 7.26. In this case, the zinc enolate resulting from the 1,4-addition is trapped in a palladium-catalyzed allyla-tion [64] to afford trans-2,3-disubstituted cyclohexanone 96. Subsequent palladium-catalyzed Wacker oxidation [82] yields the methylketone 97, which in the presence of t-BuOK undergoes an aldol cyclization. This catalytic sequence provides the 5,6-(98) and 5,7- (99) annulated structures with ees of 96%. 

For the use of ZnEt2 as a base and structural studies of the resulting sulfoximine-containing zinc enolates, see C. Bolm,). Muller, M. Zehnder, M. A. Neuburger, Chem. Eur. J. 1995, 1, 312. 

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. 

FIGURE 85. Schematic structures of zinc enolates stabilized by intramolecular nitrogen coordination... 

On the other hand, the O-metallated structure E is also observed, for example when a zinc enolate is prepared by transmetallation of a lithium enolate with a Zn(II) salt5. 

An efficient asymmetric synthesis of / -lactams, reported by van Koten and coworkers128, was based on a-aminoesters as chiral auxiliaries and on the preformed imine-ZnCl2 complex (e.g. structure 72 in equation 45), which was then added to the glycine zinc enolate 73. The presence of ZnCl2 as Lewis acid was essential to ensure complete lactamization. 

Studies of the reactions of chromenes with carbene or carbenoid structures have been described, including zinc enolates of dibromoalkanones <2005RJ0527>. Ring-opening reactions of naphthopyrans in the presence of ethyl-diazoacetate and Rh(OAc)4 have also been recorded (Equation 10) <2004TL6151>. 

Interest in zinc enolates , compared to alkaline or alkaline earth enolates, is growing due to their ideal combination of reactivity and tolerance of many functional groups. The form and structural motifs of complexes depend on the type of metal, solvents and bulkiness of the ligands. Generally, enolates of electropositive metals prefer O-coordination however, even C-coordinated metal enolates usually rearrange to O-bonded ones. 

Chelated structures analogous to (19) and (20) were first proposed by House and coworkers to explain the increased anti selectivity observed for lithium ketone enolates following addition of ZnCh (equation 30). Heathcock and coworkers determined the rate of equilibration as well as the equilibrium composition for a number of aldolates derived from benzaldehyde and zinc ketone enolates (equation 31). Again, the preference for anti aldolates is in accord with zinc-chelated structures. 

Goel, S. C., Chang, M. Y., and Buhro, W. E., Preparation of soluble and volatile zinc dialkoxides. X-ray crystal structures of an (amido)zinc alkoxide and a ho-moleptic zinc enolate Inorg. Chem., 29, 4646 (1990). 

Compound 136 can be transformed to the 3,4-trans isomer 138, which has the same configuration as ( + ) thienamycin, according to the procedure described by Chiba and coworkers in related structures [74]. The method involved oxidation of the hydroxyl group in 136 with activated manganese dioxide with concomitant isomerization at C-3, followed by K-selectride reduction of the resulting methyl ketone 137. Zinc enolates of iV,iV-disubstituted a-aminoacid esters have also been recently utilized for the synthesis of 3-amino P-lactams [75]. 

NATURE OF THE REFORMATSKY REAGENT /. 82.1 Isolation and Stability of Zinc Enolates 1.822 Structure of Zinc Enolates... 

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