Abstract
Trimethylsilyl ketene acetals derived from 8-phenylmenthyl phenyl-, p-fluorophenyl, and p-(trifluoromethyl)phenylacetates, methyl phenylacetate, and isopropyl phenylacetate have been studied. It was shown that it is not possible, under kinetic conditions, to form exclusively the desired E-isomer and that the thermodynamic Z-isomer is also obtained as the major product under reaction conditions which can be considered kinetic. The results could be rationalized using the Ireland's and Dauben's models on the basis of the changes in the acidity of the proton to be abstracted and the strength of the base. Dimethyl-tert-butylsilyl ketene acetals have also been studied. It appeared that reaction of TBDMSCl onto the Li-enolates favored the E-isomer. We proposed that, at the temperature (above -20°C) at which the reaction occurs, the E-Z equilibrium between the Li-enolates starts to play a role, and thereof the silyl ketene acetals E/Z ratios are kinetically controlled but by the second step (reaction of the reagent onto the Li-enolate)
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