STEREOCHEMICAL CONTROL IN RADICAL REACTIONS BY COMPLEXATION WITH LEWIS ACIDS

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Stereoselectivity in the radical-mediated allylation of alpha-bromo-beta-silyloxy esters yielding alpha-allyl-beta-silyloxy esters was remarkably affected when the reaction was conducted in the presence of Ln(fod)_3. In the allylation of alpha-bromo-beta-silyloxysuccinate esters affording preferentially syn diastereomers through the chelated trasition states was required stoichiometric amount of the Lewis acid [Eu(fod)_3 or La(fod)_3] to maximize the stereoselectivities, whereas in the reaction of alpha-bromo-beta-silyloxybutanoateesters and alpha-bromo-beta-silyloxy-beta-phenylpropanoate ester the effect induced by the coordination of Eu(fod)_3 to the ester group was catalytic.Diastereoselectivity in the radical mediated allylation and deuteration reactions of beta-chiral beta-alkoxy-alpha-bromoaldehyde dimethyl acetals and a related alkoxy bromide were remarkably enhanced when the reactions were conducted in the presence of Lewis acid (MgBr_2-OEt_2 for allylation ; MgI_2 for deuteration).