1,2-Asymmetric induction in radical-mediated allylation of diethyl (2S,3S)-3-bromo-2-oxysuccinates: efficient stereoselectivity enhancement by complexation with Eu(fod)3
Stereocontrol in radical-mediated allylation of acyclic α-bromo-β-siloxy esters by complexation with lanthanide shift reagents Ln(fod)3
摘要:
Stereoselectivity in the radical-mediated allylation of alpha-bromo-beta-siloxy esters 2 yielding alpha-allyl-beta-siloxy esters 3 (syn) and 4 (anti) was remarkably affected when the reaction was conducted in the presence of Ln(fod)(3) [= tris-(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionato)lanthanide]. In the allylation of alpha-bromo-beta-siloxysuccinate esters 2c and 2d affording preferentially syn-diastereoisomers 3c and 3d through chelated transition states a stoichiometric amount of the Lewis acid [Eu(fod)(3) or La(fod)(3)] was required in order to maximize the stereoselectivities, whereas in the reaction of alpha-bromo-beta-siloxybutanoate esters 2g and 2h and alpha-bromo-beta-siloxy-beta-phenylpropanoate ester 2i the effect induced by the coordination of Eu(fod)(3) to the ester group was catalytic.