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.
A revised mechanism for chemoselective reduction of esters with borane-dimethyl sulfide complex and catalytic sodium tetrahydroborate directed by adjacent hydroxyl group
The plausible mechanism for the reduction of the ester groups with a strong preference for one located α to the hydroxyl groups of S-malates and R,R-tartrate-based derivatives has been proposed together with some results with regard to its applications to the syntheses of chiral synthons.