A chiral silyl ether as auxiliary for the asymmetric nucleophilic addition to α- and β-silyloxy carbonyl compounds

Chiral silicon groups, attached as protective group in proximity to a prostereogenic functionality by means of an ether linkage, can act, at least in specific cases, efficiently as stereochemical directors. The addition of Grignard reagents to α- and β-silyloxy carbonyl compounds (silyloxy is the stereogenic (Me3C)(BnOCH2)MeSiO-group) afforded the respective products with stereofacial selectivities of up to 92%. The source of the selectivities is discussed and their dependence upon structural parameters. The potential of the described principle might be increased by the structural optimization of the auxiliary group, which has not been performed yet.