Facial-Selective Allylation of Methyl Ketones for the Asymmetric Synthesis of Tertiary Homoallylic Ethers

The stereoselective allylation of methyl ketones is described to give tertiary homoallylic ethers, which can easily be transformed into homoallylic alcohols by a Birch reduction. Reaction of methyl ketones 4 with allylsilane 5 in the presence of the chiral TMS ether 3a and a catalytic amount of trifluoromethanesulfonic acid led to homoallylic ethers 6 in high yield with a selectivity of 9:1 to >20:1. The TMS ether 3a was prepared from inexpensive mandelic acid, which is commercially available in both enantiomeric forms, in four steps.