Mechanistic Investigations on the Highly Stereoselective Allylation of Aldehydes with a Norpseudoephedrine Derivative

Abstract

Reaction of aliphatic aldehydes 1 and allylsilane 3 at −78 °C in the presence of the norpseudoephedrine derivative 2a and catalytic amounts of trifluoromethanesulfonic acid trimethylsilyl ester gives the homoallylic ethers 4 with >98% diastereoselectivity. The ethers 4 can be transformed into the corresponding homoallylic alcohols 5 having an enantiomeric excess >98% using sodium in liquid ammonia. On-line NMR spectroscopy indicates that the mixed acetal 7 and the oxazolidinium ion 12 are intermediates in the formation of 4 . At higher temperature proton transfer occurs to give the oxazolidinium ion 18, which does not react with allylsilane, but gives the oxazolidine 17 on aqueous workup. Protonation of 17 with trifluoromethanesulfonic acid leads to 18 but not to the oxazolidinium ion 12 . Thus, an allylation starting from 17 is not possible.