Facial-selective allylation of methyl ketones for the asymmetric synthesisof alpha-substituted tertiary homoallylic ethers

The asymmetric synthesis of enantiomerically pure alpha -substituted tertia ry homoallylic ethers 4a, 11 and 12a-c by the allylation of ethyl methyl ke tone (1a) with gamma -substituted allylsilanes 9a-h is described. The allyl silanes were obtained by a nickel-catalysed Grignard cross-coupling reactio n of (E)- and (Z)-(3-iodoallyl)trimethylsilane with various Grignard reagen ts. The reaction of the allylsilanes with 1a in the presence of the trimeth ylsilyl ether of N-trifluoroacetylnorpseudoephedrine (3), and catalytic amo unts of a mixture of trimethylsilyl triflate and trifluoromethanesulfonic a cid led to the homoallylic ethers 4a, 11 and 12a-c with two new stereogenic centres, with a selectivity of 1:9 to >20:1 for the homoallylic and of 1:9 9 to >60:1 for the allylic centre. The facial selectivity does not depend o n the configuration of the allylsilane, and in all reactions the anti produ ct is preferentially formed. Interestingly a pronounced switch of facial se lectivity takes place with increasing length of the alkyl group of the ally lsilane.