title: Facial-selective allylation of methyl ketones for the asymmetric synthesis of alpha-substituted tertiary homoallylic ethers.
abstract: The asymmetric synthesis of enantiomerically pure a-substituted tertiary homoallylic ethers 4a, 11 and 12a-c by the allylation of ethyl methyl ketone (la) with gamma-substituted allylsilanes 9a-h is described. The allylsilanes were obtained by a nickel-catalysed Grignard cross-coupling reaction of (E)- and (Z)-(3-iodoallyl)trimethylsilane with various Grignard reagents. The reaction of the allylsilanes with la in the presence of the trimethylsilyl ether of N-trifluoroacetylnorpseudoephedrine (3), and catalytic amounts of a mixture of trimethylsilyl triflate and trifluoromethanesulfonic acid 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 1:99 to >60:1 for the allylic centre. The facial selectivity does not depend on the configuration of the allylsilane, and in all reactions the anti product is preferentially formed. Interestingly, a pronounced switch of facial selectivity takes place with increasing length of the alkyl group of the allylsilane.
authors.affiliation: Institut für Organische Chemie der Georg-August-Universität Göttingen, Germany. ltietze@gwdg.de
authors.forename: L F
authors.initials: LF
authors.lastname: Tietze
authors.forename: L
authors.initials: L
authors.lastname: Völkel
authors.forename: C
authors.initials: C
authors.lastname: Wulff
authors.forename: B
authors.initials: B
authors.lastname: Weigand
authors.forename: C
authors.initials: C
authors.lastname: Bittner
authors.forename: P
authors.initials: P
authors.lastname: McGrath
authors.forename: K
authors.initials: K
authors.lastname: Johnson
authors.forename: M
authors.initials: M
authors.lastname: Schäfer