title: Enantioselective Lewis acid-catalyzed Mukaiyama-Michael reactions of acyclic enones. Catalysis by allo-threonine-derived oxazaborolidinones.
abstract: allo-Threonine-derived O-aroyl-B-phenyl-N-tosyl-1,3,2-oxazaborolidin-5-ones 1g,n catalyze the asymmetric Mukaiyama-Michael reaction of acyclic enones with a trimethylsilyl ketene S,O-acetal in high enantioselectivity. A range of alkenyl methyl ketones is successfully employed as Michael acceptors affording ee values of 85-90% by using 10 mol % of the catalyst. The use of 2,6-diisopropylphenol and tert-butyl methyl ether as additives is found to be essential to achieve high enantioselectivity in these reactions. The effects of the additives are discussed in terms of the retardation of an Si(+)-catalyzed racemic pathway, which seriously deteriorates the enantioselectivity of asymmetric Mukaiyama-Michael reactions. A working model for asymmetric induction is proposed based on correlation between catalyst structures and enantioselectivities.
authors.affiliation: Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan.
authors.forename: Xiaowei
authors.initials: X
authors.lastname: Wang
authors.forename: Shinya
authors.initials: S
authors.lastname: Adachi
authors.forename: Hiroyoshi
authors.initials: H
authors.lastname: Iwai
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Takatsuki
authors.forename: Katsuhiro
authors.initials: K
authors.lastname: Fujita
authors.forename: Mikako
authors.initials: M
authors.lastname: Kubo
authors.forename: Akira
authors.initials: A
authors.lastname: Oku
authors.forename: Toshiro
authors.initials: T
authors.lastname: Harada