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Enantioselective Lewis Acid-Catalyzed Mukaiyama−Michael Reactions of Acyclic Enones. Catalysis by allo-Threonine-Derived Oxazaborolidinones

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journal contribution
posted on 26.12.2003 by Xiaowei Wang, Shinya Adachi, Hiroyoshi Iwai, Hiroshi Takatsuki, Katsuhiro Fujita, Mikako Kubo, Akira Oku, Toshiro Harada
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.

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