Tetrahedron p. 10253 - 10264 (1998)
Update date:2022-09-26
Topics:
Yanagisawa, Akira
Kikuchi, Tetsuo
Kuribayashi, Takeshi
Yamamoto, Hisashi
New chiral proton sources possessing an asymmetric 2-oxazoline ring, (S,S)-imide 1 and related imides, were synthesized from Kemp's triacid and optically active 2-amino alcohols. With these chiral imides, various lithium enolates of α-monoalkylated cycloalkanones were effectively protonated with excellent to moderate enantioselectivity. An increase in enantioselectivity was observed in the asymmetric protonation of prochiral enolates with (S,S)- imide 1 using lithium salt as an additive. For example, (R)-enriched 2-n- pentylcyclopentanone 35 was obtained in high yield with 90% ee when the silyl enol ether 33 was treated with n-BuLi in the presence of 5 equiv of LiBr in Et2O, and the resulting lithium enolate 34 was then protonated by a solution of (S,S)-imide 1 in THF. In contrast, the product 35 obtained without LiBr exhibited a lower enantiomeric excess (74% ee).
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