Enantioselective protonation of prochiral enolates with chiral imides

✍ Scribed by Akira Yanagisawa; Tetsuo Kikuchi; Takeshi Kuribayashi; Hisashi Yamamoto

• Publisher: Elsevier Science
• Year: 1998
• Tongue: French
• Weight: 801 KB
• Volume: 54
• Category: Article
• ISSN: 0040-4020
• DOI: 10.1016/s0040-4020(98)00482-7

✦ Synopsis

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 ehiral imides, various lithium enolates of ct-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 3[; was obtained in high yield with 90% ee when the silyl enoi ether 33 was treated with n-BuLl in the presence of 5 equiv of LiBr in Et20, 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).