Asymmetric protonation of ketone enolates using chiral beta-hydroxyethers: acidity-tuned enantioselectivity |
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Authors: | Kim B Moon Kim Hyunwoo Kim Woosung Im Keun Young Park Jin Kyoon |
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Affiliation: | School of Chemistry, Seoul National University, Seoul 151-747, South Korea. kimbm@snu.ac.kr |
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Abstract: | New chiral hydroxyethers 1a-f were prepared for asymmetric protonation of achiral enolates prepared from prochiral ketones. The enantioselectivity of protonation was highly dependent upon the acidity of the chiral alcohols, the highest enantioselectivity (90% ee) being achieved with 3,5-dichloro-substituted beta-hydroxyether 1c. A salt-free enolate generated from trimethylsilyl enol ether 4 provided product of the highest ee. Unlike other reagents, chloro-substituted alcohols provided almost consistent enantioselections throughout the reaction temperatures examined (-25 to -98 degrees C). Protonation of other aromatic ketones showed selectivity similar to that of 2-methyl-1-tetralone. |
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