Asymmetric protonation of ketone enolates using chiral beta-hydroxyethers: acidity-tuned enantioselectivity.
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.
Kim
B Moon
BM
School of Chemistry, Seoul National University, Seoul 151-747, South Korea. kimbm@snu.ac.kr
Kim
Hyunwoo
H
Kim
Woosung
W
Im
Keun Young
KY
Park
Jin Kyoon
JK