Efficient asymmetric protonation of enolates with readily accessible chiral α-sulfinyl alcohols

Recent Studies on the Regioselective C-Protonation of Enol Derivatives using Carbonyl-Containing Proton Donors

Enolate protonation can occur by two different pathways. With strong acids regioselective protonation generally occurs on the oxygen of the enolate to give an enol and then acid catalysed tautomerisation leads directly to the thermody-namically preferred carbonyl motif. However, with much weaker acids kinetic protonation on carbon can occur to give directly the carbonyl derivative. This review discuss recent developments into the use of chelating acids to promote this stereochemically important C-protonation and we comment on factors that appear to promote this selectivity.

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.

Mechanism of enantioselective protonation of an enolate in the presence of a chiral phenolic additive

The synthesis of enantiomerically enriched (+)-(R)-2-methyl-1-tetralone 1 (up to 29% e.e.) was achieved by enantioselective protonation of the achiral enolate such as 2, using the oxazoline (S)-3 as the source of chirality. Copyright 1999 Wiley-Liss, Inc.