New Methods and Reagents in Organic Synthesis 89. Studies on Reaction Conditions and New Entry to Chiral Ligands in the Chiral Lithium Amide-Mediated Enantioselective Aldol Reaction.

Abstract

Reaction conditions for the enantioselective aldol reaction of 2, 2-dimethyl-3-pentanone (3) and benzaldehyde using the chiral lithium amide 1b as a chiral auxiliary were thoroughly investigated. All three procedures, that is, (1) the combined use of lithium diisopropylamide and the chiral lithium amide 1b, (2) the use of an excess of the chiral lithium amide 1b, and (3) the regeneration of the chiral lithium amide 1b, afforded the aldol 4 in about 90% yield and 70% enantiomeric excess (ee). Investigation of the effects of solvent by utilizing 1-naphthaldehyde revealed that in tetrahydrofuran, (S, S)-aldol 5 of 77% ee was obtained as the major product, while in ether (R, R)-5 became the major isomer (38% ee). Furthermore, addition of hexamethylphosphoric triamide caused a dramatic change of stereoselectivity, and (S, S)-5 of 70% ee was obtained in ether with 20 eq of hexamethylphosphoric triamid. The aldol 4 of 74% ee was obtained when the new chiral lithium amide 6b was used.