Enantioselective Aldol and Michael Additions of Achiral Enolates in the Presence of Chiral Lithium Amides and Amines

Abstract

It is now well established that lithium enolates and analogous derivatives generally exist as complex structures held together by noncovalent bonds ("supramolecules"). In particular, Li enolates aggregate to give dimers, tetramers, and higher oligomers, whose metal centers may be complexed by solvent molecules or chelating ligands. In addition, the anionoid part of the enolates may hydrogen-bond to weak acids such as secondary amines. Furthermore, such supramolecules can be product-forming species in synthetic reactions of Li enolates. This paper describes our observations of the temporary incorporation of chiral amines or chiral lithium amides into achiral lithium enolate aggregates (an interaction which is simply broken during aqueous workup!) to give enantiomerically enriched products. In particular, enantioselective aldol and Michael additions between achiral enolates and achiral aldehydes or achiral nitroolefins have been achieved in the presence of several chiral amines (or their lithium amides) derived from (S)-valine or (R,R)-tartaric acid. Finally, this report demonstrates the potential usefulness in asymmetric synthesis of ortho lithiation directed by chiral α-aminoalkoxides.