Carbonyl-Ene Reaction: An Emerging Tool for Acyclic Stereocontrol

The ene reaction involing a carbonyl compound, an aldehyde in particular, as an enophile should, in principle, constitute a more efficient alternative to the carbonyl addition reaction of allylmetals which has now become one of the most useful methods for acyclic stereocontrol. We describe herein the new efficient methods for acyclic stereocontrol based on the carbonyl-ene reaction promoted or catalyzed by the Lewis acid. The carbonyl-ene reactions are shown to provide high levels of diastereocontrol, diastereofacial control, ole-finic stereocontrol, andJor catalyst-based enantiofacial control, depending heavily on the characteristic feature of the achiral or chiral Lewis acid employed. Thus, the carbonyl-ene methodology is more advantageous than the allylmetal methodology for acyclic stereocontrol in terms of the easy availability of the olefin (ene) and the operational simplicity, along with the high levels of stereoselection.