Enantioselective Oxazaborolidinium‐Catalyzed Diels–Alder Reactions without CH⋅⋅⋅⋅O Hydrogen Bonding

All cis-tems go! The first detailed computational investigations into the title reaction validate Corey's two pre-transition-state models and reveal a third Lewis acid coordination mode (see picture), which operates for esters having s-cis CCCO groups. The new pre-transition-state model explains the unexpected enantioselectivity witnessed for several Diels–Alder reactions and does not involve a CH⋅⋅⋅O hydrogen bond.