Aldol Reactions between L-Erythrulose Derivatives and Chiral α-Amino and α -Fluoro Aldehydes: Competition between Felkin–Anh and Cornforth Transition States

Both matched and mismatched diastereoselection have been observed in aldol reactions of a boron enolate of a protected l-erythrulose derivative with several chiral α-fluoro and α-Amino aldehydes. Strict adherence to the Felkin–Anh model for the respective transition structures does not account satisfactorily for all the observed results, as previously observed in the case of α-oxygenatedaldehydes. In some cases, only the Cornforth model provides a good explanation. The factors that influence this dichotomy are discussed and a general mechanistic model is proposed for aldol reactions with a-heteroatom-substituted aldehydes. Additional support for the model was obtained from density functional calculations.