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 a-fluoro and a-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 a-oxygenated aldehydes. 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 alpha-heteroatom-substituted alclehydes. Additional support for the model was obtained from density functional calculations.