Enantio-differentiating reactions of a racemic γ-lactone enolate with chiral esters. A DFT investigation

摘要

The acylation of lithium (±)-spiro-g-lactone enolate 5 by the O-protected methyl (-)-(S)-lactate or the O-protected methyl (+)-(S)-mandelate occurs through enantio-differentiating reactions. The (S,S)-enolate 5 is the most reactive with the lactate whereas the (R,R)-enolate 5 selectively reacts with the mandelate. According to theoretical calculations at the B3LYP/6-31G(+ +)(d,p) level of theory of 40 intermediates of this Claisen condensation, the experimental results are compatible with a previous chelation of the ester by an auxiliary cation lithium arising from the medium. The addition reaction occurs through a chelation process mediated by the counterion of the enolate. More stable tetrahedral intermediates including two lithium cations result from an antiperiplanar transition state. These results clearly demonstrate that the presence of a second lithium cation (the first lithium cation is solvated by di-isopropylamine and the second one is solvated by a THF molecule or a di-isopropylamide anion) stabilizes the tetrahedral intermediate and is compatible with an antiperiplanar transition state according to the Felkin-Anh model.