Conformational analysis of MαNP esters, powerful chiral resolution and ¹H NMR anisotropy tools - aromatic geometry and solvent effects on Δδ values

The MαNP acid method is very powerful for the preparation of enantiopure alcohols by resolution and the simultaneous determination of their absolute configurations by the ¹H NMR anisotropy effect, where the syn-syn conformation is taken as the preferred conformation of MαNP esters. However, the syn-syn conformation of MαNP esters looks unstable, because two electronegative oxygen atoms {CH₃O and C=O} are close to each other. To solve the problem of why the MαNP esters take such a syn-syn conformation, the aromatic geometry and solvent effects on the ¹H NMR anisotropy data were studied, leading to the following conclusions: i) the hydrogen-bonding-like interaction among the H-8′ of the naphthyl group, the ester carbonyl oxygen, and the methoxy oxygen supports a triangular intramolecular force to stabilize the syn-syn conformation; ii) triangular hydrogen bonding among a hydrogen atom of protic solvents, the ester carbonyl oxygen, and the methoxy oxygen also supports the syn-syn conformation. This hydrogen bonding, as the solvation effect implies, suggests that a similar hydrogen bonding between a MαNP ester and a hydroxy group of the silica gel surface would make a dominant contribution to the excellent Discrimination of diastereomeric MαNP esters observed in the HPLC on silica gel.