Change of Conformation of MTPA Derivatives by Chelation with La (III) Ion and Application to Determination of Absolute Configuration

Abstract

The modified Mosher's method has often been employed to determine the absolute configurations of chiral substrates such as secondary alcohols and primary amines. In this method, the absolute configuration is assigned using the correlation model. The validity of the correlation model can be judged by examining the chemical shift differences between diastereomeric MTPA derivatives for as many protons as possible. Therefore, if a substrate has only few protons, the validity cannot be verified. We here report that the reversal of the relative position of ¹H NMR signals observed for the diastereomers upon addition of La(hfaa)₃ can be used for verification of the validity of the correlation model. This verification extends the scope of the determination of absolute configurations using the Mosher's method to substrates having only few probes.