Determination of the absolute configuration of 6-alkylated alpha-pyrones from Ravensara crassifolia by LC-NMR

The absolute configuration of asymmetric centres of two α‐pyrones isolated from Ravensara crassifolia was determined using the Mosher method. The conventional analysis of the purified ester derivatives by 1H‐NMR was replaced by a rapid and sensitive method in which the α‐pyrones were analysed under isocratic reversed‐phase LC‐NMR conditions prior to and after derivatisation reactions. Comparison of the LC‐1H‐NMR spectra of the actual α‐pyrones with those of the corresponding Mosher's esters recorded in the acetonitrile:deuterated water solvent system exhibited shifts comparable with those obtained using conventional deuterated solvents. Based on the shifts recorded, determination of the absolute configuration was possible by application of Mosher rules. The use of LC‐NMR has permitted a direct analysis of crude reaction mixtures containing less than 50 µg of the starting material. Completion of the reaction was checked by LC‐MS and the crude reaction mixture was analysed by stop‐flow LC‐NMR. This methodology seems very promising for the determination at the micro‐scale level of the absolute configuration of natural products which are available only in very small amounts. Copyright © 2003 John Wiley & Sons, Ltd.