NMR determination of absolute configuration of butenolides of annonaceous type.

We report herein the first determination of the absolute configuration of the annonaceous butenolides by a NMR method. This technique uses a chiral solvating agent (CSA), the so-called Pirkle's reagent, at low temperature and low concentration, allowing one to apply this method to other natural products as well. Indeed, the presence of basic sites (e.g. tetrahydrofuran, hydroxyl) did not interfere with the major solvation of the reagent with the lactone moiety. A new model is proposed which allowed us to confirm the (S) absolute configuration of the butenolide of annonaceous acetogenins. Furthermore this method can be successfully applied to the measure of the diastereomeric (or enantiomeric) excess of the same butenolide containing compounds.