Mechanism of Wittig reaction with cyclic anhydrides

The reactions of stabilized phosphoranes with cyclic anhydrides give enol-lactones as final products. The initial condensation, however, leads to the formation of acyclic adducts that are observable by NMR, can be easily trapped, and, in some cases, can be isolated. A study of the mechanism of these condensations by NMR spectroscopic methods and by various trapping experiments is described and the reaction pathway is proposed.