On the mechanism of Wittig reactions with cyclic anhydrides. II.

Abstract

A study by NMR spectroscopic methods and trapping experiments of the mechanism of Wittig reactions between stabilized phosphoranes and unsymmetrically substituted cyclic anhydrides suggests that two reactions are involved: (1) a low-energy, reversible formation of acyclic adducts; and (2) a higher energy "Wittig olefination" reaction leading to enol-lactones. The latter, more selective, transformation requires a more highly organized transition state in which π-stacking and stabilizing complexations are important factors.