ON THE MECHANISM OF WITTIG REACTIONS WITH CYCLIC ANHYDRIDES .2.

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 pi-stacking and stabilizing complexations are important factors.