On treatment with MgBr2 the β-spirolactones 7 and 10 undergo ringenlargement to the γ-lactones 8 and 11, whereas the β-spirolactones 13 show diastereoselective β-elimination to form the β,γ-unsaturated acids 14. The Lewis acid influence on the migratory aptitude of Me versus H is studied for β-lactone 18.