A new and rapid access towards exo-methylene-δ-valerolactones from (cyclopropyl)methylstannanes

Lewis-acid catalysed ring-opening of functionalised (cyclopropyl)methylstannane 10, in the presence of aldehydes or ketones, was found to afford the corresponding aldol adducts in high yields. Subsequent lactonisation under acid catalysis led quantitatively to substituted exo-methylene-δ-valerolactones, some of them possessing a unique spirocyclic structure.