[2 + 2] Cycloaddition of cyclic vinyl phosphonates with ketenes †

Highly substituted cyclic β-alkoxyvinyl phosphonates underwent thermal [2+2] cycloaddition with activated ketenes to afford bicyclic phosphonates. Fragmentation of the central polarized bond of this bicyclic system occurred readily upon treatment with zinc in acetic acid, to give rise to ring expanded products. This method provides access to cycloheptane-1,3-diones substituted at the 4-position with a phosphonate group.