A plausible mechanism was proposed in Scheme 4 by utilizing the formation of 3a as an example. Initially, the BCB 1a coordinates to the BF3 catalyst to afford species A, which undergoes enolization to intermediate B. The subsequent nucleophilic addition of B to the benzofuran-derived oxadiene 2a (which might also be activated by coordination to the BF3 catalyst) leads to the formation of carbocation species C. This key intermediate then undergoes intramolecular cyclization to form the desired adduct 3a along with the regeneration of the BF3 catalyst.