Divergent Aromatization of α-Halobenzyl γ-Butenolides Initiated by Selective Enol Protonation to Benzo[]fluorenones and Naphthalenes.

An unprecedented divergent aromatization reaction of α-halobenzyl γ-butenolides has been described for the selective and concise synthesis of highly substituted benzo and higher π-extended fluorenones, and 1,3-disubstituted naphthalenes depending on the migration ability of the quaternary α-substituent. This aromatization switch from Ag-mediated planarization to ylidenebutenolides likely originates from selective protonation on the enolic double bond rather than the benzyl halides by TfOH.