= Solution
Base first gives <phenoxide>. Use its para-carbanion/quinoid resonance contributor: the para <carbon> bearing the tether attacks the terminal chloromethylene <carbon> in an intramolecular substitution, ejecting <chloride>. This forms a five-membered ring at that <carbon> and produces a \b[para-spirocyclohexadienone], with the <phenoxide> <oxygen> becoming a <carbonyl> <oxygen>. The first stage is a <dearomatizing spirocyclization of a phenoxide>.
On acidification, <carbonyl> protonation makes the dienone electrophilic. A tether C–C bond attached to the spiro centre migrates to the adjacent dienone <carbon>, with the accompanying pi-bond shifts; loss of a proton restores aromaticity. This <dienone-phenol rearrangement> converts the spiro five-membered ring into the fused six-membered ring while regenerating the phenolic OH. The apparent bond formation meta to OH is thus explained by \b[para spirocyclization followed by skeletal migration], rather than requiring the final fused connectivity to form directly in the base step.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-spiro-dienone.png]
{title=Dearomatizing spirocyclization followed by a dienone-phenol skeletal migration}
{description=Original numbered connectivity scheme. The highlighted tether bond changes attachment from carbon 4 to carbon 3; proton transfers and bond-order changes complete rearomatization.}
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