An expeditious construction of polycyclic ketals: synthesis of fused seven-membered rings and substituted naphthalenes

Treatment of allylic alcohols 5, easily prepared from dioxene and methoxyacetophenones, with 1-(trimethylsilyloxy)cyclopentene in the presence of a catalytic amount of TMSOTf in acetonitrile, afforded a polycyclic acetal 6 by domino nucleophilic substitution, annulation and intramolecular Friedel–Crafts alkylation reaction sequence. Acid-mediated cleavage of the acetal moiety led to fused seven-membered carbocyclic rings or to substituted naphthalenes.