Synthesis of spiro[4.5]cyclohexadienones with an allene motif via a base-promoted intramolecular ipso-Friedel–Crafts addition of phenols to propargyl bromides

We developed a novel synthetic method for allenyl spiro[4.5]cyclohexadienone derivatives based on a base-promoted intramolecular ipso-Friedel–Crafts addition of phenols to propargyl bromides. The present spirocyclization proceeded in a CH2Cl2–tert-BuOH mixed solvent system using potassium tert-butoxide as the base, and produced the corresponding spiro[4.5]cyclohexadienone derivatives with an allene motif in up to 99% yield. This-type allenyl spirocycle was also accessible through Pd-catalyzed intramolecular ipso-Friedel–Crafts alkylation when a propargyl carbonate derivative with a naphthol unit was used as a substrate. Acid-promoted skeletal rearrangement of the reaction adducts was also examined.