Catalytic Intramolecular Cyclization of Alkynyl Cyclic Acetals via Chemoselective Activation Leading to a Phenanthrene Core

An intramolecular cyclization reaction of alkynyl cyclic acetals to synthesize various phenanthrene derivatives in the presence of silver triflate (AgOTf ) or boron trifluoride etherate (BF3·OEt2) as catalysts has been developed. By using AgOTf or BF3·OEt2 appropriately depending on the substituents various alkynyl cyclic acetals were converted to the corresponding phenanthrene derivatives. Investigations of reaction mechanisms, X-ray structure of the product, and computational density functional theory (DFT) study suggested that the reaction proceeds via alkyne activation (π-activation) or acetal activation (σ-activation) pathways based on π- or σ-Lewis acidities.