Indium-Catalyzed Synthesis of Benzannulated Spiroketals by Intramolecular Double Hydroalkoxylation of Ortho-(Hydroxyalkynyl)Benzyl Alcohols

[Abstract] The novel indium-catalyzed synthesis of benzannulated spiroketals by a double intramolecular hydroalkoxylation reaction of o-(hydroxyalkynyl)benzyl alcohols is reported. The reaction proceeds under mild reaction conditions using a low catalyst loading of indium triiodide (2–5 mol%) with a variety of benzyl and homobenzyl alcohols, allowing the regioselective synthesis of a set of naturally occurring and new benzannulated spiroketals (5,5-, 5,6-, 5,7-, 6,6- and 6,7-) in good yields. The synthetic transformation involves the indium(III)-catalyzed electrophilic alkyne activation followed by the regioselective intramolecular hydroalkoxylation reaction. The method is the first example of spiroketal synthesis using main group catalysis representing an economical alternative to precious transition metals. In addition, it provides a complementary strategy for the regioselective synthesis of spiroketals and spiroaminals.