Ag/Brønsted Acid Co-Catalyzed Spiroketalization of β-Alkynyl Ketones toward Spiro[chromane-2,1'-isochromene] Derivatives.

A new Ag/Brønsted acid co-catalyzed spiroketalization of β-alkynyl ketones with para-quinone methides (p-QMs) has been established, enabling multiple C-C and C-O bond-forming events to access densely functionalized spiro[chromane-2,1'-isochromene] derivatives with generally excellent diastereoselectivity and good yields. A reasonable mechanism for forming these 6,6-dibenzannulated spiroketals involving 6-endo-dig oxo-cyclization and 1,6-addition-cyclization cascades is proposed.