Enantioselective Access to Tetrahydroxanthones via Copper-bis(oxazoline)-Catalyzed [4 + 2] Cycloaddition.

Highly enantioselective access to tetrahydroxanthone compounds was achieved through copper-bis(oxazoline)-catalyzed [4 + 2] cycloaddition of chrom-4-one dienophiles and Danishefsky's diene. Oxo-dihydroxanthone (enone) adducts, containing a quaternary stereocenter, are generated in up to 98% yield and 89% ee. Cycloadducts are utilized in the synthesis of tetrahydroxanthones, featuring a novel organotin-mediated quasi-Krapcho decarboxylation of β-keto esters, with retention of stereochemistry. Tetrahydroxanthone is a versatile intermediate to a diverse array of biologically relevant, saturated xanthones.