Copper Bis(oxazoline)‐Catalyzed Enantioselective Alkynylation of Benzopyrylium Ions

The stereocontrolled construction of biologically relevant chromanones and tetrahydroxanthones has been achieved through the addition of alkynes to benzopyrylium trilfates under the influence of copper bis(oxazoline) catalysis. Excellent levels of enantiocontrol (63–98 % ee) are achieved in the addition of a variety of alkynes to an array of chromenones with a hydrogen in the 2‐position. Promising levels of enantiocontrol (54–67 % ee) are achieved in the alkynylation of chromenones with esters in the 2‐position, generating tertiary ether stereocenters resembling those frequently found in naturally occurring metabolites. Highly enantioenriched chromanones are prepared by the copper bis(oxazoline)‐catalyzed addition of terminal alkynes to benzopyrylium ions, reactive heterocycles generated in situ from chromenones (see scheme). This methodology can be an attractive key step in the synthesis of biologically relevant tetrahydroxanthones.