Enantioselective Copper-Catalyzed Alkynylation of Benzopyranyl Oxocarbenium Ions

We
have developed highly enantioselective, copper-catalyzed alkynylations
of benzopyranyl acetals. By using a copper­(I) catalyst equipped with
a chiral bis­(oxazoline) ligand, high yields and enantioselectivities
are achieved in the alkynylation of widely available, racemic isochroman
and chromene acetals to deliver α-chiral oxygen heterocycles.
This method demonstrates that chiral organometallic nucleophiles can
be successfully used in enantioselective additions to oxocarbenium
ions.