Direct Enantio- and Diastereoselective Vinylogous Addition of Butenolides to Chromones Catalyzed by Zn-ProPhenol

We
report the first enantio- and diastereoselective 1,4-addition
of butenolides to chromones. Both α,β- and β,γ-butenolide
nucleophiles are compatible with the Zn-ProPhenol catalyst, and preactivation
as the siloxyfurans is not required. The scope of electrophiles includes
a variety of substituted chromones, as well as a thiochromone and
a quinolone, and the resulting vinylogous addition products are generated
in good yield (31 to 98%), diastereo- (3:1 to >30:1), and enantioselectivity
(90:10 to 99:1 er). These Michael adducts allow rapid access to several
natural product analogs, and can be easily transformed into a variety
of other interesting scaffolds as well.