Rhodium(III)-Catalyzed Asymmetric Reductive Cyclization of Cyclohexadienone-Containing 1,6-Dienes via an Anti-Michael/Michael Cascade Process

Ring
strain plays an important role in metal-catalyzed cyclization
of 1,6-dienes. Herein, we report a rhodium­(III)-catalyzed asymmetric
reductive cyclization of cyclohexadienone-tethered α,β-unsaturated
compounds (1,6-dienes), including α,β-unsaturated ketones,
esters, amides, sulfone, and phosphonate. The reactions undergo an
unusual anti-Michael/Michael addition process, affording cis-bicyclic frameworks with good to high yields and good diastereo-
and enantioselectivities. Furthermore, several transformations of
the products and a one-pot preparation of bridged polycyclic structure
are also presented. Finally, DFT calculations show that the enantioselectivity
is determined by the initial olefin insertion step and that the ring
strain controls the overall regioselectivity and favors the formation
of 5,6-bicyclic products.