Asymmetric Photoenolization/Diels–Alder Reaction of 2-Methylbenzaldehydes and 2-Alkylbenzophenones with Chromones

The asymmetric photoenolization/Diels–Alder reaction provides a straightforward and atom-economical route to complex chiral polycyclic rings. In comparison with well-developed transformations of 2-alkylbenzophenones, the enantioselective photoenolization/Diels–Alder reaction of 2-methylbenzaldehydes was challenging due to the shorter-lived and unstable photoenol intermediates. Herein, we present a highly enantioselective photoenolization/Diels–Alder reaction of 2-methylbenzaldehydes with chromones. Chiral N,N′-dioxide/ScIII and YbIII complexes were found to interact with both photoenol intermediates and chromones simultaneously, accelerating the Diels–Alder reaction in an efficient and stereoselective manner. Experimental studies and DFT calculations were carried out to understand the reaction mechanism and the origin of stereoselectivity. In addition, 2-alkylbenzophenones were suitable substrates. A series of chiral fused polycyclic rings with vicinal multisubstituted stereocenters were afforded in good yields and high diastereo- and enantioselectivities.