Catalyst- and Substituent-Controlled Switching of Chemoselectivity for the Enantioselective Synthesis of Fully Substituted Cyclobutane Derivatives via 2+2 Annulation of Vinylogous Ketone Enolates and Nitroalkene

The first regioselective, diastereoselective, and enantioselective organocatalyzed Michael-Michael cascade of vinylogous ketone enolates and nitroalkenes for the construction of fully substituted cyclobutanes is achieved by the deployment of the appropriate chiral squaramide catalyst and the pertinent substituent on the substrate. The domino reaction provided cyclobutanes with four contiguous stereocenters, including a quaternary center in good yields with diastereomeric ratio of >20:1 and with enantioselectivities of mostly up to 98% enantiomeric excess (ee). The structures and the absolute configurations of the adducts were confirmed by single-crystal X-ray crystallographic analyses of the appropriate products.