Double Strain-Release (3+3)-Cycloaddition: Lewis Acid Catalyzed Reaction of Bicyclobutane Carboxylates and Aziridines.

A (3+3)-cycloaddition to afford 2-azabiyclo[3.1.1]heptanes was realized by reacting highly strained aryl bicyclo[1.1.0]butane (BCBs) carboxylates with tosylated aziridines. Under Sc(OTf) catalysis the transformation proceeded smoothly under mild conditions to the corresponding bicyclic products bearing at the bridgeheads of the embedded four-membered ring aryl and ester moieties, respectively. This reaction is a rare example of a cycloaddition of two highly strained ring systems under Lewis acid catalysis. Mechanistic experiments provided insights into the reaction mechanism.