Lewis Acid Catalyzed Divergent Annulation of Bicyclo[1.1.0]butanes and Quinones for the Synthesis of Various Polycyclic Molecules

Bicyclo[1.1.0]butanes have been harnessed in a divergent Lewis acid catalytic platform to access various polycyclic architectures. A variety of bicyclo[2.1.1]hexanes, spiro-cyclobutene-benzofuran-2(3H)-ones and tetrahydrocyclobuta[b]benzofurans were selectively accessed by tuning the reaction conditions. In particular, when Sc(OTf)₃ is employed as a catalyst, pyrazole-substituted BCBs efficiently carry out formal (3+2) cycloaddition reactions with quinones in dichloromethane, producing highly substituted bicyclo[2.1.1]hexanes. Remarkably, pyrazoles were eliminated and spiro-cyclobutene-benzofuran-2(3H)-ones were produced when carbon tetrachloride was employed as the solvent. On the other hand, a tandem isomerization-(3+2) cycloaddition occurs when disubstituted BCB esters react with quinones in the presence of catalytic quantities of Bi(OTf)₃ and one equivalent of diphenyl phosphate, resulting in the selective synthesis of tetrahydrocyclobuta[b]benzofuran products. A preliminary catalytic asymmetric variant for the synthesis of bicyclo[2.1.1]hexanes has also been demonstrated.