Four‐Membered Cycle Formation Challenge: GaCl₃‐Promoted Formal [2+2]‐Cycloaddition of Donor–Acceptor Cyclopropanes to Bicyclobutylidene

Abstract

The first selective process of formal [2+2]‐cycloaddition of donor–acceptor cyclopropanes (DAC) to multiple C–C bonds to form a four‐membered ring in the product has been developed. Bicyclobutylidene is used do demonstrate this reaction. The process is promoted by GaCl 3 and occurs through generation of 1,2‐zwitterionic intermediates. A carbocationic rearrangement occurs in the bicyclobutylidene moiety. It results in expansion of both cyclobutane rings into five‐membered ones and formation of a [2.3.3]propellane skeleton in the final product.