Recent advances of Pd-π-allyl zwitterions in cycloaddition reactions

Palladium-catalyzed cycloaddition reactions via Pd-π-allyl zwitterions have been established as significant synthetic transformations to enable numerous carbon- or heterocycles compounds that are key constituents of various biologically active natural products and pharmaceuticals. In addition to the well-known Pd-π-allyl zwitterions, including palladium-trimethylenemethane and Pd-1,3/1,4-zwitterions, chemists have recently discovered new applications of several long ago reported but less-studied Pd-π-allyl zwitterions, which can straightforwardly and efficiently construct novel cyclic architectures. Meanwhile, some impressive newly designed zwitterions have been also developed. Those zwitterions are diverse and can serve as transient and highly reactive intermediates for the subsequent cyclization with various acceptors. In this review, we highlight recent advances in applications of these two types of zwitterions in the synthesis of complex polycyclics and medium-sized cyclic compounds.