Catalytic Electrocyclization

Researchers have developed the first Lewis acid-catalyzed 6-π electrocyclization reaction, expanding the repertoire of synthetic techniques for efficiently cyclizing organic compounds (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200803336). In electrocyclizations, a single bond is formed between the ends of a system of conjugated double bonds, yielding a cyclic compound with one fewer double bond. Catalytic electrocyclizations have generally proven elusive, but Robert G. Bergman and Dirk Trauner of the University of California, Berkeley, and coworkers have now developed one for 6-π-electron starting materials (hexatriene systems). Previous groups had found that electron-withdrawing groups at the 2-position of hexatrienes made it easier to electrocyclize the compounds. The researchers therefore synthesized hexatrienes with an electron-withdrawing carbonyl group at the 2-position. After trying a variety of Lewis acids, they found that dimethylaluminum chloride enhanced the carbonyl group's electron-withdrawing effects and accelerated electrocyclization of the derivatized compounds by a substantial factor—55 in the case of the reaction shown. According to the researchers, the findings set the stage for milder electrocyclizations and could lead to asymmetric versions of such reactions.