palladium(0)-catalyzed cyclization of 2,7-octadienyl acetates and analogs

The palladium(0)-catalyzed cyclization of 2,7-octadienyl acetates and analogs proceeds in acetic acid solvent and generally consists of an intramolecular insertion of an alkene counterpart into a π-allylpalladium(II) unit to give a 2-vinylcyclopentylmethylpalladium as the key intermediate. The process is referred to as a 'pallada-ene' reaction after Oppolzer. Depending on the structure of the substrate, the key intermediate undergoes β-hydride elimination, carbonylation and/or further intramolecular Heck reaction. In the last two cases, the stereochemical outcome of the primary cyclization to form a five-membered ring is found to be predominantly trans in terms of an intraannular diastereoselection. When 2,7,8-nonatrienyl and 1,6,7-octatrienyl acetates are subjected to the palladium(0)-catalyzed cyclization-carbonylation, a novel intramolecular reaction of the π-allylpalladium intermediates with the allene counterpart gives rise to two modes of primary cyclization again to form five-membered carbocycles, which are controlled by the length of a carbon tether (either two or three) between them. The reactions offer effective means for constructing condensed carbocycles by further tandem reactions. Salient features of the stereochemical outcome of these reactions are discussed. Finally, the palladium(0)-catalyzed cyclization of 2,7-octadienyl carbonates containing an allylsilane moiety is described. This reaction relies on the nucleophilic nature of the allylsilane to proceed with a novel 6-endo mode of cyclization.