Bisdiene-enediene cycloisomerization

The intramolecular version of Hagihara s dimerization of 1,3-butadiene to 1,3,7-octatriene, that is, a bisdiene to enediene cycloisomerization, is also feasible. In the absence of any trapping reagent, bisdiene 15 (Scheme 5) undergoes Pd-catalyzed... 

The palladium complexes are the catalysts of choice for the cycloisomerization of the bisdienes, 9 which lead to the formation of either five- or six-membered enedienes with frarct-stereorechemistry for the vicinal substituents in the newly formed ring in high yields (Scheme 102).370... 

At first glance bisdiene substrates in which the two 1,3-diene subunits are substituted differently (e.g.. Scheme 6,17) appear to be improper candidates for Pd-catalyzed cycloisomerization, as they would probably lead to a mixture of isomers (e.g., 18). This is known for the linear dimerization of simple substituted 1,3-dienes (e.g., isoprene or piperylene). The attempted selective cross-coupling of different 1,3-dienes usually affords a complex mixture of isomeric products (vide infra). Nonetheless, the Pd-catalyzed cy-clization of bisdiene 17 does not form any of the isomeric 18 structures, but instead affords the single enediene 19 in near quantitative yield (95%). 

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