Metal-Assisted Terpenoid Synthesis. V. The Catalytic Trimerization of Isoprene to <i>trans</i>-β-Farnesene and Its Synthetic Applications for Terpenoids

Abstract

Abstract Isoprene oligomerization was effected with a catalyst system Ni(OR)(η3-C3H5)PPh(NEt2)2 (R=n-C11H23, n-C15H31) to give a linear trimer fraction above 70% yield. The major component of the trimer fraction is (E)-7,11-dimethyl-3-methylene-1,6,10-dodecatriene (trans-β-farnesene) (1). The minor constituents of the fraction are (E)-2,6-dimethyl-10-methylene-1,6,11-dodecatriene. As slightly higher boiling point fractions, two cyclic trimers 1,5,9-trimethyl-1,5,9-cyclododecatriene and 1,5,10-trimethyl-1,5,9-cyclododecatriene were detected. Factors that influence the reaction path leading to 1 were discussed. Sesqui- and diterpenoids, including farnesyl acetate, farnesylacetone and isophytol were synthesised starting from 1. A regioselective tail to tail coupling of 1 was achieved with a palladium catalyst to give the dimer C30H48 which, upon catalytic hydrogenation, gave perhydrosqualene in 85% yield.