Mild Ti^III- and Mn/Zr^IV-Catalytic Reductive Coupling of Allylic Halides:  Efficient Synthesis of Symmetric Terpenes^†

Two new efficient methods for the regioselective homocoupling of allylic halides using either catalytic Ti^III or the combination Mn/Zr^IV catalyst have been developed. The regio- and stereoselectivity of the process proved to increase significantly when the Mn/Zr^IV catalyst is used as the coupling reagent and when cyclic substituted allylic halides are used as substrates. The use of Lewis acids such as collidine hydrochloride allowed the quantity of catalyst to be lowered up to 0.05 equiv. We have proved the utility of these protocols with the synthesis of different terpenoids such as (+)-β-onoceradiene (1), (+)-β-onocerine (2), squalene (5), and advanced key-intermediates in the syntheses of (+)-cymbodiacetal (3) and dimeric ent-kauranoids as xindongnin M (4a).