Reductive coupling of terpenic allylic halides catalyzed by CP2TiCl:: A short and efficient asymmetric synthesis of onocerane triterpenes

Titanocene chloride catalyzes the regioselective alpha,alpha '-homocoupling of terpenic allylic halides. This process has been employed in the short and effective synthesis of terpenoids such as beta-onoceradiene (1), beta-onocerin (2), and squalene (3). Evidence is presented for eta(1)-allyltitanium species being involved in the coupling.