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  • Titanocene-Catalyzed Cascad...
    Justicia, José; Rosales, Antonio; Buñuel, Elena; Oller-López, Juan L.; Valdivia, Mónica; Haïdour, Ali; Oltra, J. Enrique; Barrero, Alejandro F.; Cárdenas, Diego J.; Cuerva, Juan M.

    Chemistry : a European journal, 04/2004, Volume: 10, Issue: 7
    Journal Article

    The titanocene‐catalyzed cascade cyclization of epoxypolyenes, which are easily prepared from commercially available polyprenoids, has proven to be a useful procedure for the synthesis of C10, C15, C20, and C30 terpenoids, including monocyclic, bicyclic, and tricyclic natural products. Both theoretical and experimental evidence suggests that this cyclization takes place in a nonconcerted fashion via discrete carbon‐centered radicals. Nevertheless, the termination step of the process seems to be subjected to a kind of water‐dependent control, which is unusual in free‐radical chemistry. The catalytic cycle is based on the use of the novel combination Me3SiCl/2,4,6‐collidine to regenerate the titanocene catalyst. In practice this procedure has several advantages: it takes place at room temperature under mild conditions compatible with different functional groups, uses inexpensive reagents, and its end step can easily be controlled to give exocyclic double bonds by simply excluding water from the medium. The total synthesis of a wide range of complex terpenoid skeletons, including monocyclic, bicyclic, and tricyclic natural products, is facilitated by employing a novel reaction cascade based on the combination of biomimetic strategies with titanocene catalysis. For example, the titanocene‐catalyzed radical cyclization of 2,3‐oxidosqualene mainly gave malabaricatrienes (see scheme). Mechanistically the reaction presumably occurs via discrete carbon‐centered radicals.